Motor Contribution During Three-Phase Short Circuit Fault

One of the major factors to be considered in selecting the ratings of electrical equipment (circuit breakers, bus bar bracing, etc.) is the level of three-phase short circuit current available at the equipment location in the system. The total three-phase short circuit current at a faulted point includes both the currents from the power source (such as electric utility company or in-plant generators) and contribution from electrical motors in the system. Presently, the calculation of three-phase fault current in a system is based on the method outlined in IEEE Standard 141-1976 in which the calculation of motor contribution within the system is defined. It will be pointed out that during a three-phase fault, only motors directly connected to the faulted bus or downstream buses will contribute fully to the fault. Other induction motors (with transformers connected between the motors and the fault) may or may not contribute current to the fault point. These motors will continue to receive power from the supply and act as motors. Therefore lower fault current values will be the result. This finding will have a significant bearing on borderline equipment selection with consequent savings in equipment cost. The conclusion is drawn that the calculated short circuit current based on IEEE Standard 141-1976, is higher than the actual value. This leads to unnecessarily high fuse and breaker interrupting ratings, bus bracing, etc. Computer studies and an analysis of this phenomenon are presented.