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There are two common ways to protect a motor feeder: a motor protection circuit breaker (MPCB), or a separate thermal overload relay used together with a contactor. Both see overload, but they differ in short-circuit protection, panel space and commissioning time.
The core difference: short-circuit protection
An MPCB combines overload (thermal) and short-circuit (magnetic) protection in a single device. A thermal relay only handles overload; a fuse or circuit breaker must be added upstream for short-circuit protection. In other words, a thermal-relay solution always needs at least two devices in the chain.
| Criterion | MPCB | Contactor + thermal relay |
|---|---|---|
| Short-circuit protection | Built in | Needs a separate fuse/breaker |
| DIN rail space | Less | More |
| Remote switching | Needs a contactor | Contactor already present |
| Spare parts | One item | Two or three items |
| Frequently switched load | Contactor required | Suitable |
When to choose which
- For pumps, fans and compressors that are not switched often, an MPCB alone is enough and simplifies the panel.
- On lines switched frequently by the automation system a contactor is needed anyway; there MPCB + contactor competes with fuse + contactor + thermal relay.
- When one panel holds many small motors, MPCBs noticeably shorten the required rail length.
- When retrofitting an existing panel, keeping the thermal-relay architecture can be faster because the wiring stays untouched.
Setting current: the nameplate is not enough
In both solutions the setting follows the motor nameplate current, but with star-delta starting it matters whether the relay sees phase or line current. A relay connected inside the delta is set to 0.58 times the line current. Missing this detail means the motor either trips early or is never protected at all.
Frequently asked questions
Is an MPCB enough on its own?
Yes, if the motor is switched manually. If the automation system or a remote command switches it frequently, a contactor must be added: an MPCB is not designed for frequent switching.
What should the thermal relay be set to?
For direct-on-line starting, set it to the motor nameplate current. For star-delta starting with the relay inside the delta, set it to 0.58 times the line current.
What is the difference between an MPCB and a fuse?
A fuse only clears a short circuit by melting and must then be replaced. An MPCB sees both overload and short circuit, is reusable, and its setting current can be adjusted to the motor.
Does a motor on a drive still need thermal protection?
Drives include electronic motor protection, and when parameterised correctly a separate thermal relay is unnecessary in most applications. Where several motors share one drive, each needs its own protection.