Exam Question
Explain why EACH of the following protective devices are fitted to a main electrical switchboard:
(a) Reverse power (4)
(b) Under voltage (4)
(c) Main circuit breaker overcurrent (4)
(d) Preferential trips (4)
Class 4 (4th Engineer)
MET
📅 Nov 2022
Reference Answer
### (a) Reverse Power Protection
**1. Purpose and Principle:**
A reverse power trip is a critical protective device fitted to the circuit breaker of each alternator intended for parallel operation. Its primary function is to protect the prime mover (typically a diesel engine) from mechanical damage. When multiple alternators are synchronized and connected to the main bus bars, they share the total electrical load. If the prime mover of one alternator fails (e.g., due to fuel starvation, governor malfunction, or mechanical failure), it will no longer be able to supply torque to the alternator rotor. Consequently, the alternator stops supplying real power (kW) to the system. However, since its field is still excited and it remains connected to the live bus bars, it will begin to draw power from the other healthy alternators, operating as a synchronous motor. This phenomenon is known as 'motoring'.
**2. Consequences of Motoring:**
**1. Purpose and Principle:**
A reverse power trip is a critical protective device fitted to the circuit breaker of each alternator intended for parallel operation. Its primary function is to protect the prime mover (typically a diesel engine) from mechanical damage. When multiple alternators are synchronized and connected to the main bus bars, they share the total electrical load. If the prime mover of one alternator fails (e.g., due to fuel starvation, governor malfunction, or mechanical failure), it will no longer be able to supply torque to the alternator rotor. Consequently, the alternator stops supplying real power (kW) to the system. However, since its field is still excited and it remains connected to the live bus bars, it will begin to draw power from the other healthy alternators, operating as a synchronous motor. This phenomenon is known as 'motoring'.
**2. Consequences of Motoring:**
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