Unpowered NST shocks

• To: tesla-at-pupman-dot-com
• Subject: Unpowered NST shocks
• From: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Sun, 05 Mar 2000 11:40:08 -0700
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

The explanation to the reported shocks from an unplugged NST was discussed
roughly a year ago on this List.  The suggested explanation is that if the
NST is switched off while the secondary is shorted, the short circuit
current circulating in it's secondary would persist for a considerable time.
Opening the secondary short would cause a voltage spike to be developed
across the secondary.

Modeling the NST as a 7422H inductor shorted  with a 4746 Ohm secondary
resistance, with a 60 mA initial currrent, Pspice shows that the current
diminishes exponentially from 60mA to 20mA in 1.7 seconds.  Should the
secondary short be opened, the voltage would be very high, certainly enough
to be uncomfortable.

However, I also tried a simple experiment on my bench.  I took a 15/30 NST
and hooked a NE-2 bulb across the 2 HV bushings as a HV spike indicator.  I
also shorted the two bushings with a separate clip lead.  I briefly applied,
then removed AC power from the primary, and within 1-2 seconds, removed the
short from the secondary.  I repeated this several dozen times, but never
once saw the bulb flicker.

Gary Lau
Waltham, MA USA

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