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RE: Counterpoise and MMC demise



Original poster: "Mudford, Chris by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chris.mudford-at-agresearch.co.nz>

Hi Pete

I do have a safety gap across the MMC which is set at just over 4 mm
using two opposed brass dome head nuts (about 18 mm diameter).  The
safety gap fired every so often with the ground rod but did not fire at
all while on the counterpoise.  Why does a static gap miss firing?  I
thought that they were more likely to suffer from premature firing.

Another comment to make about my MMC is that both capacitor failures
have been in the string closest to the connector points and although
they only just get warm the other strings (those away from the
connection point) don't seem to have an elevated temperature.

Thanks for the advice.

Chris (NZ)

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Friday, 20 June 2003 2:09 a.m.
To: tesla-at-pupman-dot-com
Subject: RE: Counterpoise and MMC demise


Original poster: "Peter Lawrence by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <Peter.Lawrence-at-Sun.COM>

Chris,
        your 6kV-30mA NST has a Res Cap value of 13.26-nF, and your cap
is (22 / 6) * 4 = 14.67-nF which is too close for a static gap - the Res
Cap value is the value at which the NST-Cap combination can resonate up
to absurdly high voltages.  Static gaps are notorious, if the gap for
whatever reason fail to fire once the voltage on the cap doubles, if it
fails to fire for twice in a row the voltage on the cap triples.  You
were probably just "lucky" that your cap did not fail while running with
the earth ground, switching to a counterpoise probably had nothing to do
with it.  My first recommendation is to use about 1.5 * the NST Res Cap
value for your primary cap when using a static gap, add at least one and
maybe two strings to your cap.  My second recommendation is to use caps
that have already been tested in TC service, I'm using the panasonic
ones that I got though Terry and have never had one fail.  Lastly, I'm
I'm using longer strings and only putting 750VAC per cap, even though
Terry and others report long life using the DC rating in TC-AC service.

good luck,
-Pete.

(ps, the Res formula for 60-Hz NSTs is nF = 2.652 * mA / kV).