Lightning Protection Manual 2.pdf

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This manual is intended to provide guidance on the principles and practices of lightning protection for a wide range of structures and systems. In general, it is not economically possible to provide total protection against all the possible damaging effects of lightning, but the recommendations in this manual will reduce the probability of damage to a calculated acceptable level. Will minimize any lightning damage that does occur. The decision to provide lightning protection may be taken without carrying out a risk assessment . Where doubt exists as to the need for lightning protection, further advice should be sought from a lightning protection consultant. First decision has to be whether lightning protection is needed or not. Realization that it is possible to provide effective protection against lightning began with Franklin and for over a hundred years international standards have been developed to provide guidance on the principles and practice of lightning protection. Until about ten years ago, risk assessment was used to determine if there was a need to provide lightning protection This selection takes into account both efficiency and cost of their provision. In the risk management approach, the lightning threats that create risk are identified, the frequencies of all risk events are estimated, the consequences of the risk events are determined, and if these are above a tolerable level of risk, protection measures are applied to reduce the risk (R) to tolerable level (Ra). This involves a choice from a range of protection level efficiencies for protection against direct (d) strikes to the structure and decisions about the extent of other measures for protecting low-voltage and electronic equipment against indirect (i) lightning stresses incident from nearby strikes. In short:Risk = Σ Rx = Σ Rd + Σ Ri ( Total risk is the sum of the direct & indirect risks) Rx = Nx Px δx Px = kx px R ≤ Ra

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where Nx is the frequency of dangerous events, Px is the probability of damage or injury, δx is the relative amount of damage or injury with any consequential effects, and kx is a reduction factor associated with the protection measure adopted and equals 1 in the absence of protection measures when Px = px . The lightning protection measures include an LPS for the structure and its occupants, protection against the lightning electromagnetic pulse (LEMP) caused by direct and nearby strikes, and transient protection (TP) of incoming services. The LPS for the structure comprises an air terminal network to intercept the lightning strike, a downconductor system to conduct the discharge current safely to earth and an earth termination network to dissipate the current into the earth. The LEMP protection includes a number of measures to protect sensitive electronic equipment such as the use of a mesh of downconductors to minimize the internal magnetic field, the selection of lightning protection zones, equipotential bonding and earthing, and the installation of SPDs. The TP for incoming services includes the use of isolation devices, the shielding of cables and the installation and coordination of SPDs.

Air terminal A vertical or horizontal conductor of an LPS, positioned so as to intercept a lightning discharge, which establishes a zone of protection.

Damage (δ) Mean relative amount of loss to a specified type of damage due to a lightning event, when damage factors are not taken into account.

Earth impedance (Z) The electrical impedance of an earthing electrode or structure to earth, derived from the earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complex function and depends on :  The resistance component (R) as measured by an earth tester.  The reactance component (X), depending on the circuit path to the general body of earth.  A modifying (reducing) time-related component depending on soil ionization caused by high current and fast rise times. 4

Earth potential rise (EPR) The increase in electrical potential of an earthing electrode, body of soil or earthed structure, with respect to distant earth, caused by the discharge of current to the general body of earth through the impedance of that earthing electrode or structure.

Finial It is a term referring to short vertical air terminals.

Lightning flash density (Ng) The number of lightning flashes occurring on or over unit area in unit time. This is commonly expressed as per square kilometre per year (km−2 year−1). The ground flash density is the number of ground flashes per unit area and per unit time, preferably expressed as a long-term (>10 years) average value.

Protection level (I to IV) Four levels of lightning protection. For each protection level, a set of maximum and minimum lightning current parameters is fixed, together with the corresponding rolling sphere radius.

CONCEPT OF RISK In this Standard, risk R is the probable annual loss due to lightning. Expressed as a number, it represents the probability of loss occurring over the period of a year. Thus a risk of 10-3 represents a chance of 1 in 1000 of a loss occurring during a year.

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DAMAGE DUE TO LIGHTNING Sources of damage The current in the lightning discharge is the potential source of damage. In this Section, the following sources of damage, relating to the proximity of the lightning strike, are taken into account :  S1—direct strike to the structure.  S2—strike to the ground near the structure.  S3—direct strike to a conductive electrical service line.  S4—strike to ground near a conductive electrical service line.  Conductive electrical service lines include electricity supply service lines and telecommunications service lines. The greater the height and collection area, the more lightning strikes will influence the structure. Tall trees and surrounding buildings may shield a structure from lightning strikes.Incoming conductive electrical service lines add to the lightning collection area as they can conduct lightning current into the building. Direct strikes to structure or incoming conductive electrical lines may cause mechanical damage, injury to people / animals and may cause fire and/or explosion.Indirect strikes as well as direct strikes may cause failure of electrical and electronic equipment due to overvoltages resulting from coupling of the lightning current.

Risk components S1 Lightning strikes directly to the structure These may generate: o Component Rh due to step and touch voltages outside the structure around downconductors causing shock to human beings o Component Rs due to mechanical and thermal effects of the lightning current or by dangerous sparking causing fire, explosion, mechanical and chemical effects inside the structure . o Component Rw due to overvoltages on internal installations and incoming services causing failure of electrical / electronic systems. 6

S2 Lightning strikes to ground near the structure These may generate component Rm due to overvoltages on internal installations and equipment (mainly induced by the magnetic field associated with the lightning current) causing failure of electrical and electronic systems .

S3 Lightning strikes directly to conductive electrical service lines. These may generate: o Component Rg due to touch overvoltages transmitted through incoming lines causing shock of living beings inside the structure . o Component Rc due to mechanical / thermal effects including dangerous sparking between external installation and metallic parts (generally at the point of entry of the incoming line into the structure) causing fire, mechanical / chemical effects on the structure and/or its content . o Component Re due to overvoltages, transmitted through incoming lines to structure, causing failure of electrical / electronic systems .

S4 Lightning strikes to ground near conductive electrical service line conductors. These may generate component Rl due to induced overvoltages, transmitted through incoming lines to the structure, causing failure of electrical and electronic systems.

PROCEDURE FOR RISK ASSESSMENT The procedure for the risk assessment requires: (a) Identification or defining of the structure / facility to be protected.. In most cases it is a stand-alone building. The structure may encompass a building and its associated outbuildings .Under certain conditions, a facility that is a part of a building may be considered as ‘the structure’ for risk assessment purposes. An example might be a communications installation at the top of an office building. This segregation of a part of a building is only valid under the following conditions: 7

(i) There is no risk of explosion in the remainder of the building. (ii) Suitable fire barriers exist around the structure being considered (fire rating of not less than 120 min). (iii) Overvoltage (SPD) protection is provided on all conductive electrical service lines at point-of-entry to the structure being considered. (b) Determination of all the relevant physical, environmental and service installation factors applicable to the structure. (c) Identification of all the types of loss relevant for the structure or facility.

Structures involved in the provision of public service utilities such as water, gas, electricity and telecommunications.

(d) For each type of loss relevant to the structure, determine the relevant damage factors δx and special hazard factors.

(e) For each type of loss relevant to the structure, determine the maximum tolerable risk,Ra.

(f) For each type of loss relevant to the structure, calculate the risk due to lightning by—

(i) identifying the components Rx that make up the risk (see Figure 2.1);

(ii) calculating the identified risk components Rx; and

(iii) calculating the total risk due to lightning, R.

(g) Compare the total risk R with the tolerable value Ra for each type of loss relevant to the structure.

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The selection of the most suitable protection measures shall be made by the consultant according to the contribution of each risk component to the total risk, and according to the technical and economic aspects of the different protection measures available. Technical considerations include addressing the highest risk components while economic considerations involve minimizing the total cost to achieve a suitable level of protection.

PERSONAL SAFETY The ‘30/30’ safety guideline. An approaching thunderstorm is treated as dangerous when the time interval between seeing a lightning flash and hearing the thunder is less than 30 s . A receding local thunderstorm is no longer a threat when more than 30 min have elapsed after the last thunder is heard. Don’t shelter under trees, particularly an isolated tree. If surrounded by trees, seek position outside the foliage , crouch, keeping feet together. If on a boat deck, keep a low profile and avoid contacts with or being close to masts, rails, or other metallic objects. Avoid unnecessary contacts with communication or navigation equipment. Do not enter the water, and in general avoid contact with it. Additional protection may be gained by anchoring under high objects :jetties and bridges, provided that direct contact is not made with them. Isolated buoys and pylons should be avoided. Do not take a bath / shower and do not wash hands or dishes. Do not use personal computers and other electronic / electrical equipment, and avoid contacts with sinks, refrigerators, metallic pipes and other large metallic objects in the house. Disconnect television sets, personal computers, and other electronic / electrical appliances from antennas, conductive telecommunication connections and electricity supply outlets to avoid damage to them. Switching off an appliance does not disconnect neutral and earth wiring. Isolation / unplugging is the safest.

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PROTECTION OF STRUCTURES Following are recommendations for installation practices and selection of equipment to prevent damage caused by lightning discharge. The recommendations apply generally to the protection of structures using LPSs comprising air terminals, downconductors, equipotential bonding and earth terminations. If, after completing the LPS risk assessment, it is evident that surge protection is required to protect internal systems within the building and services at entry , an investment shall not be ignored.

LPS DESIGN RULES It is obvious that air terminals must be installed providing conductive paths for the lightning current from the air terminals to the earth . The downconductors should assist in preventing side-flashes to nearby metal elements. This is best done by locating downconductors immediately below the air terminals used to protect the most vulnerable parts. Rules for air terminals (a) First, provide air terminals to protect the most vulnerable parts (points and corners);second, use the RSM method to check if the less vulnerable parts(edges) are protected and, if not, add more terminals to protect them; third, also check if the least vulnerable parts (such as flat surfaces) are protected and, if not, add more terminals. (b) Air terminals shall be placed close to the most vulnerable parts; if a strip conductor is used, it shall be directly on the part it is to protect; if a vertical rod is used, its length shall be not less than 50cm, and it shall preferably be mounted within 1 m or 1/2 its length . Maximum allowable length of a rod terminal is 6 m. (c) If the structure has horizontal or gently sloping upper parts that are essentially cylindrical or oval in shape, then the edges are the vulnerable parts and shall be protected by air terminals; if a strip conductor is used, it shall be run along the edge(s); if vertical rods are used, there shall be a minimum of two evenly spaced terminals.

