Chapter 4 Electrical Installation Design

Chapter 4: Protective Measures in Electrical Installations

What is protection? The meaning of the word protection as used in the electrical industry, is no different to that everyday used. Electrical system need the following: – To be protected against mechanical damage, the effects of the environment and electrical overcurrents – To be installed in such a fashion that persons and/or livestock are protected from dangers that such an electrical installation may create.

Protection against mechanical damage Serious electrical overcurrent left uninterrupted for too long can cause distortion of conductors and degradation of insulation; both of these effects are considered to be mechanical damage. However let us considering ways of preventing mechanical damage caused by physical impact and the like.

Cable construction – Comprise of one or more conductors each covered with insulating material – This insulation provide direct shock by direct contact and prevent the passage of leakage currents between conductors. – Clearly, insulation is very important and itself should be protected from damage. This may achieved by covering the insulated conductors with a protective sheathing during manufacture, of by enclosing them in conduit or trunking at the installation stage. – type of sheathing chosen and/or installation method depend on the environment in which the cable is installed

Protection against corrosion – Mechanical damage to cable sheath and metalwork of wiring system can occur through corrosion, and hence care must be taken to choose corrosion-resistant materials and avoid contact between dissimilar metals in damp situation.

Protection against thermal effects – This basically requires common-sense decisions regarding the placing of fixed equipment, such that surrounding materials are not at risk from damage by heat.

Polyvinyl chloride – PVC is a thermopastic polymer used widely for cable insulation, conduit and trunking. – All PVC are degraded or reduced in quality by heat and light

Protection against ingress of solid objects and liquid – In order to protect equipment from damage by foreign bodies or liquid, and also to prevent person from coming into contact with live or moving parts, such equipment is housed inside an enclosure. – The degree of protection offered such an enclosure is indicated by an index of protection (IP) code. For example IP56 is dustproof and waterproof.

Protection against electric shock There are two ways of receiving an electric shock: by direct contact and by indirect contact.

Protection against direct contact – This is contact of person or livestock with live parts which may result in electric shock – IEE regulations recommend five ways of minimizing this danger By covering the live parts with insulation which can only be removed by destruction, e.g. cable insulation By placing the live part behind barrier io inside an enclosed providing protection to at least IP2X or IPXXXB. By placing obstacles to prevent unintentional approach to or contact with live parts. This method must be only used where skilled person are working.

By placing out of arm’s reach: for example, the high level of the bare conductors of traveling cranes. By using RCD. Whilst not permitted as the sole means of protection, this is considered to reduce the risk associated with direct contact

Protection against indirect contact – This is the contact of persons or livestock with exposed conductive parts made live by a fault. – IEE regulations suggest five ways of protecting against indirect contact. Earth equipotential bonding and automatic disconnection supply use of class 2 equipment Non-conducting location Earth free local equipotential bonding Electrical separation

Types of protection devices Consumer unit contains devices for the protection of the final circuits against: – Overload – Short-circuit – Earth fault

Overload and short circuit are an overcurrent where there is a current greater than the rated current of a circuit. All of these three condition need to be protected against in order to avoid damages to circuit conductors, person and equipments.

Overload – Overload are overcurrents occurring in healthy circuits. – They may be caused for example by faulty appliances or by surges due to motors starting. – There are many reasons for overloading of healthy wiring, including: An electric motor undertaking mechanical duty in excess of the design parameters Faulty running of machinery caused by bearing failure or uneven loading A motor starting current Loss of a phase from a three phase load Excess loading added to a socket outlet circuit An underestimate of the maximum demand of an installation

– Overload often rise gradually and at the early stages may not be apparent when testing is carried out.

Overload protective devices Rewireable fuse HBC fuse – HBC fuse will be fitted by the supply authority in sealed meter heads

Circuit breakers

Overcurrent/fault current Fault current is invariably caused by some failure in the installation. This may be because: – – – –

Cable insulation has been damage by heat or abrasion Water has entered into a badly protected connection A motor has burned out A metal tool has fallen across bus-bars

The consequences of a fault may be a fire or explosion The initial surge of fault current is usually measured in thousands of amps (kA) and is only limited by the impedance of the supply up to the fault position. This is known as Prospective Fault Current (pfc). The purpose of fault current protection is to disconnect the supply speedily and to restrict damage and danger as far as possible.

