Re: NST failures... and UL-2161

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

All,

Apparently, lead length is a very real problem, and is associated with
stray capacitance tending to partially cancel the internal current
limiting function within the NST. The output current can then climb to
levels higher than intended, eventually causing overheating and burnout
of the winding under continuous operation. There may also be additional
voltage stresses.

I found an excellent description of how NST's work at Magnetek's site in
the "Neon Transformer Q & A" section:
http://www.magnetek-dot-com/ballast/support/trouble_shooting/neon_qa.html

Included is a description of how NST's are different than other
transformers, power factor, how magnetic shunts work, etc. The problem
of excessive lead length is covered in the discussion about "capacitance
effect". 

Of even more interest is a discussion of the infamous UL-2161
requirement for "secondary fault protection", why it arose (in the 1996
National Electrical Code), and Magnetek's approach to meeting this
requirement. Interestingly, Magnetek is converting to "isolated"
secondaries, where the midpoint of the secondary windings are NOT
grounded - the secondaries would be truly isolated from the core or
case. Higher-voltage units may use a center bushing (the center-tap of
the output) that's NOT connected to ground. Some interesting wrinkles
for future Tesla Coil designs! Even better, there's NO mention of
GFI's... there may be hope yet!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com


Tesla list wrote:
> 
> Original poster: "Trent Mullins" <neontrent-at-earthlink-dot-net>
> 
> Hello Brian and Dan,
> 
> For whatever reason, NST failure IS DEFINITELY directly related to lead
> length.  As mentioned below, the side with the longer lead will fail first
> (if failure is inevitable).  Most installers and designers configure NST
> placement in the middle of the series loop.  One common way to avoid long
> lead length is to "mid-point ground" the NST.
> 
> If you can visualize an NST placed dead center between a length of neon
> tubing, connected with short leads to the unit on each side, then the end
> unit on each side is wired directly to ground.  This setup works well with
> properly loaded NSTs and a good ground.
> 
> Overall, I would recommend keeping lead lengths as short as possible for
> neon applications, but as far as Tesla applications, I doubt of any
> significant benefit.
> 
> Trent Mullins
> "Bendin' Glass 'n Passin' Gas"
> 
> ----- Original Message -----
> From: Tesla list <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Wednesday, August 02, 2000 8:17 PM
> Subject: RE: NST failures
> 
> > Original poster: "Basura, Brian" <brian.basura-at-unistudios-dot-com>
> >
> > Dan,
> >
> > Sounds like an Urban Legend to me. I can't see any way lead length would
> > affect the reliability of a NST. But maybe Neon Trent can shed some more
> > light on the subject (Neon light that is)...
> >
> > Regards,
> > Brian B.
> >
> > -----Original Message-----
> > From: Tesla list [mailto:tesla-at-pupman-dot-com]
> > Sent: Wednesday, August 02, 2000 5:37 PM
> > To: tesla-at-pupman-dot-com
> > Subject: NST failures
> >
> >
> > Original poster: "Dan Kunkel" <dankunkel-at-hotmail-dot-com>
> >
> > I spent a little time talking the guys at the neon shop a few days ago.
> One
> > guy told that a major cause of failure for NST is when rookie sign
> > installers use different wire lengths coming out of each terminal from the
> > secondary connecting to the neon tube. The side with the longer wire will
> > fail.
> >
> > I am wondering if this has any bearing in the world of coiling?
> >
> > dan
> > ________________________________________________________________________
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> >
> >
> >
> >