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Rules for downconductors (a) Main conductors shall interconnect all air terminals and shall form one or more paths to earth via downconductors, such that the spacing between the downconductors does not exceed 20 m. (b) A downconductor shall be connected directly below an air terminal used to protect most vulnerable parts.If air terminal is on an exposed roof corner, its downconductor will also act as a continuation of air terminal to protect vertical edge below it, as required for tall structures.

Rules for earth terminations (a) Low earth resistance is desirable and all practical measures should be taken to achieve 10 Ω or less for the whole interconnected LPS earth termination network.There shall be equipotential bonding at ground level for all metallic surfaces. (b) There shall be one earth termination per downconductor.

ZONES OF PROTECTION FOR LIGHTING INTERCEPTION The protection efficiency against direct lightning strikes is achieved by installing an LPS in such a way that its air terminals establish zones of protection enclosing the whole structure. For the calculation of these zones of protection, the RSM,with a modification for large flat surfaces, is used. The RSM generally ensures that for lightning striking distances determined by the radius of the rolling sphere, the shortest distance between a lightning leader tip and any part of the structure is an air terminal.This method of analysis is suitable for conventional lightning terminals, which may be vertical rods, horizontal wires or strip conductors, railings, metal sheets, fascias and so on.

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In the ‘rolling sphere’ technique of determining zones of protection, a sphere of specified radius (a) is theoretically brought up to and rolled over the total structure. All sections of the structure that the sphere touches are considered to be exposed to direct lightning strokes and would need to be protected by air terminals. In general, air terminals need to be installed so that the sphere only touches their interception surfaces.

This is illustrated below in next page, showing that the top corner/edge of the structure requires protection by an air terminal but the sides and lower section do not.

The values of the rolling sphere radius (a) for the four protection levels (PL) I, II, III, IV are given in Table 4.2 together with the corresp.min. lightning current that will be intercepted.

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ZONE OF PROTECTION ON A STRUCTURE BY A ROLLING SPHERE

It is common to consider that PL III using a sphere of radius a, 45 m provides ‘standard’protection. PL I and II with a, 20 and 30 m provide higher degrees of protection and should be used if required by the risk management calculations. Conversely, PL IV with a, 60 m provides a lower degree of protection. For PL III, the protection ensures that, for striking distances of 45 m or more, the shortest distance to the structure is to an air terminal. From table in previous page, such striking distances correspond to peak currents of 10 kA , and interception effic of 91%, there being only of the order of 9% of strikes having a lower current. In the RSM, lightning is considered most likely to follow the path of shortest distance. This path will have the highest average electric field produced by the potential difference between the tip of the lightning leader and the structure .

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Buildings without structural steel frames The required conditions of protection for non-metallic buildings are generally met by placing metal air terminals on the uppermost parts of the building or its projections, with conductors connecting the air terminals to each other and to earth. By this means a relatively small amount of metal properly positioned and distributed can afford a satisfactory degree of protection and, if desired, the material may be placed so as to give minimum interference to the appearance of the building. A typical LPS is shown in

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Reinforced concrete buildings The following recommendations apply to the use of steel reinforcement in reinforced concrete buildings as part of the LPS. As far as possible, the steel reinforcement should be made electrically continuous in all concrete elements having a structural purpose, e.g. columns, beams and also in non-structural concrete elements, e.g. concrete wall panels, where the element, or a part of it, if dislodged, could endanger persons below. Where the steel reinforcement is used as the downconductor system, an effective electrical connection should be made from the air terminal network to the steel reinforcement at the top of the building. Such connections should be made, by means such as welding or clamping to a minimum of four vertical and/or horizontal bars, to ensure a multiplicity of conductive paths for the discharge of lightning current. NOTE: Steel reinforcement that is overlapped and tied by means of wire is not considered to provide an effective electrical connection for the purpose of air termination connection.

MATTER CONSIDERED WHEN PLANNING PROTECTION Design considerations (a) Metal used in the roof, walls, framework or reinforcement above. . NOTE: For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal immediately beneath the roof should be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor . (b) Available positions for downconductors providing the required number of low impedance paths from the air terminal network to the earth termination. (c) The resistivity of the soil to design a suitable earth termination. (d) Services entering the structure above ground. (e) Radio and television antennas and microwave communications antennae. (f) Flag masts, roof plant rooms:- lift rooms, boiler rooms, and water tanks . 15

(g) The provision of bonding connections to steel frame, reinforcement rods or internal metalwork to allow for the free passage of the lightning conductor. (h) The choice of metal most suitable for the conductor, e.g. aluminium conductors for structures where aluminium is employed externally. (i) Accessibility of test joints; protection by non-metallic casing from mechanical damage and hazard ; lowering of flagmasts / tall chimneys. (j) Preparation of a drawing detailing and showing positions of the main components to form the lightning protection system .

Route for conductors Conductors should be installed with a view offering the least impedance to the passage of discharge current between the air terminals and earth. The impedance to earth is approximately inversely proportional to the number of widely separated paths, so that from each air terminal there should be as many paths to earth as practicable. The number of paths is increased and the impedance decreased by connecting the conductors to form a cage enclosing the building.

Economy of installation Economy of installation can be effected by taking advantage of constructional features already installed as far as practicable.

Corrosion Corrosion resulting from contact of dissimilar metals can exist where a conductor is held by fixing devices on or against external metal surfaces of a building or structure. Bonding connections between air terminal and down conductor should be sealed against the ingress of moisture.

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The design of the earth termination network should assume that each earthing electrode will be bonded, directly to the following : the electrical installation earthing system , the building structural steelwork , any water, sewer and fire system supply pipes ( if metallic ) and pipelines for gaseous or liquid fuels( if metallic ). There is a hazard arising from the bonding of other service earthing electrodes. The electricity supply service has many loads connected to it that generate a direct current component; this direct current is an electrolytic hazard to other earthing systems to which the electricity supply service earth is bonded. The amount of direct current is limited but it is still sufficient to place at risk some types of earthing electrodes. In particular, steel rods clad with copper or stainless steel suffer premature failure when a small amount of direct current such as this perforates the cladding, initiating a process of selfdestruction of the rod core. It will be clear that the selection of any common metal or alloy for the earth termination network places either itself or other systems or services at some risk from galvanic corrosion.For lower cost installations ,the use of one of the common metals or alloys may be satisfactory. The extent to which the material combination ‘can be damaging’ is related to soil moisture. Soil resistivity typically below 30 Ω. Expert advice on the selection of an appropriate earth termination network should normally be where such soil conditions exist.

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FORM AND SIZE OF DOWN CONDUCTORS Factors influencing selection The form and size of the down conductors of the LPS should be selected having regard to their minimum cross-sectional area required of a main current carrying component of a LPS is 35 mm2. The dimensions of typical conductors are given below in table.

Electrical and thermal considerations Air terminals and downconductors that may carry the full lightning current, should have an adequate cross-sectional area and conductivity such that they are able to carry without attaining temperatures that may give rise to risk of fire. Copper conductors having a cross-sectional area of not less than 35 mm2 will normally be necessary for this purpose. 19

Conductors, which because of their arrangement in the LPS, will carry only a proportion of the lightning current, may have a cross-sectional area that is proportionately reduced but should be not less than 1/5 th of the cross-sectional area to carry the full lightning current, or 6 mm2, whichever is the greater.

JOINTS Effectiveness of joints The LPS should have as few joints as possible. Joints and bonds should be mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped, riveted or welded. All mechanical connections should be inspected on a regular basis in accordance to ensure integrity of the connection over time. Particular attention should be given to joints of dissimilar metals. Fasteners. Conductors should be securely attached to the building or other object upon which they are placed. Fasteners should be substantial in construction and not subject to breakage, and should be, together with the nails, screws, or other means by which they are fixed, of the same material as the conductors, or of such nature that there will be no serious tendency towards galvanic corrosion in the presence of moisture because of contact between the different parts. Downconductors should be fastened at spacings not exceeding 1.0 m on horizontal runs and not exceeding 1.5 m on vertical runs. Plastics may be used for the fixing of conductors provided such materials are suitable long-term exposure to the outdoor environment against the harmful effects of ultraviolet radiation .

AIR TERMINALS May consist of a vertical rod , a single horizontal conductor as on the ridge of a small dwelling, or a network of horizontal conductors with vertical rods for the protection of roofs of large horizontal dimensions . Protection may also be provided with a horizontal overhead wire supported, if necessary, independently of the building to be protected or by a vertical air 20

terminal network .The upper portions of the downconductors on tall buildings should be regarded as a continuation of the air terminal network and should be positioned so as to intercept side strikes to the building. Preference should be given to placing downconductors as near as possible to the exposed outer vertical corners of a building. All metallic projections, on or above the main surface of the roof, should be bonded to, and form part of, the air terminal network. In the case of telecommunications antennas, which have to be insulated from earth, a spark gap connection to earth or an SPD should be provided. Where roof construction consists of electrically continuous metallic materials, such metallic roofs may form part of an LPS, obviating the need for air terminals. If portions of a structure vary considerably in height, any necessary vertical air terminal or air terminal network of the lower portions should, in addition to their own downconductors,be bonded to the downconductors of the taller portions Air terminals may be of any form provided the section used and the means of attaching it to the building structure have adequate mechanical strength to withstand the expected winds.

