20th September 2020

Circuit Breaker

What is Circuit Breaker?

A circuit breaker is a mechanical switching device which is capable to make or break an electric circuit either manual or automatically under normal operation or fault conditions, faulty conditions can be short circuit or overcurrent. A circuit breaker is used for switching and protection purpose.

The breaker is a link between load/consumer and supply and the associated current is large. Hence the ideal functional requirements of the Circuit breaker shall be as follows:

  • The breaker should isolate the faulty section in the Circuit without effecting the healthy section of the circuit.
  • The breaker should isolate the section with minimum time possible in the event of a fault.
  • The breaker should withstand mechanically and thermally in the even of fault(withstand time should be more than the tripping time).
  • Difference between normal working current and short circuit current should be properly intercepted by a breaker.

Working Principle of Circuit breaker

Breaker consists of two contacts, Fixed contact and moving contact(which moves into the sliding contact) as shown in the below figure.

Circuit breaker working principle

It is important to know that the e.m.f produced in the secondary CT coil is energizing the trip circuit.

The basic operation of the circuit breaker is in any abnormal conditions, Relay(trip coil is part of the relay) will sense and initiate the tripping mechanism. Accordingly, contacts will be in an open position as shown in the below figure.

From the above figure, we can see that the automatic mechanism is mechanically connected to handle by gear and liver arrangement. The automatic mechanism has a mechanically stored potential energy, this energy can be by means of deformed spring, compressed air or hydraulic pressure.

During any abnormal conditions and after the trip coil is energised, the automatic mechanism which has the mechanically stored potential energy will be released in the form of kinetic energy to move the handle by gear and liver arrangement. Accordingly moving contact will move apart from a fixted contact and will slide into the sliding contact.

Arc is produced between the moving coil and fixture coil while breaking the circuit, the arc will delay the current interruption and will also dissipate a large amount of heat, which can damage the breaker. It is very important to have a possible minimum time of arc by quenching the arc by various method.

The fixed and moving coils are placed in a closed chamber and separated by the medium of oil, air, vacuum and gas(Sulphur Hexa Flouride(SF6)).

Types of Circuit Breakers

Classification of Circuit Breaker based on the medium used for Arc extinction:

  1. Oil Circuit Breaker
  2. Air Circuit Breaker(ACB)
  3. Vacuum Circuit Breaker(VCB)
  4. Sulphur Hexa Flouride Circuit Breaker(SF6)

Commonly Breaker below 1000V is generally known as Low voltage breakers.

LV Switchgear or Low Voltage Switchgear

Miniature Circuit Breaker(MCB) construction and Types