RE: Ungrounded coil/toasted filter

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Ed:

I should first say that I agree 100% with the way you had the filters
hooked up, although hearing the result of your mishap is not exactly a
hearty endorsement!  Beyond hooking up the RF ground next time, the only
improvement I would suggest is to hook up a pair of MOV's at the input
end of your control cabinet from both Hot and Neutral to mains-ground
(still on my to-do list).  The nasty stuff coming back is common-mode,
not differential (i.e. relative to ground, not hot-neutral), so the MOV
that you had wired Hot-Neutral had no affect.

It's again interesting to note that not having any RF ground had no
impact on spark performance.  Most of us (including myself until just
recently) believed that a poor RF ground will result in poor
performance.  The real impact is one of how much EMI gets presented to
your mains wiring.

Normally with an RF ground connected, the voltage developed between the
RF ground and mains-ground is small enough that the chokes in the EMI
filter(s) can absorb it.  But the base of the secondary HAS TO "push"
against something, and with no RF ground, the full secondary base
current was flowing through your hot & neutral lines between the two EMI
filters.  The filter in your control cabinet has a pair of caps going
from both hot & neutral to mains ground, and after the chokes in the
filters arced over, the caps were seeing some very high voltages across
them until one of them let loose.

Live and learn, and thanks for sharing this!

Regards, Gary Lau
MA, USA


>Original poster: "Ed by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<edtrind-at-hotmail-dot-com>

>Hi, 
>I was experimenting with different RF ground routes with my 12/60 NST
powered
>coil, and inadvertently tested a setup that involved none.
>I have an 8' cu ground rod down 7 feet before hitting bedrock, .. which
seemed
>to work well, and recently moved my coil and can only sink the new rod
2'.
>(this is in agreement with the poured depth of the house foundation
btw).
>I noticed no noticeable difference in spark output (about 30 inches) so
I
>decided to try running a longer line to the other one just for the heck
of it.
>No change there either.
>I'm not sure what this means... probably that I could get better output
with a
>shorter better RF ground path or counterpoise?
> 
>Anyway, ..  with wanton disregard for the pre-launch checklist that I
carefully
>wrote up, I fired up my coil without hooking it up to either RF ground,
....
>and....  still no change in spark length for 9 seconds... . and then, 
>a bright glow out of my controller cabinet followed by a percussive pop
that
>shook the cabinet pretty well and then a fountain of sparks shooting
out from a
>small hole right next to one of the terminals of the 30 amp line filter
before
>blowing the fuse I had in there (I had the spark fountain image burned
into my
>eyes for a while and could see the exact pattern when I shut them for a
few
>hours... (photographic memory without burdening the brain)).
> 
>I think I have a bit of a grasp of what happened, ...   but, rather
than give
>my cloudy image of the source/path/mechanics of the failure, I was
wondering if
>I could throw out the details and get a comment or two from those more
>knowledgeable than I.
> 
>details:
>The mains ground stops in the cabinet.
>The filter that fried is in the cabinet, hooked up in reverse, and
grounded to
>the mains.
>There is another identical line filter (that didn't fry) just before
the NST at
>the coil, hooked up in the 'normal' direction (after reading Gary Lau's
>analysis of filter hookup... .. )  - this filter is grounded to my RF
'Buss'
>(can't call it a ground for this case).
>The other things also hooked up to the RF buss: NST case, safety gap,
strike
>rail, bottom of secondary (and, unfortunately, not the RF ground line).
>Again, ... no decrease in spark output for 9 seconds until the event,
.. and
>the failure is in a mains grounded EMI filter hooked up in reverse at
the cab.
>and it manifested itself as a small hole on the coil side of the filter
>(labeled 'line' side) that spewed forth sparks, apparently as that line
arced
>to the case and ate a hole in it.
>(the damaged filter is on the mains side of the variac for the record
and I
>have a 130V metal oxide varistor that didn't kick in across the
terminals on
>the same side that failed.
> 
>I get the feeling I should consider my house wiring and all the stuff
connected
>to it lucky.
>The filter is not shorted with no load now, . but does the same thing,
sparks
>and all, when a bit of current is drawn through it.
> 
>Any chance anyone can go for a quick 'circuit analysis' approach of
what
>happened for me?
>Thanks in advance, sorry for the loquacious post. 
> 
>Ed. (waiting for more cut-off wheels for my die grinder to finish
opening the
>deceased filter).