Protection of roofs

The parts of roofs most likely to be struck by lightning are corners and edges of flat roofs, chimneys, and the ridges and eaves of sloping roofs. Preference should be given to positioning the air terminals so as to protect these highly exposed parts.The height of a vertical air terminal should be such that the tip will be not less than 50 cm above the object to be protected. On large flat and gently sloping roof areas , a number of vertical rods of greater than 50 cm in height may be needed to establish a zone of protection over the whole roof . Horizontal conductors such as strap on metallic objects such as flagpoles, metal railings, steel plant surrounds and roof access ladders may be used as air terminals to protect a planar roof surface. When positioned at a height of more than 50 cm above the area to be protected , the conductors or objects will be at a suitable height to achieve the selected interception efficiency.

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All metallic objects at roof level such as sheeting, plant, plant screens, tanks, gutters,walkways, ladders, antennas, masts, poles, vents, chimneys, conduits, piping, cable trays, ..etc should be bonded to the air termination network.

DOWNCONDUCTORS Structures Downconductors should be installed at each external corner of the building and additional downconductors , as necessary, at spacings not exceeding 20 m. Any extended metal running vertically through the structure should be bonded to the lightning downconductor,. A structure on bare rock, to be with at least 2 downconductors equally spaced.

EARTH TERMINATIONS The design of earth terminations should be such that lightning currents are discharged into the earth in a manner that will minimize step and touch potentials and also side-flashing . This can be achieved by ensuring that the potential with respect to earth at each earth termination is limited by enough low resistance to earth, so that the discharged current flows in as close to uniform manner as possible in all directions away from the structure. Ionization of the soil near an earthing electrode carrying lightning current tends to reduce the potential of the earthing electrode relative to remote earth to a lower value than the potential that would be calculated using the earth resistance measured at low currents.

Recommended values for earth resistance. In general, the whole of an interconnected LPS should have an earthing resistance not exceeding 10 Ω before any bonding is effected to services that are not part of the LPS. Where the installation has two or more air terminal networks not directly interconnected,such as a twin-tower building, then for the purpose of determining the required earthing resistance, it should be considered as consisting of separate LPSs. 22

Where buildings are used for telecommunications services or sensitive electronic equipment, an earthing resistance not exceeding 5 Ω should be required.A reduction of earthing resistance can be achieved by extending or adding to the earth termination network or by interconnecting the individual earth terminations of downconductors. Notwithstanding the above recommendations, earthing electrodes complying with either of the following, need not comply with the 10 Ω criterion: (a) For a substantial structure effectively encircled by a buried earthing electrode, an earthing resistance not exceeding 30 Ω should be satisfactory. A buried earthing electrode covering at least three sides of the structure may be regarded as effectively encircling the structure. (b) For any system incorporating two or more downconductors, it should not be necessary to install a total length of more than 50 m of widely separated horizontal or vertical earthing electrodes per downconductor, regardless of the earthing resistance.

Common earthing electrode Where conditions permit potential equalization techniques to be used, a common earthing electrode may be installed to serve the LPS and other appropriate services. The earthing electrode should comply with the recommendations in standards and with any regulations that may govern the appropriate services . Where isolation is required, a common earthing electrode should not be used, but the separate earthing electrodes should be bonded via an SPD to minimize potential differences between the LPS earth termination network and other earthing systems in the event of a lightning strike. Communications earths Where a communications earth, such as a Telecommunications Functional Earthing Electrode (TFEE), is required to be isolated from other earths, because of noise or direct current conduction considerations, this earth should be bonded through a normally nonconducting protector or SPD.

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Joints Joints between earthing conductors and earthing electrodes should be of adequate strength and current-carrying capacity, and be arranged so as to ensure that there will be no failure of the connection under conditions of use or exposure that can reasonably be expected.

GENERAL CONSIDERATIONS FOR PROTECTION

Each building will require specific consideration of the protective measures that should be applied. Particular attention should be given to possible entry and exit points for lightning current, which may include one or more of the following: (a) Rooftop or external structures : TV antennas, dishes, metals, gutters , pipes , metal windows, frames, and ventilation outlets not protected by the LPS for the building structure . These features will invariably be possible entry points for a lightning discharge. (b) The electricity supply service entry .This will normally be an entry point for lightning if the service is aerial or overhead. (c) The telecommunications services entry .This may be an entry point if the service is overhead using a dropwire or aerial cable. The service is more commonly underground and in such cases could be either an entry or exit point. (d) Gas supply systems These are usually exit points for lightning . (e) Metallic water supply . This is usually exit points for lightning. (f) The LPS for the building (if provided) By design these systems provide both an entry and exit point for a lightning discharge but, because of bonding, will present an EPR condition to other services.

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An illustration of possible entry and exit points for a lightning discharge.

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Installation of equipotential bonding Consequently, bonding conductors should not be grouped with other cables that are sensitive to induction unless the other cables are also bonded to the LPS. If the bonding conductor is long (some tens of metres) it shall be considered as an impulse transmission line, in which mode the protection afforded by the bonding will be limited. (a) Rooftop antennae and dishes. The bonding conductor should be attached to the most substantial part of the structural metal supporting the equipment consistent with it fulfilling the requirements of an air terminal for the LPS of a building. The bonding conductor to the antenna or communications hardware should be insulated , if run within the building, but may be uninsulated if run externally. The cross-sectional area of the bonding conductor should be not less than 16 mm2 if made of copper. (b) The electricity supply service entry .There are two distinct considerations that apply.Firstly, the electrical installation earth should be bonded to the LPS earth termination network with a copper conductor of not less than 6 mm2 cross-sectional area. Secondly, SPDs should be installed for each active conductor of the electricity supply service. Where an SPD is mounted on, or in, the building, its earthing system should be bonded to the LPS by a conductor having a cross-sectional area of not less than that utilized for its own earthing conductor. Where SPD equipment is separated from the building (e.g. mounted on a customer’s electricity supply service pole), the SPD earth should not be used as the earthing termination for the building LPS, however, the LPS earth termination network and the SPD earth may be bonded together, if desired. (c) The telecommunications service entry . The service should be regarded as a potential entry point for lightning and an SPD should be fitted, subject to the requirements of the telecommunications regulatory authority. The telecoms service earthing system shall be bonded to the LPS earth termination network. The bonding conductor should have a cross-sectional area of not less than 6 mm2 if made of copper. (d) Metallic water supply should be bonded to the LPS and connected to the electricity supply service earth. Metallic piping systems associated with fire sprinklers, water, hot water or flammable liquid, that are unavoidably in contact with the exposed conductive parts of wiring enclosures, cable components or other electrical equipment shall be connected to such equipment by means of an equipotential bonding conductor. 26

(e) Building earthing systems frequently have several earthing systems that may be installed independently at different times : Electricity supply service earthing system, telecoms earthing system some times more than one, the LPS earth termination network and other special purpose earthing systems. It is generally desirable to bond all such earthing systems but there may be specific reasons for not doing so. (f) The LPS earth termination network .Where an LPS is in place all of the services earth should be bonded to the LPS earth termination network.

PROTECTION OF EQUIPMENT Lightning induces overvoltages in electrical lines, telecom lines, signalling, data, and coaxial lines. Equipment overvoltages may be experienced in the following ways: (a) By direct conduction of lightning current on the conductors feeding into the building. Ex. would be lightning striking overhead lines. This mechanism is of lower probability, but involves higher surge currents. (b) Indirectly, (through magnetic induction, or electrostatic coupling) where lightning strikes nearby, and surges are induced on conductors feeding building. This mechanism has higher probability, but results in lower surge currents. (c) Lightning striking the LPS or other nearby objects, resulting in an EPR. This can cause potential differences in earthing systems, causing flashover and equipmentdamage. (d) Temporary overvoltages at mains a.c. system frequency that can occur for a number of reasons. Strategies to deal with involve equipotential bonding of the earthing systems, and the provision of SPDs.

Equipotential bonding for equipment protection Voltage differences that are insufficient to cause injury to persons can be extremely damaging to equipment. It is possible to have voltage drops in bonding conductors that are carrying lightning surges in excess of 1kV per metre. It is important that bonding conductors be kept short to reduce this voltage difference, and to achieve this, all services should enter in close 27

proximity. For protection of equipment, this concept can be extended to particular areas within the building. For example, consider the case of a multistorey building with incoming underground services, and a telephone system installed on an upper floor. On the lower level, the required equipotential bonding will be performed, and primary surge protection to both power and telecom lines can be fitted at that location, and will connect to the same equipotential earth system. On this lower level, it may well be that the bonding conductor lengths are not ideal, depending on where the services enter, and thus the protection provided is compromised.However, on upper floor where the telephone system is installed, the same concept can be repeated, but at this point more control is possible over wiring and equipment locations. That is, at this location all the services should enter the room at the same point, and secondary surge protection to both power and telecommunications lines can be provided at this location with short, direct connections to the common earthing point.

Surge protective devices (SPDs) An SPD is a device intended to mitigate surge overvoltages . That is, it behaves in a benign state until a voltage or current exceeds a designed value, then the SPD acts to reduce the voltage or current, in order to prevent damage to equipment in protection.SPDs often have features, such as mechanisms to indicate their operational status,normally indicated in colour.

VARISTORS. These are made from metal oxide and are known as metal oxide varistors (MOVs) or Voltage Dependent resistors (VDRs). The resistance of varistors drops significantly when the voltage exceeds a limit thus clamping the voltage near the limit. It normally respond in nanoseconds.

SOLID STATE DEVICES. . Consists of special zener diodes that exhibit voltage limiting characteristics and are optimized to handle surge currents. The breakdown voltages of such devices are typically in the range 5 V to 200 V. They have current ratings up to several hundred amperes and response times of the order of 10 picoseconds. Another form consists of thyristors, that switch when their operation voltage is exceeded, 28

and act to clamp overvoltages. Their reduced voltage during conduction means they can handle higher surge currents, compared with the zener diode type.

