Lv Switchgear Lv Cable Sizing

LV Switchgear & LV Cable sizing Asif Eqbal

CONTENTS

INTRODUCTION • Area of application • Leading manufacturers • Business size of LV Switchgear Industry

LOW VOLTAGE SWITCHGEAR • Governing standards • System parameters • Construction • Busbars and Other components • Wiring and Schematics • Layout aspect & EHS ASPECT

LV CABLE • Sizing criteria

INTRODUCTION Area of Application All Industrial LV load distribution • LV Motor (up to 200kW) – For pump, grinder, crushers etc….. • VFD – For application requiring speed control

Why use VFD?

• Auxiliary & control supply of control & protection equipments

Domestic load distribution- For residential consumers • Lighting • Small power • HVAC

Commercial consumers Printer & Xerox machine Scanner Elevators and escalators Public address system

Vending machines Baggage handling system Fire detection and alarm system Illumination

LOW VOLTAGE SWITCGEAR Governing standards IEC 60947, 61439, 60694

IS 13947 IS 10118, 8623, 11353

ANSI/IEEE C37.20.1.1987, C37.010 & C37.013

Switchgears with rated voltage up to 1000V AC and 1500V DC are termed as low voltage Adopted from IEC 60947 Any difference in ANSI and IEC?

Low voltage circuit breakers

LOW VOLTAGE SWITCGEAR Definitions

Switchgear

A general term covering switching devices and their combination with associated control, measuring, protective and regulating equipments enclosed inside a enclosure.

PCC & MCC

Any switchboard directly connected to transformer and which usually feeds other downstream switchboards is technically called PCC (power control center). A switchboard, which does not feed any downstream boards and directly feeds to motor or other loads, is MCC (Motor control center). A switchboard which feeds both downstream boards and motor loads or one incomer connected to emergency DG than they are called PMCC.

Metal enclosed & Metal clad

As per ANSI/IEEE: Completely enclosed on all sides and top with sheet metal except for ventilating opening. In addition to above requirement if the main switching and interrupting device is draw out. Metal clad is applicable term for medium voltage. For LV application metal enclosed + compartmentalized is metal clad

Check point: Specification needs to be checked if it contains the term metal clad

LOW VOLTAGE SWITCGEAR System Parameters Why non condensing?

Design Ambient

50C

Altitude

<1000Mtr MSLD

Humidity

<100%RH non condensing

Main system/Frequency Rated operational voltage Power frequency withstand voltage dry Rated current Fault level & duration

Why humidity?

3Ph, 4W, 50Hz 415V ± 10% 2.5kV for power circuit Rated current of switchgear is bus bar rating Fault level is normally 50kA for 1 Sec Check point: Design ambient and fault level are specification check points

LOW VOLTAGE SWITCGEAR Construction  In the pioneering days of electrical distribution, switching medium & low voltage supplies was a challenge.  Since then there have been major changes to switchgear.  Fig shows old style open switchgear

LOW VOLTAGE SWITCGEAR Construction Fixed or draw out type is decided taking into account a function a switchgear has to perform, Safety, Criticality of load and Overall cost of ownership.

Fixed type

In a fixed construction, all the feeders in the switchboard, feeding the various load points, are securely mounted in the assembly and rigidly connected to the main bus.

Draw out

In this construction each feeder is mounted on a separate withdrawable chassis. Circuit breaker is installed on a carriage which can be pulled out. Disconnecting of main incomer is not required The modules of identical types can also be easily interchanged and defective modules replaced by spare modules in the event of a fault. A draw-out assembly can be designed only in a cubicle construction and is totally compartmentalized.

LOW VOLTAGE SWITCGEAR Construction

Semi-draw-out type In this design the incoming and outgoing power contacts are of the draw-out type, but the control terminals are the plug-in type. The control terminals are to be disengaged manually first, when the trolley is to be drawn out.

LOW VOLTAGE SWITCGEAR Construction

Fully draw-out type In this construction the control terminals are of the sliding type. The moving contacts are mounted on the trolley while the fixed matching contacts are mounted on the panel frame. These contacts engage or disengage automatically when the trolley is racked-in or racked-out of the module respectively.

LOW VOLTAGE SWITCGEAR Construction Single front

Single sided arrangement of feeders.

Double front

Two sided feeder arrangement on front as well as rear side. Panel depth increases

Mounting

Floor mounting with integral base frame. . .

Check point: Drawout, fixed type and Single front, double front type are specification check points

LOW VOLTAGE SWITCGEAR Construction Cross sectional view of double front panel with common horizontal bus and separate vertical bus (Vertical bus not visible)

Common horizont al bus  Maintenance on horizontal bus can be done by opening top cover or door of bus bar compartment  Point on vertical bus where male contact of bus meets with female contact is most vulnerable to fault and same can maintained through breaker compartment or cable alley  A busbar alley is required after every feeder to access the vertical busbar

LOW VOLTAGE SWITCGEAR Construction Summary

Draw out requires fully compartmentalized arrangement

EDO: Electrical draw out

Single front

Double front

Fixed type can be non compartmentalized also depending upon feeder rating

MDO: Mechanical draw out

Single front

Double front

Compartmentalized

Single front

Double front

Non Compartmentalized (for MCB feeders)

Single front

Double front

LOW VOLTAGE SWITCGEAR Construction Sheet steel thickness of various parts

Load bearing members-2.0mm CRCA Non load bearing members & Doors -1.6mm CRCA .

What are load and non load bearing members? What is cold rolled?

LOW VOLTAGE SWITCGEAR Construction Degree of protection

IP-54 < 1600A and IP-42 for ≥ 1600A. Except, switchboard where soft starter panels or APFC capacitor banks are present wherein IP-40 will be applicable. Developed by the European Committee for Electro Technical Standardization (CENELEC) (NEMA IEC 60529 Degrees of Protection Provided by Enclosures - IP Code), specifying the environmental protection the enclosure provides. The IP rating normally has two (or three) numbers: Protection from solid objects or materials Protection from liquids (water) Protection against mechanical impacts (commonly omitted, the third number is not a part of IEC 60529) An "X" can used for one of the digits if there is only one class of protection, i.e. IPX1 which addresses protection against vertically falling drops of water e.g. condensation .

LOW VOLTAGE SWITCGEAR Construction IP Third number - Protection against mechanical impacts (commonly omitted, the third number is not a part of IEC 60529). IEC 60529 does not specify degrees of protection against mechanical damage of equipment.