Short circuit rating Under short circuit condition there will be considerable current surge For standard 100A supply the pfc may be as high as 16000A (1kA). Much depend on the supply authority network and distance from the substation. More usually figure will be less than 5kA

Circuit breaker The use of circuit breaker is more reliable compared to fuse. Disadvantage of fuse compared to circuit breaker are: – Fuse has delay operation – Fuse has to be replace every time its operate

To overcome this problems, circuit breaker is used. Circuit breaker can operate automatically when over current occurs and it can be adjusted to operate at certain over current limit easily. Circuit breaker did not to be replaced every time its operate. Circuit breaker usually rated between 15A to 200A protection current. Protection relay will be used if the circuit breaker has rated current more than 200A. Circuit breaker can be in form of oil circuit breaker, gas circuit breaker, air circuit breaker and vacuum circuit breaker.

Oil-filled circuit breaker – This circuit breaker usually used for over current protection rate from 40VA to 2500MVA and usually installed in substation and factory. – Mineral oil used to extinguish arc when circuit breaker operated – Contact immersed in oil to suppress the arc – This circuit breaker function as over current protection and isolation switch at the same time.

115 kV oil CB

41kV oil CB

Gas circuit breaker (SF6) – Size of this circuit breaker is larger compared to other circuit breaker. – It can handle 35000MVA to 50000MVA. – This device usually used in generation station and substation – arc is extinguish by releasing electronegative gas (sulfur hexafluoride) to breaker contact when break contact occur. – Advantage – time duration for arcing is short, so breaker contact doesn’t need to be change regularly – Operation is simpler, not poisonous, reliable and easy to operate (less force need), did not require large space – Disadvantage - expensive

400 kV Sulfur hexafluoride (SF6 )live tank CB

Air blast circuit breaker – Smaller size – 400VA to 25MVA – Arc produce from this breaker is more compare to gas circuit breaker, because of this breaker contact need to be replaced regularly – Air blast circuit breaker is more expensive than oil circuit breaker, but its operate faster, less arc, less maintenance and less possibility to create fire. – Arc produce while opening contact or make contact will be blow out by compress air

Vacuum circuit breaker – With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. – These can only be practically applied for voltages up to about 35,000 V, which corresponds roughly to the medium-voltage range of power systems. – Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

Air circuit breaker – Rated current up to 10,000 A. Trip characteristics often fully adjustable including configurable trip thresholds and delays. – Usually electronically controlled, though some models are microprocessor controlled. – Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Front panel of a 1250 A air circuit breaker manufactured by ABB. This low voltage power circuit breaker can be withdrawn from its housing for servicing. Trip characteristics are configurable via DIP switches on the front panel.

MCCB (Moulded Case Circuit Breaker) – rated current up to 1000 A. – Thermal or thermal-magnetic operation. – Trip current may be adjustable.

Miniature circuit breaker (MCB) – Small – suitable to replace fuse in distribution box – rated current not more than 100 A. Trip characteristics normally not adjustable – Advantage compare to fuse Less time need to install Easy to check whether circuit is ON state or OFF – easy while doing maintenance works. Long-lasting, can be reused after break contactUnlike a fuse, which operates once and then has to be replaced Did not affect by ambient temperature

MCB operation – Thermal operated use a bimetallic strip, which heats and bends with increased current, and is similarly arranged to release the latch. This type is commonly used with motor control circuits. Thermal breakers often have a compensation element to reduce the effect of ambient temperature on the device rating.

– Magneto-hydraulic operated – Electromagnet

Advantage circuit breaker – Operate more efficient and faster – Can use even after operate (need minor repair) – Easily can recognize whether the circuit breaker is operated or not – Delay time can be adjust

Disadvantage of circuit breaker – Expensive

Residual Current Device