Re: capacitor failing

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

Larry,

In parallel, the voltage rating remains the same as when using
a single cap.  For example if the rating of one cap is 24kV,
then the rating for 4 caps in parallel is still 24kV.  There are a
number of other things to consider however.  For example
a single sheet of 60 mil poly can withstand only about a 10kV
transformer input.  Building the dielectric thickness with multiple
sheets as you have done, will raise this rating, but I don't know
by how much.  The reason a single sheet of 60 mil poly can
withstand only a 10kV transformer voltage is because of; 1)
the peak voltage is 1.4 times the rms voltage, 2) the transformer
voltage may rise above it's peak voltage rating due to 
resonant charging effects, 3) the ratings you mentioned may
be for high quality polyethylene sheets (special for capacitor
construction), which you may not have used.  4)  Any sharp
screw-edges, etc in the cap, may compromise it's voltage
rating.  5)  Corona effects and other effects in a single dielectric
cap, may promote voltage breakdown.  The output from
a 15/30 NST for example can reach something like 30kV peak
or more, depending on various factors.  I always use two rolled
60 mil poly caps in series for use with a 12/30, or 15/30 NST
powered TCs.

John

>
> there are four caps in the tank. the caps are connected in parallel.
> assuming 500v/mil dielectric strength for PE, that gives 30kv rating.
> some have said 400v/mil for PE, which gives 24kv rating. whatever.
> each cap at 7.2nf.
>
> I had thought the cap was rated at a minimum of 24kv.
>
> (since i've already proved i'm weak in the ohms law area :)
>
> the cap (since it was wired in parallel) was THEN actually rated
> at 6kv.  24kv / 4 = 6kv.
>
> am i correct that it was actually 6kv instead of 24kv?
>
> following that the voltage rating is an inverse relationship
> to capacitance...  well sort of.