SPD configuration SPDs are configured as being either shunt or series protectors : (a) Shunt protector . Known as a one-port SPD, connected in shunt with the circuit to be protected, as shown in Figure 5.3(a). A one-port device may have separate input and output terminals without a specific series impedance between these terminals, as shown in Figure 5.3(b). The latter arrangement is sometimes known as a ‘series connected, shunt protector’. A shunt protector with just two terminals (i.e. does not have separate input and output terminals) has no limitation with regard to the load current of the circuit to which it is applied. However, its ability to clamp overvoltages is reduced by the additional voltage drop that occurs across its connecting leads. For this reason, some shunt protectors are manufactured with separate input and output terminals. This arrangement substantially reduces the connecting lead voltage drop problem, but does mean that the full load current passes through the device, which needs to be designed to handle this current. FIGURE 5.3 EXAMPLES OF SHUNT (ONE-PORT) PROTECTORS

(b) Series protector known as a two-port SPD, is an SPD with two sets of

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terminals, input and output. A specific series impedance is inserted between these terminals. Typical examples are shown in Figure 5.4.

The series current limitation of the series protector will typically be determined by the series impedance. Sometimes a protector is referred to as an n-stage protector, and although this term is not applied, the ‘n’ should refer to the number of shunt overvoltage stages that are separated by series.

Multi-service surge protection device (MSPD) An MSPD is a combination protector that combines both power protection and signalling/telecommunications protection in one . Is an effective way of protecting IT and sensitive equipment that has more than one connected service. By including all the protection in the one device, the distance between the SPD earth connections is very short, which greatly reduces the potential difference between these services under incident surge conditions. A general diagram of an MSPD is shown in Figure 5.5.

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Parameters of an SPD. The primary parameters relate to how well an SPD limits overvoltages, how much surge current it handles, and what voltage system is it designed for. The following parameters are listed together with the IEC symbols for these parameters. Maximum continuous operating voltage (Uc)—This is the max voltage that can be continuously applied to protector. For the power system, this should be at least 275 V for SPDs connected between the phase and neutral conductors. Rated load current (IL)—Max continuous rated r.m.s. or d.c. current that can be supplied to a load connected to the protected output of an SPD. Maximum surge current (Imax)—This is the peak value of the 8/20 μs waveshape current impulse that the protector can handle. The protector only has to be able to withstand this surge current once. Known as the single shot rating. Nominal surge current (In)—This is the peak value of the 8/20 μs waveshape current impulse that protector can handle many times. A protector must be able to withstand at least 15 impulses at In. Voltage protection level (Up)—This is the peak voltage that the protector protects to (limits the voltage to). It is sometimes referred to as the let-through, , voltage. Some SPDs include shunt capacitance that provides a filtering effect at EMI/RFI frequencies, but provides little benefit at typical lightning surge frequencies. Similarly, surge current levels may cause inductor saturation in standard EMI/RFI filters, which will degrade the filter action.

Temporary overvoltage (TOV) It is when the power voltage on an a.c power system rises above its normal value. This can be caused by many factors including poor regulation, faults on the LV or HV distribution system (including phase shorts to neutral or earth, and loss of neutral conductor), capacitor switching, HV contact on LV circuits. These events typically last from 0.2 s up to 5 s. And considered a surge.

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Application of SPDs

The following aspects should be considered : (a) Modes of protection . With any signal or electrical transmission system employing two lines and a separate protection earth, two types of transients can occur. The first type appears as a difference between the two lines, independent of their potential differences to earth; this is known as a differential mode transient , where the transient voltage source is superimposed onto the normal signal carried by the lines. The second type appears as a transient between each line and the earth, and is known as a common mode transient , where the transient voltage sources are superimposed onto the normal potentials between the lines and earth.

The use of two non-earthed lines is common. Telephone lines use two wires over which the signal is transmitted, with neither line tied to earth. RS-422 signalling for computer data uses two lines for each data channel, which is known as balanced-pair signalling.When protective equipment is connected to such lines, both differential and common mode transients must be suppressed. Placing a protective device across the two signalling lines alone is not sufficient. The high potentials to earth created by common mode transients can cause insulation breakdown and arc-over, and damage electronic components. Protection against transients can be achieved by the provision of voltage clamping or diversion devices between the lines, and between the lines and earth. These will shunt common mode transients to earth before they are allowed to reach breakdown potentials.

Equipment to be protected is typically more robust to transients from line to earth . Experience has shown that mains equipment is more easily damaged from line to neutral transients, and although protection could be provided in all modes , good protection is usually obtained by providing L–N and N–E protection modes only.

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SPD location. There are many possible locations where SPDs could be installed within a facility. The aim is to provide effective protection to the equipment nominated for protection, and to do so economically. The most effective is to provide SPDs at the building service point-of-entry (known as primary protection), and then, if necessary, to provide additional surge protection within the building closer to the equipment to be protected (known as secondary protection). Primary protection is important because the main function of such an SPD is to keep most of the surge current from entering the building, by diverting it directly to earth. When considering mains power circuit protection, the concept of location categories can be used . Building point-of-entry is where primary protection would be installed. Further within the building are for secondary protection, to be fitted as required, especially in the following situations: (i) Where sensitive equipment is present; cctvs , Pcs, telecoms equip,etc. (ii) Where the distance between the SPD located at the entrance and the equipment to be protected is too long (iii) Where there is internal equipment generating switching surges, or other internal interference sources, inside the building.

Surge ratings A lightning surge, travelling within a building is attenuated by the SPDs. Thus higher levels of surge current are likely to be encountered at the building pointof entry, compared to the distant end of a branch circuit; The lightning surge current to be handled by a point-of-entry SPD has traditionally been considered to come into the building via the service conductors. If lightning strikes the building LPS, or even the ground or an object nearby, a local EPR occurs. The incoming service conductors are typically referenced to a distant earth (such as the neutral conductor grounded at the secondary transformer some distance down the street, with the phase conductor also being referenced to that distant earth by virtue of the transformer winding).

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The effect of the local EPR is that a proportion of the lightning current flows out through the point-of-entry SPDs on its way to reaching the distant earth. The surge current in the SPDs in this case is very large, being a significant proportion of the lightning current itself. While Table below gives a surge rating for SPDs in this case (Category C3) using the 8/20 μs waveshape, it should be acknowledged that the IEC standards make reference to a 10/350 μs waveshape for use in this case, and the symbol given to the current rating using this waveshape is Iimp. For example, an SPD withstanding a 100 kA 8/20 μs impulse might be expected to withstand a 10 kA 10/350 μs impulse. Given this discussion, for mains power system SPDs, the following surge current ratings are recommended, where the surge rating is the Imax, or single shot, 8/20 μs value, and apply for each SPD from the phase to neutral conductors.

Coordination Often the approach taken is to have the primary SPD handle the bulk surge current. A secondary protector that will not need to handle such a high value of surge current,can be installed close to the equipment and can be chosen to have an acceptable Up value. However, to achieve this result, careful coordination between the two devices needs to be undertaken. This is quite a complex matter, and a total examination of the issues is beyond the scope of this Standard. However, simple rule is to ensure there is at least 10 to 20 m of electrical cabling between two . If this cannot be achieved, purpose built inductors are available that can be placed in the circuit to achieve this effective separation. 34

Wiring considerations. It is essential to provide a fuse or circuit breaker ahead of the SPD to provide for the safe disconnection of a failed SPD. SPDs should be installed after the main switch but prior to any RCD. If the terminals of the SPD are rated for the required load current level, a configuration as shown in Figure below (B) is preferred.

This wiring method is known as Kelvin connection . Failing this, twisting the connecting conductors together as shown in above ( C ) can have a substantial impact on reducing the voltage drop.

Failure modes of SPDs Typically an SPD initially fails to a low impedance state, and the resulting current that then flows into the SPD either causes the SPDs internal fusing to operate, or causes external fusing/protection to operate. Consideration needs to be given to the most desirable location for the external fuse, and whether it is desirable for power to be disconnected from the load when the SPD fails. If the external fuse/protection is in series with the load current, power will be disconnected from the load when the SPD fails. In some applications this is considered beneficial, since the SPD is no longer protecting the load. However, locating the fuse in the non-load carrying SPD connection wiring, means that a ruptured fuse will isolate the SPD, but allow power to continue to the load. SPDs are fitted with a visual indication to show their operational status, and may additionally be fitted with contacts to allow for remote monitoring.

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Withstand voltage of equipment. The equipment to be protected may have a level of resistability to surges designed into it. In particular, a resistance level to electromagnetic interference (EMI) disturbances is mandated. Ideally, the Up of the externally provided SPDs should be lower than the Up of any equipment internal surge protection components, otherwise the internal components may be damaged instead of being protected by the external SPDs. Lightning surges need not physically damage equipment for it to experience problems. Erratic operation, that may or may not require manual resetting, can occur and lead to data loss at surge levels which are lower than those required to cause hardware failure. Rapid changes in the voltage supply, even those for which the amplitude does not exceed the normal mains power peak, can cause problems. This dv/dt problem can be reduced by utilizing SPDs with filters.

Magnetic shielding and line routing. Magnetic shielding reduces electromagnetic fields, and can also provide a reduction in the emissions from electrical noise. The complexity of shielding can range from the use of metallic conduits, to simple metal enclosures or cabinets, and up to whole rooms being comprised of shielding materials. Such shields need to be earthed to be effective, and any SPDs provided to conductors entering the shielded area need to be effectively connected to the shield. The amount of surge energy directly induced into building conductors from nearby lightning strikes depends on the closeness of the current source, and the loop area formed by the conductors. Conductors of the same service should be run together, along with an earthing conductor, or otherwise run in close proximity to other earthed components, such as earthed cable trays. To reduce the inductive loop area, such cables should be neatly tied together and not allowed to splay out over the whole tray width.

