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The three do not replace each other; they protect the motor against different types of fault. The motor protection circuit breaker and the thermal relay measure current to estimate how hot the winding is. A temperature sensor makes no estimate; it reads temperature directly from inside the winding.
| Device | What it senses | What it protects against | Limitation |
|---|---|---|---|
| Motor protection circuit breaker (MPCB) | Current (thermal and magnetic trip) | Overload, short circuit, phase loss on most models | Does not see winding temperature directly |
| Thermal relay + contactor | Current (thermal trip) | Overload, phase loss | Needs a fuse or breaker upstream for short circuits |
| PTC / PT100 winding sensor | Actual winding temperature | Blocked cooling, hot ambient, frequent starts, low speed on an inverter | Does not detect short circuits; needs a separate evaluation device |
Current-based protection assumes the motor is cooled normally. When the fan cover is clogged with dust, ambient temperature rises or the motor runs at low speed on an inverter for long periods, current looks normal but the winding overheats; only a winding sensor catches this. Conversely, the sensor does not react to a short circuit; a fuse or breaker is essential for that.
The HEM catalogue requires the MPCB/fuse and thermal protection to be selected and set according to rated current; PTC or PT100 is an order option. Our article on motor protection devices helps when preparing the panel list.