How to adjust the current of a relay protector

The current of a relay protector is adjusted by setting the relay's pickup current, typically using coil taps or plug settings, expressed as a percentage of the CT's rated secondary current.

Understanding Relay Current Settings

Pickup Current (Ip): This is the minimum current at which the relay begins to operate, overcoming the controlling force of the relay coil. It ensures the relay only trips when the current exceeds a predefined threshold, protecting equipment from overcurrent or faults . Current Setting / Plug Setting: The pickup current can be adjusted by changing the number of active turns in the relay coil using a plug bridge or taps. This is expressed as a percentage of the CT's rated secondary current. For example, if a relay is connected to a 1 A CT and you want it to operate at 125% of rated current, the pickup current is set to 1.25 A . Plug Setting Multiplier (PSM): PSM is the ratio of the actual fault current in the relay to its pickup current. A higher PSM results in faster relay operation according to the inverse time-current characteristic curves .

Step-by-Step Adjustment

1. Verify CT Rating: Ensure the current transformer ratio matches the system load and fault levels. The CT secondary current should align with the relay's rated pickup current .
2. Determine Desired Pickup Current: Decide the current level at which the relay should operate. For overcurrent relays, typical settings range from 50% to 200% of the CT secondary current; for earth fault relays, 10% to 70% is common .
3. Set the Relay Plug or Coil Taps: Adjust the plug bridge or coil taps to achieve the desired pickup current. Each tap changes the number of active turns, modifying the deflecting force required to operate the relay .
4. Adjust Time Settings (Optional): Use the Time Setting Multiplier (TSM) to scale the operating time according to coordination requirements. Lower TSM values result in faster tripping, while higher values provide backup protection .
5. Test the Relay: After adjustment, simulate fault conditions to verify that the relay operates at the intended current and time settings. Ensure proper coordination with upstream and downstream relays to avoid unnecessary tripping .

Overload Relay Considerations

For motor overload relays, the setting is usually based on the motor's full-load amperes (FLA). The relay should be set slightly above the FLA (typically 115–125%) to prevent nuisance trips while still protecting the motor from overheating .

Key Tips

• Always ensure the pickup current is above the maximum load current but below the minimum fault current.
• Follow manufacturer manuals and IEC/IEEE standards for accurate settings.
• Regularly maintain and test relays to ensure reliable operation over time. By carefully adjusting the pickup current and related settings, a relay protector can effectively safeguard electrical equipment while maintaining proper system coordination.