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Arc Flash Warning!

The most deadly locations in an electrical distribution system, that are almost always readily identifiable without
performing an
Arc Flash Hazard Analysis, are the secondary of large distribution transformers (> 125 kVA) and the
first disconnect or overcurrent protective device (i.e., circuit breaker or fusible switch) downstream of the same

These locations have the perfect ingredients for extremely high arc flash incident energy (cal/cm2) and arc blast force:

  • Relatively high fault current (short-circuit current)
  • Relatively long fault clearing time

At the aforementioned locations, we have usually just derived a new source (a separately derived system in NEC
terms), or at least have transformed the voltage to a lower level for distribution and utilization.  In doing so, the new
"source" creates a very high magnitude available fault current at the secondary terminals, limited only by the source
impedance (i.e., utility or generator available fault current) and the impedance of the transformer (e.g., nameplate %Z
or % Imp.) being studied.

Contrary to most intuitive thinking, very low fault current can generally translate to
higher arc flash incident energy.  
This phenomenon occurs because of the inverse nature of the circuit breakers or fuses attempting to limit the fault -
the less current the circuit breaker of fuse has through it, the longer it will take to open and clear the fault.  (Similar to
the concept of an "overload" vs. a "short-circuit")

This effect is most evident at the secondary of these large distribution transformers and the first disconnect
downstream.  At these locations, the next overcurrent protective device upstream that will sense and limit the fault
current is the circuit breaker or fuse on the primary side of the transformer.  With regard to a utility service entrance,
this device is controlled by the Utility Company and generally is a medium-voltage, fuse-link in a cut-out switch or an
E-rated fuse.  A worker being subjected to short-circuit current at the aforementioned transformer secondary locations
would appear as only a small "load" increase to the primary overcurrent device.

In the many Arc Flash Hazard Analyses we have completed, these locations are very often determined to be greater
than 40 cal/cm2 of incident energy, meaning the arc blast could be more lethal than the arc flash.

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