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Re: Steel structure - appropriate ground?

Subject:         Re: Steel structure - appropriate ground?
       Date:     Fri, 2 May 1997 09:25:26 +1200
       From:     "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:    Wellington Polytechnic, NZ
         To:     tesla-at-pupman-dot-com


Hi all,

>        From:   Phillip Jarvis <jarvisp-at-bee.d93.k12.id.us>
> Organization:  Hillcrest High School
>          To:   Tesla List <tesla-at-pupman-dot-com>
>  References: 
>             1
> 
> 
> Tesla List wrote:
> > 
> > Subject:  Steel structure - appropriate ground?
> >   Date:   Sat, 26 Apr 1997 16:25:35 -0400 (EDT)
> >   From:   ADatesman-at-aol-dot-com
> >     To:   tesla-at-pupman-dot-com
> > 
> > My Tesla coil is set up in a school building which has the steel girder
> > frame
> > very typical of institutional buildings.  Instead of sinking copper
> > pipes
> > into the ground, couldn't I just attach my ground wire to these steel
> > girders?  It seems the same idea to me, but possibly there is some
> > aspect of
> > this idea I don't see.  I don't want to blow up any computers in the
> > building, for instance.
> >     Thanks for the wisdom.
> > 
> >             Aaron Datesman
> 
> 
> Aaron,
> 
> The girders may not be at ground potential - or at the same potential as
> the power company ground. There can be a significant voltage difference
> between the two. 
> 
> Phil Jarvis

I once ran a coil (1.5kW) as part of an advertising demo for our 
electrical dept at an education expo. The only true ground available
was the mains earth (lousy at RF though). The show building had 
interconnected iron girders everywhere so I decided to earth the 
secondary to them. I figured the high capacitance should make for a 
reasonable counterpoise at that power level even if the structure 
was not well grounded (if at all). It worked fine and didn't disturb 
computers away from the immediate vicinity of the coil (5 ft 
discharges). As a precaution, I arranged for a strike barrier 
connected to the girders to ensure good secondary loading. The only 
effect noticed was that the e.s. potentials on someone standing near 
the coil did elevate them with respect to the mains ground somewhat
so small sparks could be drawn from a hand to the earthed case of 
the circuit breaker.
    BTW, the demo completely overshadowed the neon Jacob's ladder of 
one institute (hardly visible under the light) and the Van de Graaf 
of a competing university. Within 10 seconds of commencing firing, a 
sizable crowd gathered to see what was making all the noise.

Malcolm