RE: apartment grounding (was: I'm a newbie coiler!- apartment coiling)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

I have seen someone get shocked this way (or a similar way) at the 
Cambridge Teslathon last year. As best we could determine, what happened was:

Coil had a flashover between primary and secondary, connecting the HV 
power supply (a 10kV DC supply powered by twin MOTs) to RF ground. 
The other end (to be pedantic the centre tap of +/-5kV)  of the power 
supply was connected to line ground.

RF ground was the frame of the building and line ground was the 
ground on a large 400 volt 3 phase cable that was run into the 
building temporarily from goodness knows where, to supply our large 
power-hungry coils. It seemed there was enough impedance between 
these grounds, that when the power supply with its large filter cap 
bank was shorted between them, it made enough potential difference to 
shock the operator who was touching the control panel (made of metal 
and connected to line ground) while standing on the concrete floor 
(sort of connected to RF ground).

The operator got enough of a jolt to give him a scare and we decided 
not to run that coil any more. (Not to mention that we were worried 
about damaging other equipment by slamming 5kV into the line.) But if 
he had been standing on damp grass and RF ground had been a stake 
pounded into the lawn, it could have turned out a lot worse.

You need to take care to make sure you aren't part of a ground loop 
like this when running a coil big enough to need separate RF and line 
grounds. As regards the original poster's question, I would say that 
the 24v circuit should have one pole connected to line ground, and he 
should wear rubber-soled shoes and stand on something dry ;-)

Steve Conner
http://www.scopeboy.com/


Tesla list <tesla@xxxxxxxxxx> wrote:

The risk (not huge, but something to think about) is the following sequence:

HV from secondary flashes over, creating moderately low impedance
connection between 110V side of transformer to 24V side of
transformer (or, worse, between TC primary with 10kV on it, to 24V
side of transformer)

Current flows from 110V (or 15kV) through you.

You lie there twitching and/or smoldering, unable to let go, as the
arc burns happily, roasting you.