Thermal overload relays are simple devices.

That is the nice part.

But when a motor keeps tripping, refuses to start, or randomly stops in the middle of production, that “simple” overload relay suddenly becomes the main suspect. Sometimes it really is faulty. Many times it is only doing its job and protecting the motor from a real problem.

That is the important thing to remember.

A tripped thermal overload relay is not always the fault. It may be the warning.

What Does a Thermal Overload Relay Do?

A thermal overload relay protects an electric motor from overload current.

It monitors the current going to the motor. If the motor draws too much current for too long, the relay trips. When it trips, it normally opens a normally closed auxiliary contact in the contactor control circuit. This de-energizes the contactor coil and stops the motor.

So the overload relay usually does not switch the main motor power directly.

The contactor switches the motor.

The overload relay trips the contactor.

Small difference, but important.

Common Symptoms of Thermal Overload Relay Problems

Thermal overload relay faults, or overload-related motor faults, usually show up like this:

• Motor does not start
• Contactor pulls in, then drops out
• Motor starts but trips after a short time
• Motor runs for a few minutes, then overload trips
• Overload relay trips immediately
• Overload relay will not reset
• Motor hums but does not rotate
• One phase current is much higher than the others
• Control circuit has no voltage after overload contact
• Fault lamp stays on
• PLC shows motor overload fault
• Relay trips even when motor current looks normal

The last one is the tricky one. If the current is actually normal but the overload trips, then you may have a bad overload relay, wrong setting, high ambient temperature, poor connection, or mechanical problem affecting the relay.

But first — measure.

Guessing gets expensive.

Tools Needed for Diagnosis

You can diagnose most thermal overload relay problems with basic electrical tools.

Useful tools include:

• Digital multimeter
• Clamp meter
• Screwdriver set
• Wiring diagram or motor starter schematic
• Motor nameplate data
• Insulation resistance tester
• Phase sequence tester, optional
• Thermal camera, optional but useful
• Contact cleaner
• Known-good spare overload relay, if available
• PLC online monitor, if overload feedback is wired to PLC
• Current transformer information, if CTs are used

The most important tools are the multimeter and clamp meter.

The multimeter checks the control circuit, contacts, supply voltage, and continuity. The clamp meter tells you what current the motor is really drawing.

And with overload relays, real motor current is the truth teller.

Safety First

Before testing, remember that motor starters contain dangerous voltage.

You may have three-phase power, control voltage, contactor coils, rotating motor shafts, and moving machine parts. Lockout/tagout should be used when working on disconnected circuits. Live testing should only be done by someone qualified and with proper precautions.

Also, do not reset an overload relay again and again without finding the cause.

That is not troubleshooting. That is bullying the motor until it burns.

Step 1: Check the Motor Nameplate Current

Start with the motor nameplate.

Find the rated current. It may be marked as:

• In
• FLA
• Rated current
• Nominal current
• A

For example, a motor nameplate may show 4.8 A at 400 V.

The overload relay current dial should normally be set according to this value, depending on the motor starter design and local standards.

Good condition:

Bad condition:

If the motor current is 6.2 A and the overload relay range is 2.5–4 A, that is wrong.

If the motor current is 3 A and someone installed a 10–16 A overload relay, that is also wrong.

Not “close enough.” Wrong.

Step 2: Check the Overload Relay Setting

Look at the small current adjustment dial on the overload relay.

The setting should be compared with the motor nameplate current. Do not just assume it is correct because the machine used to work. Someone may have changed it. Happens more often than it should.

Good setting:

• Relay range matches motor current
• Dial is set near the motor rated current
• Setting is not turned fully up just to avoid trips
• Setting is not below normal motor current
• Reset mode is suitable for the machine

Bad setting:

• Dial set much lower than motor rated current
• Dial set much higher than motor rated current
• Relay current range does not match motor
• Automatic reset selected where manual reset is required
• Setting changed without checking motor current

A wrong setting is one of the easiest faults to find.

And also one of the most embarrassing.

Step 3: Check If the Overload Relay Is Tripped

Most overload relays have a trip indicator or reset button position that shows if the relay has tripped.

Check the relay status.

Good condition:

Bad condition:

After a real overload trip, the relay may need time to cool before it can reset. That is normal.

But if it cannot reset after cooling, suspect mechanical failure or a damaged overload relay.

Step 4: Check the Auxiliary Contacts

Thermal overload relays normally have at least two auxiliary contacts:

• NC contact for stopping the contactor
• NO contact for overload fault indication

Common terminal numbers are often:

• 95–96 = normally closed overload contact
• 97–98 = normally open fault contact

Always check the actual device marking.

With power off, use a multimeter in continuity mode.

Good readings when relay is not tripped:

Good readings when relay is tripped:

Bad readings:

A closed auxiliary contact should have very low resistance. Usually close to 0 Ω or at least well under 1–2 Ω, depending on meter leads and contact condition.

If you see unstable readings, high resistance, or open circuit when it should be closed, the contact may be faulty.

Step 5: Check Control Voltage Through the Overload Contact

Now check the control circuit.

