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 
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
decrease in operation or in storage?