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Re: the physics of a neon sign
Ted writes, responding to Jeff...
>We had quite a go-round early-on in the list about installation techniques.
>At that time I went on at great length about my own "tried-and-true"
>bulletproof techniques, which pretty much make the same claim as yours: many
>signs in the field without failure, never a fire, six years in business
>doing signs night-and-day, and a good living made by "curing" the ills of
>chronically troubled (competitor's) signs. The techniques I use are pretty
>much the same as yours (smaller transformers, short transformer leads, load
>to 80% max with a meter, etc) with one notable exception: I ALWAYS use metal
>flex, either 3/8" or 1/2", and I ALWAYS use generic bottom-of-the-line GTO,
>and I've not had any GTO burnout on any signs ever (oops I take that back:
>just before I left Hawaii I had a GTO failure -- it was a two-year old sign
>where I inexplicably used a 15k on an indoor PK housing -- musta been out
>of twelves and in a deadline rush). (And, of course, this doesn't count the
>sign installations done for other companies, who typically overload the 15k
>trafo and then require servicing once or twice per year).
>
>This leaves me to conclude that it's not so much the conduit (be it metal
>or plastic) but the other techniques that have been discussed (though
>clearly the type of casing makes a big difference when using large
>transformers, or when a long run from the transformer to the first tubes is
>unavoidable).
Ted, *right on, right on!*
Most of what you say here would read just like our neon installation
guidebook. We always recommend the most-conservative practice if it is
possible. Our objective is to assure a safe, longer-lasting installation.
Metal conduit can be used quite well if other factors of the application are
handled correctly.
As you do, we typically recommend (in a perfect world, of course) using
lower voltage txs, attaching slightly less tube that called for by the
chart, keeping high-voltage wiring as short as possible, etc., etc. Most of
the group probably already knows all this.
In closing, I wanted to reinforce the most important point you made. Most
of the nasties which take place in a neon installation happen in that first
run of gto coming from the secondary terminals. These are the ones at
highest potential since there is a voltage drop across each successive tube
in the load. That's where prospects for capacitive current are greater
since this phenomena is, among other things, a function of voltage. By
definition, it does not take place below 1000 Volts. However, using 15KV
txs, that first little bit of gto sees operating current of 4X that.
Sometimes, under real harsh conditions, it will see spikes of15-25X that.
Net result is not too good for wire insulation.
Thanks for your suggestions,
Eric