The overload NC contact is usually wired in series with the contactor coil. If this contact is open, the contactor will not energize.

With the control circuit powered, measure voltage before and after the overload NC contact.

Good readings:

Examples:

For a 24 V DC control circuit, you should usually see around 24 V DC before and after the closed overload NC contact.

For a 230 V AC control circuit, you should usually see around 230 V AC before and after the closed overload NC contact.

Bad readings:

A closed overload contact should not create a noticeable voltage drop.

If you measure a voltage drop across a closed NC contact, that is suspicious. It may mean a bad contact or loose connection.

Step 6: Test the Trip Button

Most overload relays have a test button.

Pressing the test button should simulate a trip and change the auxiliary contacts.

Good result:

• NC contact opens
• NO contact closes
• Contactor drops out
• Fault lamp or PLC input activates
• Relay can be reset afterward

Bad result:

• Nothing changes
• Contactor stays energized
• Fault indication does not work
• Relay cannot reset after test
• Test button feels jammed

This test confirms the control circuit reaction. It does not prove the thermal part trips at the correct current, but it is still useful.

If the test button opens the NC contact and the contactor drops out, the control wiring is probably correct.

Step 7: Measure Motor Current With a Clamp Meter

This is the big one.

Clamp each motor phase one at a time while the motor is running.

Compare the measured current with the motor nameplate current and overload setting.

Good readings:

Bad readings:

As a rough guide, phase currents should usually be reasonably balanced. A small difference is normal. A large difference is not.

If one phase is much higher or much lower than the others, do not ignore it.

That is often where the real fault is hiding.

Step 8: Compare Current to Overload Setting

Let’s say the motor nameplate current is 5.5 A.

If the motor is running at 5.0–5.4 A, that is usually okay.

If it is running at 6.5 A continuously, the overload relay may trip because the motor is overloaded.

If the overload relay is set to 4.5 A, it may trip even though the motor is not overloaded according to its nameplate.

Good comparison:

Do not raise the overload setting above the proper value just to stop nuisance trips.

Find out why it is tripping.

Yes, slower. But cheaper than a burned motor.

Step 9: Check for Phase Loss

Phase loss is a common reason for overload trips.

A three-phase motor that loses one phase may continue running, but the current in the remaining phases can increase heavily. This creates heat and can quickly damage the motor.

Check voltage phase-to-phase at the motor starter output and motor terminals.

Typical good readings in a 400 V system:

Bad readings:

Phase loss can be caused by:

• Blown fuse
• Bad contactor pole
• Loose terminal
• Damaged cable
• Broken motor winding
• Faulty breaker
• Bad connection in junction box

If overload relay trips and one phase current is missing, do not just reset it. Find the lost phase.

Step 10: Check Contactor Contacts

A bad contactor can cause overload trips.

If one main contact is burned, loose, or high resistance, the motor may receive poor voltage on one phase. This can create current imbalance and overheating.

Check voltage across each contactor pole when closed.

Good condition:

• Very low voltage drop across each closed contact
• Similar voltage on all three output phases
• No overheating at terminals
• No visible burning

Bad condition:

• High voltage drop across one contact
• One phase voltage lower after contactor
• Burned or pitted contacts
• Terminal discoloration
• Hot contactor pole
• Motor current imbalance

A thermal camera can be very helpful here. A hot terminal or contactor pole often tells the story before the meter does.

Not always. But often.

Step 11: Check Terminal Tightness and Heating

Loose terminals create heat.

Heat affects overload relays. It also damages cables, contactors, and motor starters.

Inspect all terminals:

• Supply terminals
• Contactor terminals
• Overload relay terminals
• Motor output terminals
• Control circuit terminals
• Motor junction box terminals

Good condition:

• Terminals tight
• No discoloration
• No melted plastic
• No burned smell
• Similar temperature between phases
• No damaged wire insulation

Bad condition:

• Loose terminal
• Brown or black discoloration
• Melted insulation
• Burned smell
• One phase terminal much hotter
• Screw damaged or stripped

A loose terminal can cause voltage drop, phase imbalance, and overload trips.

It can also become a fire risk.

Not a small thing.

Step 12: Check the Motor Mechanically

If the motor current is high, the overload relay may be doing exactly what it should.

Check the mechanical load.

Possible mechanical causes include:

• Jammed conveyor
• Blocked pump impeller
• Fan obstruction
• Gearbox problem
• Worn bearings
• Misalignment
• Too much belt tension
• Product jam
• Machine running overloaded
• Dirty filter or blocked airflow

Good mechanical condition:

• Motor turns freely when safe to check
• Load moves normally
• Bearings sound normal
• No abnormal vibration
• Current drops when load is reduced

Bad mechanical condition:

• Motor difficult to turn
• Pump blocked
• Conveyor jammed
• Gearbox hot
• Bearing noisy
• Belt too tight
• Current rises with load
• Motor trips only under production load

If the motor runs fine uncoupled but trips under load, the problem is probably not the overload relay.

The machine is asking too much from the motor.

Step 13: Check Motor Insulation

If a motor repeatedly trips overload or draws abnormal current, check insulation resistance.