LOW VOLTAGE SWITCGEAR Construction Older enclosure specifications are sometimes seen with an optional third IP digit denoting impact resistance. German standard DIN 40050‐9 extends the IEC 60529 rating systems with an IPK rating. Newer products are likely to be given an IK rating instead.

LOW VOLTAGE SWITCGEAR Construction NEMA Rating (enclosures) The NEMA Standard for Enclosures for Electrical Equipment does test for environmental conditions such as corrosion, rust, icing, oil, and coolants.

Apart from NEMA 6P, NEMA 12 & 13 are also present

LOW VOLTAGE SWITCGEAR Bus bars and other components Bus bars NEMA Phase arrangement

A bus is a low impedance conductor or set of conductors that serves as a common connection for two or more circuits. NEC Article 408 requires three-phase bus bars to have phases in sequence R, Y and B vertical left right when viewed from front and horizontal right to left.

The order of the busbars is usually N, L1, L2 and L3: - from the front to the rear of the switchboard, - from the left to the right of the switchboard.

LOW VOLTAGE SWITCGEAR Bus bars and other components Main bus bar or Horizontal bus bar Vertical or distribution busbars

Main busbar that distributes power horizontally between the various switchboard columns. It may be installed on the top, middle or bottom of the switchboard depending on the type of switchboard, customer specifications and/or local practices. Vertical or distribution busbars are connected to the main busbar. They provide power to outgoing devices

LOW VOLTAGE SWITCGEAR Bus bars and other components Bus bars material Sizing criteria

Step-1 is Input data required

Copper or Aluminium Bus bar is sized for its cross section by correlation of electrical and thermodynamic equations.

Ambient Temp Initial Busbars Temp before Fault Permissible Final Temp after Fault Busbars Material Conductivity Details of Busbars Support : Material Tensile Strength Flexual Strength Compressive Strength Di-electric Strength

400C 900C 1000C AL E91E 31.9m/Ω.mm SMC 750Kg./sq.cm 1650Kg./sq.cm 1900Kg./sq.cm 10KV/mm

LOW VOLTAGE SWITCGEAR Bus bars and other components Step-2: Minimum cross section requirement as per fault level

where S =cross sectional area, I=fault level, t=fault withstanding time, k=constant. For formula reference & value of constant k refer IS 3043 or INDAL Al BUSBAR HAND BOOK

LOW VOLTAGE SWITCGEAR Bus bars and other components 2

Step-3: Finding ampacity of minimum cross section area

Heat generated in electrical terms 1

Ambient as per IEC 60694 is 40oC. If project specific ambient is used than no temperature derating required at later stage

2

Where Q = Heat dissipated in thermodynamic terms as per Stephen Boltzmann’s law

Q1 Heat generated by radiation 36.8 x 10‐12 x Emissivity Factor x Final Temperature 4‐ Ambient Temperature 4

Is Q2 in still air?

2







0.0022 x Air Pressure 0.5 x Temperature Rise above new ambient 1.25 / Height of cross section of busbar mounted along thickness 0.25

LOW VOLTAGE SWITCGEAR Bus bars and other components Electrical and thermodynamic equation can be equated (because of rate of heat generated = rate of heat dissipated in steady state) to give following expression for ampacity. {24.9(θ− θn) 0.61. S0.5. P0.39} √ρ [1 + α (θ−20)] This ampacity expression is called Melson & Both equation l θn (θ - θn) S p ρ α

: permissible current expressed in amperes (A) : ambient temperature 40 °C : permissible temperature rise as per IEC 439 50 °C : busbar cross-section/bar in Sq centimeter as obtained in step-2 : busbar perimeter/bar in centimeter : resistivity of the conductor at ambient temperature. Copper: 1.83 μΩ cm : temperature coefficient for the resistivity Copper 0.004 at 20 degree Celsius.

LOW VOLTAGE SWITCGEAR Bus bars and other components Can this current value be used for selection of bus bar?

Derate the busbar as per the derating factor, K which is product of 6 other factors k1, K2, k3, k4, k5 Hence k = k1xk2xk3xk4xk5

Coefficient k1 is a function of the number of bar strips per phase for: 1 Bar k1= 1 (This derating is due to proximity effect) For 2 or 3 Bars, see table below: e/a

LOW VOLTAGE SWITCGEAR Bus bars and other components

Emissivity depends upon surface condition of bus bars that is painted or bare

Painting is done to improve emissivity and radiation losses

Painting may be worthwhile for very wide bars (where convection is less effective) operating at large temperature rises (where radiation is more effective).

Paint the bus bars for better heat dissipation and possibly increase the current carrying capacity

Pain layer acts as an insulator reducing the efficiency of convection process and possibly reduce the current carrying capacity

Convection heat loss

Heat loss by radiation is directly proportional to emissivity

Radiation heat loss

Coefficient K2 is a function of the surface condition of the bars:Colored tape at bare: k2 = 1 suitable locations Painted bus bars are painted: k2 = 1.15 as per IS 11353 uprated?

LOW VOLTAGE SWITCGEAR Bus bars and other components Coefficient k3(optional) is a function of the busbar position: edge-mounted busbars: k3 = 1 1 bar flat-mounted: k3 = 0.95 several flat-mounted bars: k3 = 0.75

Insulator yp type?

Coefficient k4 is a function of where the bars are installed: Calm indoor atmosphere: k4 = 1 Check point: Calm outdoor atmosphere: k4 = 1.2 Insulator type is a bars in non-ventilated ducting: k4 = 0.80 specification check points

LOW VOLTAGE SWITCGEAR Bus bars and other components SMC (Sheet molding compounds) for the busbar mounting supports. They are basically fiber- or glass-reinforced thermosetting plastics (FRP or GRP) and possess good physical and thermal stability, high mechanical strength and excellent. The improved properties, particularly its strength, over DMC is a result of reduced degradation of the glass and the ability to use longer fiber. In DMC, this is usually 6-25 mm, while in SMC it is about 25-50 mm.

LOW VOLTAGE SWITCGEAR Bus bars and other components Coefficient k5 is a function of the artificial ventilation, busbar area & enclosure area: Enclosure factor = Busbar area/ Enclosure area

LOW VOLTAGE SWITCGEAR Bus bars and other components Mechanical criteria to be checked:

Why mechanical check?

