RE: Common mode voltage & Safety Gaps

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

Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>

 >Original poster: "Steve K by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<teslainnovations-at-yahoo-dot-com>
 >So you are saying that when arc touches the
 >primary, there will be a stress potential between the
 >insulation of the NST secondary and grounded core? If
 >that's the case, then wouldn't the potential rather
 >arc across either side of the safety gap instead of
 >through the secondary's insulation?

Exactly - this is one of the reasons that we use safety gaps across the NST 
secondary.  In the event of a strike to the primary, the safety gap that is 
across the NST secondary will likely fire, so the NST case-ground and NST 
terminals are all at essentially the same potential.  One might also say 
that the differential secondary voltage (both core to HV terminal and HV 
terminal to HV terminal) is shunted or clamped by the firing of the safety 
gap(s).  But the common mode NST secondary voltage, with respect to the AC 
mains and mains ground may be very high, since there is nothing that clamps 
this.

 >I guess having
 >only one side of the safety gap fire would explain
 >your common mode voltage explanation, since it is
 >attracted to ground instead of the other high voltage
 >lead. I would think that safety gaps would be
 >sufficient protection against primary stikes, so long
 >as they were properly spaced. The capacitors shouldn't
 >be stressed as you explained before, so the only other
 >thing that should be bad about primary stiking is
 >connecting the high voltage to the 60hz of the
 >primary. Let's forget about RF interference too...

I've not paid a great deal of attention to whether one or both sides of the 
safety gap fire, but I would suspect that both sides fire.  It may be that 
the strike causes just one side to fire, but then this somehow triggers the 
other side.  But in any case, the NST core, at "RF ground", will see a high 
voltage transient relative to the AC mains and mains ground.  There will be 
strong capacitive coupling from the core to the NST primary, resulting in a 
common mode signal on the primary, and out to the mains.

 >Steve Klec

Regards, Gary Lau
MA, USA


 >> Original poster: "Lau, Gary by way of Terry Fritz
 >> <teslalist-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>
 >>
 >> Consider the two wire mains cable - hot and neutral.
 >>  Common mode refers to
 >> a signal that is pretty much the same on hot &
 >> neutral.  The voltage of
 >> interest is relative to ground, not between the two
 >> conductors.
 >>
 >> The opposite of common mode is differential.  Here
 >> the voltage of interest
 >> is measured between the two conductors, and ground
 >> is irrelevant.
 >>
 >> What I was getting at is that high voltage
 >> transients exist not between the
 >> two NST primary leads, but rather equally between
 >> each of the two leads and
 >> ground.
 >>
 >> My "definition" may not be clinically precise, but I
 >> hope it helps.
 >>
 >> Regards, Gary Lau
 >> MA, USA