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 the 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?
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 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) 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. How the over load 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 thermal trip unit of MCCB.
Thermal trip unit:
It is made up by a bimetallic thermal device which actuates the opening of a circuit breaker with delay depending on the over current value. Thermal trip unit allow high but not too much 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 magnetic trip unit of MCCB.
Magnetic trip unit:
Made up by an electromagnetic device, with fixed or adjustable threshold, which actuates the opening of a circuit breaker on a predetermined over current 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?
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.
5. 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.
3. The function of ELCB is violated as it won’t break the circuit due to no difference between amount of live and neutral currents.