The relay itself has aged and trips at a lower current than it should. The motor is fine, the power is fine -- the relay is just old and unreliable.

What you'll see

Intermittent overload trips with no apparent cause. Motor current is normal and balanced when measured, voltage is good, element turns freely -- but the relay keeps tripping. More common on older machines. The relay might trip after running for a while (heat-related sensitivity) or randomly.
Before you assume this is the problem

Verify that the motor current is truly normal before blaming the relay. Use a clamp meter during loaded operation on all three phases. If current is genuinely within nameplate rating, the relay is the likely culprit.

See all causes of overload relay trips →

How to diagnose

  1. Measure actual motor current

    Clamp all three phases during loaded running. Compare to the motor nameplate rating. If current is within spec (or below) and the relay still trips, the relay is faulty. The bimetal elements inside age and become more sensitive over time.
    Result: Normal current + trips = worn relay.

  2. Check the relay setting

    The relay has an adjustable current range. Verify it's set to match the motor's full-load amperage (FLA from the nameplate). Someone may have set it too low.
    Result: Setting too low = adjust to correct FLA.

How to fix it

  1. Replace the overload relay

    Install a new relay with the same current range. Set it to the motor's nameplate FLA. This is a very common and satisfying fix on older machines -- new relay, problem gone. These are also called 'thermal blocks'. Remember: BLUE button is reset, RED button is test.

Common mistakes

Never replace the relay with a higher-rated one to 'prevent nuisance trips'. The relay is sized to protect the motor. A higher-rated relay won't trip when it should, and the motor burns out. Replace with the same rating.

Parts & tools

Replacement thermal overload relay (same current range as original). Clamp meter for verifying motor current before replacing.

Review safety precautions before starting →