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Re: A new cap failure mode?



Tesla List wrote:
> 
> Original Poster: Mark Finnis <mefinnis-at-medicine.adelaide.edu.au>
> 
> At 21:29 5/02/99 -0700, Bert Hickman wrote:
> 
> ... snip ...
> 
> >> The suggestion comes up that perhaps a safety gap placed directly
across a
> >> primary cap needs a little resistance in the circuit to keep this current
> >> to a "safe" level.
> 
> ... snip ...
> 
> >So THAT's why it makes such a loud bang! :^)
> >
> >While the instantaneous currents can be quite high, the duration is also
> >very short, and the physical inertia of the elements in either a rolled
> >or flat-plate cap should prevent damage from electromagnetic forces. An
> >exception might be the very thin metalization in a ceramic doorknob cap,
> >but even here, the dielectric itself will generally limit the maximum
> >rate that energy can be "extracted".
> 
> Hi Bert,
> 
> Hopefully my other post will appear at some stage, but why not take the
> energy through the primary ?
> 
> That is,have the safety gap discharge into the circuit by having it in
> parallel with the RSG, not across the cap.  This will provide the same
> protection in the charging phase at 50-60 Hz surely ....... or is my lack
> of EE training showing through YET AGAIN ;-)
> 
> I would put to the assembled masses that:
> 
> 1.  Safety gap across the cap is superfluous in a static gap system.  If
> the safety gap breakdown V is significantly greater than the SG (as it
> would be), then I can't see how it would ever function.
> 
> 2.  For RSG systems, a parallel static SG is the appropriate protection
> circuit.
> 
> Donning iron clad asbestos suit & ducking for cover ;-)
> 
> cheers
> 
> Mark
> 
> http://www.cobweb-dot-com.au/~dkfinnis

Mark,

No need for a flamesuit, Mark - it's certainly a reasonable question!
The main reason is that this method provides an added measure of
protection for the tank cap(s) and power transformer in the event of a
direct streamer hit to the primary circuit. Under this condition, the
transient overvoltage will be searching for a way to get to ground, and
will get there through whichever happens to be the "weakest link". In
this event, a safety gap across the RSG will not adequately protect the
tank capacitor(s), or the input transformer for that matter, since this
gap does not provide a solid shunt to ground around the tank cap or the
HV transformer. The resulting high voltage transient that occurs in the
primary HAS been known to even take out a pig (rated at 125 kV Basic
Impulse Level!) - an event that happened to a fellow coiler on this list
last year.

It's essential to have a front-end safety gap on either HV leg from the
transformer to ground. A safety gap across the tank cap(s) then
completes the transient protection. In the event of an accidental
streamer hit, these gaps will break down before the expensive parts of
your system, providing a safe, solid path to ground. A static gap across
the RSG can also be used to protect the tank cap and HV transformer from
RSG misfires. Ideally, each gap is adjusted to trigger at a voltage
significantly below the breakdown voltage of the device you're trying to
protect. Utility-style lighning arrestors can also be used to perform
this function (but they tend to be physically large). Coordination of
insulation breakdown voltages is the secret behind the way utility
companies protect their equipment from switching or lightning-induced
transients.

Hope this answers your questions, and safe coilin' to you!

-- Bert --