Switchgear Protections MCQ Questions & Answers | Electrical Engineering

Distance relays are primarily used for the protection of transmission lines in power systems.

They operate based on the ratio of voltage to current (i.e., impedance) seen at the relay location. When a fault occurs, the impedance between the relay and the fault point drops. The relay measures this drop and operates if the impedance falls within a preset value.

Impedance relays are designed to detect this change in impedance and hence are best suited for distance protection.

Split-phase relays are used for other specialized protection schemes and are not suitable for distance protection.

Reactance relays are a type of distance relay, but they are more sensitive to resistive faults and are mainly used in specific conditions like high resistance ground faults. They are not as commonly used for general distance protection as impedance relays.

Therefore, impedance relays are the standard and most widely used type of distance relays.

A fuse in a motor circuit primarily provides protection against short-circuit and overload conditions.

Short-circuit Protection:
In the event of a short-circuit, where a low-resistance path is created between two conductors, a large current surge flows through the circuit. This can damage the motor and other components. The fuse interrupts the circuit when the current exceeds its rated value, preventing damage by quickly melting and opening the circuit.

Overload Protection:
Overload conditions occur when the current drawn by the motor exceeds its rated value for an extended period. This can happen due to mechanical issues, such as increased friction, or electrical issues, such as voltage fluctuations. The fuse protects against overload by melting and opening the circuit when the current exceeds its rated value for a sustained period.

Therefore, Option B correctly identifies that a fuse in a motor circuit provides protection against both short-circuit and overload conditions.