We have to check if the selected bus bars can withstand the electrodynamic forces The electrodynamic forces following a short-circuit current are given by the equation: What is peak 2 -8 F1 = (2 L/d) x Idyn x 10 value? F1 : Force expressed in daN : is the peak value of short-circuit expressed in A Idyn L : distance between insulators on the same phase cm d : phase to phase distance cm

LOW VOLTAGE SWITCGEAR Bus bars and other components Mechanical criteria to be checked: Indicates direction of short circuit current Idyn Indicates phase to phase distance d in cm Indicates F1 force expressed in daN Distance L between insulators on the same phase cm Force at head of support: F = F1 (H+h)/H H = Insulator height h = Insulator top from bus bar CG F = This force is deflection force and should be ≤ insulator bending resistance

LOW VOLTAGE SWITCGEAR Bus bars and other components Mechanical criteria to be checked: Since bus bar are rigidly fixed to insulator and insulator has sufficient cantilever strength to resist the deflection so bus bar will be subjected to bending moment whose resultant strain is: ∏ = F1xLxn/12 ∏ is Resultant strain F1 is Electrodynamic force L is distance between insulators on the same phase n is modulus of inertia of given bus bar size and arrangement arrangement ∏ should be ≤ Permissible strain of bus bar material In case of failure of any of the above two condition than insulator type or support span or type or both.

LOW VOLTAGE SWITCGEAR Bus bars and other components What is current density? What is role of current density in bus bar sizing?

Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square metrer

LOW VOLTAGE SWITCGEAR Bus bars and other components Current density is indicative of distribution of current over surface and shape of a conductor. You need to determine the minimum cross sectional area first, then find its resistance and then correlate the thermodynamic heat loss by convection and radiation at (allowable temperature rise) with electrical heat loss.

To save time and avoid the multiple repetition of above process a very approximate starting point is to assume an average current density of 2 A/mm² in still air for Copper and iterate either up or down. The more popular thumb rule being followed in India is to assume current density of 1.0 Amps / Sq.mm for Aluminium and 1.6 Amps for Copper for any standard rectangular conductor profile.

LOW VOLTAGE SWITCGEAR Bus bars and other components: Forms of separation inside switchboard There are several reasons for partitioning a switchboard: Partitioning rules are defined in standard IEC 61439-2. This definition is subject to agreement between the switchboard manufacturer and the end user.

Form 2: Partition of bus bars and functional unit inside switchboard

LOW VOLTAGE SWITCGEAR Bus bars and other components: Forms of separation inside switchboard

Form 3: Each functional unit is partitioned from another. Draw-out type boards possible with form 3 and higher

LOW VOLTAGE SWITCGEAR Bus bars and other components: Forms of separation inside switchboard

Form 4: Each functional unit as well as there terminal connecters is partitioned from another. Draw-out type boards with cable alley is possible with form 4

LOW VOLTAGE SWITCGEAR Bus bars and other components: Sleeving or bare Bus bars can be bare or sleeved. For safety to personnel during maintenance and to protect the live system from lizards and rodents the busbars may be covered with PVC tape or heat shrinkable PVC sleeve. The joints and the tap-offs can be protected through FRP shrouds as shown in picture.

Disadvantages of Sleeving a bus bar It is possible that one may not be able to provide a true skin-fit sleeve through the length of the busbars, which may affect its cooling. At certain places, it may have air bubbles from where it will provide a reduced heat dissipation. For higher rating systems, say 2500 A and above, Sleeving is normally not used. L&T and Schneider can offer both sleeved and bare bus bars with type tested design. Siemens has sleeved bus bar as only type tested design. If panel is type tested with sleeved bus bar than bare bars cannot be offered but reverse is possible Check point: Bare or sleeved bus bar is specification check point

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB

Circuit-breaker: a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short-circuit (IEC 60947-1 def. 2.2.11) Current-limiting circuit-breaker: a circuitbreaker with a break-time short enough to prevent the short-circuit current reaching its peak value (IEC 60947-2 def. 2.3)

L&T H range ACB from 800A to 4000A L&T C Range ACB from 800A to 6300A

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.

When a live circuit is interrupted, an arc is formed between the parting contacts. the intensity and magnitude of which would depend upon the quantum and the quality (p.f.) of the current being interrupted. The insulating material ( may be fluid or air) used in circuit breaker should serve two important functions. It should provide sufficient insulation between the contacts when circuit breaker opens. It should extinguish the arc occurring between the contacts when circuit breaker opens At low voltage level di electric strength of air provides sufficient insulation between the contacts as well as arc quenching medium

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc. During interruption, the arc is formed producing N2 (80%) and O2 (20%) and metallic vapors

Figure shows the dielectric properties of different mediums at different contact gaps There are two methods by which interruption is done.

High resistance method. Low resistance method or zero interruption method.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: In high resistance method we can increase the electrical resistance many times to such a high value that it forces the current to reach to zero and thus restricting the possibility of arc being restruck. Proper steps must be taken in order to ensure that the rate at which the resistance is increased or decreased is not abnormal because it may lead to generation of harmful induced voltages in the system. The arc resistance can be increased by various methods like lengthening or cooling of the arc etc.

This method is advantageous in dc power circuit breaker, where there is no natural current zero so forcing the current to reach zero is achieved through this method. This is also employed in low rating AC breakers.

During arc discharge most of the energy is received by CB itself. Hence circuit breaker require more mechanical endurance/ mechanical strength to withstand sudden change of large energy.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Restriking voltage: Voltage at the terminal of CB, after the circuit interruption. Restriking voltage is what appears across the contacts at current zero during arc period. The current interruption in the circuit depends upon restriking voltage

Here v = restriking voltage. V = value of voltage at the instant of interruption. L and C are series inductor and shunt capacitance up to fault point. Thus from above equation we can see that lower the value of product of L and C, higher the value of restriking voltage.

Recovery voltage: It is the normal frequency (50 Hz) r.m.s voltage that appears across the contacts of the circuit breaker after final arc extinction.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rate of Rise of Restriking Voltage (RRRV): It is defined as the ratio of peak value of restriking voltage to time taken to reach to peak value. It is one of the most important parameter as if the rate at which the dielectric strength developed between the contacts is greater than RRRV, then the arc will be extinguishes.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Low resistance method is applicable only for ac circuit and it is possible there because of presence of natural zero of current. The arc gets extinguished at the natural zero of the ac wave and is prevented from restricting again by rapid building of dielectric strength of the contact space.

There are two theories which explains the phenomenon of arc extinction: Energy Balance Theory: When the contact of circuit breaker are about to open, hence generated heat would be zero and when the contacts are fully open there is no production of heat. So maximum generated heat is lying between these two cases, this theory is based on the fact that the rate of generation of heat between the contacts of circuit breaker is lower than the rate at which heat between the contact is dissipated. Thus if it is possible to remove the generated heat at a high rate than the generation, than arc can be extinguished.

