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Re: the physics of a neon sign

In a message dated 96-04-28 13:40:19 EDT, you write:

>
>I think the NEC has very little to do with reliability for neon signs...
>it's purely an attempt at safety... there may be gobs of GTO at high
>potential, running thru 3/8" conduit, just waiting to arc thru.   Though
>the sign will invaribly fail in short order, the arcing will (hopefully) be
>contained, and you'll be left with a buzzing, flickering sign instead of a
>fire.
>
>The older, metal signs did rely on physics.  Fewer components (and of them,
>rugged and simple in design), and inch or more of air as HV insulation,
>ventilation of the trans. compartment, etc.
>
>A look thru the Neon Installation Manual reveals a very different picture.
>Some installations are hopelessly complex. There are hundreds of individual
>components:  GTO inside GTO-Sleeving inside conduit, PK housings with
>complex strain-relief mechanisms and gaskets and conduit-plug-assemblies
>with o-rings, rubber boots that attempt to make a "water tight" seal around
>the entire electrode and GTO, and more and more.  The backs of some of
>these signs look like a conduit octopus.
>
>It makes me wonder if _anything_ from the lessons from the past are being
>used.
>
>

Thanks, John, I think we agree on many things here. 

You're right, NEC is not referring to safety. The old metal flex conundrum,
thank God we're now officially able to use plastic conduit without needing to
do legal research to justify it. That was tortous circular reasoning. You
needed the metal conduit to protect from the wire burnouts that were caused
by the metal conduit, but didn't need it if you didn't use it because the
only reason for the burnouts and arcing was the existence of a ground plane
hugging a 15kV wire. Sort of like overloading a plane with parachutes in case
the plane went down from being overloaded, or something like that. About time
someone would recognizet that the only way the wires get burned out is from
exposure to a gnarly ground surface only 1 millimeter away.

Have made a good living replacing 3/8" metal flex conduit by 1/2" plastic
smurf (indoors), which, working with Carlon's VP I got listed this year,
finally. Outdoors, I use Carflex liquidtite.The metal's not the solution,
it's the problem. I've never had any arcing problems as long as I have used
it. I also do a nice service I call "dividing down" the circuit for one of
those buzzing, wire burning, overloaded (tubing wise) signs, where I take out
the 15kV transformer and install 2 9kV transformers, splitting the circuit in
half. I usually do this in connection with removing all the metal flex EMT
 and replacing it with ENT. This has never failed to turn a problem sign into
a faithful performer. Never a fire, unless someone else messes with it. The
customers nervously pay big bucks for it but are always satisfied not to have
the aggravation of having to call the sign company back to come out again,
with their sign making them look unpolished meanwhile.
For those outdoor metal signs, I just try to repeat what was done before, you
can't improve on 3" of air as an insulator. I tried!
Jeff GOlin