Re: Single-bushing 14.7kV pigs. Thoughts?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net> 

 > Original poster: Dave Leddon <dave-at-leddon-dot-com>

[cut]

 > >This seems like false safety. I measured the capacitance of my 10kVA
 > >transformer, from the case ground to the HV terminals (both shorted). I
read
 > >1.14nF. This would come from interwinding capacitance and maybe the giant
 > >porcelain terminals.
 > >
 > >That's an impedance of around 1.4kohms at 100kHz and 2.3Mohms at 60Hz.
You
 > >woun't want to touch any terminal with such a low effective impedance to
 > >ground. I once got a hell of a shock off one terminal of a cap that was
 > >"floating" and charged to 8kV. There's nothing "floating" about these
things
 > >at AC voltages, even DC half the time. For fun last night, I connected a
 > >frequency generator and high impedance voltmeter to the transformer I was
 > >testing, and swept from 50 to 500kHz. The transformer is definetly a
giant
 > >capacitor, and conducts quite well between the windings.
 > >
 > >KEN
 > Ken,
 >
 > You make an interesting point.  I cranked up the Visible Pig (in which
 > everything, including the core, floats) to 14400 kV and attempted to draw
 > an arc to ground.  Sure enough, I got a very weak, quarter-inch
 > spark.  Reading the current and voltage to ground I got 2.3 ma at 7200
 > volts which works out to about 3.1 megohm, which is not too far from your
 > measured impedance. So, while you're absolutely right in saying that
 > capacitive coupling between the primary and secondary will create a
current
 > path to ground, coming in contact with this doesn't present much of a
 > hazard, particularly when to compared to coming in contact with the output

This possible, assuming you have no tesla coil connected to the transformer.
It's possible to get an arc between your coil and the primary circuit, and
at the frequencies the tesla coil operates at, nothing is really floating
anymore. Everybody should fear the large distribution transformers, even in
under prefect conditions you can run up to it and start grabbing it. Hoping
there are still a few megs of resistance is a bit optimistic too,
transformers fail, weird things happen.

It does seem sort of nice to have a transformer that's less dangerous some
of the time, vs. all of the time. It should still be treated as if it's
completely dangerous though.

KEN


 > of a single bushing transformer. That said, however, I have to agree with
 > Dr R's assessment that with an enclosed power source having no exposed
high
 > voltage up to the point of connection at the coil, it's unlikely that
 > anyone could get near enough to touch anything in the tank circuit of an
 > operating pig-powered coil without bursting into flames first.
 >
 > Dave
 > Pleasanton, CA