What is a Motor Protection Circuit Breaker?

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

It is a protective device that offers protection to electrical equipment.

Electrical motors play huge roles in electrical products as they are used to driving 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 electrical motors are fitted with some protective device.

The working principle of the engine circuit protector

MCPs work like other circuit breakers, but while circuit breakers protect electrical circuits and appliances from damage, MCP protects electric motors.

MCPs offer both magnetic and thermal protection against short circuits and overload, respectively.

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, the bimetallic strip bends away, activating the trip mechanism.

But unlike the magnetic protection 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 other high electrical faults. It disconnects the faulty current instantly to prevent damage to the system.

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 electrical 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 the electric motor circuit breaker

Electric motor circuit breaker regulates the amount of light that goes in and out of the motor. When the current becomes high, the breaker cuts it 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 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.