Motor Protection: Thermal Relay, MPCB and PTC Compared
Motor protection: Overload, phase loss, locked rotor and frequent starting are different risks, and each is seen by a different device.
ContentsMotor protection is often treated as a single-device problem, yet in practice it has to be built in layers. A winding is rarely destroyed by a sudden short circuit; far more often it fails from heat accumulated over days of mild overload. A sound design starts by knowing which risk is detected by which device.
Five scenarios that damage a motor
- Continuous overload: current slightly above rating, winding temperature drifts past the limit
- Phase loss or unbalance: the remaining phases carry more current and heating accelerates
- Locked rotor: starting current persists and dangerous temperature is reached within seconds
- Frequent starting and stopping: accumulated heat has no time to dissipate
- Undervoltage: current rises for the same power, torque falls and starting takes longer
Thermal overload relay and trip class
A thermal relay imitates the heating effect of current and is defined by a current-time curve. The trip class (Class 10, 10A, 20, 30) sets the tripping time at locked rotor current: pumps and fans that accelerate quickly are served by Class 10, while high-inertia loads need Class 20 or 30. Choosing a class too high leaves the motor unprotected; too low produces nuisance tripping during start. The relay is set to the motor nameplate current, not to the cable or the breaker rating.
When an MPCB is the better choice
A motor protection circuit breaker combines adjustable thermal and magnetic release in one body. Together with a contactor it forms a compact outgoing way, saves panel space and reduces nuisance tripping because the magnetic setting can be matched to the starting current. Compared with fused solutions it allows quick restoration after a short circuit; at high prospective short-circuit currents, however, the coordination type (Type 1 or Type 2) must be verified from the manufacturer tables.
Direct temperature monitoring with PTC thermistors
Current-based protection only estimates winding temperature indirectly. When a cooling fan fails, an air inlet is blocked or ambient temperature is high, current can look normal while the winding overheats. A PTC thermistor or PT100 embedded in the winding measures temperature directly and closes that gap. On critical and expensive motors, adding temperature monitoring alongside current protection is standard practice.
Which device sees which risk
| Risk | Device that detects it | Typical setting or note |
|---|---|---|
| Short circuit | Magnetic release or aM fuse | Set above starting current |
| Continuous overload | Thermal or electronic relay | Motor nameplate current |
| Locked rotor | Trip class selection | Class 10 / 20 / 30 |
| Phase loss, unbalance | Electronic relay or phase monitor | Unbalance band of 5 to 10 per cent |
| Winding over-temperature | PTC or PT100 monitoring | According to insulation class |
| Earth leakage | Earth fault relay or type B RCD | Requires care with drives |
For the effect of the starting method on protection see motor starting methods, and for coordination between stages see selectivity and protection coordination.
Key takeaways: motor protection
- Thermal overload relay and trip class — A thermal relay imitates the heating effect of current and is defined by a current-time curve.
- When an MPCB is the better choice — A motor protection circuit breaker combines adjustable thermal and magnetic release in one body.
- Direct temperature monitoring with PTC thermistors — Current-based protection only estimates winding temperature indirectly.
- Which device sees which risk — For the effect of the starting method on protection see motor starting methods, and for coordination between stages see selectivity and protection coordination.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is it wrong to set the thermal relay above nameplate current?
Yes. The setting follows the rated current on the motor nameplate. If tripping occurs during start, the answer is to review the trip class or the starting method, not to raise the setting.
Does a motor fed by a drive still need a thermal relay?
The drive contains its own electronic motor protection, so a separate thermal relay is usually unnecessary. If the drive can be bypassed to the mains, however, the bypass path needs its own overload protection.
Can PTC monitoring stand alone?
No. A PTC sees temperature but does not provide fast protection against a short circuit or the first seconds of a locked rotor. It is used together with current-based protection.
Related articles
- Motor Starting Methods: DOL, Star-Delta, Soft Starter and VFD
- IK Impact Rating Explained: IK07, IK08 and IK10 Compared
- VFD Panel Design: EMC, Filters and Thermal Management
- Type 1 and Type 2 Coordination on Motor Feeders
- All technical articles
Related Devpan solution: DevControl MCC panel.
Devpan designs motor outgoing ways around the load characteristic and starting method, and supplies coordination tables and relay settings in the handover file. Send us your motor list and we will set the protection stages together.
