Motor Protection

Motors can be very dangerous if installed incorrectly, and expensive to replace if not protected properly.

The electrical requirements for motors is outlined in AS/NZS 3000 4:13. This has been written as a brief and basic overview of motor installations and protection devices.


Motor Switching Devices

Every motor needs to be provided with a switching device or multiply devices, capable of preforming the following functions;

  • Starting and stopping the motor; and
  • Emergency stopping (A manually operated device capable of breaking full load, with the means to be restrained in the OFF position and not re-energise upon release of the device. This can include the switch of a socket outlet.)
  • Isolation of the motor for mechanical maintenance (An isolator located near the motor, the circuit breaker, or the plug and socket.)
Starting and stopping devices & switches

Starting-and-stopping devices need to be located as to provide easy operation by the person in charge of the motor. In particular, the equipment responsible for the stopping of the motor needs to remain effective in the event of a failure in the control circuit. This can be achieved with the use of a latching control circuit and  a contactor with normally open contacts, therefore in the event of control circuit failure, the contact will return to the open state, and remove power to the motor. This also satisfies the requirement to prevent against unexpected restarting.

If the control of the motor is remotely, there is also a need to provide stopping devices (E-Stops) at locations where danger is likely.

The switching device must have a rating of not less than the full load current of the motor, and shall be capable of safely interrupting the locked-rotor current of the motor. Motor equipment and switches are often marked with a utilisation category such as AC 23, indicating that it suitable for motor switching applications.


Protection against overload

Electric motor exceeding 0.37kW need to be provided with equipment that protects against overload of the motor.  This places almost an motor you might find in an industrial space requiring overload protection. This protection is commonly achieved with a circuit breaker. The use dedicated motor protection devices such as soft starters, or motor CB/TOL’s is becoming increasingly popular as it affords a greater level of protection to the motor and increases the lifespan of equipment.


Protection against overtemperature

For all motors grater then 2.25kw, overtemperature protection is required. This is most commonly achieved by the means of a thermal overload or TOL in conjunction with a motor contactor.

Additional means of protection such a VSD’s, Soft starters, Motor CB’s, etc. are also used and with increasing popularity due to the additional protection afforded to the motor.

There are some cases where protection against overtemperature is not required, such as; motors associated with fire protection systems, or those under 2.25kW.

Thermal Overload (TOL)

A TOL is designed to open the starting circuit in the event that the motor is drawing excessive current for an extended period of time. There are two main types of TOL’s, the bi-metallic strip and electronic.

TOL’s have an adjustable current setting in order to suit a few different motors. Electronic TOL’s have the greatest range of adjustment, thus catering for a range of motors within the same device.