The rating of the circuit breaker is given on the duties that are performed by it. For complete specification standard ratings and various tests of switches and circuit breakers may be consulted. Apart from the normal working of circuit breakers, the circuit breaker is required to perform following three major duties under short circuit conditions.
- It is capable of breaking the faulty section of the system. This is described as the breaking capacity of the circuit breaker.
- The circuit breaker must be capable of making the circuit in the greatest asymmetrical current in the current wave. This refers to making the capacity of the circuit breaker.
- It must be capable of carrying fault safely for a short time while the other breaker is clearing the fault. This refers to the short-time capacity of a circuit breaker.
In addition to the above rating, the circuit breakers should be specified in terms of
- The number of poles
- Rated voltage
- Rated current
- Rated frequency
- Operating voltage
These terms are explained below in details.
Rated voltage – The rated maximum voltage of the circuit breaker is the highest RMS voltage, above nominal voltage for which the circuit breaker is designed and is the upper limits for operation. The rated voltage is depicted in KVrms and used phase to phase voltage for three phase circuit.
Rated current – The rated normal current of the circuit breaker is the RMS value of the current with which the circuit breaker shall be able to carry at rated frequency and at rated voltage continuously, under specified conditions.
Rated Frequency – The rated frequency of a circuit breaker is the frequency at which it is designed to operate. Standard frequency is 50 Hz
Operating Duty – The operating duty of a circuit breaker consists of the prescribed number of unit operations at stated intervals. The operating sequence refers the opening and closing operation of circuit breaker contacts.
Breaking Contact – The terms expressed the highest number of short-circuit current that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in KA RMS at contact separation. The breaking capacities are divided into two types.
- Symmetrical breaking capacity of a circuit breaker
- Asymmetrical breaking capacity of a circuit breaker.
Making Capacity – There is always the possibility that the circuit breaker is closed under short circuit conditions. The making capacity of the circuit breaker is its ability to withstand under the effect of electromagnetic forces which are directly proportional to the square of the peak value of the making current of a circuit breaker.
The making current of the circuit breaker, when closed on a short circuit, is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker.
Short Circuit Current – The short circuit current of a circuit breaker is the RMS value of current that a breaker can carry in a fully closed condition without damage, for the specified time interval under prescribed condition. It is normally expressed regarding terms of KA for 1 seconds or 4 seconds. These ratings are based on thermal limitation.
Low voltage circuit breaker does not have any such short circuit current because these are normally equipped with straight acting series overload trips.