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From: Lew Hodgett (no email)
Date: Sat Jul 02 2005 - 23:34:24 EDT
wrote:
> I have seen shore power connectors afire, a startling sight.
More common than you might think, it's known as an arcing ground fault.
Typically, the fault current is less than that required to trip the
circuit protective device (either fuse or c'bkr) and a fire often results.
The following is an example of arcing ground fault that one of my
customers experienced years ago that caused a lot of damage, and put a
bunch of people out of work for a while.
Customer had a 1,000KVA transformer serving the business which requires
a 1,200A switch board for 480Y/277V/3PH/60HZ service.
One cold, snowy, weekend, the switchboard caught fire, melted down to
zip and started other parts of the business on fire.
Fire dept had to get someone from the utility to disconnect the high
voltage primary of the 1,000KVA transformer to make sure power was cut
off to the building.
In the final analysis, it was determined that rats had gotten into the
bottom of the switchboard, built a nest and had some how contacted a
couple of the phase bars creating a dead short of several hundred amps,
but far less than the 1,200 amps required to trip the main c'bkr.
This fault current ultimately started the fire.
Even pulling all the strings available to get a new switchboard built
and shipped, my customer was closed down for almost 6 weeks which meant
no paycheck for a lot of people.
Today, most plant engineers recognize the benefits of fault current
monitoring, but there are still those facilities.
Guess the point to this little story is to demonstrate the need to
recognize that a healthy respect for electrical energy is required.
There are no short cuts, there is no free lunch.
Lew
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