Electrical engineer interview question - Switch gear company


Interview Questions and answers - Electrical engineer,Electrician and Electrical apprentice trainee.

1.What is meant by reach point of a relay?

Ans - The farthest point from relay location,which is still inside the zone of protection,is called the reach point.

2.What is difference between fuse and breaker?

Ans - Fuses are burnt when over current flows in the circuit but circuit breaker will just open during over current.Thus fuses are used for only once but breakers can be used by multiple number of times.

3.What is the difference between isolators and electrical CB?

Isolators are mainly for switching purpose under normal conditions but they cannot operate in fault conditions. Actually they used for isolating the CB for maintenance. Whereas CB gets activated under fault conditions according to the fault detected.

4.Explain the working of an OC relay and mention type of it?

An over-current relay has a single input in the form of ac current.The output of the relay is normally open contact,which changes over to closed state when the relay trips.

The relay has two settings.These are the time setting and the plug setting.The time setting decides the operating time of relay while the plug setting decided the current required for the relay to pickup.

Three types of over current relay.
1.Instantaneous over current relay
2.definite time current over relay
3.Inverse time current over relay.

5.Explain the working of an IDMT relay?

In an IDMT (Inverse definite minimum time) relay,the operating principle is based on the idea that with the more severe a fault is,the faster it should be cleared to avoid damage to the apparatus. It is inverse in the sense,the tripping time will decrease as the magnitude of fault current increase.


6.How to select pickup value of a relay?

We can set the pick-up value of the relay,keeping in mind that the relay should allow normal load as well as a certain degree of overload to be supplied.

Thus the pick-up value of the relay should be more than the allowable maximum load.At the same the relay should be sensitive enough to respond to the smallest fault.Thus,the pick-up value should be less than the smallest fault current.

7.How to set the operating time of a relay?

A relay must get an adequate chance to protect the zone under its primary protection. Only if the primary protection does not clear the fault,the back-up protection should initiate tripping.Thus as soon as the fault takes place,it is sensed by both primary and back-up protection.

Naturally the primary protection is the first to operate,it’s operating time being less than that of the back-up relay.We have to allow for the overshoot of the primary delay so that there is a proper coordination the primary and the back-up


8.When do we use IDMT relay and DTOC relays ?

IDMT relays have better fault clearing times over DTOC relays.But DTOC relays are used for lines which are short in length as Z greater then Z I and if remains almost same.