The death of a classic - Second look

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Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Under the scope, each of the blown areas had a little hole in the center
were it shot through.  So it does appear to be all over voltage related.  I
saw no evidence for #3 below.  The metalization in non-arced areas was
perfectly normal.  There is some corrosion inside the caps, but I think it
is due to all of that polypropylene being burned when the arcs occurred.
The burnt poly seems to have an oily residue that attacks metal a bit.
This is a secondary thing that would not be an issue if the caps had not
arced so much.

These caps do show ionization damage to the dielectrics.  I can't rule that
out as a cause of the problem.  The edges of the plates have damage lines.
It is possible that long term ionization damaged the poly to where it
started arcing through.  It is also possible it is a chemical reaction of
normal ionization and the chemicals from the burnt poly.

So we have two possibilities:

1.  The caps were run over voltage which blew thousands of holes in the
dielectric and released chemicals inside the cap.  As time went on, these
chemicals and normal ionization continued to degrade the cap abnormally
even though the voltage was no longer high.  There is a chance that if it
were not for the corrosive chemicals, the cap would have survived a normal
life.

2.  Ionization at the edge of the plates cut away at the dielectric
eventually causing a large number of punctures.  Then, the above chemical /
ionization process eventually destroyed the caps catastrophically.

So, either the high voltage damage in the past finally caught up with it,
or long term (10 hours) ionization did.  Over voltage damage in the past is
just one of those things...  However, if it is ionization damage, that
would be a real concern.  There is not much I can do to prove or disprove
either with Finn's caps.  One would need long term run caps that were not
subjected to over voltage breakdown to rule that out as a cause.

I am going to send the caps on to Scott so he can look at them too.  It
would take a relatively simple experiment of running a few caps at
1400VACrms (2000Vpeak) for 10 hours and checking the dielectrics afterwards
to see how badly they were damaged.  Just an NST and variac could do that
but you would have to be careful not to resonate the NST/cap combination.
I have a lot going on these days ;-) but sounds like an experiment that
needs to be done.  It would be nice to use 'new' caps like Finn's MMC used
since ionization damage could differ significantly from manufacturer to
manufacturer.  I don't know if Finn has like 5 unused caps left?  But I
have Panasonics here too.

Cheers,

	Terry


================
Hi Finn and All,

Today I got the failed strings from Finn's cap.  It sure does smell!  I
love the smell of burning electronics :o)))

I tested a number of the resistors and they were all perfectly in spec.
They showed no signs of stress.  The resistor I was concerned with before
was in the middle of a pretty bad fire.  The fire damage to the boards is
very profound.  Seeing the boards first hand and testing the resistors, I
can find no evidence the resistors caused the failure.  They seemed to be
perfectly good and adequate.

I disassembled a number of caps that were not involved in the fire or were
in the "good" string.  The is no evidence of end plate or current heating
damage.  They appeared to run cool without any thermal stress.

However.........  All of the cap's internal plates and dielectrics are
"machine gunned".

http://hot-streamer-dot-com/temp/finn-001.jpg

There are literally thousands of holes blown in the dielectric layers that
have self healed.  Some areas are very bad.  Considering the large number
of caps in Finn's MMC, catastrophic failure was almost certain.  One cap
had a section blown out of the case but did not burn up.  It was a disaster
that never happened..., but it tells a story of serious problems...

I will have to get to a high power scope tomorrow, but there are three
possibilities:

1.  Over voltage on the cap caused millions of dielectric breakdowns.  A
"few" did not self heal and ignited the caps

2.  Ion damage to the dielectrics caused weak spots that eventually failed.

3.  High currents in the internal metalized plate degraded and failed the
thin metal center internal layer.  

Finn stated a very possible clue:

"Gap voltage is 32 kV (most of the time, but I am sure I`ve hit it with up
to 45kV for quite some time as well)"

45kV is 40% over the caps rating.  "I" would only go to 25% on EMMCs.  If a
few caps in a string breakdown and go to zero volts, it can put even more
voltage stress on the others in the string causing further breakdowns.  I
note that some areas have slight corrosion suggesting that the damage is
older than a few days.

When I can see the areas under high magnification tomorrow, I can determine
which it was.  But it sure "looks" like the first possibility.

BTW - No doubt about MMC's ability to self heal now!!!  Even though it
"eventually" failed.  Finn's MMC easily has millions of self healing events!

Unfortunately, the rest of Finn's cap is certainly heavily damaged too.
Time to take it out of service and put it in a trophy case so It can be
admired for generations to come :-))  Maybe if you explained to the museum
that it was one of the first of it's kind, they would accept it as a
display item.  100 years from now, it will be a big piece of Tesla coiling
history!

Cheers,

	Terry