NST protection

From:  Sulaiman Abdullah [SMTP:sulabd-at-hotmail-dot-com]
Sent:  Monday, April 13, 1998 6:25 AM
To:  tesla-at-pupman-dot-com
Subject:  NST protection

Hello all,  I had a 15 kV 30 mA NST to start my first Tesla Coil.
I'd been "playing" with various capacitors and inductors,
a simple ball-bearing spark-gap, Jacob's Ladder etc. ... no problems.
Trying to be "smart" I made a 'protection' circuit/filter and used
the ball bearings as safety gaps.

For each side of the NST;
Safety-spark-gap to ground ( 5 mm gap, 1" steel balls)
1.1 nF Ceramic to ground   (two parallel sets of 4 x 2.2 nF each)
7 kOhm 30 W wirewound resistor in series with
18.5 mH air-core inductor (0.4mm magnet wire, 109mm dia, 750 turns)
Great (in theory).
Surprisingly the safety-gaps fired before 100% on the variac,
with no filter the gaps didn't fire.
No problem, simulated resonant-rise on PC so decided to adjust gaps.

During the adjustments (each adjustment involved 'firing' the gaps)
one side of the NST died :-(

After consideration I beleive that safety gaps SHOULD NOT be across
the NST as they will cause 'nasty' voltages, I've always seen the
safety gaps shown across the NST, and I've read many sad tales of
NST's forming 'tracks' internally. I think that all gaps should be
isolated from the NST by at least a resistor, preferably a filter
such as above.

Was it coincidence that the NST failed under the above conditions?
or is a spark-gap (even a "safety" gap) directly across an NST a
bad idea? Comments ? ............
Bye ... Sulaiman