Electrical Interview Questions: Series-2

Electrical Interview Questions Series-2

In this series of Interview questions, we will cover some Protection Electrical-related questions.

Protection Electrical Interview Questions:

For any Electrical job, knowledge of protection is essential. We can anticipate these protection-related questions in most interviews.

1. Why is under-voltage protection required in a power system network?
2. How are overload and short-circuit protections provided in a Molded Case Circuit Breaker (MCCB)?
3. What is the purpose of motor thermal protection?
4. Why is a neutral link used in electrical circuits?
5. What is the problem when Earth is using a Neutral path?

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

i) Failure of voltage regulating equipment.

When the system load increases, the voltage falls below the rated, in this case, corrective action would be taken by the voltage regulating equipment such as AVRs 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) Coordination with other voltage and current-based protection devices.

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

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

When a fault occurs on the incomer or bus bar itself results in a 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 the supply voltage is restored, the load is not connected. Hence, the automatic tripping of a CB on a feeder upon detection of a complete loss of voltage on the bus bar may be required. This is achieved by a three-phase undervoltage protection relay.

2. How the overload and short circuit protections will provide in a Molded Case Circuit Breaker (MCCB)?

Molded case circuit breakers are used to provide circuit protection for low-voltage distribution systems. They will protect connected devices against both overloads and short circuits.

• Overload protection is provided by the thermal trip unit of an MCCB.

Thermal trip unit:

It is made up of a bimetallic thermal device that actuates the opening of a circuit breaker with a delay depending on the overcurrent value. Thermal trip units allow high but not too many currents for the given delay time. So it is possible to avoid unnecessary tripping due to current spikes that last for a brief moment.

• Short-circuit current protection is provided by the magnetic trip unit of the MCCB.

Magnetic trip unit:

Made up of an electromagnetic device with a fixed or adjustable threshold, which actuates the opening of a circuit breaker on a predetermined overcurrent value. If the current is very high even for a short time it will trip the breaker.

3. What is the purpose of motor thermal protection?

The purpose of motor thermal protection is to allow the motor to start and run within the given manufacturer-specified limits but trip if the motor heat energy exceeds those ratings because of overloads, negative-sequence current, or locked-rotor starting.

4. Why is a neutral link used in electrical circuits?

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

5. What is the problem when Earth is using a Neutral path?

1. The earth fault current reflects as overcurrent in the primary winding of the distribution transformer. If the circuit is using the same earth for neutral, then in any case of fault, the overcurrent flows through the transformer. It leads to the transformer windings overheating.
2. In case of any fault, high currents flow through the earth wire also, which is not intended for safety reasons.
3. The function of the ELCB-Earth Leakage Circuit Breaker is violated, as it won’t break the circuit due to no difference between the amount of live and neutral currents.