What is the difference between a motor protection circuit breaker and a thermal overload relay?
Compare MPCBs and thermal overload relays in motor starter design: short-circuit protection, main power contacts and contactor coordination.
What is the main difference between a motor protection circuit breaker (MPCB) and a thermal overload relay? An MPCB provides both adjustable thermal overload protection and magnetic short-circuit breaking in one device, directly interrupting all three motor phases. A thermal overload relay provides overload and phase-loss detection only; it has no short-circuit breaking capacity and trips the motor by opening its auxiliary NC contact in the contactor coil circuit.
MPCB vs Thermal Overload Relay comparison table
| Criterion | Motor Protection Circuit Breaker (MPCB) | Thermal Overload Relay |
|---|---|---|
| Overload Protection | Yes (adjustable current dial) | Yes (adjustable current dial) |
| Short-Circuit Protection | Yes (built-in magnetic trip and kA breaking capacity) | No (requires upstream fuse or breaker) |
| Main Power Contacts | Directly interrupts 3-phase motor current | No main contacts; trips contactor via NC auxiliary contact |
| Standalone Manual Operation | Can start, stop and protect a motor on its own | Must be mounted onto a contactor with upstream short-circuit protection |
| Category Link | Motor Protection Circuit Breakers | Thermal Overload Relays |
Frequently asked questions
Do I need a thermal overload relay if I already use an MPCB?
No. An MPCB already includes adjustable thermal overload and phase-failure protection. When remote PLC control is needed, pair the MPCB directly with a power contactor.