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Practical installation examples. Determine protection needs. Simple methodology for determining where SPDs are required. Method asks what requires protection , and then drawing an imaginary box around it. Each location where electrical lines cross the box is a potential location for SPDs, and they should be provided where the particular electrical line is prone to having surges on it. Then the earths of the SPDs are connected together to the earths of the equipment, and taken to earth. Typically it will be necessary to consider the following points for SPDs:

(a) At point-of-entry of external services e.g. electricity supply and telecoms. (b) At the connection of the external services to the equipment. (b) At the connection of long internal cabling to the equipment e.g. communications and LAN. The two latter mentioned can damage equipment:

(i) An excessive voltage/current enters the building via a service due to either a lack of protection or incorrectly installed protection. Both the mains and telecomms point of entry SPD’s earths are bonded to the main earth bar by conductors of 1.5 m or less where SPDs are installed. (ii) An excessive voltage/current is induced into the internal wiring loop. The procedure for protecting equipment is as follows: (a) Install secondary protection at equipment when risk of damage due to induction into the external service conductors and the building conductors exceeds an acceptable level. (b) Install point-of-entry protection when the risk of damage due to a direct strike to the structure or service conductors exceeds an acceptable level. A prime role of the point-of-entry protection, apart from preventing dangerous discharges is to protect the secondary protection from damage. 37

Protection examples. Example 1. A central PLC and remote sensor . It is determined that a particular industrial process must be protected. It consists of a central controller (PLC), and various sensors and controls. For simplicity, the example will show two sensors, one at a considerable distance from the PLC, and another close to the PLC. The arrangement is shown in Fig. below.

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EXAMPLE OF CENTRAL PLC AND REMOTE SENSOR. The PLC has a console and modem to allow communication, and consequently has electrical and telephone services. The signal line is over 1 km in length. Since the operator console is close to the PLC there is no reasonable subjecting of surges impinging on that connection. Likewise, any sensors or controls close to the PLC would not normally require protection. SPDs should be installed close to equipment they are to protect, and must have their earths connected together, and connected to the PLC earth, and taken to earth. The PLC and modem are mounted next to each on a rack. Just below , them, the required SPDs are mounted in a row, with an earthing busbar immediately below them, connected at each end to the rack frame. Each SPD will have a short direct connection to the busbar not exceeding 10 cm, and the PLC will also have a direct connection to the busbar. In this manner, effective equipotential bonding will be achieved . The SPD on the signal line at the PLC end does not provide protection for the remote sensor. The only line here is the signal line, and so an SPD is provided on it. This SPD is earthed with a direct short connection to the sensor earth (and associated pipe ), and taken to a local earth. There is no point in trying to connect this very distant earth to the PLC earth via a bonding conductor. In addition to secondary SPDs installed closed to the equipment, considerations as whether it is run in metallic conduit or not, it may be prudent to include primary point-of-entry protection on the PLC signal lines where they enter the building, in addition to the secondary protection shown at the PLC. Example 2.

A video surveillance system . This example is similar in eg 1. The DVR will have surge protection applied to the long camera feeds and to the a.c. power line. The video feeds that are selected for protection will need all their signal lines protected, and this may include the video feed, the power supply leads, and any pan and tilt control signals. The earths of these SPDs will connect to each other and to the equipment earth. Video cameras that are located nearby in the same building may not need surge protection fitted. Those video cameras that are located a long distance from the central monitoring and recording equipment will need the same protection fitted at the remote end as was fitted at the central end. 39

The earths of these SPDs must be connected to the video camera earth, and then be taken to a local earth. Although all earth connections should be as short as possible, it is particularly important to keep the length of the SPD earths to video camera earth short. Example 3 A multistorey building with PABX on upper floor . his is a multistorey building with services entering on grnd floor, and PABX on an upper flr. Regardless of whether the point-of-entry location of the telecoms and power services are co-located , there can be induction into the internal wiring between the grnd floor and PABX. Therefore the electricity supply, the exchange line and the outdoor extensions will require protection at the PABX. The local handsets do not need protection . To ensure adequate protection of the PABX it is necessary to have a DB and a telecoms distributor (IDF) co-located with the PABX. SPDs are installed in the DB and the IDF. Where direct strike protection is required SPDs need to be installed at the point-of-entry .

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Example 4

A domestic computer and ADSL modem .

The simplest way to provide SPDs with short bonding conductors to a common earth point is to use an MSPD so this has been used, along with a power board to provide additional protected outlets. A Fax machine at the same physical location has also been protected using a second MSPD.

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Example 5 A rooftop cellular base station. In this example a cellular base station is located on the roof of a multistorey building.Fig belowshows an effective means of providing lightning, earthing and surge protection.

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The a.c. power supply is fed to the base station dist.board from a main switch board located in the basement of the building. An SPD (diverter )would be installed at the MSB. A second SPD (surge filter) is installed on the a.c. power feed to the rooftop distribution board. All other metallic services, for example antenna feeders should be bonded and fitted with SPDs. All equipment should be referenced to a common earth bonding bar in the cabin and this in turn bonded to the tower and building LPS. There will be a connection to the main a.c. earth via the earth conductor in the power cable to the roof. Good practice suggests that all metallic services should enter the cabin on the same side and the common earth to the tower and LPS should exit the same side. It should be noted that in the event of a lightning strike to the tower, conducted currents will flow through the power earth conductor, and care should be taken to segregate cables if possible.Antennas installed on the tower that contain electronics may require additional protection measures.

PROTECTION OF MISCELLANEOUS S T R U C T U R E S A N D P R O P E R T Y. STRUCTURES WITH ANTENNAS Indoor antenna system. This may be indoor radio and television receiving antennas . Outdoor antennas on protected structures This may be outdoor radio and television receiving antennas without further precautions, provided that every part of the antenna system, including any supporting metalwork, is within the zone of protection of the LPS. Where these conditions cannot be fulfilled, precautions should be taken to ensure that the lightning current can be discharged to earth without damage to the structure or injury to its occupants with an antenna system fitted : -

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(a) Directly onto a protected structure. This can be done by connecting the antenna bracket structure to the LPS at the nearest point accessible below the antenna installation; or (b) On a metallic support structure that projects above the LPS. This can be accomplished by connecting the antenna support structure to the LPS at the nearest point accessible below the antenna installation. Consideration should be given to the fitting of SPDs in the conductors connected to the antenna system. Antennas on unprotected structures.

Before installing an antenna on an unprotected structure, the need to provide an LPS should be assessed . The earthing electrode of the LPS may also be used for the purpose of earthing a radio system.

STRUCTURES NEAR TREES When a tree is struck by lightning, a voltage drop develops along its branches, trunk and roots. The side-flash clearances between the tree and adjacent structures are set by taking 100 kV/m as the flashover strength of unseasoned wet timber and 500 kV/m as the breakdown strength of air. If tree is less than the height of the structure , its presence can be disregarded. If taller , the following clearances between the structure and the tree may be considered as safe: (a) For normal structures; one-third of the height of the structure. (b) For structures with explosive or highly-flammable contents; the height of the structure.If the clearances cannot be met then the structure should be fitted with lightning protection in such a manner that the side-flash always terminates on the protection system. If the tree is fitted with LPS, no further protection is necessary for the structure , provided that conditions for zone of protection and separation are fulfilled. PROTECTION OF TREES. Protection of trees needs to be considered only where the tree is desired for historical or other reasons. 44

A main downconductor should be run from the topmost part of tree to the earth termination . Large upper branches should be provided with branch conductors bonded to the main conductor. Allowance should be made for swaying in the wind and the natural growth of the tree. The earth termination should consist of two rods driven into the ground on opposite sides of, and close to, the trunk of the tree. A strip conductor should be buried to a depth of 3mtrs to encircle the roots of the tree at a minimum distance of 8 m radius from the centre of the tree or at a distance equal to 1 m beyond the spread of the foliage, whichever is the greater. This conductor should also be bonded to the rods by two radial conductors. Where two or more trees are so close together that their encircling earth conductors would intersect, one conductor adequately connected to the earth rods should be buried so as to surround the roots of all the trees. The recommended is to protect the roots of the tree and to reduce the potential gradient, in the event of a lightning discharge to the tree.

CHIMNEYS, METAL GUY-WIRES OR WIRE ROPES Metal guy-wires or wire ropes attached to steel anchor rods set in earth may be considered as sufficiently earthed. Other guy-wires or wire ropes should be earthed . Metal chimneys or flues need no protection against lightning other than be properly earthed.

Metal ladders . Where chimneys have a metal ladder , they should be connected to the LPS at their upper and lower ends.

Chimneys Chimneys consisting partly reinforced concrete should be electrically connected together and electrically connected to the downconductors at the top and bottom of the concrete.

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PROTECTION OF BOATS A boat is at risk both because of its method of construction (except for metal-hulled boats) and because it forms a marked protrusion above the surrounding water surfaces. Overseas statistics show that in excess of 10 percent of fatalities occurring on cruising sailing boats are due to lightning. While the principles to be applied will not differ from those for land-based structures, the methods employed will depend on the form of construction and the type of boat to be protected.

Elements of the protection system Air terminal A metal mast or the metal fitting on a timber mast will act as an adequate air terminal. The mast, if metallic will both act as downconductors and each should be connected to an earth termination. Stays as small as 3 mm diameter steel wire will serve as effective downconductors, but may be damaged under severe lightning discharges. Earthing Any metal surface that is normally submerged in the water will provide adequate earthing. Propellers, metal rudder surfaces and metal keels may be used. The earth plate for the radio transmitter may also be used, providing that it is constructed of solid material and not of the porous type. A metal or a ferrocement hull also constitutes an adequate earth.

Metallic objects Metallic objects that are permanent parts of the boat and whose function would not seriously be affected by earthing should be made part of the LPS by interconnection with it. The purpose of interconnecting the metal parts of a boat with a downconductor is to prevent side-flashes to metal objects . A general rule is, that if the non-conducting part of the alternative path through such objectis less than one-eighth of the length of downconductor bridged out, then that object shouldbe electrically interconnected with the downconductor.

