Fault Current Calculations At Medium Voltages

The method of calculating short circuit currents in medium-voltage systems differs from the procedures for low-voltage systems in the following respects:

1. Medium-voltage breakers are slower acting in their operation. The result is that the maximum current which the breaker must be capable of closing and latching, and the maximum current it must be able to interrupt do not occur at the same time and may be different values.
2. Medium-voltage breakers are rated based upon the maximum asymmetrical current in one phase. Low-voltage breakers use symmetrical short circuit ratings.

Medium-voltage breaker manufacturers generally show a closing and latching capability which is 1.6 times the maximum symmetrical short circuit (interrupting) capability. However, at operating voltages near nominal nameplate kV, the maximum ratio of closing and latching capability to the symmetrical short circuit capability exceeds 1.6.

A circuit with an X/R of 25 has a 1.6 ratio between first cycle asymmetrical and symmetrical RMS currents. However, in a circuit whose X/R ratio is 15 the ratio of AC high-voltage circuit breaker symmetrical short circuit (interrupting) capabilities and associated asymmetrical short-circuit (interrupting) capabilities is approximately the same as the ratio of symmetrical and asymmetrical currents.