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.
Key Performance Duties of a Circuit Breaker
In addition to its regular operation, a circuit breaker is expected to perform three major duties in the event of a short circuit.
- Breaking Capacity: 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.
- Making Capacity: 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.
- Short-Time Withstand 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.
Standards for Circuit Breaker Ratings
Circuit breaker ratings are governed by international standards such as:
- IEC 60947-2 – for Low-Voltage Circuit Breakers
- IEC 62271-100 – for High-Voltage Circuit Breakers
These standards ensure safe operation, consistent performance, and global compliance.
Standard Circuit Breaker Ratings
Apart from the performance-based ratings discussed earlier, circuit breakers are also defined by a set of standard parameters that ensure proper selection and coordination within electrical systems. These include:
- The number of poles
- Rated voltage
- Operating Voltage
- Rated current
- Rated frequency
- Operating duty
- Breaking Capacity
- Making Capacity
- Rated Short-Time Current
Below is a detailed explanation of each rating term.
Number of Poles
The number of poles refers to the number of separate circuits that a circuit breaker can control. In three-phase systems, circuit breakers are typically 3-pole or 4-pole. A 3-pole breaker interrupts all three phases simultaneously, while a 4-pole breaker also interrupts the neutral wire along with the three phases.
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.
Operating Voltage
The operating voltage of a circuit breaker refers to the voltage level at which the breaker is intended to function under normal conditions. It must be suitable for the system voltage to ensure reliable operation and safe interruption of the circuit during fault or maintenance conditions.
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.
Rated Short-Time Current
The rated short-time current of a circuit breaker is the maximum RMS value of current that the breaker can safely withstand in the closed position for a specified short time interval, under defined conditions, without suffering damage. This rating is typically expressed in kA for 1 or 4 seconds and is based on the thermal withstand capacity of the circuit breaker.
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