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Radio transceivers A whip antenna consisting of a fine wire embedded in a glass fibre tube cannot be considered a satisfactory lightning conductor and should be folded down during a lightning storm. All radio equipment or other navigational equipment with exposed transducers such as radar, wind speed/direction indicators, and the like, should be fitted with effectively-earthed spark gaps or SPDs. Alternatively, input cabling should be disconnected from the equipment if there appears to be imminent danger of the boat being struck by lightning.

Corrosion Care should be taken that the design of the LPS does not promote the occurrence of electrolytic or galvanic corrosion. Bonding of dissimilar metals and interconnection of the earth terminals of different pieces of electrical equipment should not be undertaken without expert knowledge .

Protection of boats with masts Sailing or power boats that have a mast or masts of sufficient height must consider if they give an adequate zone of protection . They may be protected by earthing the lower ends of the standing rigging and the base of a metallic mast, or the lower end of a continuous metal sail track on a timber mast. Where the mast of a boat is stepped on deck, particular care should be taken to ensure that the conductor from the base of the mast follows a direct route if it passes through the accommodation section of the boat. A typical small sailing boat with aluminium mast stepped on deck, glass fibre hull with the metal ballast encapsulated in the glass fibre and with chainplates moulded into the hull provides something of a problem.In such cases, it is suggested that some protection be sought when necessary by temporarily connecting the mast and stays together at deck level by a length of chain or other flexible conductor and allowing a short length of chain or the conductor to hang in the water at each chainplate.

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Protection of boats without masts Boats without masts do not constitute as high a risk as boats with masts. However, where the size of the boat is such as to cause a marked protrusion above the surrounding water surfaces, should be fitted with air terminals that will give at least the protection for land-based structures. Precautions for persons and maintenance suggestions During a lightning storm, persons should remain inside a closed boat and avoid contact with metallic items such as gear levers . Persons should stay as far as practicable from any parts of the standing rigging or other items forming part of a downconductor. No person should dangle arms or legs in the water. If a boat has been struck by lightning, compasses and navigation instruments should be checked for calibration. Protective coatings on steel hulls and glass fibre sheathing over ballast keels should also be checked for damage. All standing and running rigging and associated fittings should be checked in detail.

Bonding the LPS to the vessel’s electrical wiring system earth. The bonding of LPS on a boat should recognize that the electrical wiring system on a boat is commonly only a final subcircuit. As such, the wiring will be very light, and neither the live conductors (whether or not energized) nor the earthing arrangements, are capable of carrying lightning discharge current. Even with a larger vessel, where the wiring is for a submain or a complete installation with a generator, this will often still be the case, though larger wire sizes would be in use. The ship installation may have common fed accessories off the isolation transformer secondary, and all systems may incorporate RCD protection. One side of the isolation transformer secondary may be a pseudo ‘neutral’ with ship earthing. When the LPS is completed, the earthing conductor at its final point connection to its chosen earth termination network should be bonded to the ship earth or ship bonding point at its termination.

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FENCES If an extended length of metal fence is struck it is raised to a high potential relative to earth. Persons or livestock in close proximity to, or in contact with, may therefore be exposed to danger.Fences that give rise to the most risk are those constructed with posts of poor conducting material, such as wood or concrete. Fences built with metal posts set in earth are less hazardous, especially if the electrical continuity is broken. Breaking the electrical continuity prevents a lightning stroke from affecting the entire length of a fence, as it can if the stroke is direct and the fence continuous, even though earthed. In addition, persons or livestock can be endangered by potential differences in the ground in the proximity of fences . The risk is greatest on rocky ground. No value can be given for the earth termination resistance, since this must be largely governed by the physical conditions encountered, but the lower the resistance to earth the less risk will result to persons and livestock. It should be borne in mind that because of large body spans and bare contact areas many types of livestock are more susceptible to electric shock than humans.

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MISCELLANEOUS STRUCTURES Metal scaffolding , including overbridges. Where metal scaffolding is readily accessible to the general public, particularly when it is erected over and on part of the common highway or may be used in the construction of public seating accommodation, it should be efficiently bonded to earth. A simple method of bonding such structures consists of running a strip of metal other than aluminium, 20 mm ×3 mm size, underneath and in contact with the base plates carrying the vertical members of the scaffolding and earthing it at intervals not exceeding 20 m. With public seating accommodation only the peripheral members of the structure need bonding to earth. Other such as those used for pedestrian bridges over main trunk roads, are frequently sited in isolated situations where they may be prone to lightning strikes and should be bonded to earth, at the approach points.

Tall metal masts, towers ,and cranes. Masts and their guy-wires, floodlighting towers and other metallic construction, particularly those to which the general public have access, should be earthed. Cranes and other tall lifting appliances used for building construction purposes, shipyards and port installations should also be bonded to earth. For cranes or revolving structures mounted on rails, efficient earthing of the rails, preferably at more than one point, will usually provide adequate lightning protection. In special cases, where concern is felt regarding possible damage by lightning to bearings, additional measures may be justified.Mobile towers, portable cranes and similar structures mounted on vehicles with pneumatic tyres can be given a limited degree of protection against lightning damage by drag chains or tyres of conducting rubber such as are provided for dissipating static electricity.

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PROTECTION OF HOUSES AND SMALL BUILDINGS Houses / small buildings vary greatly in the degree to which their construction provides inherent lightning protection. Small buildings with mainly nonmetallic materials offer little or no protection against lightning, whereas a building with a metallic roof, metallic gutters, and metallic downpipes leading into the ground has a high degree , since the main elements of an LPS are already present. If lightning strikes a house with little lightning protection, the lightning is likely to penetrate the roof and attach to electrical wiring in the roof area. This will usually result in damage to electrical equipment in the house, and in some cases, may result in a fire, or in hazard to persons within the house. The objective in protecting small buildings should be to provide conductors to intercept the lightning, to provide a low-resistance path to earth, and to provide at least two earth earthing electrodes for taking lightning current to the earth.

Air terminal network for the building If building roof consists mainly of metallic materials, then it will serve as the air terminal network. It is necessary to ensure that there is electrical continuity between the various parts of the roof. If building roof consists mainly of non-metallic materials, then an air terminal network should be provided. Copper wire and copper strip are recommended for their durability. At least one conductor should be run along the highest parts of the roof, eg. the highest ridge of the building. If the roof has a complicated shape, it may be necessary to run additional conductors along the highest parts of each section of the roof. All conductors should be joined together. The cross-sectional area of the conductors should be not less than 35 mm2, achieved, for example, by copper strip 25 mm × 1.5 mm. However, it should be realized that much thinner conductors are able to carry most lightning currents without damage. Even if the conductor were to melt, it would have carried out its function for that one strike, as the lightning current would flow through the path of the molten metal, rather than penetrate below the roof of the house.

For a large, flat roof of non-conducting material, the simplest form of air 51

terminal network may be a series of vertical metallic rods above the roof level, all connected together. Metallic gutters may become a strike attachment point. If there are metallic gutters around the roof, these should be connected to the air terminal network. With metallic roofs, these connections may already exist in the fastenings of the guttering to the roof. With non-metallic roofs, the guttering should be connected to the air terminal network at no less than two points.

Provision of downconductors for the building There should be at least two low-resistance paths to convey the current from any lightning strike to the roof down to earth. Metallic downpipes from metallic gutters may be used for this purpose, provided they afford a direct electrically continuous path for the lightning current. In the absence of any low-resistance path from roof to earth, at least two conductors should be provided to serve as downconductors. These may be continuations of the conductors forming part of the air terminal network.

Provision of earthing electrodes A path to earth for the lightning current should be provided at no less than two well separated points, Eg. at opposite ends of the house. Preference should be given to areas that are usually damp, such as gardens. A metallic water pipe buried in the ground would be a satisfactory earthing electrode provided that the water pipe is also connected to the electricity supply service earth. Each downconductor should be connected to an earthing electrode by the shortest possible route, with the proviso that downconductors and earthing electrodes should not be placed close to entry doors, or places where persons are likely to stay for long periods. Eg. close to swimming pools. Earthing electrodes and their connected conductors should be examined periodically to ensure that they are intact, and not suffering corrosion or mechanical damage.

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I N S T A L L A T I O N & M A I N T E N A N C E P R A C T I C E. WORK ON SITE

Throughout the period of erection of a structure, all large and prominent masses of metalwork,Eg. steel frames, scaffolding and cranes, should be effectively connected to earth. Once work has commenced on the installation of an LPS, an earth connection should be maintained at all times. INSPECTION

All LPSs should be inspected after completion, or alteration . A routine inspection should be made at least every two years. More frequent inspections may be warranted in some circumstances. Such circumstances include, but are not limited to— (a) Areas subject to severe weather and lightning activity. (b) Structures located in areas where aggressive soil or other conditions may accelerate corrosion or other aspects of system degradation. (c) Changes in technology use within the structure that may necessitate a review of the protection means and their continued effectiveness. (d) Any other time where it is deemed necessary to update the original risk assessment for lightning damage.

TESTING On the completion of installation or any modification to it, and at time of any maintenance inspection, the resistance to earth of whole installation and of each earth termination should be measured, and the electrical continuity of all conductors, bonds and joints and their mechanical condition verified. Where regular testing during maintenance reveals that the earthing resistance is substantially unchanged, the frequency of maintenance testing may be reduced to each alternate inspection.

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RECORDS The following records should be kept on site, and by the person responsible for the upkeep of the installation: (a) Scale drawings showing the nature, dimensions and position of all component parts of the LPS. (b) The nature of the soil and any special earthing arrangements. (c) Date and particulars of salting, if used. (d) Test conditions, date and results . (e) Alterations, additions or repairs to the system. (f) The name and contact details of the persons responsible for the installation or for its upkeep. Detection of occurrence of flashes to structure and magnitude of discharge current may be estimated by magnetic links, or magnetic tape . While the use of instruments to count the number of strikes intercepted by the protection system is highly recommended, in practice, this may be impractical to achieve on multiple downconductor LPSs.