Voltage Race Theory : The arc is due to the ionization of the gap between the contact of the circuit breaker. As the contact separates the resistance starts increasing. If we remove ions at the initial stage either by recombining them into neutral molecules or inserting insulation at a rate faster than the rate of ionization, the arc can be interrupted.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing The rated currents of the circuit-breakers, the cross-sectional areas of cables, the transformer power and the various other characteristic quantities of electrical equipment are represented by a series of numbers 10, 12.5, 16, 20, 25, 32, ...630, 800 not immediately understandable. French army engineer Charles Renard invented Renard series which are geometric progressions with common difference n Start with 100. Multiply that by 1.2589. Rating is 125.89. Round that to 125. Multiply that by 1.2589. rating is 158.48. Round that to 160. Multiply that by 1.2589. Rating is 199.52. Round that to 200. Multiply that by 1.2589. Rating is 251.19 Round that to 250 And so on…….

Charles Renard (1847-1905), a French army engineer

User wants a ‘fine’ production in order to find always the product suitable for his own requirements, whereas the manufacturer tends to rationalize the production in a ‘discrete’ way. Renard invented his series to meet both the requirements.

LOW VOLTAGE SWITCGEAR What is

Bus bars and other components: ACB, MCCB, MPCB, MCB,thermal ELCB current?

ACB: Rating & Sizing

As per IS 13947 part-1, clause number 4.3.2.2, the value of the conventional enclosed thermal current shall be at least equal to the maximum value of the rated operational current of the enclosed equipment in eight-hour duty. The conventional enclosed thermal current is the value of current stated by the manufacturer to be used for the temperature-rise tests of the equipment when mounted in a specified enclosure.

Further IS 13947 part-1 clause number 4.3.2.2 says that this current (Thermal current) is not a rating and is not mandatorily marked on the equipment.

As per clause number 4.3.2.3 of IS 13947 part-2, Rated current for circuit-breakers, is the rated uninterrupted current and is equal to the conventional free-air thermal current (Ith).

Why free air thermal current, enclosed thermal current and rated current?

Specification check point: Is it in panel rating or free air rating?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing  Transformer size and Circuit breaker size are considered based on maximum demand and not connected load. Hence demand factor can be used.  For the load calculation, load factors to be considered shall be 0.9 for continuous loads, 0.4 for intermittent loads.  Diversity in operation of loads can be considered in sizing of transformer but not circuit breaker. Continuous de-rated current of ACB/MCCB due to enclosure shall be more than transformer current rating.

Any margin required on transformer full load current?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, Icu, Ics & Icw  Icu - Ultimate short-circuit breaking capacity. Icu is subject to an O-t-CO sequence.  As per duty cycle if the breaker happens to break its full rated ultimate short circuit breaking current,"Icu", twice within a time interval of 3 minutes, the breaker cannot be re-used again, even for carrying its normal rated continuous operating current and must be replaced immediately.  Ics - Service short-circuit breaking capacity. Ics is subject to an O-t-CO-t-CO sequence. The breaker is then subject to both dielectric withstand and temperature rise tests.  This means that the breaker can break its rated service short circuit breaking current three times within a gap of 6 minutes. And, even after this, the breaker can be used as a switch, capable of making, carrying & breaking its normal rated continuous current and overload currents. Only, it would not be able to break any further short circuit currents. As such, the breaker can be continued in service, albeit, with a short circuit protective device back-up, until the replacement arrives.  Icw - Rated short-time withstand current. Circuit breakers may be subject to through fault which they are not intended to clear. While not clearing these faults, the breaker will still need to withstand the thermal and mechanical stress imposed by the fault current.  The more ‘Icw’, that switchgear has got, the more co-ordination interval that one has got.  Many switchgear may have breaking capacities of, say, 50kA, but their one-minute ‘Icw’ may be lower say, 35kA only. So, this point must be paid attention to, while specifying switchboard.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, Icu, Ics & Icw  The Ics rating of all LV circuit breakers should at least be equal to short circuit current at the point of installation. This will ensure that in worst case, the breaker is able to clear the full magnitude of fault current (full 3 phase ‘bolted’ short circuits at the CB terminals, which is rare of the rarest fault) three times and remains operational.  Majority of faults tend to be towards the load end and cable impedance substantially reduce the fault level. The faults are more likely to be a single phase rather than a three phase thereby reducing the fault further.

Icw = Ics = 100% Icu will suffice the requirement

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, Short circuit making current of CB

Short circuit making current = 2.5 x short circuit breaking current

RMS Value of Short circuit current I ≤ 5 kA

cosΦ

n

0.7

1.5

5kA
0.5

1.7

0.3

2

0.25

2.1

0.2

2.2

As per Indian standard 8623-part-1 for low voltage switchgear and controlgear assembly peak value multiplier is as per table above

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, TP, TPN & 4P CB 1. TP 2. TPN 3. 4P

Triple Pole Triple Pole and Neutral Four Pole Neutral link R Y B

TP

R Y B

TPN

N

R Y B

4P

How to decide between 4P, 3P and TPN CB? N

Specification check point: What type of CB is asked in specification?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, TP, TPN & 4P CB Where to use 4 Pole CB instead of TPN CB In case of parallel operation of DG (For DG and mains or two DG: IS 3043 clause number 23.2.2) In case of multiple incomer to a switchboard:  DG and transformer  Two transformer If one DG is failed and other DG sets are in running condition to feed the loads, and there is some unbalance in loads then depending on degree of unbalance, there will be flow of current through neutral. During this time, if any technician is attending on failed DG incomer and if he touches the neutral conductors (which is earthed) he will get electric shock depending on the potential rise in common neutral due to flow of current through neutral conductor. This problem will also be there if there is standby DG with transformer and their location is different. Three pole breaker with direct neutral earthed at their respective end will cause hazard during maintenance being done at any one of them with other in running condition.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Rating & Sizing, TP, TPN & 4P CB Where to use 4 Pole CB instead of TPN CB

What is unrestricted earth fault protection?

3. If unrestricted ground fault protection is fitted to the transformer neutral, then the bus section circuit breaker should have 4-poles and preferably incomer circuit breakers should also have 4-poles.

