Re: Secondary Circuit Ground Fault Protection

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

Original poster: "Jeremy Scott by way of Terry Fritz <teslalist-at-qwest-dot-net>" <supertux1-at-yahoo-dot-com>

I read those articles and it looks like the SGFP
is very sensitive to anything other than doing
what it's supposed to do -- exciting tubes of
gas.

It appears that the temporary NST short circuit
that any spark gap in parallel creates would almost
certainly trip the things.  (not just safety gaps
firing.) Also, Terry's NST filter forms an imperfect
voltage divider that would cause some minor
imbalances.

I pulled up Ye Olde pspice model of my coil
and threw a current probe on one side of the
NST. The main gap firings cause a little over
1 amp of current to flow through the secondary's
winding, after going through 5K of power resistors
on either side. (Part of Terry's NST filter)
This is normally ok because these shorts
only happen at the rate the spark gap fires.

To keep a 15/60 from drawing no more than 60mA
would require 160K power resistors in the filter.
But, according to my pspice model that would make
your 400KV peak sparks only 80KV peak sparks..bah.

The sad part is that UL/CSA is now
recommending/requiring this in all NSTs.

So... any ideas for ballasting a potential transfomer?

:)


BTW:

Here's a screen shot of the current primary design.

http://sunny.doit.wisc.edu/tesla/acad_coil.bmp

Make sure you have IE expand the full bitmap
so you can see the detail.



--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > 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?
 > >
 > >.
 >
 >