Circuit Breaker Ratings

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High Voltage Circuit Breakers

Based on IEC 62271-100 (HV circuit breakers) and IEC 62271-1 (HV switchgear):

Ir (Rated normal current)

The rated continuous / uninterrupted current that the circuit breaker can carry.

Ik (Rated short-time withstand current)

The rms value of the current which must be carried by the circuit breaker in the closed position during a specified short time under prescribed use and behaviour. The specified time is called the rated "duration of short circuit" or tk, and has a standard value of 1s. Other recommended values are 0.5s, 2s and 3s. The rated short-time withstand current is equal to the rated short-circuit breaking current (Isc).

Ip (Rated peak withstand current)

The peak current associated with the rated short-time withstand current, and is defined according to the dc time constant (where a time constant of 45ms covers the majority of cases and corresponds to a rated peak withstand current of 2.5 x Ik for 50Hz and below or 2.6 x Ik for 60Hz systems). The rated peak withstand current is equal to the rated short-circuit making current (Imc).

Isc (Rated short-circuit breaking current)

The highest short-circuit current that the circuit breaker shall be capable of breaking under the conditions of use and behaviour prescribed in IEC 62271-100. It is characterised by two values:

  • DC time constant - the time constant that results in a percentage of the dc component of the short-circuit at contact separation. A standard value of 45ms covers the majority of all cases. Special time constants can also be used depending on voltage (e.g. 120ms for <52kV, 60ms for <420kV and 75ms for >550kV)

Imc (Rated short-circuit making current)

The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.

TRV (Transient recovery voltage)

The reference voltage which constitutes the limit of the prospective transient recovery voltage of circuits which the circuit breaker shall be capable of withstanding under short circuit conditions.

Low Voltage Circuit Breakers

Based on IEC 60947-2 (LV switchgear and controlgear - Part 2: Circuit breakers):

In (Rated current)

The rated continuous / uninterrupted current that the circuit breaker can carry.

Icm (Rated short-circuit making current)

The short-circuit current that the circuit breaker can withstand as it is closing where the act of closing initiates the fault.

Icu (Rated ultimate short-circuit current)

The maximum symmetrical short-circuit current the circuit breaker can interrupt.

Ics (Rated service short-circuit current)

The breaking capacity such that the circuit breaker is tested to carry its current continuously. The test sequence verifies that the breaker can be returned to service. Ics is the maximum current the breaker can interrupt multiple times and be returned to service without being damaged and is expressed as a % of Icu.

Icw (Rated short time withstand current)

This is the steady state symmetrical fault current the breaker has to be able to carry for a duration of 0.05s to 3s without exceeding its thermal integrity.

Other Ratings

Ith (Thermal short-circuit current)

Ith is the joule integral and thermal equivalent short-circuit current and is a measure of the energy generated in the resistive element of the system by the short-circuit current (see IEC 60909-0 Clause 4.8).

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