How to Test a Generator Overload Relay

What the overload relay does

The overload relay is the component that protects the alternator from damage when the connected load exceeds the generator's rated output. When current through the relay coil rises above a threshold, the contacts open and interrupt the circuit, which typically trips the main breaker or disconnects the output.

Because the relay's job is to open under stress, a relay that sticks closed is a safety hazard. A generator that never trips under overload can overheat the winding insulation, and the generator may continue producing power into a fault until something fails. See Generator Overload Protection for the full picture.

• Normally closed (NC) contacts pass current during normal operation.
• The coil senses line current; at a set point it pulls the contacts open.
• Some relays also provide a manual reset lever or push button.
• The relay is separate from the output breaker, though both may sit in the same panel.

Safety first: carbon monoxide and electricity

Work on the relay only with the generator shut off and the battery disconnected if present. Relays are low-current devices, but the output circuit they control carries lethal voltage. Do not remove the control panel cover with the engine running.

If you run the generator as part of the test, operate it outdoors away from windows, doors, and vents. The CPSC reports that portable generators produce large amounts of carbon monoxide, which is odorless and invisible, and can kill in minutes. The CPSC and CDC both direct that you never operate a generator inside a home or garage, and always use a battery-powered CO alarm nearby. See Generator CO Poisoning Risks and Safe Placement Distances and What Does a CO Sensor on a Generator Do and How It Works for more.

Because the trip test involves a live generator, keep a working fire extinguisher rated for electrical equipment within reach, and confirm the generator sits on a stable, dry, level surface.

Multimeter checks on the relay

Before testing the trip function, verify the relay itself is sound. A relay that passes voltage when it should and opens when it should may still have internal contact resistance that causes overheating under sustained load. Set the multimeter to the lowest ohms range and measure resistance across the NC contacts. The reading should be near zero ohms, typically below 1 ohm. A reading above 2 ohms suggests pitted or dirty contacts.

Measure the coil resistance across the two coil terminals. The expected value varies by design, but a coil that shows zero ohms or an open circuit is defective. Compare your reading to the value printed on the relay body if one is present. A coil reading far from the marked value usually means the winding is damaged.

Inspect the relay case for heat discoloration, cracks, or a swollen appearance. Any of those signs indicate the relay has been subjected to repeated overload trips or sustained overcurrent, and replacement is the safer choice.

For reference on other electrical checks you can perform on the generator, see How to Test Generator AVR Output with a Multimeter and How to Test a Generator Outlet with a Multimeter.

Low-voltage trip test

The low-voltage trip test checks that the relay opens when the coil sees an overcurrent condition. With the generator off, disconnect the relay from the wiring harness. Apply a controlled low-voltage DC signal across the coil terminals. The relay should click audibly and continuity across the NC contacts should break within a second.

If the relay does not click, the coil is likely faulty or the armature is seized. If the relay clicks but the NC contacts still show continuity, the contact mechanism is stuck. Both conditions require replacement.

To simulate an actual overload rather than a simple coil activation, connect the relay in series with a test load that draws current above the relay's rated trip point. The exact trip curve varies by relay design, so consult the relay body markings or the generator service manual for the rated current and time delay. If the relay fails to open within several seconds of sustained overcurrent, replace it.

After the trip test, manually reset the relay if it has a reset lever, then confirm the NC contacts close again. A relay that does not reset cleanly or that sticks in the open position should be replaced.

See Checking Generator Fuses and Relays and How to Load Test a Generator for related checks.

• A healthy coil should produce an audible click when energized.
• NC contacts should open within the relay's trip time under overcurrent.
• After tripping, the relay should reset to closed position.
• If the relay trips below or above its rated threshold, it is out of calibration and should be replaced.

Contact surface inspection and cleaning

Even if the relay passes the continuity and trip tests, the contact surfaces may be eroded from arcing. Arcing happens every time the contacts open under load, and over many trips the silver-alloy surface develops pits or a dark oxide film. Heavy pitting reduces the effective contact area and increases resistance, which causes further heating and faster wear.

If the contacts are only lightly darkened, you can dress them with a fine contact burnishing file or a piece of 400-grit abrasive paper. Do not use a metal file, because it removes too much material and leaves rough edges. After cleaning, wipe the contacts with a lint-free cloth and recheck the contact resistance.

If the contacts are deeply pitted or the silver layer is worn through to the base material, the relay is at the end of its service life. Replace it rather than risk a welded contact that leaves the generator without overload protection. See What Size Breaker Does My Generator Need for breaker sizing, and How to Replace a Generator Circuit Breaker when the breaker itself is suspect.

Replacing the relay and checking the system

If the relay fails any test, order a replacement with the same model number or an electrically equivalent part. Match the coil voltage, contact current rating, and trip curve to the original. A relay with a higher coil voltage or a slower trip curve will not protect the generator the way the original did.

After installing the new relay, perform the same continuity and trip tests before bolting the panel back on. Then run the generator under a known load that does not exceed the rated output to confirm the relay stays closed during normal operation.

An overload relay that trips frequently even under a load within the generator's rating may indicate a different problem, such as a failing alternator bearing, a shorted winding, or an undersized breaker. Check the generator's rated output against the connected load, and see Rated Watts vs Max Watts and Generators with Wattage Meters for how to measure actual draw. If the load is correct and the relay still trips, inspect the alternator and the output wiring for shorts.

Record the relay checks in your maintenance log. A simple note with the date, the resistance readings, and the trip test result helps you spot drift over time before it becomes a failure. See Generator Maintenance for how to schedule that.

What to pick for your use

Questions

How do I know if my generator overload relay is bad?

Signs include: the generator trips the breaker for no apparent reason, the relay does not click when the coil is energized, the NC contacts show high resistance in continuity mode, or the relay does not open when subjected to an overcurrent. If the relay never trips under a load that should cause it to trip, the contacts are likely welded closed.

Can I test a generator overload relay without running the generator?

Yes. With the generator off and the relay disconnected, you can measure coil resistance and contact continuity with a multimeter. You can also apply a low-voltage DC signal to the coil to confirm the armature moves and the contacts open. To test the actual trip point, you need to pass current through the coil, which is easiest to do with a test bench supply.

What resistance should a generator overload relay coil have?

There is no universal value. The coil resistance depends on the relay design, coil voltage, and trip current rating. Compare your reading to the value stamped on the relay body or given in the generator service manual. A coil that reads open (infinite resistance) or zero ohms is defective.

Can I clean the contacts on an overload relay?

You can dress lightly darkened contacts with a fine burnishing file or fine abrasive paper, then wipe away any residue. Do not use a metal file or sandpaper coarser than 400 grit, as that removes too much silver alloy. If the contacts are deeply pitted or the silver is worn through, replace the relay.

Why does my generator trip the overload relay even with a small load?

A trip under a small load points to a fault elsewhere. Check the alternator windings for a short to ground, confirm the output breaker is the right size, and verify the relay matches the original rating. A failing bearing or a seized rotor can also draw excess current that makes the relay trip.

Is the overload relay the same as the circuit breaker?

No. The circuit breaker protects the wiring and the connected load from high fault currents, and it usually has a visible switch you can reset. The overload relay senses sustained overcurrent and protects the alternator from thermal damage. Some generator control panels integrate both functions, and testing them separately keeps the system reliable.