Motor Protection Circuit Breaker or MPCB

A motor protection circuit breaker (MPCB) is a compact electrical protection device for electric motors. It is a disconnect with a thermal overload trip plus a magnetic short-circuit trip, often built to IEC 60947-2 and IEC 60947-4-1. Motors drive fans, pumps and machines, so they need that combined protection. MPCBs are common on these commercial and industrial drives:

• Rooftop air conditioners, chillers, compressors, heat pumps and cooling towers.
• Extraction and injection fans, as well as air handling units.
• Water pumping systems.
• Elevators and other hoisting devices.
• Industrial conveyor belts and other machinery used in manufacturing processes.

In those motor circuits the MPCB is the device that interrupts overload and short-circuit current.

What is a Motor Protection Circuit Breaker and what are its Functions?

A motor protection circuit breaker, or MPCB, is a thermal-magnetic device used on 50 Hz and 60 Hz motor circuits. Typical functions are:

• Protection against electrical faults such as short circuits, line-to-ground faults and line-to-line faults. The MPCB can interrupt any electrical fault that is below its breaking capacity.
• Motor overload protection, when a motor draws electric current above its nameplate value for an extended period of time. Overload protection is normally adjustable in MPCBs.
• Protection against phase unbalances and phase loss. Both conditions can severely damage a three-phase motor, so the MPCB will disconnect the motor in either case as soon as the fault is detected.
• Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down. An overheated motor can be permanently damaged if it is turned back on.
• Motor Circuit Switching – MPCBs are normally equipped with buttons or dials for this purpose.
• Fault Signaling – Some motor protection circuit breakers show a trip flag. A few electronic models add an LED. Treat that mark as a trip: clear the fault before closing the breaker again.
• Automatic Reconnection – A few electronic MPCB models can wait a cool-down time after overload and then reclose. Most thermal-magnetic units stay open until someone closes them by hand.
  Motors are costly. Correct MPCB settings reduce rewind and replacement work and help the motor last its design life.

Motor Protection Circuit Breaker Working Principle

The motor protection circuit breaker can be considered a subtype of a thermal magnetic circuit breaker, but with additional functions that are specially designed to protect electric motors. The basic working principle is similar to all other circuit breakers.

• Thermal protection guards the motor against overload. A bimetal (or equivalent) opens after a delay, so the start inrush currents can pass. If the motor stalls and the high current continues, that same thermal element trips.
• Magnetic protection covers a short circuit or other high-current electric fault. Unlike thermal protection, the magnetic trip is instantaneous and opens the dangerous current at once.
• Unlike a distribution breaker, an MPCB also covers phase loss and severe current unbalance. A three-phase motor needs three live conductors. NEMA MG-1 treats voltage unbalance above about 1% as a derating case; 2% is already severe heating. Phase loss is worse because the motor keeps running on two phases. Typical thermal-magnetic MPCBs detect this from current in the three poles (IEC 60947-4-1), not from a voltage-unbalance meter. Current unbalance is normal on three-phase systems that also feed single-phase loads. It is not acceptable when the three-phase circuit powers only the motor.
• MPCBs are also equipped with a manual interruption mechanism, allowing disconnection of electric motors for replacement or maintenance.
• MPCBs come in diverse current ratings, and many models feature adjustable settings. This versatility allows a single MPCB to protect motors of various capacities.

Asynchronous Motor Protection

Most industrial motors are asynchronous motors, also called squirrel-cage induction motors. three-phase power produces a rotating magnetic field that induces rotor current and torque. Motor protection therefore has to allow starting inrush and still trip on stall, overload and phase loss, unlike ordinary electrical circuits.

• Direct-on-line asynchronous motors draw a high inrush while the rotating field builds. Typical values are 5 to 8 times rated current for a short time. MPCB magnetic trips are therefore set above about 10 times rated current. Some miniature circuit breakers (B-curve) trip near 3 to 5 times rated current and would open on a healthy start. A reduced-voltage starter can be added when inrush must be cut.
• The three supply voltages should stay close to equal. NEMA MG-1 treats voltage unbalance above about 1% as a derating case. A motor circuit breaker that watches the three pole currents will open on the severe unbalance that follows phase loss.
• If one phase opens, the motor can keep running while the other two currents rise and overheat the windings. IEC 60947-4-1 thermal MPCBs trip on that current unbalance. A large drop on one pole, or a pole current falling to zero while the others remain, is the usual phase-loss signature.

Use an MPCB (or a motor-starter combination) sized for the motor voltage and full-load current. Makers sell ranges of thermal-magnetic and electronic units rather than one universal trio.
A contactor is often added for remote start and stop. An undervoltage release can open the motor if supply voltage falls well below rating.

Motor Protection Circuit Breaker Sizing (Selection Guide)

Size the MPCB from the motor nameplate voltage and the nameplate full-load current.

• The MPCB voltage rating must cover the motor nameplate voltage. Catalogue units are commonly listed for 230 V, 380 V, 415 V, 440 V, 500 V and 660 V AC. Check the maker’s Ue for the site voltage.
• Then read the nameplate full-load current. Running current can be lower at light load. Still select the MPCB from the nameplate current so a full-load start does not trip it. Example: a 20 A nameplate motor may draw less at part load, but the breaker still has to be set for 20 A.
• Motor protection circuit breakers can then be calibrated to the exact current value that is adequate for the electric motor being protected. They typically have an adjustment range. For example, a MPCB rated at 32 amperes might be usable for motors with rated currents as low as 22 amperes. This is very useful if an electric motor is replaced with a more efficient model that requires a lower current, since it will not be necessary to replace the motor breaker.
• Correct motor protection still needs cable that can carry the rated current. Undersized cable overheats, insulation fails and a fault can occur even with a breaker fitted.

Motor Protection Circuit Breaker Specification Chart

MPCB manufacturers typically provide charts where the technical specifications of the circuit breaker are presented, in order to simplify the selection process. The following chart, provided as an example, is for the motor circuit breaker SGV2-ME model manufactured by CGSL.

The thermal-release and magnetic-release columns list those trip currents. Confirm voltage and current ratings against the motor nameplate before you fit the MPCB.

Conclusions of Motor Protection Circuit Breaker

An MPCB is the usual compact protector for commercial and industrial motors.
Induction motors need that motor-specific trip curve. An undervoltage release, a timer or a starter can be added when the scheme requires it.
Set the thermal dial to nameplate current. An undersized MPCB will trip on start. An oversized MPCB may not trip on a real overload.