Testing a motor starter involves checking each of its major components individually: the contactor coil, the main contacts, the overload relay, and the auxiliary contacts. You need a digital multimeter for most of these tests, and some checks require the starter to be energized. The process is straightforward once you understand what each part does and what normal readings look like.
A motor starter has two core components. The contactor is an electrically controlled switch that connects and disconnects power to the motor. The overload relay sits downstream and protects the motor from drawing too much current. Together, they form a system that both controls and safeguards the motor, and each piece can fail independently.
Safety Before You Open the Panel
Motor starters live inside electrical panels that can produce arc flash, a release of energy intense enough to cause severe burns. Before doing any work, verify the arc flash PPE category labeled on the panel. Category 1 requires arc-rated long-sleeve clothing with a minimum rating of 4 cal/cm², a face shield, hard hat, safety glasses, hearing protection, and heavy-duty leather or arc-rated gloves. Category 2 raises the clothing requirement to 8 cal/cm². Higher categories exist for larger systems and demand progressively more protection, up to 40 cal/cm² for Category 4.
For any test you can perform with the power off, lock out and tag out the disconnect upstream of the starter first. Verify zero voltage at the line-side terminals with your multimeter before touching anything. Some tests, like voltage drop across the contacts, require the circuit to be live and the motor running. Only perform those with appropriate PPE and with someone else present.
Visual Inspection of the Contacts
Start with a visual check of the main contacts inside the contactor. Open the starter and look at the contact surfaces for signs of heavy pitting, material loss, or welding (where the contacts have fused together). Welding is a serious failure that means the contactor can no longer open the circuit and needs immediate replacement.
One important detail that surprises many technicians: rough, discolored contacts are not necessarily bad. ABB’s maintenance guidelines note that contacts with blackened, uneven surfaces often perform better than a brand-new set because the initial surface roughness creates stable contact points. Replacing contacts based on appearance alone is a common mistake. The real indicator of failure is material loss severe enough that the contacts no longer make firm, full-face contact when closed, or voltage drop readings that exceed acceptable limits (more on that below).
Testing the Contactor Coil
The contactor coil is an electromagnet. When it receives a control signal, it pulls the contacts closed. When a coil fails, the contactor either won’t close at all or closes weakly. You test the coil with your multimeter set to resistance (ohms).
With power locked out, disconnect the coil wires and place your meter leads across the two coil terminals. A healthy coil will show a resistance value that varies based on the coil’s voltage rating. Lower-voltage coils have lower resistance: a 24V coil might read around 23 ohms, while a 120V coil might read around 126 ohms, and a 240V coil around 483 ohms. The exact value depends on the manufacturer and model, so check the datasheet for your specific contactor. Your reading should be within 5% of the listed value.
If you get an “OL” or infinite reading, the coil is open (the wire inside has broken) and the contactor needs replacement. If you read near zero ohms, the coil is shorted. A shorted coil draws excessive current and can also damage a control transformer upstream, so check the transformer too if you find a short.
Checking the Main Contacts
The main contacts carry the full motor current. Over time, they pit, erode, and eventually fail to conduct properly. There are two ways to test them: a continuity check with power off and a voltage drop test with power on.
Continuity Check (Power Off)
With the starter de-energized and locked out, set your multimeter to continuity or low-ohms. Place one lead on the line-side terminal and the other on the load-side terminal of the same phase. With the contactor open (not pressed in), you should see infinite resistance, meaning no connection. Manually press the contactor armature in to close the contacts, then check again. You should now see very low resistance, ideally under one ohm. Repeat for all three phases. If any phase shows high resistance or no continuity when closed, that contact set is damaged.
Voltage Drop Test (Power On)
This is the more revealing test because it shows how contacts perform under actual load. With the motor running, set your multimeter to AC voltage and measure across each set of contacts (line side to load side on the same phase). Ideally, you want to see nearly zero volts. The rule of thumb is that anything exceeding 5% of the applied voltage is excessive and means the contacts need replacement.
For a 240V circuit, 5% is 12 volts, so any reading above that is a problem. For a 120V control circuit, the threshold is about 6 volts. Even readings well below the 5% limit can indicate developing issues if they’re significantly different from one phase to another. Uneven voltage drop across the three phases suggests one contact set is deteriorating faster than the others.
Testing the Overload Relay
The overload relay protects the motor from sustained overcurrent that would overheat the windings. It uses two mechanisms: a thermal element (a bimetallic strip or heater coil) that responds to prolonged high current, and in many models, a magnetic element that catches sudden extreme spikes the thermal element would be too slow to detect.
Most modern overload relays have a built-in test function. On Schneider Electric’s TeSys D LRD relays, for example, there’s a slot labeled “Test” on the front face. Insert a small flathead screwdriver and slide it across to manually trip the device. When tripped, a small yellow indicator flag appears and the overload’s auxiliary contacts change state. This confirms the mechanical trip mechanism works.
To verify the electrical side, use your multimeter to check the overload’s auxiliary contacts. These typically include a normally closed (NC) contact that opens on trip (breaking the control circuit to the coil) and a normally open (NO) contact that closes on trip (used for alarm signals). With the overload in its normal, non-tripped state, the NC contact should show continuity and the NO contact should show infinite resistance. Trip the overload manually and confirm both contacts reverse state. If either contact doesn’t switch properly, the overload relay is faulty.
After testing, press the reset button to return the overload to its normal state. Also verify that the current setting dial on the overload matches the motor’s full-load amp rating from its nameplate.
Testing the Auxiliary Contacts
Auxiliary contacts are low-current switches attached to the contactor that change state whenever the contactor opens or closes. They’re used for control logic: interlocking with other starters, status indication, or holding circuits. They come in normally open (NO) and normally closed (NC) configurations.
Test them with your multimeter on the continuity or resistance setting. With the contactor de-energized (open), NO auxiliary contacts should show infinite resistance and NC contacts should show continuity (near zero ohms). Then manually press in the contactor armature or energize the coil. The readings should reverse: NO contacts show continuity, NC contacts show infinite resistance. Any contact that doesn’t cleanly switch in both directions needs replacement. Auxiliary contact blocks are usually modular and can be swapped without replacing the entire contactor.
Insulation Resistance Testing
If you suspect insulation breakdown between phases or between a phase and ground, an insulation resistance test (commonly called a megger test) gives you the answer. This uses a specialized meter that applies a high DC voltage and measures resistance in megohms.
For motors rated 230V or 460V, the test is performed at 1,000 volts DC. The minimum acceptable reading is 5 megohms. For higher-voltage motors (2,300V or 4,160V), the test voltage increases to 5,000 volts DC, and the minimum acceptable reading is 25 megohms. You test between each phase conductor and ground, and between each pair of phases, with the starter’s load-side cables disconnected from the motor so you’re testing the starter and wiring in isolation.
Readings below the minimum threshold indicate deteriorated insulation that could lead to a ground fault or phase-to-phase short. Moisture, heat damage, and age are the most common causes.
Putting the Results Together
A motor starter that passes all these checks, coil resistance within spec, contacts with low and even voltage drop, overload trip mechanism functioning, auxiliaries switching cleanly, and insulation above minimum megohm values, is in good working order. When a starter fails one test, that usually points directly to the failed component. Coil failures and contact wear are the most common issues in starters that have been in service for years. Overload relay failures are less frequent but more consequential since they leave the motor unprotected. If you find a problem with any single component, re-test after the repair to confirm the fix before putting the motor back into service.