Question

6) Compare and contrast the electrical protection devices discussed in this class: Fuses, Circuit breakers, Lightning arrestors and Surge Protectors. Include method of operation, relevant electrical ratings and indicate if a device is re- useable.

Answer 1

Fuse:

Fuse is protection device which consists of a wire having capabability to cutoff the circuit at large currents. By the properties it is widely used in most of domestic application. Operation of fuse is simple. It is connected between starting of circuit and other circuit element. Whenever high current flows then it melts and isolate the healthy part of circuit. It is usually made alloy of tin and lead.

Its rating generally 125% of the rated current. If this much flow then it blows.one drawback is it need replacement after every operation.

Circuit Breakers:

It generally used in high voltage circuits. Its operation is isolate the circuit when a fault occurs. Unfortunately it cannot detect the fault. The additional equipment needed to sense the fault called relay. If fault occur then relay will send the information to circuit breaker then it will begin its operation.

Rated voltage depends upon insulation system.

For below 400KV circuit breaker is designed to withstand 10% above the system voltage.

For above or equal to 400 KV system insulation of circuit breaker should be capable of withstand 5% above the normal system voltage.

It doesn't need to replacement after every operation. It open the circuit during fault and closes when the fault is cleared.

Lightning Arrester :

It is protecting device which protects electrical equipments and other apparatus from lightning overvoltage.

When a travelling wave reach the arrester, its sparks over at a certain prefixed voltage. The arrester provides a conducting path to the waves of relatively low impedance between the line and ground.

Its rating generally depends on line to ground voltage and maximum voltage. The rating is chosen nearest to this value. For 132 kv substation ,

L-G voltage is 76.21kv and Maximum voltage is 108 kv. So by taking 10% as margin the rating of lightning arrester for 132 kv substation is nearly 120 kv. It doesn't need replacement.

Surge protectors:

Surge protector or surge suppressor is device thet designed to protect electrical devices from voltage spikes

A surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting to ground any unwanted voltage above a safe threshold.

The surge protector is rated in joules. For least rated protection 200 to 400 joules is needed and for better protection its rating should be 600J or more.

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