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Re: R.I.P. one 14kV pole pig! (fwd)

---------- Forwarded message ----------
Date: Fri, 03 Oct 1997 00:53:11 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: R.I.P. one 14kV pole pig!

Tesla List wrote:
> 
> ---------- Forwarded message ----------
> Date: Thu, 2 Oct 1997 20:35:24 -0400 (EDT)
> From: Charles Brush <cfbrush-at-interport-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: R.I.P. one 14kV pole pig!
> 
<SNIP>
> long before I discovered that the pig was now acting like a dead short.  No
> mistake.  With nothing connected to the secondary bushings, it now draws
> full current.  Shorted secondary perhaps?    I opened the pig up, and did
> some tests with the tap switch set to various positions, but no luck.  I am
> actually relieved that it was the transformer and not the caps, since it
> cost less than a quarter of what they did!  I can tell you that from now on
> I will be certain that the pig's safety gap is connected, and may add some
> bypass caps as well.  A grounded strike shield over the primary is also
> called for if the power levels are going to keep increasing (and they are!
> ;-).
> 
> I've posted some shots of the interior of this pig at the following
> address.  There are also some recent pictures of the coil, new rotary gap,
> etc.
> 
> http://www.foundrygroup-dot-com/cbrush/temp
> 
> So has this ever happened to anyone else here?  Yes this pig should have
> had a safety gap and should not have been subjected to such abuse, but
> should it have failed?  I am wondering if it could have had an internal
> weakness, or if the same thing would have happened to any 14.4kV pig in
> that situation.  I was only getting 6-7 foot streamers, but then again that
> kind of voltage is way beyond the transformer's BIL rating.  Lastly, I was
> using some fairly long RG213 to connect the pig to the tank (grounded to
> the RF ground at the tank end).  Is the jury still out as to whether this
> can be detrimental or not?   Any thoughts would be welcome.
> 
> Thanks for reading this long message, and for any replies!
> 
> Charles Brush

Charles,

Congratulations, Charles - you may have the dubious distinction of being
the first to actually to take out a pig!  :^(

A 14.4 KV pig is normally in the 125 KV BIL class... but you were
probably hitting it with at least several hundred kilovolts during the
strikes to the power buses. Even so, most pigs would have still
withstood this - your pig may have had a little less margin than others.
Bummer! 

However, even if you only had a safety gap going across the bushings, it
might not have prevented this disaster. A direct strike to the primary
circuit is seeking ground, and it takes a safety gap that includes a
path to the RF ground to protect you (sort of like that used on neons).
Arrestors used by utilities go from the high voltage bushings to ground
for lightning protection for this reason. BTW, safety gaps across the
cap(s) are also a good idea for an additional measure of protection. If
you can anticipate where stikes could hit, and then provide a safe path
to ground via safety gaps (or even power distribution arresters!) so
that neither bushing sees a voltage above its Basic Impulse Level (BIL),
you'll have a bulletproof system. This "insulation coordination" is
exactly the process utilities use to protect their equipment. Thnaks for
an informative and  sobering post!

And, safe coilin' to you, Charles!

-- Bert --