Contribution of single-phase motors to system fault level

System designers need to know the maximum current which may flow in any system component to ensure that otherwise healthy plant is not put at risk due to a fault in other equipment. This is particularly the case for switchgear. Large three-phase induction motors make a contribution to both break and make duty. Due to the rapid decrement of their contribution, the 2.55 factor is inadequate. It is shown by means of calculation and test results that even small single-phase induction motors make a contribution to switchgear make duty. It is necessary to calculate both the make and break duty, but it is suggested that, for the longer term, the relationship between break and make should be raised, possibly to a factor of 3. Failure to allow for the higher make duty may lead to dangerous conditions on a system