Ground fault located at the load side of a feeder have two return paths. As shown above, a ground fault on a feeder at the bus section “A” will have a current return path in both the incomers, thus tripping both Bus. The sensitivity of the unrestricted ground fault relay is reduced due to the split current paths.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Auxiliary supply ACB Spring charge motor ACB Closing & tripping & relays Panel illumination & space heater Meters & Transducers Control circuit of O/G feeders Motor space heater

: 240V AC, Source - Internally tapped from incomer before breaker : 220V DC, Source - External : 240V AC, Source - Internally tapped from incomer before breaker : 240V AC, Source - Internally tapped from incomer before breaker : 240V AC, Source - Internally tapped from incomer before breaker : 240V AC, Source tapped from respective feeder after breaker

Specification check point: What type of control and auxiliary supply is asked in specification? Selection of control and auxiliary supply inside switchboard should be done carefully as it will have direct impact on: 1. Size of battery and chargers 2. Amount of cabling 3. Requirement of control transformer

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ACB: Protection through relay or integral release Protection in LV network can either be achieved through external protection relays or integral releases in CB There are two types of release: 1. Thermal Magnetic Release - Thermal element for over current and magnetic element for short circuit. 2. Microprocessor Release - The Microprocessor release works on monitoring of current True R.M.S value. It is simulated and calculated from peak values, which installed microprocessor, can detect. 1.

Integral releases consume lesser wiring, thus saving on cost, complexity and vulnerability to failures. 2. Operating times of releases are lesser when compared to external relays. 3. There are more components - Main Protection Relays, Aux. Relays, Master Trip Relays, DC Sources, Shunt Trip Coils, wiring & connections between them, etc. - that can go wrong in arrangement with external relays. Whereas in arrangement with integral releases, these are minimized to a great extent. 4. Energy savings - Coil consumption wattage of relays, coils, etc. reduces thereby reducing the capacity requirement of the DC Source.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB What is an MCCB? “A mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit condition such as those of short circuits.” Same definition as given in ACB in slide no-43 than what is the difference?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB MCCB differs with ACB on account of following: Governing standards- For ACB: 60947 & For MCCB: IEC 60898, BS EN 60 947-1 General Rules, BS EN 60 947-2 Circuit Breakers & BS EN 60 947-3 Switch Disconnector

Construction- Housing of the contacts structure are contained within a molded case. Application- At what level of distribution system, intended CB is to be applied as MCCB.

The principle of operation of ACBs & MCCBs have common features. A contact system with arc-quenching, a mechanism to operate the breaker, a system to provide a means of protection, control and indication. However, there are some fundamental differences in application that should be considered.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB Where to use MCCB? This image cannot currently be display ed.

 A normal low voltage installation for an industrial or commercial application would consist of an ACB incomer connecting the low voltage side of the distribution transformer to the main switchboard.  The area known as final distribution would consist of MCBs feeding the loads directly.  The area in between is normally where an MCCB would be located within the distribution network.

Why intermediate level is MCCB and not ACB?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB Selection between ACB & MCCB? Difference between ACB & MCCB? Application wise difference

What are application wise difference between ACB & MCCB?

Where to use MCCB? Selection between ACB & MCCB

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB

Load

Selection between ACB & MCCB? Fault level

Discrimination

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB Selection between ACB & MCCB? Category A This is for MCCBs with no intentional time delay and are therefore not specifically intended for high selectivity applications, which tend to be a thermal magnetic MCCB. These MCCBs would not have a Icw rating. Category B These MCCBs have a time delay which makes them more suited to applications that demand higher selectivity .The majority (but not all) of microprocessor MCCBs have a short time withstand Icw rating. For example a 1250A MCCB may have an Icw of 15ka rms for 300msec

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB Thermal-magnetic circuit breaker – Contains a thermal element to trip the circuit breaker for overloads and a faster magnetic instantaneous element to trip the circuit breaker for short circuits.

Electronic trip circuit breaker – Contains a solid-state adjustable trip unit. These circuit breakers are extremely flexible in coordination with other devices.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB MCCB is constructed in five major components: 1. Frame (Molded Case) 2. Contacts 3. Arc Chute Assembly 4. Operating Mechanism 5. Trip Unit  MCCB do not have Icw rating.  As practice used upto a rating of 630-800A  Draw-out feature is only possible in microprocessor based MCCB

The frame provides an insulated housing to mount the circuit breaker components. The construction material is usually a thermal set plastic such as glass-polymer.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Contacts MCCB use a straight-through contact as well as blow apart contact arrangement. The electrical path through the contacts is a straight line.

 The two contact arms are positioned parallel to each other as shown.  Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other.  During normal current conditions, the magnetic field is not strong enough to  force the contacts apart.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Contacts  When a fault develops, current increases which increases the strength of the magnetic field.  The increased strength of the opposing magnetic fields open the contacts faster by forcing them apart.  In comparison, the blow-apart contact design helps to open the contacts faster than the straight-through arrangement.  I2T is greatly reduced since arc extinguishment in less than 4 milliseconds is common with blow-apart contacts.  Electrical equipment is exposed to less heat over a shorter period of time.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Arc chute assembly  As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts.  This forms arc and if isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture.  An arc chute assembly is used to quench the arc.  This assembly is made up of several “U” shaped steel plates that surround the contacts.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Operating handle  An operating handle is provided to manually open and close the contacts.  Molded case circuit breakers (MCCBs) are trip free, meaning that they cannot be prevented from tripping by holding or blocking the operating handle in the “ON” position.  There are three positions of the operating handle: “ON” (contacts closed), “OFF” (contacts open), and “TRIPPED” (mechanism in tripped position).  The circuit breaker is reset after a trip by moving the handle to the “OFF” position and then to the “ON” position.

Can this operating handle remotely operated?

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Operating Mechanism The operating handle is connected to the moveable contact arm through an operating mechanism.  Operating handle is moved from the “OFF” to the “ON” position.  In this process a spring begins to apply tension to the mechanism.  When the handle is directly over the center the tension in the spring is strong enough to snap the contacts closed.

To open the contacts, the operating handle is moved from the “ON” to the “OFF” position.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Trip Unit for thermal magnetic release type The trip unit is the “brain” of the circuit breaker. It consists of components that will automatically trip the circuit breaker when it senses an overload or short circuit.  MCCBs are Compact. They save considerable panel space  MCCBs minimize downtime. Unlike in a fuse-based system, there’s no searching for a replacement fuse. MCCBs can be Reset & Switched On immediately after clearing the fault that caused the tripping.  MCCBs minimize inventory. Unlike fuses, they are not "consumables" and hence there is no need to stock MCCBs the way fuses have been stocked. Erosion of contact buttons results in higher contact resistance and subsequent overheating/nuisance tripping of the MCCBs. To evaluate the extent of contact erosion, milli volt drop should be measured across the contacts. In case these values exceed the limit suggested by the manufacturer, the MCCB needs to be replaced.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Type of Trip Unit Trip Unit or Release type Electromechanical

Electronics

Thermal magnetic

Static

Magnetic

Digital µp based

Numerical

Difference between digital & numerical?

