Circuit Breaker

As the name itself suggests that a circuit breaker is used to break the circuit. Now, in this particular post we will see that How it breaks the circuit? When it breaks the circuit?and Why it breaks the circuit?I have also attached a video at the end of this post.

During operation of power system, it is often desirable and necessary to switch on or off the various circuits (e.g. generating plants, transmission lines, distributors etc.) under normal and abnormal conditions.

The function of tripping the circuit was performed by the fuses and switches in earlier days, but there are few disadvantages as follows:

  1. A switch cannot break the circuit automatically. It needs manual operation to break the circuit.
  2. When a fuse blows out, it takes quite some time to replace it and restore supply to the costumers.
  3. A fuse cannot successfully interrupt the heavy fault currents that result from faults on large capacity circuits and high voltages.

To overcome these disadvantages, we need a more dependable means of control and that is obtained by the use of Circuit breaker.

Definition of circuit breaker

 A circuit breaker is device which is used for switching and protection of various parts of power system. It can open or close the circuit, either manually or automatically under all conditions such as no-load, full load, and short-circuit conditions.

A circuit breaker can-

  1. Make or break a circuit either manually or automatically under normal conditions.
  2. Breaks a circuit automatically under fault conditions.
  3. Make a circuit either manually or automatically under fault conditions.

The automatic control employs relays and operates only under fault conditions.

Working of a circuit breaker

A circuit breaker consists of two contacts, a fixed contact and a moving contact, called electrodes. An illustrative fig. of an oil circuit breaker is shown below.

working of an oil circuit breaker

Under normal operating conditions, these contacts remain closed and will not open automatically until and unless any fault occurs in the system. If anyone wants to open these contacts then, these can be opened either manually or by remote control anytime.

As we told that for the automatic operating operation of circuit breakers in a faulty condition, it needs a relay. A relay is an instrument which is used to sense the fault in the system and it sends signal to the circuit breaker.

Functioning of relay and circuit breaker

We will understand the functioning of relay and circuit breaker with the help of a circuit diagram below.

operation of relay and circuit breaker

A circuit breaker is connected with the main line and the contacts are closed in the normal operation.

A current transformer is also connected as shown. The primary winding of current transformer is connected with the main line in which fault occurs, and secondary winding is connected with the relay.

Current transformer is used to step down the current value and make it suitable for the relay connected with the secondary of the current transformer. Let us suppose a relay can work on 5A current value, and the line has 200A flowing in it, then current transformer will convert it from 200A to 5A, so that relay can work properly.

Relay coil is connected with the trip circuit as shown. Trip circuit has a trip coil which is connected to the circuit breaker.

If any fault occurs in the system by any reason like short circuit, overheating etc., then the primary current of current transformer will increases and the secondary current will also increase which finally energises the relay coil.

When a relay coil gets energised then it creates a mmf around it and behaves like a magnet, which will pull the relay electrode downwards and the trip circuit will get closed.

Now, the electrode which is connected with the relay coil comes down, it will complete the trip circuit and current will start flowing. In this case, the trip circuit which will energise the trip coil (the electrode is connected with the C.B in normal condition).

When the trip coil gets energised, it will also behave like a magnet and will pull the electrode, and the circuit breaker will operate and finally the circuit will open.

In this way, with the help of a circuit breaker and relay, healthy part of a system can be protected from faulty part.

Arc phenomenon

Now, when the moving contact of circuit breaker begins to apart from fixed contact, then the current density will increase at their tip and this will increase the heat.

The heat generated will ionize the medium present around (may be air, oil etc.). Ionization of that medium will give a path to current to flow and it will lead to create an arc between the contacts.

This arc can do severe damage to the system if it is not interrupted in time. The categories of circuit breakers are basically dependent on arc interruption methods. We will read the arc phenomenon, and methods of arc extinction in detail in another post. 

Classification of circuit breakers

There are several ways of classifying the circuit breakers, but as I told you that the most general way of classification is on the basis of medium used for arc extinction. Broadly, oil, air, sulphur hexafluoride (SF6) and vacuum are used as arc quenching medium.

Types of circuit breakers:

  1. Oil circuit breakers: these employ some insulating oil (e.g. transformer oil) for arc extinction. Read the full post on oil circuit breakers here. Click here
  2. Air-blast circuit breakers: high pressure air blast is use for the extinguishment of arc.
  3. Sulphur hexafluoride circuit breakers: In this circuit breaker, sulphur hexafluoride (SF6) gas is use for arc extinction.
  4. Vacuum circuit breakers: Vacuum is use for the arc extinction process.

Each type of circuit breaker has their own advantages and disadvantages, their construction, operation and every other detail, you can read in the upcoming posts.

You can also visit our YouTube channel “Electrical Paathshala”.

Watch out the video of circuit breaker in Hindi for better understanding.

Circuit breaker pdf available here, click on download button below

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Also read,

Switchgear and protection

Fuses and its types

Oil circuit breaker

Protection of transformer with buchholz relay