Thermal relay protection device setting value

Thermal relay protection devices are typically set based on motor full-load current and service factor, with pickup values around 115–125% of FLA to ensure proper overload protection.

Key Setting Parameters

1. Pickup Current (Overload Setting): The pickup current is the threshold at which the thermal relay begins to operate. It is usually set as a percentage of the motor's full-load amps (FLA). For motors with a service factor of 1.0, the pickup is typically 115% of FLA, while for motors with a service factor of 1.15 or higher, it is 125% of FLA . Setting the pickup too high may fail to protect the motor from moderate overloads, while too low a setting can cause nuisance trips . 2. Time Setting (Thermal Time Constant): Thermal relays operate based on the I²t principle, where the tripping time depends on the square of the current over time. The relay's thermal time constant defines how long it takes to trip at a given multiple of the rated current. Modern microprocessor relays calculate accumulated heat and trip before the motor reaches its hot stall limit . 3. Plug Setting Multiplier (PSM) and Time Setting Multiplier (TSM): For IDMT (Inverse Definite Minimum Time) relays, PSM indicates how many times the actual current exceeds the relay pickup, affecting the operating speed, while TSM scales the base operating time to coordinate upstream and downstream relays . Higher PSM results in faster tripping, and lower TSM reduces trip delay.

Practical Adjustment Guidelines

• Mid-range setting: Position the current potentiometer near the motor's rated current within the adjustable range to balance protection and tolerance .
• Voltage considerations: If the network experiences voltage drops, recalculate the maximum motor current and adjust the setting slightly below the nameplate value to prevent nuisance trips .
• Monitoring: Watch for warning signs such as tripping without overload (too low setting) or failure to trip under real overload (too high setting) to fine-tune the relay .

Calculation Example

For a 15.2 A motor with a service factor of 1.15:

• Pickup current = 125% × 15.2 A = 19 A
• Set the relay potentiometer to 19 A (mid-range of the device)
• Verify that the thermal time constant allows tripping before the motor exceeds its thermal limit .

Summary

Correct thermal relay settings ensure motor protection without unnecessary downtime. The main steps are:

1. Determine motor FLA and service factor.
2. Set pickup current at 115–125% of FLA depending on service factor.
3. Adjust thermal time constant to match motor thermal characteristics.
4. Verify settings against network conditions and relay coordination requirements. Following these principles ensures reliable operation, prevents motor damage, and avoids nuisance tripping in industrial and commercial electrical systems .