JohdotaIf a motor feeder is built with a motor protection circuit breaker (MPCB), the MPCB is the first device in the motor supply. It protects the motor and the cable against overload and short circuits, and it can be used to switch and isolate the motor by hand.
Protection
- Overload protection. A thermal release whose current is set on the dial to the rated current on the motor rating plate. An overcurrent that lasts a long time trips the breaker.
- Short-circuit protection. A magnetic release that trips at once when a short circuit makes the current many times higher than normal.
- Phase loss protection. Many MPCBs also trip if one phase is missing.
Wiring
The supply comes in at terminals 1/L1, 3/L2 and 5/L3, and the output 2/T1, 4/T2 and 6/T3 continues to the contactor. The contactor does the starting and stopping, and the MPCB does the protection.
Another common way is a switch-fuse, a contactor and a thermal overload relay. The fuses then protect against short circuits and the thermal overload relay against overload.
Auxiliary contacts
An auxiliary contact block is fitted to the breaker and follows the position of the breaker:
- The NO contact 13–14 in series with the contactor control: when the protection trips, the contactor drops out and the self-holding is released.
- The NC contact 21–22 can switch on a fault light.
For tripping there are also separate signalling contacts, which tell a tripped breaker apart from one that has been switched off by hand.
In your region
Showing notes for
United States, Canada
UL and CSA list these devices in several ways. The two main ones: as a manual motor controller, the device needs a fuse or circuit breaker upstream for short-circuit protection. As a Type E self-protected combination motor controller, it needs nothing upstream, but larger line-side spacings and a visible trip indication are required: the ABB MS132, for example, then needs a line-side feeder block. Type E devices have a slash voltage rating such as 600Y/347 V, so above 347 V they can only be used on solidly grounded (wye) systems.
Marked "Suitable as Motor Disconnect", the device can also be the motor disconnect when it is on the load side of the branch-circuit protection.
On ladder diagrams it is marked CB, for circuit breaker. We found no North American symbol made for these devices. The English game draws them as a circuit breaker with thermal trip, from the Square D symbol table.
Source: ABB: manual motor starter guide, North American applications · NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations) · Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
How do you set a motor protection circuit breaker?
Set the current dial to the rated current on the motor rating plate, for the voltage and connection the motor is used with.
What is the difference between an MPCB and a thermal overload relay?
Both are common. An MPCB has overload and short-circuit protection in one device and can also isolate the motor by hand. An overload relay needs fuses for short-circuit protection, for example a switch-fuse.
How does a motor protection circuit breaker work?
The thermal release trips it when an overcurrent lasts too long, and the magnetic release trips it at once on a short circuit. Many MPCBs also trip if a phase is missing.
How do you wire the auxiliary contacts of an MPCB?
NO 13–14 goes in series with the contactor control, so the contactor drops out and the self-holding is released when the breaker trips. NC 21–22 can switch on a fault light.