[Explained] Motor Circuit Protection System and Devices


An electrical motor is a machine that converts electrical energy into mechanical energy. An electrical motor has a huge application from domestic to industrial everywhere. From smaller to heavier, in all categories, the electric motor came. For larger or high power motor protection system is very important. High power motors need much time to manufacture and to do maintenance. Also, they are so much costly. So running a motor without a protection system can cause a huge loss of money, time, even accidents also. So, in this article, we are going to discuss everything about motor circuit protection.


What is Motor Circuit Protection?


The proper arrangement, selection, and commissioning of different types of protective devices for the protection of motor circuits can be called the motor circuit protection system. So, a motor circuit protection system includes 

  • the proper understanding of motor circuit and its nameplate details
  • proper selection of protective devices with appropriate ratings
  • Compact Circuit Design
  • Smooth operation
  • high efficiency
  • to minimize the loss and chances of an accident

Basically, a motor circuit implies it's winding only. Because an electric motor does have not any special and complex circuitry. So, protecting the winding of the motor, internal connections, insulation can be called motor circuit protection.




What are the protections required for a motor circuit?


Here is the list of protections, we can provide electrically for a motor,
  1. Overload Protection
  2. Over Current Protection
  3. Short Circuit Protection
  4. Ground Fault Protection
  5. Overvoltage protection
  6. Single phasing protection

Motor Protection devices


Here, the list of protective devices used for motor circuit protection,
  1. MCB
  2. MCCB
  3. Overload Relay(OLR)
  4. RCCB(Residual Current Circuit Breaker)
  5. ELCB(Earth Leakage Circuit Breaker)
  6. Earth Fault Relay
  7. Earth Leakage Relay(ELR)
  8. MPCB(Motor Protection Circuit Breaker)
  9. Single Phasing Preventer




Overload VS Overcurrent Fault


Although, we understand overload and over-current fault are the same but as per the electrical standard it is wrong.

Overload fault is that when the motor is overloaded due to excess loading, mechanical fault such as bearing jammed or continuously running a motor with full load current for a long time. But it is true that an overload fault can cause an over-current fault.

When a motor consumes current more than its full load current due to any internal fault, internal short circuit, overloaded, then it is called an overcurrent fault.


Overload Protection


For the overload protection of the electrical motor, thermal overload relay(OLR), circuit breaker, MCCB can be used. But in most of the cases, thermal overload relay is used because it is easy to use with contactor and other control circuits.

Here, you can see the connection diagram of the Overload Relay with a contactor for motor circuit protection.

Motor Circuit Protection


The circuit breaker or thermal relay, any protective device used for the overload protection of the motor should have a current rating of less than 125% of the full load current. For, example if your motor has a full-load current rating is 40A, then the protective device should have the current rating of 40 x 1.25 = 50A




Short Circuit Protection


MCB(Miniature Circuit Breaker) for low power motors and MCCB(Moulded Case Circuit Breaker) for high power motors are mostly used for short circuit protection. MCCB provides the function to vary the tripping current setting or adjustment.


Ground Fault or Earth Leakage Protection


For the ground fault or earth leakage fault protection purpose RCCB(Residual Current Circuit Breaker), Earth Leakage Circuit Breaker(ELCB), Earth Fault Relay, Earth Leakage Relay(ELR) can be used.




Single Phasing Protection


In a three-phase motor circuit, when anyone phase is broken or disconnected among the three phases, the motor will draw a very high imbalance current, this is called single phasing. Single phasing can cause the burning of the motor coil, affect the whole power circuit. So, to give protection against single phasing, a single phasing preventer is used.




Motor Protection Circuit Breaker(MPCB)


A Motor Protection Circuit Breaker or MPCB is a specially designed protective device to protects the electrical motor from different types of faults. An MPCB gives protection against all types of faults possible to happen with an electrical motor. Also, the MPCB has auxiliary contacts that help to make other control and indication circuits.
An MPCB can give protection against,

  1. Overload Fault
  2. Short Circuit Fault
  3. Line to Line Fault
  4. Line to Ground Fault
  5. Voltage Unbalance Fault
  6. Current Unbalance Fault
  7. Ground Fault
  8. Earth Leakage Fault
  9. Single Phasing


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[Explained] Motor Circuit Protection System and Devices [Explained] Motor Circuit Protection System and Devices Reviewed by Author on 9/03/2021 Rating: 5
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