Relay protection settings are typically defined using parameters such as Plug Setting Multiplier (PSM), Time Setting Multiplier (TSM), overload (OL), earth leakage (EL), and coordination with system fault currents.

Key Relay Settings

1. Plug Setting Multiplier (PSM) PSM determines how many times the actual current exceeds the relay's pickup current. It is crucial for inverse definite minimum time (IDMT) relays, affecting the operating speed: higher PSM values result in faster tripping. Typical PSM values are set based on the maximum load current and expected fault currents, often ranging from 1.0 to 2.0 times the rated current for overcurrent relays, depending on system coordination requirements and IEC 60255-151 standards . 2. Time Setting Multiplier (TSM) TSM scales the relay's base operating time. It is used to achieve proper coordination between upstream and downstream relays. Lower TSM values result in faster tripping. In practice, TSM is selected to provide adequate grading time, often 0.5 to 1.5 seconds for definite time relays or adjusted according to the inverse time curve for IDMT relays . 3. Overload Setting (OL) Overload protection is typically set slightly above the maximum continuous load current to prevent nuisance tripping. Commonly, OL settings are 110–125% of the rated load current, depending on the thermal capacity of the protected equipment . 4. Earth Leakage / Earth Fault Setting (EL) Earth fault relays are set to detect ground faults while avoiding false trips from normal leakage currents. Typical EL settings are 10–30% of the phase current, depending on system grounding and sensitivity requirements . 5. Multiplying Factor (MF) MF or metering factor adjusts the relay input to account for current transformer ratios. It ensures that relay pickup corresponds to actual primary currents. MF is calculated based on CT ratios and is usually 1.0 for direct secondary connections or adjusted proportionally for scaled measurements .

Coordination and Time Grading

• Time-graded protection ensures the relay closest to the fault operates first. In radial networks, inverse time relays are preferred, with operating times decreasing as fault current increases .
• Definite time relays operate at a fixed delay, typically 0.8–1.0 seconds for backup overcurrent protection .
• Distance relays for transmission lines often set Zone 1 reach to 80–90% of line impedance, with Zone 2 and 3 providing backup coverage .

Practical Considerations

• Relay settings must consider maximum load current, minimum fault current, and system voltage levels to ensure sensitivity and selectivity .
• Coordination with downstream relays prevents unnecessary tripping and ensures reliable backup protection.
• Settings are validated during commissioning and may be adjusted based on measured system parameters . In summary, typical relay protection values are carefully calculated to balance speed, selectivity, and reliability, using PSM, TSM, OL, EL, and MF, along with proper time grading and coordination across the network .