Why Circuit Breakers?

Defining Circuit Breaker

A piece of equipment which can:• Make or break a circuit either manually or by remote control undernormal conditions

• Break a circuit automatically under fault conditions• Make a circuit either manually or by remote control under faultconditions

Requirements Of Circuit Breaker

1.Normal working current and the short circuitcurrent must be safely interrupted by the circuit

breaker

2.Faulty section of the system must be isolated ASAP.

3.Should not operate with flow of overcurrent duringhealthy conditions.

4.Faulty circuit should only be isolated withoutaffecting the isolated one.

Principle Of Operation

Arc Phenomenon

ARC Current- Depends upon Arc Resistance

•Degree of ionization- Decreases with increase in no. ofionized particles

•Length of the arc-Increases with increase in length or gapbetween electrodes

•Cross section of the arc-increases with decrease incross section

Principles Of Arc Extinction

Factors responsible for maintenance of arc:

1. Potential Difference between the contacts(Not feasible in high voltage operation)

2. Ionized particles between contacts- By cooling the arc orremoving the ionized particles

Methods Of Arc Extinction

• High Resistance Method• Low Resistance Or Current Zero Method

High Resistance Method• Lengthening the arc- achieved by separation of contacts.

• Cooling the arc- By a gas blast• Reducing the X-section- achieved by letting the arc pass through anarrow opening or having smaller area of cross section

• Splitting the arc into a no. of small arcs in series

Low Resistance Or Current Zero Method

• Recovery Rate Theory (Slepain’s Theory)• Energy Balance Theory (Cassie’s Theory)

Recovery Rate Theory (Slepain’s Theory)

Energy Balance Theory (Cassie’s Theory)

Important Terms

Circuit Breaker Ratings

• Breaking Capacity• Making Capacity• Short-time Capacity

Breaking Capacity

Making Capacity

Short-time Capacity

Normal Current Rating