Ground Faults and Phase to Phase Faults
An electrical explosion is the sudden release of energy due to a short circuit between power phases or a phase to ground. Before discussing design issues, it is important to realize that a very large amount of energy is available on the bus of most electrical panels when they are operating within their normal design parameters. For example a phase to phase short circuit on a 400 Amp, 120/208 Volt panel can instantaneously release over 84,000 watts of energy during a fault condition and not blow the protective fuse or circuit breaker. An explosion of this magnitude will destroy the panel and can start a fire and can cause severe injuries or even death.
Far more serious explosions occur when a short circuit causes the collapse of the magnetic field in the transformer feeding the building. When this happens, there is an instantaneous release of the energy stored in the transformer in the form of a fault current. The most important aspect of designing an electrical system for a building is to make sure that the protective fuses and circuit breakers are capable of handling the maximum available fault current. When fault current ratings are exceeded circuit breakers weld together and are unable to clear the fault.
Being unable to clear ground faults is a serious electrical hazard. Conversely, the cost for a circuit breaker increases exponentially with the fault current rating. Under normal operating conditions, circuit breakers with low fault current interrupting capability work as good as the more expensive ones with high fault current ratings. There are economic pressures on electrical contractors bidding a job and on the owners maintenance staff to use the lower cost breakers. Verification of adequate fault capability should be part of the commissioning process. The true cost of an inadequate circuit breaker will be shocking after a ground fault.
The maximum available fault current at a transformer is calculated as follows:
Isca = FLA * (100/%Z)
FLA = Full Load Amps = (KVATransformer * 1000)/ (VLine to Line *1.73)
%Z = Transformer Impedance
The transformer impedance is a measure of the transformers efficiency. The lower the impedance, the more efficient the transformer. Examining the above equation it is clear that the available fault current also increases with the efficiency of the transformer. Power companies, in their quest to improve efficiency have been systematically replacing old transformers with high impedance with low impedance ones. The power companies are not required to notify the owners of the change even though they may have doubled or tripled available fault current.
For Help or more information, call John Bass at 952/544-6079.
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Copyright 1998 Bass Associates Inc. Last modified: March 31, 2010