MAINTENANCE Some system components will lose their effectiveness over time because of weathering, corrosion, and stroke damage. Both physical and electrical characteristics of the LPS must be maintained . The periodic inspection and tests described in report of maintenance,shall indicate works done. Particular attention should be paid to any evidence of corrosion of earthing and to any alterations or extensions to the structure that may affect the LPS. Examples of such alterations or extensions are as follows: (a) Changes in the use of a building. (b) The erection of radio and television antennas. (c) Installation / alteration to electrical, telecoms or It within the building. A good maintenance program should also contain provision for the following: (i) Inspection of all system components. (ii) Tightening of all accessible clamps and splices. (iii) Measurement of system resistance, including earth resistance of terminals. (iv) Inspection or testing of SPDs.

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EXAMPLE OF LIGHTNING RISK CALCULATIONS : Two storey house.

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NATURE OF LIGHTNING & PRINCIPLES OF PROTECTION THE NATURE OF LIGHTNING Thunderstorms occur under particular meteorological conditions, and partial separation of electrical charges within the thundercloud usually results in regions with negative charge mainly in the lower parts of the thundercloud, and regions with positive charge in the upper. A complete ground flash consists of a sequence of one or more high amplitude short duration current impulses. About 40% of ground flashes have more than one ground termination, usually separated by distances up to a few kilometres . The currents are unidirectional and usually negative, i.e. a negative charge is injected into the object struck. For all practical purposes the stroke can be considered to be generated by a current source whose waveshape and magnitude are unaffected by the characteristics of the ground termination.

The lightning attachment process The first stroke of a ground flash is normally preceded by a downwardprogressing low-current discharge that commences in the negatively charged region of cloud, progresses to the earth. When the lower end of the leader is roughly 100 m from the earth, electrical discharges are likely to start at protruding earthed objects. Several streamers may start, but usually only one is successful in reaching the downcoming leader. The high current phase (return stroke) commences at the moment the upward moving streamer meets the downcoming leader. The position in space of the lower portion of the lightning discharge channel is therefore determined by the path of the successful streamer, i.e. the one that succeeded in reaching the down leader. Primary task in protecting a structure is therefore to ensure a high probability that the streamer originates from lightning protection wires , and not from a part of the structure that would be adversely affected by the lightning current that subsequently flows. It is therefore possible to provide protection for a large volume with a relatively small number of correctly positioned conductors. This is the 56

basis for the concept of a zone of protection provided by an elevated earthed conductor, and provides the basic principle underlying interception lightning protection. Thus the basic protection system consists of air terminals to provide launching points for streamers, and downconductors and earthing electrodes to deliver the lightning current into the earth.

EFFECTS OF LIGHTNING The risk of side-flash is increased at any deeply re-entrant bend or loop in a downconductor due to the local increase in inductance. If such a flashover occurred, part of the lightning current would be discharged through installations with consequent risk to occupants and building. The amount of energy deposited in any object carrying lightning current may be calculated by multiplying the action integral by the electrical resistance of the object. From this, the temperature rise may be calculated. It should be noted however, that the resistance of most objects other than metallic conductors, e.g. wood, masonry or earth, is very non-linear for the large currents associated with lightning. It should also be noted that the passage of ightning current through moist resistive materials such as masonry or wood can convert the moisture to high-pressure steam, causing the material to explode or shatter. The thermal effect of a lightning discharge is confined to the temperature rise of the conductor through which the lightning current is discharged. Although the amplitude of a lightning current may be high, its duration is so short that the thermal effect on an LPS, or on the metallic parts of a structure where this is included in the LPS, is usually negligible. This ignores the fusing or welding effects that occur locally consequent upon the rupture of a conductor that was previously damaged or was of inadequate cross-section. Cross section of a normal lightning conductor is determined primarily by mechanical and secondarily by thermal . At the point of attachment of a lightning discharge channel to a thin metal surface, a hole may be melted in the surface.

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The passage of lightning current through a conductor causes a force on the conductor given by the equation:

F = B × l × i . . . B3 Where

F = the force on the conductor, in newtons (N) B = the component of the magnetic flux density at right angles to the conductor, in teslas (T) l = the length of the conductor, in metres (m) i = the current through the conductor, in amperes (A) 58

Earth currents At the point where the lightning current enters the ground, the current density is high.Hazardous earth potential gradients may be generated. Earth electrodes should be distributed symmetrically, preferably outside and around the circumference of structure. This minimize earth potential gradients near the building, and cause lightning current to flow away from building rather than underneath it.In addition, with earth connections properly distributed, the current from a lightning flash to ground near the building will be concentrated at the outer extremities. Thus current flow underneath the building, as well as earth potential gradients, will be minimized.

Side-flash If an LPS is placed on a building and there are unbonded metal objects of considerable size nearby, there will be a tendency for side-flashing to occur between the conductors and unbonded metal objects. To prevent damage from side-flash, interconnecting conductors should be provided at all places where sideflashes are likely to occur. This is referred as equipotential bonding, although complete equalization of potential is never achieved. As the currents required to equalize potentials are considerably less than the full lightning current, conductors of relatively small cross-section are adequate for this .

Potential (voltage) differences The impedance of earth termination to the rapidly changing lightning current influences the potential rise of the LPS. This in turn affects the risk both of sideflashing, and of dangerous potential gradients in the ground. The potential gradient around the earth termination on the other hand, depends on the physical arrangement of the earthing electrodes and soil resistivity. In Figure , a lightning flash is assumed to occur to the LPS of a building. For the purposes of the illustration, no equipotential bonding is shown although such bonding is required . As the lightning current is discharged through the downconductor and the earthing electrode, the conductor system and the surrounding soil are raised, for the duration of the discharge, to a potential with respect to the general mass of the earth.

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NOTES: 1 Person X is in contact with the ground at a and b. Person Y is in contact with the ground at c and the conductor at d. Person Z is in contact with conductor at e and a metallic handrail f shown earthed at g. 2 Person X is subject to ‘step’ potential. 3 Person Y is subject to ‘touch’ potential. 4 Person Z is subject to ‘transferred’ potential. 5 The potential depends on the current magnitude and the impedance of the path of the lightning discharge. 60

6 Step potential increases with the size of step a-b in the radial direction from conductor and decreases with increase in distance between person X and the conductor. 7 The transferred potential increases with increase in the radial distance between the downconductor and the ground at g. 8 The diagram does not show equipotential bonding that may be necessary to protect persons from hazardous potential differences of the type described in this diagram.

PRINCIPLES OF LIGHTNING PROTECTION Purpose of protection The purpose of lightning protection is to protect persons, buildings and their contents, or structures in general.

Interception of lightning The function of an air terminal in an LPS is to divert to itself the lightning discharge that might strike a vulnerable part of the object to be protected. It is generally accepted that the range over which an air terminal can attract a lightning discharge is not constant, but increases with the severity of the discharge. The path of a lightning discharge near a structure is determined by the path of thesuccessful streamer that will usually be initiated from a conducting part of the structure nearest to the downcoming leader. The initiation of streamers is also influenced by the local electric field. The upper outer edges and corners of buildings or structures, especially protruding parts. When the down coming leader is within about 200 m of the building, the electric field at these protruding parts and corners will exceed the breakdown field strength of air, resulting in corona currents that cause these parts to be surrounded by ionized air. The resulting space charges influence the electric field in such a manner that the field is limited to the breakdown strength of air. The rolling sphere method is a reliable guide to the most probable strike attachment points.Hence, if air terminals are placed at all locations where high electric fields and streamer initiation are likely, there will be a high probability that the discharge will terminate on some portion of the LPS. 61

Determination of lightning strike attachment points to buildings The rolling sphere method The procedure for determining lightning strike based on RSM by a sphere of specified radius is imagined to be rolled across ground towards the building , up the side, and over the top of the building, and down the other side to ground. This can be carried out in various orientations with respect to the building. Any point on the building touched by the sphere is a possible lightning strike attachment point. The rolling sphere will tend to touch salient points, and the method therefore provides a geometric means of identifying such points.

The striking distance The striking distance, ds, is between the leader tip and the eventual strike attachment point at the moment when it has become inevitable that the gap, of dimension ds,will be bridged by the lightning discharge channel. The RSM is closely related to the electrogeometric method developed for predicting lightning attachment to electricity supply service lines, whereby the lightning leader is supposed to progress until it comes within the distance ds of an earthed object, when the final discharge path is determined to that object. There are theoretical and observational grounds for a relationship between ds and the imax.,where imax. is the peak return stroke current. The following relationship has been proposed. ds = 10 × imax. Where ds = the striking distance, in metres (m) imax. = the peak current of the return stroke, in kiloamperes (kA) The advantage of the RSM is that it is easy to apply, even to buildings of complicated shape. The limitation of the method is that no account is taken of the influence of electric fields in initiating return streamers, and the method therefore does not distinguish between likely and unlikely lightning strike attachment points. In particular, the enhancement of electric fields at the upper outer corners of a building makes these corners the most probable strike 62

attachment points, whereas return streamers are unlikely to be initiated from a flat surface away from a corner or edge, even if on the roof and touched by the sphere. Some qualitative indication of the probability of strike attachment to any particular pointcan be obtained if the sphere is supposed to be rolled over the building in such a mannerthat its centre moves at constant speed. Then the length of time that the sphere dwells on any point of the building gives a qualitative indication of the probability of that point being struck. Thus for a simple rectangular building with a flat roof, the dwell time would be large at the corners and edges, and small at any point on the flat part of the roof, correctly indicating a high probability of the corners or edges being struck, and a low probability that a point on the flat part of the roof will be struck. The RSM with its modification of an increased radius for plane surfaces is now applied with some account of electric field enhancement effects in mind, so that high priority is given to providing air terminals at the more probable attachment points. For a building of rectangular shape with a flat roof, this means giving top priority to providing air terminals around the periphery of the roof. This could take the form, for example, of a metallic perimeter handrail.

