kAIC Rating of Circuit Breakers | Calculation in Low Voltage System

Low voltage circuit breakers, unlike their medium- and high-voltage counterparts, respond to short-circuit instantaneously. Therefore, the calculation of the interrupting short-circuit duty for the selection or verification of kAIC rating of circuit breakers is based on the one-half cycle network. The reactances of the rotating equipment to be used in the calculation of the short-circuit current is presented in IEEE Std C37.010 and IEEE Std C37.13.

To learn more about rotating equipment reactances and their effect on short-circuit current contribution, click here.

The multiplying factors in determining the first cycle asymmetrical peak and RMS value of the short circuit current are based on the ‘half-cycle’ current and a purely reactive circuit assumption with the source voltage phase angle of 0° at the time of fault.

These multiplying factors become the basis in calculating the multiplying factors for low voltage circuit breakers .

kAIC Rating of Circuit Breakers: Unfused Circuit Breakers

According to IEEE Std C37.13, the basis of the rated short-circuit current of an unfused circuit breaker is the rms symmetrical current value at a power factor of 15% lagging (X/R_test ratio of 6.6). By applying this, we obtain the multiplying factor as shown in the equation below.

kAIC Rating of Circuit Breakers: Fused Circuit Breakers

According to IEEE Std C37.13, the basis of the rated short-circuit current of a fused circuit breakers is the rms symmetrical current value at a power factor of 20% lagging (X/R_test ratio of 4.9). By applying this, we obtain the multiplying factor as shown in the equation below.

This multiplying factor is also applicable to fuses. If the fault point X/R is not available, IEEE Std C37.13 recommends an X/R ratio of 20. It also important to note that small impedances such as cable impedances should be taken into account because their effect to the calculation of circuit interrupting duty is very significant.

The kAIC rating for circuit breakers depends on the manufacturer test power factor. The following table shows maximum test power factor for each type of low voltage circuit breakers.

Motor Contributions

The following recommendations were provided by IEEE Std C37.13 for motor contributions.

• For induction motors, if impedance is not known, use 3.6 times FLC. For synchronous motors, use 4.8 times FLC.

• If motor load installation is not known,
  • System voltages of 120V and 208Y/120V, assume 50% lighting and 50% motor load. Translates to 2 times FLC.
  • System voltages of 240V to 1000V, assume 100% motor load, 25% synchronous, 75% induction motors. Translates to 4 times FLC.

References

IEEE Std 551-2006 [The Violet Book]: Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems. (2006). S.I.: IEEE.

IEEE Std C37.010-2016: IEEE Application Guide for AC High-Voltage Circuit Breakers > 1000 Vac Rated on a Symmetrical Current Basis. (2016). S.I.: IEEE.

IEEE Std C37.13-2015: IEEE Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures. (2015). S.I.: IEEE.

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