You get called to a panel. The motor is stopped. The contactor has dropped out, the main contacts are open, and the overload relay flag is showing tripped. The operator says the motor "just stopped." No one pressed STOP. No one hit the E-stop. The control circuit de-energized itself, and the operator wants the motor running again. This is the most common motor control troubleshooting call in an industrial facility, and the answer starts at the overload relay.
What an Overload Relay Does
The overload relay monitors motor current and trips when that current stays above the relay's set point long enough to threaten the motor windings. It sits in the starter between the contactor and the motor, with sensing elements in series with the power conductors. Its job is a narrow one: detect sustained overcurrent and shut the motor down before the windings overheat.
The overload relay is not a short-circuit protection device. A dead short across two phases produces thousands of amps in milliseconds. The circuit breaker or fuses upstream handle that. Their magnetic trip elements respond in fractions of a cycle. The overload relay handles the slower, quieter threats that a breaker would never see: a jammed conveyor that doubles the motor current for three minutes, a worn bearing that adds enough drag to push current 20% above nameplate, a single-phasing condition where one supply fuse has blown and the remaining two phases carry elevated current trying to maintain torque. These are the conditions that cook motor windings over minutes and hours, not milliseconds.
How Thermal Overload Relays Work
A bimetallic thermal overload relay uses heater elements wired in series with the motor leads in the power circuit. Motor current passes through these heaters, generating heat that warms bimetallic strips inside the relay. As the strips absorb heat, they deflect. When the deflection reaches a calibrated trip point, the mechanism actuates.
The trip point is set by an adjustable dial on the relay, matched to the motor's nameplate full-load amperes (FLA). A motor rated at 14.0 FLA gets the relay dial set to 14.0. The heater elements themselves are selected from the manufacturer's table to match the FLA range. Allen-Bradley, Eaton, Square D, and Siemens all publish selection tables that cross-reference motor FLA against specific heater catalog numbers.
The trip characteristic is inverse-time: the higher the overcurrent, the faster the trip. A motor drawing 115% of FLA might run for several minutes before the bimetallic element accumulates enough heat to reach the trip point. A motor at 600% of FLA (locked-rotor current) trips in seconds. The relay does not know why the current is high. It accumulates heat and trips when the thermal integral crosses the threshold.
Trip class defines how fast. Class 10 relays trip within 10 seconds at 600% of FLA. Class 20 relays trip within 20 seconds. Class 30 allows up to 30 seconds. These ratings matter for motor starting: a high-inertia load that needs 15 seconds to accelerate through starting current will trip a Class 10 relay on every startup, regardless of heater sizing. Class 10 covers most general-purpose motor applications. Class 20 and 30 are specified for fans, pumps, and other loads with extended acceleration times.
How Electronic Overload Relays Work
Electronic overload relays replace the bimetallic mechanism with current transformers and a microprocessor. CTs around each phase conductor measure the motor current continuously. The microprocessor calculates an I-squared-t thermal model, simulating the heat accumulation that a bimetallic strip would experience.
The result is the same inverse-time trip characteristic, but with more precision and more features. The FLA setting is a digital parameter or dial, not a heater element selection. The trip class is selectable without changing hardware. The relay can detect phase loss, phase current imbalance, ground faults, and motor stall conditions that a bimetallic relay cannot sense. Many electronic relays display per-phase current, thermal capacity used, and the specific trip cause on a digital readout.
The control circuit interface is identical. Electronic relays still use a 95-96 NC output relay contact to signal a trip to the control circuit. Whether the overload relay is a $30 bimetallic unit or a $300 electronic unit, the wiring from 95 and 96 to the contactor coil circuit is the same.
Why the Motor Stops: The 95-96 Contact
The overload relay does not disconnect motor power directly. It has no contacts in the power circuit large enough to break motor current. Instead, it opens a small NC auxiliary contact at terminals 95 and 96 (IEC convention) that is wired in series with the contactor coil in the control circuit.
Under normal operation, the 95-96 contact is closed. Current flows through it, through the contactor coil, and the contactor stays energized. The motor runs.
When the overload relay trips, the 95-96 contact opens. The series path to the coil breaks. The coil de-energizes. The contactor drops out, and its main power contacts open. The motor stops.
This is the same fail-safe series-NC principle that makes the STOP button work in a standard motor starter. The OL relay contact, the STOP button, and any other protective device in the coil circuit are all NC contacts wired in series. Any one of them opening breaks the coil path. A tripped overload, a pressed STOP button, or a broken wire all produce the same result: the coil loses power and the motor stops. The overload relay tells the control circuit to shut down. The contactor does the disconnection.
