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RE: Grounding Question



Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


Gary -

The NST case has to be grounded to the AC Mains ground for safety reasons
and to avoid violating the rules of the National Electrical Code.

The function of the safety gap is to fire when a transient high voltage is
on the NST HV line. This hazard is best taken care of by shorting the
transient to the NST case and the AC Mains ground. However, when a spark
comes off the TC topload the best way to take care of this spark is to short
it to a separate RF ground. Note that this topload spark is actually a short
circuit between the two sides of a capacitor due to overvoltage between the
plates. Forget about streamers.

The RF ground in a TC system is also the other side of the TC secondary
topload capacitance. At some point both the RF ground and AC Mains ground
are connected. This connection introduces an impedance that creates a
voltage between the RF ground and AC Mains ground when either the safety gap
or the output spark fires! The concern of the coiler is to be certain that
this condition does not injure or kill anyone in addition to other hazards.
In the Tesla Coil Notebook I show a schematic diagram of how these currents
can occur with Tesla coils. The diagram shows only part of the grounding
network of the typical large Tesla coil system.

There is much more to the TC grounding problems. The fact that there are
only about 3 TC deaths in over a hundred years would indicate that Tesla
coils are dangerous but not very dangerous. Actually it is because coilers
and most people are very cautious around sparks. As I say in the disclaimer
in my books TC construction and operation should be done only by persons
with the proper knowledge of electricity. However, this would be a great
topic of discussion in a court of law.

John Couture

-------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, October 29, 2003 8:49 PM
To: tesla-at-pupman-dot-com
Subject: RE: Grounding Question


Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>

  >Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>
  >
  >The ground on your NST is a safety ground.  You should ground it to your
  >120VAC ground.
  >Not RF ground ! ! ! !

  >Remember, the ground on the NST is primarily for safety.  You don't want
  >the chassis to become electrically hot
  >due to internal short.  Ground the NST to your 115VAC ground.

I can't see touching the case of the NST, ever, when the coil is operating,
as I'd likely be hit with streamers from the top load.  Even without
streamers, it's just not a good place to put your hands.

But more importantly, consider what happens when the safety gap fires.  The
center terminal of the safety gap is connected to the NST case, so you're
suggesting connecting it to AC mains ground.  Should a streamer hit the
primary and cause the safety to fire, the connection you propose would
discharge that streamer to your mains ground.  I think it's better to
channel such events through the RF ground, which is why I advocate
connecting everything to RF ground.

Gary Lau
MA, USA