Strike rails/safety gaps IMPORTANT

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

Original poster: "Mr Gregory Peters by way of Terry Fritz <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

Hello all,


I am SICK SICK SICK and tired of people saying "well, I don't use 
safety gaps and strike rails and have had no problems". You are playing 
Russian Roulette here. These devices don't only protect your equipment, 
they protect YOU!! Don't forget that a nice 200kV streamer makes a very 
nice conductive path for the output of your pole pig, or even the 240v 
mains! You DON'T want it getting into your control/power circuitry. The 
little effort in making a simple safety gap and strike rail are well 
worth the bother. These devices don't HAVE to look good. Fencing wire 
will do. I have included part of an old post from Richard Quick (A very 
successful old school coiler) below. I urge you all to read it: 

"Well I have seen a coiler or two who did not use contactors to
remotely switch their 60 cycle mains. I have operated coils at
1.5 kVA without contactors. But let me tell you about the one
time I had an incident that "resolved itself" inside the power
cabinet. The experience changed my mind.

Everything happened too fast to really give a "blow by blow"
description, but basically what started it was the secondary
discharge struck a primary that was not equipped with an RF
grounded strike shield. The secondary discharge shorted the
primary turns, and the main tank circuit ran away. By "run
away" I mean that the oscillator shifted to a much higher
frequency due to the arc shorted primary. The primary/secondary
field flux collapsed when the tune was lost. All of the input
energy, the field flux energy, and the energy already in the
secondary, appeared to feed back into the tank circuit; in any
case it looked like discharger had become a vacuum cleaner and it
sucked in the five foot streamers. All of the conductors in the
tank circuit sprouted six inch long sparks and corona at the same
time the safety gap and capacitor terminals went to flames. 

What happened next I do not rightly know, but just when my finger
hit the switch to shut the runaway oscillator down, there was a
tremendous explosion inside the power cabinet. It was so bright
that I had spots in my eyes even though I did not directly see
the flash. The smell of ozone and electrical burn were strong in
the cabinet when I opened the back to peer inside. At first I
could not see anything wrong. But given the intensity of the
flash, I was determined to disassemble everything if required. 

A careful examination of every inch of the wiring revealed a
small hole in the insulation on the bottom side of a 60 cycle
main. The hole was located about 1/2 inch from the first line
filter, closest to the coil. Apparently kickback had made it
inside of the cabinet and the impedance of the filter was
sufficient that the kickback blew out the insulation and left the
conductor entirely. The line filter, having been bypassed, was
undamaged! Looking around some more I found fine arc tracks
scored into the surface of the polyurethane coated plywood mount
board. The arc had left the wire and traveled nearly four inches
across a surface of poly-coated plywood while spreading into a
"fan" of fine arcs that struck all along the main ground strap.
The delicate fan-like spark pattern was nearly the size of a
man's hand! 

As it turns out nothing in the cabinet was damaged. I tried to
power up later, but a neon in my bank had gone south. I assume 
that the neon broke down internally, and the kickback had jumped 
to the 60 cycle primary.  

After all that I completely re-thought, re-designed, then re-
wired everything. BTW, I just happened to be standing on an
insulated platform, which I rarely do when I am at the controls.
Had I not, I firmly believe I would have been jolted to my
backside.

Richard Quick



Greg Peters
Department of Earth Sciences,
University of Queensland, Australia
Phone: 0402 841 677
http://www.geocities-dot-com/gregjpeters