High Fault Contribution may not be a bad thing
October 26, 2009 by Jeff
Filed under Arc Flash, Electrical Safety
In the past you determined the bus withstand of equipment by the bolted fault. In some cases the system would be designed to limit the available fault energy on any one bus by installing higher impedance transformers, multiple small transformers, etc.
Today there is as much concern regarding the Incident Energy (IE) released in an arc fault as there is regarding if the equipment can withstand a bolted fault. In regards to IE more fault current may actually lower the energy when the protection is in its instantaneous range. The amount of time required to clear a fault decreases as the magnitude of fault current increases. The IE equation, energy is related to the square of time. If the time to clear the fault is halfed, the IE is 25% of the original.
Higher fault energy typically represents a stiffer overall electrical system. This leads to better coordination as the designer will be better able to predict what will happen during an incident.
Better coordination, of course, assumes a bolted fault.
I suppose that’s right. Typically however when you are doing coordination for a 3 phase fault this is assumed.
Most internal faults within equipment will approach a bolted fault when there is enough arcing between the busses (one “s” or two, I never know) from ionized air and vaporized copper if the fault is large enough.