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 breaking capacity, making capacity and short time capacity.
MAKING CAPACITY
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.
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