Re: Single-bushing 14.7kV pigs. Thoughts?

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

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

At 04:37 PM 2/11/04 -0700, you wrote:
>Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net>
>----- Original Message -----
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: <tesla-at-pupman-dot-com>
>Sent: Monday, February 09, 2004 9:03 AM
>Subject: RE: Single-bushing 14.7kV pigs. Thoughts?
>
>
> > Original poster: "Paul Marshall" <klugmann-at-hotmail-dot-com>
> >
> > The only down side is that you have a 14.7kV potential to ground. On a two
> > bushing pig, if you happen to contact one live bushing you'll live to tell
> > the tale. On a single bushing pig accidental contact with that bushing and
> > we will all be reading about you in the obituary column of the local
> > newspaper...
>
>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 
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