Overload relay or motor protection circuit breaker: which one?

Both protect a motor against overload, which is why they get mixed up. The key difference is this: a motor protection circuit breaker (MPCB) also protects against short circuits and breaks the current itself; an overload relay only watches for overload and, to switch off, opens the control circuit so the contactor drops out.

The difference in a table

	Overload relay	Motor protection circuit breaker
Protects against	Overload and, on most models, phase loss	Overload, short circuit and nearly always phase loss
How it switches off	Opens its 95-96 contact and the contactor disconnects the motor	Opens its own main contacts
Also needs	A contactor, plus fuses or a breaker for short circuits	A contactor only if the motor is controlled remotely or from a PLC
Set to	The motor’s nameplate current	The motor’s nameplate current
Reset	Reset button, manual or automatic	Switched back on with its own handle
Standard	IEC 60947-4-1	IEC 60947-2 (and 60947-4-1 as a starter)

In North America the same pair appears as the overload block of a magnetic motor starter (in older NEMA starters, with replaceable "heater" elements) and the manual motor protector or MPCB.

How each one is wired

Overload relay: mounted under the contactor, with its three poles in series with the motor phases. Its 95-96 contact (normally closed) is wired in series with the contactor coil: when it trips, the coil loses power and the motor stops. The 97-98 contact (normally open) is used to tell the PLC or to light a "tripped" lamp.
MPCB: fitted before the contactor, like a circuit breaker designed for motors. It breaks the three phases directly. If you need to know it has tripped, add an auxiliary contact block.

In this simulator you can trip the overload relay with the motor running and watch its 95-96 contact cut the power to the contactor coil:

Try it right here. Open it full screen, with the explanation.

Which one to choose

MPCB and contactor: the most common choice in machine panels with small and medium motors. Compact, protects against everything and resets without replacing fuses.
Fuses, contactor and overload relay: for higher currents, when the installation already uses fuses or when the manufacturer gives the coordination with fuses. Also in star-delta starters, where the overload relay is usually in the winding circuit and set to 0.58 times the nameplate current.
MPCB on its own: to start small motors by hand, such as a pump or an extractor fan, with no automation.

How to set them

Both are set to the motor’s nameplate full-load current, not to a value "with some margin". If you do not have the nameplate, the motor current calculator gives a good estimate. Overload relays also have a trip class: class 10 is for normal starts; class 20 or 30 for machines that take a long time to start, such as large fans or centrifuges.

Typical mistakeTurning the overload setting above the nameplate current "because it keeps tripping". If it trips, something is wrong: a mechanical overload, a lost phase, low voltage or a start that takes too long. Raising the setting just lets the motor burn out without warning.

Other guides

PNP or NPN sensor: the difference and how to tell which you have · What 4–20 mA means and why it does not start at zero · What is a contactor and what is it used for?

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