Electrical Interview Questions: Series-2

Electrical Interview Questions Series-2

In this series of Interview questions, we will see 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 of the interviews.

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

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 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 decreased depends on several factors like the type of fault, 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 the fault occurred 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 under-voltage 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 MCCB.

Thermal trip unit:

It is made up of a bimetallic thermal device that actuates the opening of a circuit breaker with delay depending on the over-current 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 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 currents are 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 Neutral Link is used in electrical circuits?

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

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

1. The earth fault current reflects as over current 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 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.