From a document by Bill Brown System Protection
static.schneider-electric.us
The time constant of the circuit is (L/R) and thus the higher the value of L/R the longer the transient will last. Instead of (L/R) power systems typically are defined in terms of (X/R), where (X/R) is the ratio of the inductive reactance of the short-circuit impedance to its resistance. Thus the higher (X/R) or the ?X/R ratio,? the longer the short-circuit transient will last. This has great implications on the rating of equipment.
Low voltage fuse AC interrupting ratings are based upon a maximum power factor of .2, corresponding to a maximum X/R ratio of 4.899. In order to evaluate a low voltage fuse?s interrupting rating on a system with a higher X/R ratio the system symmetrical fault current must be multiplied by a multiplying factor