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Re: NST isolation level



Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: Gregory Hunter <ghunter31014-at-yahoo-dot-com>
 >
 > Hi Rik,
 >
 > I'm not in the neon sign trade, but I'll share what I
 > know. My understanding of NSTs it that their HV
 > isolation is marginal. They are not really designed to
 > withstand their own maximum voltage for very long,
 > because they don't need to. The 15kv faceplate voltage
 > is only required for the moment when the sign is first
 > turned on. The high voltage ionizes the neon, which
 > quickly becomes conductive. The sign goes from being
 > an open to a near-short very quickly. From that point
 > on, only 800vac or so is needed to keep the sign lit,
 > and the NST shunts clamp the current at 30 or 60ma (or
 > whatever) to prevent the NST from overloading and
 > tripping the mains breaker. The NST can idle like this
 > 24/7, for years if neccessary, since it is only
 > supplying ~800v-at-30/60ma, a small fraction of it's
 > faceplate rating, into a simple resistive load. The
 > NST is a transformer, but it is also valid to view it
 > as a luminous tube ballast with a built-in starter.
 > The bottom line? NSTs often fail in Tesla coil service
 > because they are continually exposed to maximum
 > voltage, dynamic reactive loads, abrupt switching, and
 > RF feedback. For any application other than powering a
 > neon sign, the lifespan of an NST is "iffy".
 >
 > Cheers,
 >
 > Greg

	I'm not sure I really understand the issues here, but my own experience
is that NST's can operate open-circuit for a very long period of time.
I have a "skunk zapper" here consisting of a pair of insulated wires
running around the back yard and connected to the secondary of a 9 kV,
60 ma NST that I found at the side of a freeway on ramp some years
back.  (Apparently someone had gone around the corner too fast and
dumped a bunch of transformers into the ice plant and my rider and I
picked up several, mostly 12 kV, 60 ma.  All had beat up insulators and
all worked OK.)  I operated this every night for about 3 years, say 10
hours per day, with the transformer sitting out in the rain and where
the sprinklers wet it down when they were run.  Never experienced a
failure and it was still in original condition when I took it out of
service.

	On the irrelevant subject of skunk zapping, 60 ma is way too much
current.  First time we got one it ended up hung up in the wires and
thoroughly cooked, with much arcing to the skin and attendent smells
that woke up the whole neighborhood in the middle of the night.  I
finally ended up running the thing with a big 1 meg resistor in series
and no more burned up skunks, although it was still lethal and the total
take is about a dozen, with an almost equal number of possums.  Took it
down when the victim was a nice big old cat that had wandered by.  Not a
neighborhood ped but someone must have loved it and missed it and we
didn't want any repeats.

Ed