Mike,
Systems can only supply so much fault current. It looks like the testers calculate an available fault current based on circuit impedance, voltage and the assumption that the supply system can supply unlimited current (infinite bus).
The calculations for selection the correct PPE require an actual available fault current and not a theoretical maximum. The incident energy produce by the arc often goes up with a lower available fault current. This is because a lower current will often result in a longer time before the OCPD operates and this results in a higher incident energy and will require a higher level of PPE protection.
Don
