z@rocksvax.FUN (Jim Ziobro,840-11H,76712,7162442148) (11/15/85)
This note is compiled from a conversation with an RG&E representative. RG&E is
the electric utility in Rochester.
The limiting current to the household will be of course a quantity that will
vary to a large extent on the what is between the short and the substation.
RG&E admits that even given specific circuits it is difficult to estimate
surge current capacity.
The first topic is protection other than normal house fuses. The response
is there is *no* mechanism intended to current limit between substation
and house. Depending on the distribution transformer it may have either
a low-side fuse or a CSP (self-contained resetable breaker) on the high side
or possibly no protection at all. Typical distribution transformer ratings
are 15, 25, or 37.5 KVA. The typical protection mechanism could be expected
to trip at about 3 times rated capacity though instantaneous surge current could
be higher.
The typical impedance between house and distribution should never result in more
than about 1 volt drop at rated capacity. Thus a typical 90 amp service would
be fed with a pair of #2 and a #4 wires. Typical loss through the distribution
transformer would be about 3-6%. After that it depends on your substation.
Typical reaction at the substation to overload is to shutdown the circuit
and recycle it back up a few moments later. This is retried until the
fault is cleared.
My own interpretation is that a short circuit could be expected to dissapate
somewhere around 50,000 watts minimum. Compare this to a typical space heater
at 1,500 watts. Also assuming most of the dissapation is at the point of
contact it will no doubt lead to a spectacular display.
Disclaimer: author assume no responsibilty for veracity of this posting
or misquotes of RG&E source.
//Z\\
James M. Ziobro
Ziobro.Henr@Xerox.ARPA
{rochester,amd,sunybcs,ihnp4}!rocksvax!z