I have made some calculations based on simplifying assumptions and the Engineer’s report. It would appear that there is a transformer vault with four 2,000 kVA transformers connected together in a bus on the secondary side (480Y/277 V). It would appear that there is no secondary protection from the transformers to the collector bus or no secondary protection from the collector bus to the busduct, which is 5,000A. There are two 5,000A busducts connected to the collector bus in the transformer vault. Furthermore, there is no mention of expulsion or partial current limiting fuses being used in the transformers themselves on the primary side. The only protection mentioned on the primary side of the transformers is the power circuit breakers in the utility substation upstream. If the utility system is considered as an infinite bus, then the available fault current at the collector bus is as follows:

Rough Three-Phase Short Circuit Calculation (based on assumption of infinite utility bus and ANSI standard impedance (5.75% + 7.5% = 5.3% to 6.2%)
Fault current from one transformer IF = 2000/0.53 = 37, 736 A
The available fault current from four transformers IFTOTAL = 37,736A x 4 = 150,944A

The arcfash incident energy potential here has to be through the roof – not only is this a stiff system with a lot of capacity, it is a system that is relying on the upstream utility substation circuit breakers to trip, which extends the tripping time. As should be known, Arcflash incident energy depends on available current (I) and time to trip (t). Without performing any calculations, I believe that would be safe to say that there is no safe level of PPE for working on the main switchboard. This means that work should not be performed on the switchboard while it is live. This is the way electrical systems for large buildings were done in the late seventies, when arcflash was not part of the design considerations.

Since then, starting in and around 1995 Arcflash considerations started being considered in electrical designs. It is now the case (required by OSHA and NFPA 70E) that an electrical safety hazard analysis be performed. Arcflash is part of that analysis requirement.

Does the facility have an Electrical Safety Program as required by OSHA and have they performed an electrical safety analysis? If they have or if they do in the future, they may need to consider making significant adjustments to the electrical distribution system based on electrical safety considerations dictated by OSHA.