Re: Secondary Circuit Ground Fault Protection

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

Original poster: "Bert Hickman by way of Terry Fritz <teslalist-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Jeremy,

Unlike a direct current sensing GFI, Allanson's Secondary Ground Fault 
Protection (SGFP) circuit senses _voltage_ imbalances between HV outputs 
through a capacitive sensor to indirectly sense current from the HV 
outputs. Voltage imbalances would occur when the current drawn by one of 
the HV outputs is significantly different than the current drawn by other 
HV output, reflecting current being diverted to ground. The SGFP circuit 
will almost certainly trip when the safety gap fires, during a 
primary-secondary flashover, a streamer hit to the primary, one of the HV 
windings going bad, and even differences in capacitive loading between HV 
outputs (versus ground). As Terry indicated, it would be very interesting 
to see how an "unmodified" transformer actually works in a Tesla Coil - the 
nuisance tripping may in fact require that the SGFP circuit be defeated.

A very good technical explanation about Allanson's SGFP approach (10 pages) 
and a paper covering installation and troubleshooting for protected 
transformers can be found on Allanson's web site in the Technical Support 
section:
http://www.allanson-dot-com/tech_support/sgfpart.pdf
http://www.allanson-dot-com/tech_support/unitrantrouble.pdf

Give it a try and let us know what you find... :^)

Best regards,

-- Bert --
-- 
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Tesla list wrote:
>Original poster: "Jeremy Scott by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <supertux1-at-yahoo-dot-com>
>
>Has anyone used an NST that has a secondary circuit
>ground fault protection built into it?
>New Allanson transformers have apparantly have these
>built in, and as far as I can guess they detect
>shorts to ground on the secondary side and cut
>the power if that happens. (Just like the outlets
>in bathrooms and kitchens.)
>I was wondering which "ground" these circuit
>protectors
>are referenced to -- the AC mains ground or the
>NST ground reference (secondary center tap)
>I don't connect the AC mains ground (ie green/bare
>wire) to my NST's but I do install saftey gaps
>between the secondary center tap ("NST ground" - zero
>voltage reference) and hot secondary connections.
>(Part of the Terry RC filter)
>I am concerned that arcing here (ie spark gap
>fails/cap goes overvoltage) will trip the ground fault
>protector
>and shut the off the power. That wouldn't be a bad
>thing in theory, but I do expect the occasional
>safety gap spark during tuning and adjustment.-- is
>this GFCI going to be a pain?
>
>.