Answer MET Question 26

Question : Compare the effectiveness of a current limiting circuit breaker with that of a HRC fuse.

 Answer:

Circuit Breaker	HRC Fuse
Circuit Breakers are used for the over current portection of the system.They can be of Air, Vacuum or gas type.	HRC fuses are used for the short circuit protection In a H.V system.
The operation of circuit breaker is based upon relays. All relay types(Magnetic, thermal or Electronic) have an inverse current time characteristic.	Fuses are not chosen for their rated current but for their inverse current/time (I/t) characteristic.
They are used for the overload protection. A 6.6kV generator generally protected with circuit breakers rated only for an Full load current of 220 A.	Fuses do not protect against normal overloads but do protect against a short-circuit fault. A typical fuse could be "32M63" which indicates a continuous rating of 32 A buta rating of 63 A for the starting period.
They are capable of repeated use. But subjected to periodic maintenance.	Fuses are used as a backup protection. They  can be used only once. Need to replace once blown.
They usually have an adjustable thermal overcurrent setting and an adjustable or fixed magnetic overcurrent trip for short-circuit protection built into the case. An undervoltage trip coil may also be included within.	Fuse operation is based on thermal effect of current. Fuses have a operating time is nearly instantenous.