Some overload relays also include a 97-98 NO contact that closes when the relay trips. This contact is used for alarm indication, pilot light circuits, or PLC inputs — it signals that a trip has occurred without carrying the coil circuit current.
Diagnosing and Resetting After a Trip
The relay tripped for a reason. Before pressing the reset button, find the reason.
Check the motor current with a clamp meter on all three phases at the load conductors. Compare the readings to the motor nameplate FLA. If the motor is drawing more than nameplate, the overload is doing its job. The problem is not the relay. The problem is whatever is making the motor work harder than it should: a jammed or overloaded mechanical load, a seized bearing, a misaligned coupling, or a process change that increased the driven load beyond the motor's continuous rating.
Check for single-phasing. If one of the three supply fuses has blown, or one contactor pole is pitted and not making contact, the motor loses a phase. The remaining two phases carry roughly 1.73 times their normal current trying to maintain torque. The overload relay responds to that elevated current and trips.
Check the heater sizing. The installed heater or FLA setting must match the motor nameplate FLA. If the motor has been replaced since the original installation, the heater may no longer be correct for the motor that is connected now. Motor swaps without heater re-selection are common in plants that have been running for decades.
Check the ambient temperature inside the enclosure. Bimetallic relays start closer to their trip point in a hot panel. A relay that holds at 25 degrees C ambient may trip at 40 degrees C with the same motor current.
Once the cause is identified and addressed, reset the relay. On a manual-reset relay, press the reset button on the front of the relay body. The bimetallic element must have cooled enough for the mechanism to latch. If the relay will not reset, it has not cooled sufficiently, or there is a mechanical issue with the relay itself.
Some relays are configured for automatic reset. The 95-96 contact re-closes on its own after the bimetallic element cools. Automatic reset is used in applications where unattended restart is acceptable, but it carries risk: if the overload condition has not been resolved, the motor restarts, overloads again, trips again, and the thermal cycling damages the windings. Manual reset is the default for most industrial motor control applications because it forces a human decision before the motor restarts.
For a deeper diagnostic sequence when the cause is not immediately obvious, see Why the Overload Relay Keeps Tripping. That article covers supply voltage measurement, phase-by-phase current analysis, insulation testing, and heater selection tables.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Trip and reset the overload relay in the motor starter trainer →Frequently asked questions
What is an overload trip?
The overload relay has detected sustained motor current above its set point. The bimetallic element (thermal relay) or I-squared-t thermal model (electronic relay) has reached the trip threshold. The 95-96 NC contact opens, de-energizing the contactor coil and stopping the motor. The relay is protecting the motor from winding damage caused by prolonged overcurrent.
How do you reset an overload relay?
Press the manual reset button on the relay body after the bimetallic element has cooled. Some relays use automatic reset, where the 95-96 contact re-closes after cooling without manual intervention. Before resetting, determine why the relay tripped. Repeated resets without diagnosing the cause subjects the motor windings to thermal cycling that accumulates damage with each trip-and-restart cycle.
What is the difference between an overload and a short circuit?
An overload is sustained current above the motor's rated capacity, typically 110% to 600% of FLA, caused by mechanical load, voltage problems, or motor faults. It develops over seconds to minutes. A short circuit is a direct low-impedance connection between phases or between a phase and ground, producing thousands of amps in milliseconds. Overload relays protect against overloads. Circuit breakers and fuses protect against short circuits. The two devices cover different fault types at different time scales.
What do the 95-96 terminals do on an overload relay?
Terminals 95 and 96 are the NC auxiliary contact on the overload relay per IEC terminal designation. This contact is closed when the relay is not tripped and opens when the relay trips. It is wired in series with the contactor coil in the control circuit. When the contact opens on a trip, the coil de-energizes, the contactor drops out, and the motor stops. The overload relay controls the motor indirectly through this contact rather than switching motor power directly.
What is trip class on an overload relay?
Trip class defines how many seconds the relay takes to trip at 600% of its rated current from a cold start. Class 10 trips within 10 seconds, Class 20 within 20 seconds, Class 30 within 30 seconds. Lower class numbers provide faster protection. Class 10 is standard for most motor applications. Class 20 and 30 are specified for high-inertia loads (large fans, centrifugal pumps, flywheels) that draw extended starting current during acceleration.
Why does the overload relay keep tripping?
Common causes: the motor is drawing more current than its nameplate FLA due to a jammed or overloaded mechanical load, worn bearings, or misaligned coupling. The heater elements or FLA setting may be sized too small for the motor. A blown supply fuse on one phase (single-phasing) forces the remaining two phases to carry elevated current. High ambient temperature inside the enclosure shifts the bimetallic element closer to its trip point. A failing motor with shorted winding turns draws excess current that increases with each thermal cycle. Measure supply voltage and motor current on all three phases before suspecting the relay.