Relay protection operating time should be fast enough to isolate faults selectively while coordinated with upstream and downstream relays to minimize system disruption.

Key Principles

Selectivity and Coordination: Relay operating time must ensure that only the relay closest to the fault operates first, isolating the faulted section without affecting the rest of the network. This is achieved through time-graded protection, where downstream relays have shorter operating times and upstream relays have longer times to provide backup protection . Time Characteristics:

• Definite Time Relays: Operate after a fixed time regardless of fault current magnitude.
• Inverse Time Relays: Operating time decreases as fault current increases, allowing faster clearing of severe faults while maintaining selectivity for smaller faults . Instantaneous Protection: Some relays, such as instantaneous overcurrent relays (ANSI 50), operate with minimal or zero intentional delay (typically 20–50 ms) for very high fault currents, providing rapid fault clearance .

Factors Affecting Operating Time

1. Plug Setting Multiplier (PSM): Determines how many times the fault current exceeds the relay pickup current. Higher PSM results in faster relay operation .
2. Time Setting Multiplier (TSM): Scales the base operating time from the relay's characteristic curve. Lower TSM values produce faster tripping .
3. Fault Current Magnitude: In inverse time relays, higher fault currents reduce operating time, enhancing protection speed .
4. Relay Type and Mechanism: Electromechanical relays have inherent mechanical travel times, while digital relays can operate faster due to electronic processing .

Practical Guidelines

• For radial networks, inverse time relays are preferred to speed up operation at high fault currents while maintaining selectivity .
• Relays should start operating when the energizing current exceeds 1.1–1.3 times the set pickup current, depending on the relay's time characteristic .
• Time grading should be applied so that downstream relays trip first, and upstream relays provide backup with slightly longer operating times .
• Instantaneous relays are used for very high fault currents to minimize damage and ensure rapid isolation . In summary, relay operating time must balance speed and selectivity, using a combination of definite, inverse, and instantaneous characteristics, coordinated through PSM and TSM settings, to protect equipment effectively while minimizing unnecessary outages .