الأربعاء، 2 فبراير، 2011

Circuit Breakers Ratings

 Circuit Breakers Ratings

A circuit breaker must operate under all conditions, but its operation becomes critical when there is a fault on the system in which the breaker is used. During fault conditions, a circuit breaker must open the faulty circuit and break the fault current. The ratings of the circuit breakers are also for breaking and making capabilities. There are three ratings for breakers as:
1- Breaking Capacity
2- Making Capacity
3- Short-time Rating

Breaking Capacity
 Breaking capacity is defined as the r.m.s. current that a circuit breaker is capable of breaking at given recovery voltage and under specified conditions (i.e. power factor, rate of rise of restriking voltage).

The breaking capacity is always stated at the r.m.s. value of fault current at the instant of contact separation. When the fault occurs, there is a considerable asymmetry in the fault current due to the presence of a d.c. component. In the Britain, it is a usual practice to take breaking current equal to the symmetrical breaking current. However, in America, the practice is to take breaking current equal to asymmetrical breaking current.
Therefore, the American rating given to a circuit breaker is higher than the British rating.
It is a common practice to express the breaking capacity in MVA by taking into account the rated the rated breaking current and rated service voltage. Thus if I is the rated breaking current in Amperes and the rated service voltage is V in volts, the breaking capacity for three-phase circuit is:
Breaking Capacity = sqrt 3 x V x I x 10-6 MVA

However, the agreed international standard of specifying breaking capacity is defined as the rated symmetrical breaking current at a rated voltage. The MVA breaking capacity is illogical in a sense that when the short circuit current is flowing there is only a small voltage across the breaker contacts, while the service voltage appears across the contacts only after the current has been interrupted. Thus MVA rating is the product of two quantities which do not exist simultaneously in the circuit.

Making Capacity
It is the peak value of current (including d.c. component) during the first cycle of current wave after the closure of circuit breaker.
There is always a possibility of closing or making the circuit breaker under the short circuit conditions. The capacity of a circuit 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. Therefore, making capacity is stated in terms of a peak value of current instead of r.m.s. value.
Mathematically stated, making capacity is given as:
Making Capacity = 2.55 x symmetrical breaking capacity

Short-time Rating
The period for which the circuit breaker is able to carry fault current while remaining closed is known as short-time rating.
This rating is needed because sometimes a fault on the system is of a temporary nature and persists for only a second or two after which the fault is automatically cleared. For the sake of continuity of the supply, the breaker should not trip in such situations. This means that the circuit breakers should be able to carry high current safely for some specified period while remaining closed. This means that they should have a specified short-time rating. However, if the fault persists for a duration longer than the specified time limit, the circuit breaker will trip, disconnecting the faulty section.
The short-time rating of a circuit breaker depends upon its ability to withstand:
1- The electromagnetic force effects
2- The temperature rise

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