A circuit breaker is required to perform the following three duties:1. It must be capable of opening the faulty circuit and breaking the fault current.
2. It must be capable of being closed on to a fault
3. Must be capable of carrying fault current for a short time while another breaker is clearing the fault.
Depending on the above duties circuit breaker has three ratings braking capacity, making capacity and short time capacity.
It is current that a circuit breaker is capable of breaking at a given recovery voltage under specified conditions. The breaking capacity is always stated at the r.m.s value of fault current at the instant of contact separation. When a fault occurs there is considerable asymmetry in the fault current due to presence of d.c component. The d.c component dies away rapidly. The contacts are separated at DD’ as shown in fig5.
At this point, fault current has
x = max value of a.c component
y = d.c component
Therefore Symmetrical breaking current = r.m.s value of a.c component
= x /  1/2
Asymmetrical breaking current = r.m.s value of a.c component
=[[x /  1/2 ]^ 2 + y^ 2 ]
Breaking capacity is expressed in MVA by taking into account the rated breaking current and rated service voltage. Thus if I is the rated breaking current in amperes and V is rated service line voltage in volts, then for a three phase circuit breaking capacity = √ 3 * V * I * 10 –6 MVA
The capacity of a breaker to make current depends upon its ability to withstand and close successfully against the effects of electromagnetic forces. These forces are proportional to the square of maximum instantaneous current on closing. So making capacity is stated in terms of a peak value of current. The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as making capacity. To find making capacity multiply symmetrical breaking current by root 2 to convert from r.m.s to peak and then by 1.8 to include the doubling effect of maximum asymmetry. Making capacity = 2.55 * symmetrical breaking capacity.
SHORT TIME RATING
It is the period for which the circuit breaker is able to carry fault current while remaining closed. The fault on the system of very temporary nature persist for 1 or 2 sec after which the fault will be cleared, so the breaker should not be tripped in such situations. This means the circuit breakers should be able o carry high current safely for some specified period while remaining closed. i.e they should have short time rating. It depends on its ability to withstand electromagnetic force effects and temperature rise.