Question

Describe, with the aid of a diagram, the operation of the following components of electrical equipment and explain the purpose of EACH:

Under voltage protection;

Miniature circuit breakers.

Its main function is to trip the breaker when a severe voltage dip (around 50%) occurs. The U/V release on a generator circuit-breaker also prevents it being closed when the generator voltage is very low or absent.

An under voltage relay, which may be magnetic or electronic, also provides back-up protection to short-circuit protection.

This is achieved by lifting the mechanical latch (which keeps the contacts closed) to allow the trip spring to function which opens the breaker contacts.

The U/V release on a generator circuit-breaker also prevents it being closed when the generator voltage

These are very small air circuit-breakers fitted in moulded plastic cases.

They have current ratings of 5-100 A and generally thermal over-load and magnetic short-circuit protection. They have a very limited braking capacity (about 3000 A) and are commonly used in final distribution boards instead of fuses. The d.b. is supplied via a fuse or MCCB with the required breaking capacity. MCBs must be replaced if faults develop — no maintenance is possible