Cl 7.5 off Al Alstom t network protection & automation guide

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Type of Trip Unit

Thermal magnetic

Magnetic

Specification check point: What type of release is asked for power & motor feeders in specification?

For Power feeders having basic protection requirement of OC & SC For motor feeders having separate motor over load relay

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCCB: Metering Module of releases Is it possible transmit metering data like current, voltage and kW from a numerical or digital release MCCB? Or Can any MCCB display metering data and save external MFM?

Option-1

Option-2

Option-3

Individual meter

Common HMI in B/C compartment

Door display through Schneider FDM 121 or L&T MTX3.0 Release

Most cost effective for non ACB feeders

Cost effective compared to individual HMI however not used in switchgears mostly in UPS or process plant switchboard

Cost effective for ACB feeders

Specification check point: Metering through release or meter or HMI?

LOW VOLTAGE SWITCGEAR MCCB: Type of Trip Unit

LOW VOLTAGE SWITCGEAR MCCB: Type of Trip Unit OL trip between 25 sec & 175 sec at 600 amps with a 40°C ambient temperature This circuit breaker has an adjustable instantaneous trip point from 900 A to 2000 A. If the trip point adjustment is set to minimum & a fault current of 900A or greater occurs, the breaker will trip within 1 cycle (16.8ms). If the trip point setting is set to maximum & a fault current of 900A occurs, the breaker will trip between approximately 12 & 55 seconds.

LOW VOLTAGE SWITCGEAR MCCB: Type of Trip Unit Frame Size – The term Frame size is applied to a group of circuit breakers of similar configuration. Frame size is expressed in amperes and corresponds to the largest ampere rating available in that group. Interchangeable Trip - The user does not have access to the trip unit on some Circuit Breakers. This means the trip unit cannot be changed with another. Interchangeable trip is a design feature that is available on some thermal-magnetic and solid state breakers. The advantage of a breaker with an interchangeable trip unit is that the user can change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating. GE Frame Sizes: three basic frame sizes up-to 800A. First frame covers 3.2A to 160 A, Second in 200 / 250 / 400A and third in 400 / 630 / 800A. Specification check point: Interchangeable trip units asked in specification or not?

LOW VOLTAGE SWITCGEAR MCCB

guides\Guides\MCCB Tech details\L&T MCCB_Technical details.xlsx guides\Guides\MCCB Tech details\Schneider MCCB_Technical details.xlsx

Fuse based system: In this system MCB & MCCB is not used at all. Till 630 Ampere fuse is used and above 630 Ampere ACB is used. • In any switchgear with two incomer and buscoupler, If fuse is used than changeover has to be manual, because with fuse you cannot attain automatic changeover. where as if MCCB is used you can attain automatic changeover. • Whenever we say fuse it means we are supposed to use SFU or SDF that is switch fuse unit or switch disconnect fuse. • In case of MCCB you get spare contacts for remote indications and if you use MCCB with shunt trip coil you can even use it for remote tripping and operation with relays. Same is exactly not possible with fuse • In case of SFU of say 132 Ampere please note that the rating should be indicated as 132/132 or 132/150 ampere where numerator indicates fuse rating and denominator denotes switch rating. Generally switch rating is next higher side to fuse rating but even if it is same no issues, it should be left to vendor as different manufacturer have different standards.

Comparison between Fuse based and fuse less system: Broadly because of following advantages fuse based system is preferred: Fuse has high rupturing capacity. Fuse has shortest operating time. Because of quick operating time of fuse we do not need to check the SC criteria at all for LT power cables Thermel magnetic MCCB is to be used below 160 Ampere which has operational history of nuisance tripping. • Only disadvantage of fuse is that once blown it has to be replaced so spare should be available. • • • •

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MCB Construction wise MCB is similar to MCCB with exception that it is available in lower kA rating hence it is of small frame size and hence the name miniature circuit breaker  As practice used up-to a rating of 125A  Motorized operation not possible because of small size hence not available with shunt trip coils for remote operation  Available with auxiliary contacts

Trippin g class

Application

B

Fast Acting. Instantaneous trip response of 3 - 5X rated current (In). Used for sensitive equipment and purely resistive loads.

C

Normal acting. Instantaneous trip response of 5 - 10x rated current (In). Used for control circuits and mixed loads

D

Slow Acting. Instantaneous trip response of 10 - 20x rated current (In). Used for motor, transformer and solenoid applications.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB MPCB This can match closely the thermal characteristics of a motor. In this case one MPCB will be sufficient to replace the HRC fuses and the thermal relay and a separate OCR may not be necessary. Motor overload protection, Thermal protection has a delayed response, to allow the high inrush currents when a motor starts. However, if the motor is unable to start for some reason, thermal protection will trip in response to the extended inrush current.

Protection against phase unbalance and phase loss. MPCB will disconnect the motor in either case as soon as the fault is detected. Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down.

ELCB

LOW VOLTAGE SWITCGEAR

Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB Difference between Earth Fault and Earth Leakage. As per IS Earth leakage current is flow of current to earth through extraneous conductive parts of any installation which is in normal condition electrically sound that is either isolated or non-current carrying. According to IEC 60947-2, Annex B, Earth fault current is the current flowing to earth due to insulation fault and Earth leakage current is the current flowing from the live parts of the installation to earth in the absence of an insulation fault. In earth fault, a phase conductor insulation is completely damage at point of fault, thus allowing large current (limited by earth resistance plus circuit resistance up-to fault location)  Earth leakage current can cause safety hazard by direct or indirect contact.  Protection against direct and indirect contact is governed by IS 732 Cl no 5.0.

ELCB

LOW VOLTAGE SWITCGEAR

Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB

 Protection against direct contact electric shock is based on normal common sense measures such as insulation of live parts, use of barriers or enclosures, protection by obstacles or protection by placing live parts out of reach. As a result, under normal conditions it is not possible to touch the live parts of the installation or equipment inadvertently.  IS 732 says that protection against direct contact electric shock:  By preventing the current from passing through the body. Live parts shall be inside enclosures or behind barriers providing at least the degree of protection IP 2X  Limiting the body current to a safe value

ELCB

LOW VOLTAGE SWITCGEAR

Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB    

IS 732 says that protection against indirect contact electric shock: By preventing the current from passing through the body Limiting the body current to a safe value Automatic disconnection from the supply mains (by RCD). The basis of RCD protection is to ensure that any voltage, exceeding 50V that arises due to earth leakage currents, is immediately disconnected. This is calculated by using a simple formula given in BS 7671 Regulation 411.5.3.

