Arc flash calculations have to start at the source of power. If a facility is powered from a generator then generator characteristics are the starting point. If a facility is powered from a utility, then certain characteristics including “fault current”, are necessary to start an arc flash evaluation.
Fault current is the amount of current which rushes into a system in the event of a fault (more discussion about fault current is covered in this article: Maximum Available Fault Current… What is it?). During an arc flash evaluation, Rozel requests this fault current value from the utility provider. The utility provider typically responds with calculated fault current values (our preference) or sometimes they will respond with a maximum fault current value.
Maximum fault current values are beneficial when designing equipment but not always great for calculating worst case arc flash values (more information about this topic can be found in our article discussing “arc flash size“). The maximum fault current value is usually determined in one of two ways. 1) The maximum fault current that the utility’s upstream generator can generate. 2) The maximum throughput from the nearest transformer, also known as infinite bus.
A transformer has an internal iron core which it’s windings are wrapped around. This iron core will saturate with enough current. After saturation, regardless of how much current is supplied on the primary side, the secondary side of the transformer will have a limit, i.e. the maximum throughput, also known as infinite bus.
The infinite bus method is simple, calculate the FLA (full load amps) of a transformer’s secondary and divide by the transformer’s impedance.
kVA/Voltage/1.73/%Z = FLA (kA)
For a 2000kVA transformer with 4.5%Z which steps down to 480V on the secondary, the calculation is:
2000/480/1.73/0.045 = 53.52kA
It is very unlikely that a facility with this transformer will ever see this amount of fault current. When Rozel receives fault current based on infinite bus method, we usually start our calculations with a range of typical primary fault current values, and provide labels based on the worst case resulting from the range.