Electrical Interview Questions - Part 2

Electrical Interview Questions Series

1. Why under voltage protection is required in a power system network?

i) Failure of voltage regulating equipment.

When the system load increases then the voltage falls than the rated, in this case the corrective action would be taken by voltage regulating equipment such as AVR or On Load Tap Changers, in order to bring the system voltage back to its nominal value. If this voltage regulating equipment is unsuccessful in restoring healthy system voltage, then tripping by means of an under-voltage relay will be required following a suitable time delay.

ii) Co-ordination with other voltage and current-based protection devices.

Faults occurring on the power system result in a reduction in voltage of the phases involved in a fault. The amount of voltage decreased depends on several factors like type of fault, method of system earthing and its location with respect to the relay installation point. Consequently, Co-ordination with other voltage and current-based protection devices is essential to achieve sufficient discrimination.

iii) Isolate the individual feeder loads from bus-bar.

When the fault occurred on the incomer or bus bar itself results in complete loss of bus bar voltage. In this case it may be a requirement for each of the feeders to be isolated, so that when supply voltage is restored, the load is not connected. Hence, the automatic tripping of a CB on a feeder upon detection of complete loss of voltage on the bus bar may be required. This is achieved by a three phase under voltage protection relay.

2. What are Boost Charging and Float Charging of Battery?

To keep a battery in healthy condition and to achieve optimum life of the battery, two modes of Charging Boost charging and Trickle/Float charging are required.

i) Boost Charging:

Boost charging means, charging of a battery from discharged condition to fully charged condition. In this mode constant current process is followed for efficient charging.

The Boost mode is basically meant for quick charging the battery, after a heavy discharge, so as to restore the capacity of the battery within minimum time.

ii) Trickle / Float Charging:

Trickle charging means, charging a battery at constant voltage to compensate local discharge.

The float mode is meant for supplying the DC continuous load at constant voltage mode and at the same time, trickle charging the battery to keep it in fully charged condition.

3. Why Neutral Link is used in electrical circuits?

Neutral link is used to provide manual isolation of the circuit, clearing faulty downstream, by means of switchgear and preventing short circuit and protecting electric circuit from damage caused by overload, in the form of circuit breaker and so on.

4. What is the problem when Earth is using as Neutral path?

1.The earth fault current reflects as over current in primary winding of the distribution transformer. If the circuit is using the same earth for neutral then in any case of fault the over current flows through the transformer. It leads to transformer windings overheating.

2.In case of any fault, high currents flow through the earth wire also which is not intended for safety reason.

  1. The function of ELCB is violated as it won’t break the circuit due to no difference between amount of live and neutral currents.