240V

Fault current = Phase to ground voltage/(Earth resistance + Equipment resistance up-to frame) I∆ = V/(Re+Ri) I∆ = 240/(1.5+20) = 11.2Amp Voltage across man touching the leaked machine = I∆ x Re = 11.2x1.5 = 16.75V Physical body current = 16.75/2000 = 8mA

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB Internal impedances of the human body Number indicates the percentage of internal impedance of human body with respect to hand to foot impedance. In order to calculate the total body impedance Zt for a given current path, the internal impedance of all parts of the body in path of current to be added as well as impedance of skin of contact area. Electrocution should not be viewed in terms of “current” alone, but in terms of “contact voltage”. A person gets electrocuted by coming in contact with an object that has a different potential from his/her own. The difference in potential causes the current to flow through the body. The human body has known limits: - Under normal dry conditions, voltage limit = 50V - In damp surroundings, voltage limit = 25V

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB Range 16A - 63A Sensitivity 30mA, 100mA, 300mA Execution Double Pole (2P) Four Pole (4P) Specification IEC 61008-1/IS 12640 - 1:2000 / EN 61008-1 RCBO has kA rating in addition to rated current In and sensitivity Residual current circuit breaker (RCCB), also popularly known as Earth Leakage Circuit Breaker (ELCB). IEC 61008/ IS 12640 part 1: Residual Current Operated Circuit Breaker without Integral Overcurrent Protection (RCCBs) IEC 61009 / IS 12640 Part 2: Residual Current Operated Circuit Breaker with Integral Overcurrent Protection (RCBOs)

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB RCDs may be distinguished by their technology, as follows. Voltage Independent RCDs. These RCDs rely on the energy of the residual current to activate the RCD. These devices are sometimes referred to as Electromechanical RCDs, and are voltage-independent in operation. Voltage Dependent RCDs. These RCDs use the mains supply voltage to power an electronic circuit and the tripping mechanism to activate the RCD. These devices are sometimes referred to as Electronic RCDs and are voltage-dependent in operation.

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB

LOW VOLTAGE SWITCGEAR Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB ELCB ELCB + MCB in incomer or one single RCBO is optimum design. Placing ELCB + MCB in each branch circuit is not viable

Specification check point: IS ELCB or leakage protection asked in wiring installation?

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes Power circuit: It consists of power equipments for switching and protection of the downstream equipment e.g. ACB, MCCB, fuse switch, power contactor, relays, CTs, etc. Power circuit is generally indicated by the use of thick lines in the scheme drawing. Such that, it can be easily differentiated from the control circuit. Control circuit: The control circuit consists of metering, protection and indication devices with necessary switches and interlocks. It includes the coil circuit of contactors, ACBs, their auxiliary contacts, shunt trips of MCCBs and all indicating lamp circuits. The supply for control circuit is either tapped from one of the phases and neutral of the feeder directly (or through a control transformer) or from external supplies. Control fuses are used for protection of control circuit. The rating of control fuses is selected according to the equipments connected in the control circuit. Cross-reference: For reading convenience, the drawing page on which scheme is drawn is divided into equal parts. The scheme drawing consists of various different equipments like contactors, relays, ACBs, MCCBs, etc. These devices consist of power contacts & auxiliary contacts. A specific tag number denotes each device & its terminals used in the scheme. The family of a device along with its contacts & terminals are drawn in one of the sections of the drawing page & the section where each terminal is used is also written alongside. Now the section in which a specified terminal is used also bears a notation alongside, which indicates the section in which the family of that specified terminal exists & the number of the device to which the terminal belongs.guides\Guides\MCCB Tech details\DOL Reference Scheme.pdf

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes

Specification check point: Wiring size for CT, PT, Annunciation and control circuit?

Contacts / Push Buttons: The push buttons are the switches, which can close or open the circuit whenever it is pressed. The contact is a part which is actually responsible for opening or closing of the circuit. The contacts are basically of two types. power contacts & the auxiliary contacts. The power contacts act in the power circuit, whereas the auxiliary contacts act in the control circuit. NO/NC contacts: The auxiliary contacts are further classified into two parts, namely NO (Normally Open) & NC (Normally Closed). This implies that these contacts are opened or closed respectively when the coil is de-energised or is in the OFF condition. Stay-put Push Button: This type is a special version of push buttons. As its name suggests these buttons once pressed stay or remain in the same position. Turning the button in the clockwise direction can open these buttons. Another version of this button can be turned to open only by using its own specific key to prevent any unauthorized use of it. This image cannot currently be display ed.

This image cannot currently be display ed.

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes Interposing relay: In auto mode, only the auto start command (coming from central control room through PLC/DCS, etc.) will be able to start the motor. In such cases, the contact coming in the circuit should be capable to carry the pick-up current of the power contactor coil. This needs special attention since the contact comes from an electronic circuit. In case the contact is not rated for that much current, we have to use an auxiliary relay to multiply this command. The contact of the auxiliary relay is then used in the circuit of the power contactor coil. The auxiliary relay in such cases is known as Interposing relay

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes Potential free contacts (also called "dry contacts") are simply contacts which are physically operated with the main device, but not electrically connected to it. For example a motor contactor often has auxiliary contacts that are operated by the main coil and open and close at the same time as the main contacts but are not used for control of the motor starter. If they are connected to an outside circuit to indicate the status of the starter without being powered by the motor supply they would be considered potential free contacts.

DCS Side

Swb Side

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes Electricity (AC or DC) has many uses. Electrical engineer Concept of DI/DO/AI/AO/RO What is Signal?

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use

electricity for power and energy. Similarly electronics engineer or telecommunication engineer use electricity in different sense that is using electricity as a tool/medium to convey information which is nothing but signal. A pneumatic (air signal) level "transmitter" device set up to measure height of water (the "process variable") in a storage tank would output a low air pressure when the tank was empty, a medium pressure when the tank was partially full, and a high pressure when the tank was completely full. This is pneumatic signaling system where air pressure signals are transmitted using tubes, easily measured (with mechanical pressure gauges), and are easily manipulated by mechanical devices using bellows, diaphragms, valves, and other pneumatic devices.

