The ratings of a circuit breaker are determined based on the duties it is designed to perform. For a comprehensive understanding of the specifications, standard ratings, and various tests of switches and circuit breakers should be consulted. In addition to its regular operation, a circuit breaker is expected to perform three major duties in the event of a short circuit.
- It should be capable of breaking the fault current. The ability to interrupt or break an electrical circuit under abnormal conditions is called the breaking capacity of the circuit breaker.
- The circuit breaker must be able to handle the highest asymmetrical current in the current wave. The highest asymmetrical breaking current capacity is called the making capacity.
- A circuit breaker must safely carry a fault for a short period while the other breaker clears the fault. This refers to the circuit breaker’s short-time capacity.
In addition to the above ratings, the circuit breaker should be specified for the following terms.
- The number of poles
- Rated voltage
- Rated current
- Rated frequency
- Operating voltage
The above terms are explained in detail below.
Rated voltage – The rated voltage of a circuit breaker is the highest RMS voltage that it is designed to handle, above the nominal voltage. This voltage is represented in KVrms and is measured phase-to-phase for a three-phase circuit. It sets the upper limit for operation.
Rated current – The rated current of a circuit breaker is the RMS value of the current that it can continuously carry, under specified conditions, at rated frequency and voltage.
Rated Frequency – The rated frequency of a circuit breaker is its designed operating frequency. The standard frequency for most circuit breakers is 50 Hz.
Operating Duty – The circuit breaker duty requires a specific number of operations at set intervals. The operating sequence refers to the opening and closing of the circuit breaker contacts.
Breaking Capacity – The term “breaking capacity” refers to the maximum short-circuit current that circuit breakers can interrupt under specific conditions of transient recovery voltage and power frequency voltage. This capacity is measured in KA RMS at the point of contact separation. There are two types of breaking capacities.
- Symmetrical breaking capacity of a circuit breaker
- Asymmetrical breaking capacity of a circuit breaker.
Making Capacity – It is possible that the circuit breaker may be closed under short circuit conditions. The making capacity of the circuit breaker refers to its ability to withstand electromagnetic forces that are directly proportional to the square of the peak value of the making current of the circuit breaker.
The making current is the peak value of the maximum current wave( including the DC component) in the first cycle of current when the circuit breaker is closed on a short circuit.
Short Circuit Current – The short circuit current rating of a circuit breaker is the maximum RMS value of current that the breaker can safely handle in a closed state for a specified time interval under specific conditions without getting damaged. This rating is usually expressed in kA for 1 or 4 seconds and is based on thermal limitations.