NST failure modes

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gerry Reynolds by way of Terry Fritz <teslalist-at-qwest-dot-net>" <gerryreynolds-at-earthlink-dot-net>

Hi everyone,

I'm new to this group and enjoy reading the posts very much.  I have an 
electrical engineering background but only limited experience with HV 
TCs.  I have built one coil 3.5"x40" using 30 guage.  I have an unknown 
transformer (single pole and maybe 5KV output that charges a glass plate 
capacitor in parallel with the xformer output.  I have a spark gap (nails) 
in series with the 18 gauge helical primary of the TC. I was happy with the 
results and got maybe 4-5 inch leaders and maybe a 9 inch spark when a 
grounded wand was moved close to the 3 inch brass top end load.

After reading some TC theory posted in this group, I want to upgrade this 
setup to a NST to get more power and longer sparks.  Next, I want to build 
an new coil 12"x36" using 21 guage and use that as a vehical to learn more.

Now for my question.  I read some of the posts about NST failures and want 
to learn more about these failure modes.  I reviewed my engineering books 
on transformer modeling (its been a long time since I have had to deal with 
transformers)  and think the appropriate model for this application is the 
"ideal xformer - modeling the appropriate turns ratio.  The primary of the 
xformer would then have an R and L in series with the primary as well as in 
series with the secondary.  There are other shunt elements in the model 
that I think can be neglected at least for now.

The NST being current limited by flux leakage in both the primary and 
secondary and by the shunts that many have talked about, is normally 
protected against shorts.  I can see that if a capacitive load is applied 
to the secondary, the load reactance can reduce the inductive reactance due 
to the leakage flux in the xformer.  If a series resonance occurs,  I can 
see how the current may increase above rating maybe damage the secondary 
windings.  I can also see how a series resonance can amplify the voltage 
out.  I have also read where the RF in the TC primary can affect the 
voltage on the NST secondary.  Can someone explain what kills NSTs, 
overcurrnet, overvoltage, resonant charging, or whatever?

Thanks from newbie,

Gerry