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Re: 50 kVA Pole Pig

Hi Jim and All,

	I happened to be talking about this very subject Thursday with the
electrical compliance engineer at work.  We occasionally see circuit
breakers on equipment welded closed due to very high currents.  An obvious
problem despite the fact that they have every certification and approval
know to mankind.  Some even blow apart...

	In industrial areas, there are typically a number of progressively larger
rated breakers along the way back to the main service transformer for the
plant.  These are (should) be sized by the electricians to handle dead
short conditions (such as those that occur when a certain switching power
supply manufacturer's H bridges cross conduct :-)). 

	What we should be aware of, is how many and what type of breakers are
between us and the closest service transformer.  The danger would come if
one were to wire into the main service box before any fuses and then use
just that old used breaker one found at the garage sale 10 years ago :-)
The old breaker could easily weld shut and be a problem if something really
shorts hard.

	The solution is to use modern (lots of safety approvals) breakers (or
fuses) in good condition that have their break rating printed on them.
This rating is typically in the tens of thousands of amperes and is the
maximum current the breaker will reliably interrupt.  The work service
breakers (that we could get near) were rated at 65000 amps, the bench
breakers were at 15000 amps, and the supply breakers are at 5000 amps.
There are "really big" breakers back in the electrical rooms.  Substations
use circuit breakers the size of a dishwasher.  I don't remember their
ratings, but they are not going to trip for our purposes unless we REALLY
screw up! :-))

	So... any fuse device should have some huge current rating that it is
guaranteed to interrupt.  So how much current do you have?  If you short
across 220 volts and have say .05 ohm of resistance in the wiring between
you and the big transformer, that works out to 4400 amps.  When I draw 100
amps on a 220 circuits at work, the voltage drops about 5 volts so that is
probably pretty typical.  I would not worry about the percent impedance of
your pig because you should assume the sort will be on the input side (like
at one's breaker :-))  As long as your house's electrical service is
installed to code and you don't bypass anything, you will be fine.
Something will safely blow, even if it is not the local breaker.  However,
if one has gotten "creative", then you want to be sure your breaker can
interrupt BIG short currents or add some BIG break rated fuses.  

	At work, with the big 100+ kilowatt inverters running on 480 3 phase (150
amps), we have knocked conduit off the ceiling from the jerking wires
inside during shorts (part of compliance testing)...  It sounds like the
pipes were hit with a sledge hammer...  We are now installing 300 amp
wiring...  but now, I really do digress... ;-)

	Terry



At 08:34 PM 5/7/99 -0400, you wrote:
>
>This is getting a bit off-topic, but since it is a something of a safety
>issue, hopefully our fearless moderator will have some patience...
>
>I don't think you really can size a circuit breaker to handle a short
>at its terminals since the current that will flow depends on the
>impedance and voltage of the source supplying the breaker.  These may
>be changed without warning by the local power company.
>
>I have not personally had the pleasure of seeing a circuit breaker
>fail to open with over 10,000 Amps flowing, but it does not seem like
>the kind of think you want to have happen accidentally.
>
>Jim Monte
>
snip........