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November 2005

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Subject:
From:
David Greig <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Greig <[log in to unmask]>
Date:
Wed, 9 Nov 2005 11:22:56 -0000
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What is the operating current and frequency?

Often caps of this type are not designed for continuous use, and even if they are not all manufacturers understand that you may
be running a constant AC current.

I had some trouble a decade ago with caps used in a resonator circuit for a high power conversion, 360nF, ~100kHz, 500Apk. The
sealed metal case devices with oil fill were offered by several manufacturers, but explosively were not suitable. Even a custom
designed part failed due to lack of metallization - turned out that over 20% of available electrons in the metallization were
required for the current and the metallization migrated rapidly over time.

The gas will most likely be oil impregnation/fill evaporating.


Best Regards

David Greig
______________________________
GigaDyne Ltd
Buchan House
Carnegie Campus
Dunfermline KY11 8PL
United Kingdom
t: +44 (0)1383 624 975
www.gigadyne.co.uk
______________________________

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - Bogert
Sent: 09 November 2005 10:34
To: [log in to unmask]
Subject: [TN] Seeking Info on Metal-Foil Film Capacitors Regarding Creation of Explosive Gas

November 9, 2005

Folks, we recently had a metal foil film power capacitor (60 microfarad) explode and cause significant damage to the cabinet it
was installed in.  The cap is manufactured using aluminum foils with polypropylene insulation between the foils.  The foils are
soldered together (qty 18 stacks of three individual stacks connected in series) then the foils are inserted into a rectangular
metal can and the can is filled with epoxy.  The sides of the can are brazed together. These caps are designed such that there
can be a number of individual capacitor cells fail and still retain the ability of the overall caps to meet their required total
capacitance value.  Although the cap can is brazed, it is not hermetic sealed since the electrical connections are made via
wires that are routed outside the can through a rubber grommet.  However, there is complete epoxy fill in the can so it is
doubtful that moisture can penetrate through to the foils.  The cap must operate in 95%, 50C humidity and temperature
environment.

We believe there was a thermal runaway within the cap (caused by individual cap cells failing) that created explosive gas that
built up pressure and cracked the can braze joint  The cap continued to generate the gas and eventually, a spark from an arc
within the cap caused the explosion.

Does anyone have technical info on thermal runaway for these type caps, and what type/quantity of explosive gas can be created?
Also, does anyone have info regarding whether the overall capacitance value can
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