A thermal overload relay is not a nuisance-trip suppressor. Its job is to interrupt an operating condition before the motor exceeds an accepted thermal limit. Raising the setting until production continues can conceal overload, excessive starts, phase unbalance or lost cooling.
Assign each protection function
List the functions in the starter or numerical relay, then list the sensors inside the motor. A current-based model estimates heating. Embedded PTCs or RTDs indicate temperature at physical points. They complement one another only when alarm, trip and reset actions are deliberately coordinated.
IEC 60034-11 addresses thermal protection incorporated in rotating machines. IEC 60947-4-1 addresses contactors, starters and motor protective switching devices. Neither reference supplies a universal ampere setting; the project still needs motor- and relay-specific data.
Build the setting sheet from evidence
| Evidence group | Required input | Error prevented |
|---|---|---|
| Motor limit | Rated current, voltage, frequency, duty and supplier limits | Applying a value to the wrong rating |
| Starting duty | Cold and hot current, time, starts per hour and restart interval | Consuming the trip margin during normal starts |
| Installation | Ambient, altitude, enclosure, cooling and load condition | Protecting an assumed cooling condition |
| Measurement | Relay model, CT ratio, phase coverage, scaling and wiring | Entering a value against the wrong primary current |
| Coordination | Curves, switching-device capability and short-circuit protection | Treating overload protection as short-circuit protection |
| Test record | Measured currents, start trace, trip time and reset behaviour | Approving an unproved trip chain |
Identify the source and revision of every value. If the motor data sheet conflicts with the protection schedule, resolve it instead of selecting the convenient number.
Check the complete thermal cycle
Plot a normal cold start, the most demanding permitted hot start and the worst credible running overload against the relay characteristic. Normal cases need clearance from the trip curve; damaging cases need a trip inside the motor limit. Review phase loss and current unbalance because heating can be severe even when average current appears moderate.
Reset policy also matters. Automatic restart after a thermal trip can reconnect a still-hot motor or start machinery unexpectedly. Define manual or automatic reset, cooling delay and process interlocks before commissioning.
Prove the path at site
Verify CT polarity and ratio, displayed primary current, phase readings, start duration, alarm and trip outputs, switching-device operation, thermal memory and reset. For embedded sensors, test the input circuit and confirm labels against the terminal diagram.
Keep the as-left settings with the measured start trace. They provide the baseline for distinguishing load growth, cooling degradation and unauthorised setting changes during later investigations.
Approval rule
Approve only when normal operation is stable, credible damaging conditions trip inside the verified motor limit, and the measurement-to-trip path has been tested. If a motor limit is provisional, the setting remains provisional.