Overload Relay Keeps Tripping: 8 Causes and How to Fix Them

An overload relay that keeps tripping is one of the most common problems in industrial motor circuits. The relay is usually doing its job. It is warning you that the motor is drawing too much current for too long. This guide explains the most common causes and gives you a safe, step-by-step way to find the real problem.

Safety first: Switch off and lock out the power before touching terminals or wiring. Work on live equipment only if you are qualified and your local rules allow it. Always follow the equipment manual and your local regulations.

What does an overload relay do?

A thermal overload relay protects a motor from running at too high a current for too long. When the current stays above the set value, a heating element warms a bimetallic strip inside the relay. The strip bends and opens a contact, which stops the contactor and the motor. This protects the motor windings from overheating and burning out.

8 common causes of overload relay tripping

1. The relay is set incorrectly

If the current setting is lower than the motor’s full-load current (FLA) on the nameplate, the relay trips even when the motor is healthy. Compare the setting dial with the nameplate value.

2. The motor is mechanically overloaded

A jammed conveyor, worn bearings, a blocked fan, a tight belt or a misaligned coupling all make the motor work harder and draw more current. Check that the load turns freely by hand with the power off.

3. Phase loss (single phasing)

If one phase is missing because of a blown fuse, a loose terminal or a worn contactor contact, the other two phases carry extra current and the relay trips. Measure the current on all three phases.

4. Voltage problems

Low supply voltage makes a motor draw more current to deliver the same power. Unbalanced voltage between phases can cause a much larger current imbalance. Measure the voltage at the motor terminals while it runs, not only at the panel.

5. Loose or corroded connections

A poor connection creates heat and resistance. Heat at the terminals can also warm the relay and cause nuisance trips. Check and retighten the terminals as the manufacturer recommends.

6. High ambient temperature

A relay inside a hot enclosure can trip earlier than expected, especially if it is not ambient-compensated. Check the enclosure temperature and the ventilation.

7. Motor winding or insulation fault

Damaged windings, shorted turns or moisture can raise the current. Test the insulation resistance with a megohmmeter (with the motor disconnected and locked out) and compare with the manufacturer’s values.

8. Frequent starts or a long starting time

Each start draws a high inrush current. Heavy loads with long acceleration times, or too many starts per hour, heat the relay and the motor. Check that the trip class of the relay suits the application.

Step-by-step troubleshooting

1. Lock out the power and note the relay setting and the motor nameplate FLA.
2. Check that the relay setting matches the nameplate.
3. Inspect the terminals, wiring and contactor contacts for heat damage or looseness.
4. Turn the load by hand to check for mechanical resistance.
5. Restore power and measure the current on each phase with a clamp meter. Compare it with the FLA and look for imbalance between phases.
6. Measure the supply voltage on all three phases at the motor.
7. If the problem remains, disconnect the load and run the motor empty. High current with no load points to the motor or its wiring.
8. If everything is normal, replace or test the relay.

Common mistake to avoid

Never raise the relay setting above the nameplate value just to stop the tripping. This hides the problem and can destroy the motor. Always find the cause.

When to call a specialist

If the motor smells burnt, the insulation test fails, or you cannot find the cause safely, stop and call a qualified electrician or motor repair specialist.

Quick summary

Symptom	Likely cause
Trips right at start	Setting too low, jammed load, phase loss
Trips after running for a while	Overload, bad ventilation, high ambient temperature
Trips at random	Loose connections, voltage problems
Trips with no load	Motor winding fault or wiring problem

Have you solved a tricky overload problem on your own equipment? Share it in the comments below.

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