What is a Motor Protection Circuit Breaker?

A motor protection circuit breaker (MPCB) is a type of circuit breaker used to protect electric motors from overload and short circuits.

It is a device that offers protection to electrical equipment.

MPCBs are UL listed as circuit breakers and verified as motor overload relays. They are used on starter applications with a contactor to control motor load conditions.

Electrical motors play some vital roles in electrical products as they are used to drive mechanical devices of any type, and such equipment needs adequate protection.

Why motor protection is necessary

Motor protection is necessary to prevent unexpected breakdown of the machine, costly repairs and frequent losses due to motor downtime.

That is why they are fitted with some protective device.

How does a motor protector work?

motor circuit protector

Motor protectors work like other circuit breakers, but the fuse protects electrical circuits and appliances from damage, MPCB protects electric motors.

MPCBs offer both thermal and magnetic protection against short circuits and motor overload.

Thermal protection

Thermal protection works with a bimetallic strip. The metallic substance expands upon exposure to heat or electricity and contracts when it is OFF.

Each time there is an excessive current in the system, it leads to a temperature rise. This causes the bimetallic strip to bend away, activating the trip unit.

But unlike the magnetic trip that is instant, thermal protection takes some time to allow the inrush of current when a motor starts.

Magnetic protection

Magnetic protection protects the motor from short circuits and overcurrents. It disconnects the faulty current instantly to prevent damage to the system.

Motor protection circuit breakers vs. motor circuit protectors

in a circuit, overcurrents occur for two major reasons: overload and short-circuits.

Overload occurs when an appliance draws slightly more than the rated current. It does not cause immediate damage, unless left for some time.

On the other hand, short circuits occur quickly, producing higher currents which can cause damage immediately. For this reason, it requires immediate attention.

The MCP provides protection against short circuits only. To protect against overloads when using an MCP, you must also install an overload relay.

While MPCBs protect against short circuits and overload, but are often used on low voltage switchgear.
MCPs are generally used in motor branch circuits, while MPCBs are used in feeder circuits.

What is the difference between circuit breaker and motor circuit protector?

Circuit breakers are multipurpose. They protect both circuits and appliances, while motor protection circuit breakers protect only motors.

Unlike ordinary breakers, MCP also protects against phase imbalance and phase-loss. Three-phase circuit motors require 3 live conductors with balanced voltage to work properly.

Any unbalance greater than 2% will surely be dangerous to the motors service life. If any of the phase voltage is lost, the motor will continue to move, causing greater damage. However, with an MCP, it will easily detect the fault by measuring the phase voltage differences, and subsequently disconnect the current.

How to size an electric motor circuit breaker

The electric motor circuit breaker regulates the amount of current that goes in and out of the motor. When the current is high, the breaker trips OFF.

Below is a step-by-step guide on sizing an electric motor protector

Step 1

Find the data sheets of the motor. For overload or over-current protection, the National Electric Code (NEC) recommends that each conductor feeding an electric motor must have an overload protection device.

If they use a circuit breaker, each conductor must have a separate breaker.

Step 2

Look for the horsepower and the service factor of the motor. You can see it on the motor face plate or data sheet.

Check out nec article 430.32 and confirm where your motor falls on their table. The table will provide you with the normal current of the motor and the overload protection it needs.

If, for example, the table says 125%, and the current rating of the motor is 20 Amp, the overload protection should be at least 25 amp.

Step 3

Go to table 430.52 in the NEC code. The table emphasizes short circuit protection. Look at the motor data sheet for its full load current.

Look for where your motor falls on the table based on your motor size and full load current. The table will give you the size of the short circuit protection as a percentage of the full load current.

For example, if the short circuit protection of the motor is 150% and the full load current is 20 amp, then the short circuit device should be 30 amps.