Use an insulation resistance tester only with the motor disconnected and isolated from drives, electronics, and control devices.

Good insulation readings depend on motor voltage, size, environment, and standards, but as a practical maintenance rule:

Bad signs:

A common quick rule used in many workshops is that readings should be in the megaohm range, not kiloohms. For critical motors, follow your company standard or motor manufacturer requirements.

Do not perform insulation tests through VFDs, PLC outputs, soft starters, or electronic protection devices. Disconnect properly first.

Step 14: Check Reset Mode

Thermal overload relays may have manual or automatic reset.

Manual reset is usually safer for machinery because someone must inspect and reset the fault.

Automatic reset can restart a motor unexpectedly if the control circuit still commands it to run. That can be dangerous.

Good setup:

Bad setup:

• Automatic reset used on machinery where unexpected restart is dangerous
• Reset mode changed without checking safety
• Overload resets and motor restarts repeatedly
• Fault is hidden because relay resets by itself

If the motor keeps stopping and restarting, check whether automatic reset is enabled. That can make the fault harder to notice and more dangerous.

Step 15: Check Overload Relay Temperature and Environment

Thermal overload relays work with heat, so the surrounding temperature matters.

If the control panel is very hot, the relay may trip earlier than expected. Some relays have compensation, but heat can still affect performance.

Check for:

• Hot control cabinet
• Poor ventilation
• Overload relay mounted near heat source
• Loose terminals generating heat
• Contactor heat
• High ambient temperature
• Dust blocking ventilation

Good condition:

• Cabinet temperature reasonable
• No local overheating
• Terminals cool and tight
• Relay not installed near excessive heat
• Motor current matches setting

Bad condition:

• Relay body very hot
• One terminal hotter than others
• Cabinet overheating
• Relay trips at normal motor current
• Heat damage visible

If current is normal but the overload trips in a hot panel, temperature may be contributing.

A thermal camera is very useful here. It quickly shows hot spots that your eyes miss.

Step 16: Check Current Transformers on Large Motors

For larger motors, the overload relay may be connected through current transformers.

In that case, check:

• CT ratio
• CT wiring
• Correct secondary current
• Correct overload relay setting
• Loose CT secondary terminals
• Open CT secondary circuit
• Wrong phase routing
• Incorrect CT orientation, if relevant to the system

Good condition:

• CT ratio matches design
• Secondary current proportional to motor current
• Overload relay setting calculated correctly
• CT wiring secure
• All three phases balanced

Bad condition:

• Wrong CT ratio
• CT secondary open
• One CT not wired
• Incorrect setting after CT replacement
• One phase current missing at relay
• Relay trips incorrectly or never trips

CT circuits must be handled carefully. An open CT secondary can create dangerous voltage. Use proper procedures.

No guessing here.

Good and Bad Readings Summary

Here is a practical quick-reference table.

Common Faults and Likely Causes

When the Overload Relay Is Actually Faulty

Thermal overload relays can fail, but they should not be blamed first.

Possible overload relay faults include:

• Auxiliary contact stuck open
• Auxiliary contact stuck closed
• Trip mechanism jammed
• Current dial damaged
• Relay will not reset
• Relay trips at normal current
• Heat damage
• Burned terminals
• Mechanical damage
• Internal bimetal fatigue after long service

The relay is more likely faulty if:

• Motor current is normal
• Phase currents are balanced
• Voltage is correct
• Mechanical load is normal
• Current setting is correct
• Cabinet temperature is normal
• Relay still trips repeatedly

At that point, replacing the overload relay is reasonable.

But replacing it before checking current is just gambling with a screwdriver.

Final Troubleshooting Checklist

Use this checklist when diagnosing thermal overload relay faults:

1. Check if the overload relay is tripped.
2. Read the motor nameplate current.
3. Confirm overload relay current range.
4. Check the current dial setting.
5. Test NC and NO auxiliary contacts.
6. Measure control voltage before and after the NC contact.
7. Press the test button and confirm contact operation.
8. Measure motor current on all phases.
9. Compare motor current with the overload setting.
10. Check phase-to-phase voltage.
11. Look for phase loss or imbalance.
12. Inspect contactor main contacts.
13. Check terminal tightness and heating.
14. Inspect mechanical load.
15. Check motor insulation if needed.
16. Verify manual or automatic reset mode.
17. Check cabinet temperature and heat sources.
18. Check CT wiring and ratios on large motors.
19. Replace the overload relay only after confirming the basics.

Final Thoughts

A thermal overload relay is not complicated, but it is very important.

It protects a motor from running too hot for too long. When current rises above the set value, the relay heats internally, trips, opens the control circuit, and drops out the contactor. The motor stops before the damage becomes worse.

When diagnosing faults, do not start by turning the current dial higher.

Start with the motor nameplate. Check the setting. Measure the real current. Compare all three phases. Check the auxiliary contacts. Follow the control voltage through the relay. Look for mechanical overload, phase loss, bad contactor poles, loose terminals, and overheating.

Most overload trips are not mysterious.

Sometimes the relay is faulty. But often, it is just the messenger — and the message is: the motor is working harder than it should.