Relay protection values are set by calculating the pickup current, time multiplier, and coordination parameters to ensure selective and reliable fault isolation.

Key Parameters for Relay Settings

1. Plug Setting Multiplier (PSM) PSM determines how many times the actual fault current exceeds the relay's pickup current. It is crucial for IDMT (Inverse Definite Minimum Time) relays to calculate operating time. Formula: PSM = Fault Current / Pickup Current Example: For a CT of 400/5 A, plug setting 120%, and fault current 10 A (secondary), Pickup = 1.2 × 5 = 6 A, PSM = 10 / 6 = 1.67. Higher PSM results in faster relay operation .
2. Time Setting Multiplier (TSM) TSM scales the base operating time from the relay's characteristic curve. It ensures proper coordination with downstream relays. Typical Values: Motor protection: 90–110% of FLA; Transformer protection: 110–130% of rated current . Formula: Operating Time = Base Time × TSM
3. Earth Leakage / Earth Fault Setting (EL) EL defines the threshold current at which the relay detects ground faults. It is often expressed as a percentage of CT primary or a secondary current value. Typical Values: LV systems: 30 mA–1 A; Industrial LV: 10–50 A; MV relays: 10–40% of CT primary . Purpose: Detects single-phase-to-earth faults that phase overcurrent relays may miss.
4. Overcurrent / Pickup Current Setting The pickup current is the minimum current at which the relay operates. It is usually set above the maximum load current to avoid nuisance tripping. Formula: I_pu = Rated CT output × %Current Setting .

Steps to Set Relay Protection Values

1. Determine System Parameters
   • Maximum load current, minimum fault current, CT ratios, and system voltage levels .
2. Select Relay Type and Characteristic Curve
   • Choose IDMT, very inverse, extremely inverse, or definite time based on application .
3. Calculate Pickup Current
   • Set above maximum load but below minimum fault current to ensure selectivity .
4. Compute PSM and TSM
   • Use fault current and relay curve to determine PSM.
   • Adjust TSM to coordinate with downstream relays, ensuring proper time grading .
5. Set Earth Fault / Leakage Values
   • Determine EL based on system sensitivity requirements and CT ratings .
6. Verify Coordination and Selectivity
   • Ensure upstream relays operate after downstream relays for the same fault.
   • Use coordination time interval (CTI) to maintain proper delay between relays .
7. Test and Validate
   • Perform secondary injection tests or simulation to confirm relay operation under fault conditions .

Practical Tips

• Always follow IEC standards (IEC 60255 series) for relay performance and IDMT characteristics .
• Document all settings and maintain a relay coordination study for future reference.
• Re-evaluate settings during commissioning or after system modifications . By carefully calculating and coordinating PSM, TSM, pickup current, and earth fault settings, relays can protect equipment effectively while maintaining system reliability and selectivity.