Arcing Fault and Relay Operation

An arcing fault occurs when an electrical current jumps across a gap or air space between conductors. This can happen due to a breakdown in insulation, a loose connection, or other factors. Arcing faults are hazardous as they can generate high temperatures, intense light, and pressure waves, leading to damage to equipment and potential fire hazards. Therefore, it is crucial to detect and isolate these faults as quickly as possible.

Impedance and Admittance Relays

Impedance and admittance relays are commonly used in power systems for fault detection and protection. These relays measure the electrical characteristics of the power system to determine the presence of faults.

- Impedance Relay: An impedance relay is designed to detect changes in impedance (a combination of resistance, inductance, and capacitance) in the power system. It operates based on the magnitude and phase angle of the measured impedance. When a fault occurs, the impedance at the fault location changes, causing the relay to initiate a trip signal.

- Admittance Relay: An admittance relay, on the other hand, measures the admittance (the reciprocal of impedance) of the power system. It detects changes in the conductance and susceptance of the system. Similar to an impedance relay, an admittance relay operates based on the magnitude and phase angle of the measured admittance.

Effect of Arc Resistance

When an arcing fault occurs, the arc resistance plays a significant role in determining the behavior of the relay. The arc resistance depends on factors such as the arc length, arc temperature, and the characteristics of the surrounding medium.

- If the arc resistance is significant (relatively high), it can cause a decrease in the measured impedance/admittance at the relay location. This decrease in impedance/admittance leads to a reduced reach of the relay.
- Consequently, the relay may not be able to detect the fault accurately, resulting in under-reaching. Under-reaching means that the relay fails to trip for a fault that is within its intended protection zone.

Reason for Option 'B': Under-reach

The correct answer to the given question is option 'B': under-reach. This is because when the arc resistance is significant, the impedance or admittance relay will exhibit reduced reach due to the decrease in measured impedance/admittance. Therefore, the relay will not be able to detect faults within its protection zone accurately, resulting in under-reaching.