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Different terminology in schemes Concept of DI/DO/AI/AO/RO DI: Digital Input- is potential free contact from any other control circuit. (Hard wired) DO: Digital output- is potential free output from controller (Hard wired) AI: Analog input- 4-20mAmp (by paired cable) or any other continuous signal like Voltage, kVA, Current etc…into controller (Hard wired) AO: Analog output- 4-20mAmp (By paired cable) or any other continuous signal like Voltage, kVA, Current etc…as output of controller (Hard wired) RO: Relay output- Hardwired voltage driven signal with some current rating. Usually 240V AC, 10Amp. Potential free relay outputs are also available.

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Basic Schemes 1: DOL Feeder

LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders Basic Schemes 2: DOL Feeder with local remote selection and remote start

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LOW VOLTAGE SWITCGEAR Wiring Schemes & type of feeders  Basic Schemes 3: DOL Feeder with star delta starter  Basic Schemes 4: DOL Feeder with Local remote selector switch + Auto manual selector switch  Basic scheme 5: RDOL Feeder with Local remote selection + Auto manual selection Process for scheme preparation

• Identification of type of drives, Motorized valve, pumps etc….

Based on process requirement & kW rating

Based on type of control & Signal exchanges • Drive control philosophy

• Field and DCS I/O list finalization • Logic diagram for respective breaker operation

Electrical control schematics can be prepared

Specification check point: Drive control philosophy, I/O list should be checked in specification?

LOW VOLTAGE SWITCGEAR Layout aspect Clearance criteria: 1. Section 51 of IE rule 2. CPWD norms 3. NEC rule 10 4. TNEB guidelines 5. Tariff advisory committee guidelines 6. CEA guidelines

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Points to check: 1. Bus duct entry or cable entry 2. Bottom entry or top entry of bus duct 3. Single front or double front panel 4. Fixed type or draw out type 5. Future space required or not 6. Phase sequence of transformer and switchgear incomer to be checked. 7. In case of space constrained possibility of making L shaped panel or placing two single front back to back can be checked 8. There should not be any doorway or cupboard behind the switch board. Board should not block the doorway.

LOW VOLTAGE CABLES SIZING CRITERIA The following three criteria apply for the sizing of cables for circuit breaker controlled feeders: I. SHORT CIRCUIT CURRENT WITHSTAND CAPACITY This criteria is applied to determine the minimum cross section area of the cable, so that cable can withstand the short circuit current. II. CONTINUOUS CURRENT CARRYING CAPACITY This criteria is applied so that cross section of the cable can carry the required load current continuously at the designed ambient temperature and laying condition. III. STARTING AND RUNNING VOLTAGE DROPS IN CABLE This criteria is applied to make sure that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

LOW VOLTAGE CABLES SIZING CRITERIA Minimum conductor size is given by the following formula: A = (Isc x √t)/K Isc = RMS Short Circuit current Value in Ampere K = Constant value is 94 A = Minimum required cross section area in mm2 t = Duration of short circuit in seconds The fault clearing time (tc) of the breakers/fuses per ANSI/IEEE C37.010, C37.013, and UL 489 are: For medium voltage system (4.16 kV) breakers, use 5-8 cycles For starters with current limiting fuses, use ½ cycle For low voltage breakers with intermediate/short time delay, use 10 cycles For low voltage breakers with instantaneous trips, use 1 cycle Si. No. 1 2 3

Parameters Relay sensing/pickup time Tolerance/Delay time Breaker operating time

4

Relay overshoot

5

Safety Margin

TOTAL TIME IN MILI SECONDS

Time in Mili Seconds 20 10

Source/Back up

40

SIEMENS 7SJ61 technical data SIEMENS 7SJ61 technical data L&T make C-Power breaker have typical opening time of 40 ms and closing time of 60ms)

20 30

GEC handbook “Network Protection & automation Guide”

120

LOW VOLTAGE CABLES SIZING CRITERIA Cable selected for a circuit breaker feeder in 415V or 400V switchgear shall be suitable to withstand the maximum rated fault current for at least 120msec. However taking allowance of 40 Mili seconds in the opening time of circuit breaker due to various reasons.

Specification check point: Any short circuit withstand time given in specification?

LOW VOLTAGE CABLES SIZING CRITERIA Standard ampacity tables are available for a variety of cable types and cable installation methods and can be used for determining the current carrying capacity of a cable for a particular application. Ampacity Deration factor = Product of applicable multiplying factors among 1 to 4 listed above. K = K1 X K2 X K3 X K4

K1= Variation in ambient air temperature for cables laid in air / ground temperature for cables laid underground. K2 = Cable laying arrangement. K3 = Depth of laying for cables laid direct in ground. K4 = Variation in thermal resistivity of soil.

What if the cable is laid in two different type of installations?

LOW VOLTAGE CABLES SIZING CRITERIA IEEE standard 525 annexure C, clause number C3 mentions- An acceptable voltage drop is determined based on an overall knowledge of the system. Typical limits are 3% from source to load center, 3% from load center to load, and 5% total from source to load. These values are indicated diagrammatically below.

When voltage tolerance is 10% why allowable drop is 5%?

Specification check point: Any voltage drop criteria mentioned in specification?

LOW VOLTAGE CABLES SIZING CRITERIA X component of voltage drop: = Vdx = AE = AD + DE = AD + BG = IRCosф + IX Sinф (Equation-1) Y Component of voltage drop: = Vdy = CE = CG-EG = CG-BD = IXCosф – IRSinф (Equation-2) X component of VS: VSx = OE = √ (OC2 –CE2) VSx = √ VS2 – Vdy2 (Equation-3) V = OE –AE = VSx – Vdx (Equation-4) Now Voltage drop Vd is: Vd = VS – V = VS – (VSx –Vdx) (Putting the value of V from equation-4) Vd = VS + Vdx – VSx Vd = VS + Vdx – VS2 – Vdy2 Equation -5 (Putting the value of VSx from equation-3) Now substituting the values of Vdy and Vdx from equation-2 and equation-1 respectively: Vd = VS + (IRCosф + IX Sinф) – √ (VS2 – (IXCosф – IRSinф)2 (Equation 6)

Vd = VS + (IRCosф + IX Sinф) Vd = IRCosф + IXSinф

Recommended Practice for Electric Power Systems in Commercial Buildings, clause number 3.6.1 and IEEE-141, Recommended Practice for Electric Power Distribution for Industrial Plants, clause number 3.11.1

VS2 – √(IXCosф – IRSinф)2

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