Set the overload relay to 115–125% of the motor's full-load amperes (FLA), adjusted for service factor and operating conditions, to protect against sustained overcurrent while allowing normal startup currents.

Understanding Overload Relays

An overload relay protects motors from overheating and winding damage by monitoring current flow and disconnecting power if excessive current persists for a prolonged period. It allows brief inrush currents during motor startup without tripping, but trips if the motor draws too much current for an extended time, signaling the contactor to cut off power . Key components include:

• Current Limit Adjuster: Sets the maximum current before tripping, usually based on the motor's FLA .
• Auxiliary Contacts (95–96 NC, 97–98 NO): Provide status signals when an overload occurs .
• Overload Signal Light: Indicates when the relay has tripped .

Calculating the Overload Setting

1. Obtain Motor Data: Record the motor's FLA, service factor (SF), and other nameplate data such as voltage, power, and insulation class .
2. Determine Base Setting:
   • For motors with SF ≥ 1.15, set the relay to 125% of FLA.
   • For motors with SF < 1.15, set to 115% of FLA .
3. Adjust for Startup Conditions: If the motor frequently starts under load, some standards allow increasing the setting up to 140% of FLA for short durations to prevent nuisance trips .
4. Consider Trip Class: Thermal overload relays have trip classes (Class 10, 20, 30) indicating the time to trip at six times rated current. Choose a class suitable for the motor's duty cycle .
5. Environmental Adjustments: Reduce the setting slightly for high ambient temperatures or altitude to enhance protection .

Example

For a motor with FLA = 100 A and SF = 1.15:

• Correct pickup = 125% × 100 A = 125 A
• Incorrectly using SF current as base could result in 132 A, allowing dangerous overloading .

Setting Procedure

1. Turn the current limit dial on the overload relay to the calculated amperage.
2. Verify the relay type (thermal or electronic) and ensure it matches the motor's characteristics.
3. Test the system under controlled conditions to confirm the relay trips appropriately without nuisance trips.
4. Perform routine maintenance to ensure reliable operation over time .

Best Practices

• Always use motor nameplate FLA for calculations, not measured operating current .
• Match the trip class to the motor's starting and load conditions.
• Adjust for ambient temperature and load variations.
• Document all settings and verify against manufacturer recommendations and local safety standards. By following these steps, the overload relay will provide effective protection against sustained overcurrent while allowing normal motor operation and startup transients.