One of the things that I do in order to earn a living is related to electrical system reliability
and power quality. This past week has featured a couple of high
profile stories regarding electrical infrastructure failure; one in
Athens
and one in Chicago.

I passed through the American terminal at O'Hare in April and it was
very clearly under-construction at the time. So in some respect the
likelihood of an accident was bound to increase. Reading the original
news dispatches it seemed clear to me that it was unlikely that a
transformer actually exploded in-service in the building; it sounded
more like a cable fault to me. It was reported that workers
inadvertently "moved wires". An updated story does identify that a
cable fault occurred, but that is not how it is referred to. Instead,
they call it a "flash":

I have no personal or first-hand knowledge to relate regarding the
Chicago event, however I can offer the following general professional
observations.



In my experience, a dead shorted cable
packing a few hundred amps does indeed give off quite a
flash and it can be lethal. The loud sound associated with a short circuited
cable is a combination of two things; the sonic component of the EMI
arc (think of it as being the "thunder" part of a mini lightning
strike)  and either (or both) of the operation of  circuit
breakers or fuses opening up to clear the fault. When large fuses blow
they can be really loud.



The pressing question for a forensic engineer on site now would be,
"Why did the cable fault occur?".