Protection of the sides of tall buildings When the RSM is applied to a building of height greater than the selected sphere radius,then the sphere touches the vertical edges on the sides of the building at all points above a height equal to the sphere radius and a sphere of the corresponding increased sphere radius touches all flat surfaces on the sides about a height equal to the increased radius. This indicates the possibility of strikes to the sides of the building, and raises the question of the need for an air terminal network on the sides of the building.

Practical experience indicates that strikes to vertical edges on the sides of tall buildings do occur but are uncommon ,ie rarely occur to flat side surfaces. A strike to the sides of a building may result in minor damages, unless there are specific reasons for side protection, as would be the case for a structure containing explosives.If it is decided that some protection for the sides of a building is justified, then conductors should be provided at the most probable lightning attachment points on the sides of building. The most probable attachment points are at protruding corners and vertical edges of the sides of the building, including surfaces with changes greater than 20°. The conductors will generally serve both as air terminals and downconductors and will generally be connected to the roof air terminal network at their upper ends, and to earthing termination network at their lower ends. 63

The conductors may be made flush with the surface, and should be placed as near as practicable to the vertical edge to be protected. Buildings including such as large metallic window frames, then these can form part of the interception protection system. It is necessary to provide electrical connections between adjacent metal objects both in the horizontal and vertical directions. This provides multiple paths for the lightning current from any point on the surface metalwork to earth, and local potential differences will be reduced to an acceptable level.

Potential equalization Lightning strikes give rise to harmful potential differences within a building. Of particular concern is differences that may exist between the local earth and in coming conductors such as metallic water services, telecom, and electricity systems.Reduction ma be achieved by bonding of all affected conductors contained in the building. This includes all incoming metallic services, protective earths of electricity supply, telecom systems, and the building lightning protection earth termination.

RESISTIVITY OF SOIL Earthing electrodes should not be located near brick kilns or other installations where soil can dry out by the temperatures involved.

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Artificial reduction of soil resistivity Chemical additives can be used to reduce soil resistivity. These additives generally are ionizable salts such as sulphates, chlorides or nitrates. Such chemical additives should not be used indiscriminately as— (a) Benefit provided will lessen with time due to leaching through the soil. (b) May increase rate of corrosion of earthing electrode material.Maintain some moisture adjacent to the earth termination network, and provide uniform and non-corrosive environment for the earthing electrodes. 65

CALCULATION OF EARTH RESISTANCE OF AN EARTHING ELECTRODE

where R = resistance, in ohms ρ = soil resistivity, in ohm metres L = buried length of earthing electrode, in metres d = diameter of earthing electrode, in metres

USE OF EARTHING ELECTRODES IN PARALLEL In situations where a desired earthing resistance cannot be achieved with one earthing electrode, a number of earthing electrodes may be used in parallel. The combined resistance of parallel earthing electrodes is a complex function of a number of factors, some of the more important being the number of earthing electrodes, their dimensions, the separation between the earthing electrodes, the soil resistivity and the configuration of the earthing electrodes.

DRIVEN OR DRILLED EARTHING ELECTRODES The use of drilled earthing electrodes combines economy of surface space with efficiency of performance, and accesses clays and other conductive layers at depth. In consequence, it is a preferred method of earthing electrode installation.

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Earthing electrode diameter Although the resistance between the earth and the earthing electrode depends to a certain extent on the area of the earthing electrode in contact with the soil, a large earthing electrode of, say, 50 mm diameter does not decrease the resistance materially compared with earthing electrodes of 13 mm or 20 mm diameter, which need to be only slightly larger to achieve the same resistance. For a driven earthing electrode, provided the required current-carrying capacity is met, the minimum diameter will usually be determined by mechanical rather than electrical considerations. The usual practice is to select a diameter that will give enough strength to enable the earthing electrode to be driven into the soil of a particular location without bending or splitting. Large diameter earthing electrodes are more difficult to drive than small diameter ones. For deep drilled earthing electrodes the size is selected in the light of available drill diameters, requirements for connections and economy.Strip electrodes are commonly used.

Depth of installation The depth to which an earthing electrode is installed is usually the most important factor affecting its earth resistance, first because the area of soil contacted increases directly with the length of earthing electrode below the surface, and secondly because the soil resistivity usually decreases with depth. Tests shown that the soil down to a depth of between 6 m and 9 m consisted of ballast, sand and gravel, below which was clay. The rapid reduction in resistance when the earthing electrode penetrated the latter . The mean resistivity up to a depth of 7 m in one case was 150 Ω.m, at 10 m the mean value for the whole depth was 20 Ω.m due to the low resistivity of the clay stratum. BURIED STRIP EARTHING ELECTRODES Buried strip earthing electrodes provide a solution to the problem of obtaining a low resistance earth connection in locations where soil resistivity is high.. For a given cross-section, strip earthing electrodes have the advantage of a greater surface area in contact with the soil. The CSA of the conductor has little effect on the resistance of the earth connection so that the strip or cable size is not important provided it affords reasonable protection against mechanical damage and corrosion, and is of adequate current surge capacity. 67

INSPECTION AND MAINTENANCE OF EARTHING ELECTRODES The scheduling of maintenance inspections is the prerogative of the system owner. However, the frequency of testing and the associated considerations are listed below as a guide to good engineering practice. Materials of construction data and electrical measurements pertaining to the original design should be prepared and preserved as a guide to later performance. Soil resistivity data are likewise useful for future comparison. The following practices are recommended: (a) Inspections should be both physical and electrical. (b) The inspections should be carried out at intervals of not less than two years. (c) The physical inspections should address corrosion or mechanical damage to visible parts of the whole system, structural alterations that may have prejudiced the design or operation of the system, or changes in the usage of the structure. (d) Electrical tests should cover the continuity of the downconductors, the integrity of bonding arrangements, and the resistance to earth of the earthing electrodes, preferably individually as well as collectively. (e) The replacement of earthing electrodes to achieve a specified resistance may be necessary, and if this is done it should be recorded along with other test results. Restoration records of clamps, joints and fittings should also be kept as a future maintenance guide.

MEASUREMENT OF SOIL RESISTIVITY, EARTHING ELECTRODE RESISTANCE AND EARTH TERMINATION NETWORK IMPEDANCE

Four-pin method of soil resistivity measurement is commonly used. Involves the use of four test pins (test electrodes) equally spaced in a straight line and driven to the same depth d, not exceeding 5 percent of their separation s and not more than 1 m in any case ,see Fig. In next page.

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Earth resistance Earth resistance is measured by applying a voltage to the earth electrode and measuring the current passing through the electrode to earth. Resistance is calculated by the earth tester using Ohm’s Law R=V/A. To create a path for the test current and to measure the voltage, a current is passed in a potential test stake, which is placed in the soil outside the potential gradient area of the electrode being measured.Conduction of current through the soil is non-linear. Consequently, earth resistance measurements will vary with different test voltages, currents and frequencies. Interference voltage and frequency can be picked up by the test stakes. An earth resistance tester ideally should provide variable test frequencies to avoid measurement errors from interference.Earth resistance can be measured by a three-pin or four-pin measurement. A three-pin test makes one connection to the earth electrode while a four-pin test makes two. By making two connections to the electrode under test, the voltage is measured at the earth electrode and not at the instrument. This eliminates any error caused by resistance in the connecting lead and is generally regarded as the preferred method.

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Instruments for earth resistance measurement The earth resistance of an earth electrode can be measured by several methods using various types of earth testers. It cannot be assumed that each method or each type of instrument is suitable for every situation. A four-pin earth tester as used for soil resistivity can also be used for measuring earth resistance. Lower cost three-pin earth testers are also suitable for measuring earth resistance but cannot be used to measure soil resistivity. Users must be aware that measurements made with the three-pin method are susceptible to error from test lead resistance.Both three-pin and four-pin earth testers require the earth being measured to be disconnection during easurement. This poses a major safety hazard should there be a lightning strike or a fault current present during test. To eliminate the inconvenience of disconnecting the earth to be measured, the selective method has been developed. Another alternative is the stakeless method. This has the advantages of not requiring the isolation of the earth electrode or the use of test stakes. This does however require the earth system to have a secondary reliable solid bond to earth, such as a bond to the electrical earth and/or metal water service. Users must be fully conversant with the earth circuitry,otherwise measurements by this method can be misleading. Each method has advantages and will produce reliable measurements if used in the appropriate situations. There is no one method which can be used for every situation. This has given rise to multi-method earth testers which combine all three methods into a single instrument.

Selective method The principle of the selective method is the use of a clip-on current transformer (CT) to measure the test current flowing in the earth electrode under test. Resistance is computed from the actual current flowing to earth via electrode. Current flowing to earth through remainder of earth network is not measured and has no effect on the result. Both the three-/four-pin and selective methods of measuring earth resistance rely on test stakes being placed, typically 50 to 100 m, or even further from the earth system being tested. This is generally of no consequence in rural areas but it is seldom possible in cities and towns where buildings are surrounded by pavement or the electrodes are in basements. 70

THE THREE-PIN AND FOUR-PIN METHOD OF MEASURING EARTH ELECTRODE RESISTANCE

HAZARDOUS CONDITIONS ASSOCIATED WITH HV POWER EPR HV sites of particular concern. A hazardous EPR may occur in the following sites: (a) Buildings near a power generating station or substation. (b) Near a HV transformer . (c) Near any HV area of high soil resistivity e.g. rocky or dry, sandy terrain.

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VOLTAGE/TIME TOLERANCE OF COMPUTING EQUIPMENT

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