The rating of a circuit breaker is determined by the duties it performs. For complete specifications, standard ratings and various tests for switches and circuit breakers should be consulted. In addition to the normal operation of circuit breakers, they are required to perform the following three major duties under short-circuit conditions:
- Breaking the faulty section of the system, defined as the breaking capacity of the circuit breaker.
- Making the circuit under the greatest asymmetrical current in the current wave, referring to the making capacity of the circuit breaker.
- Carrying fault current safely for a short time while another breaker clears the fault, known as the short-time capacity of the circuit breaker.
In addition to the above ratings, circuit breakers should be specified in terms of:
- Number of poles
- Rated voltage
- Rated current
- Rated frequency
- Operating duty
Detailed explanations of these terms:
Rated Voltage
The rated maximum voltage of a circuit breaker is the highest RMS voltage (above nominal voltage) for which it is designed, serving as the upper limit for operation. Rated voltage is expressed in kVrms and uses phase-to-phase voltage for three-phase circuits.
Rated Current
The rated normal current of a circuit breaker is the RMS value of the current it can continuously carry at rated frequency and voltage under specified conditions.
Rated Frequency
The frequency at which a circuit breaker is designed to operate, with the standard frequency being 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 to the opening and closing operations of the circuit breaker contacts.
Breaking Capacity
This term denotes the highest short-circuit current a breaker can interrupt under specified conditions of transient recovery voltage and power-frequency voltage, expressed in KA RMS at contact separation. Breaking capacities are categorized into:
- Symmetrical breaking capacity
- Asymmetrical breaking capacity
Making Capacity
When a circuit breaker closes under short-circuit conditions, its making capacity is the ability to withstand electromagnetic forces (directly proportional to the square of the peak making current). The making current is the peak value of the maximum current wave (including the DC component) in the first cycle after the breaker closes the circuit.
Short-Circuit Current Withstand Capacity
This is the RMS value of current a breaker can carry in a fully closed state without damage for a specified time interval under prescribed conditions, typically expressed in KA for 1 second or 4 seconds. These ratings are based on thermal limitations. Low-voltage circuit breakers generally lack such short-circuit current ratings, as they are typically equipped with straight-acting series overload trips.
