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From: gnome@oliveb.UUCP (Gary Traveis)
Newsgroups: net.analog
Subject: Re: Varistor failure modes (funny you should ask)
Message-ID: <984@oliveb.UUCP>
Date: Mon, 14-Jul-86 20:28:51 EDT
Article-I.D.: oliveb.984
Posted: Mon Jul 14 20:28:51 1986
Date-Received: Wed, 16-Jul-86 02:00:50 EDT
References: <3937@decwrl.DEC.COM> <774@eneevax.UUCP>
Organization: Olivetti ATC; Cupertino, Ca
Lines: 31

> 
> In article <3937@decwrl.DEC.COM> white@euclid.dec.com (Robert White) writes:
> >
> >In a recent posting, it was stated that when varistors have seen one
> >too many surges, they explode.  In my experience, varistors explode
> >only when drastically overstressed.  The more typical failure mode
> >is that they quietly become an open.

Evidently, not always.
I just had my pinball machine (FLASH) go down because of a blown fuse.

Since I wasn't sure if the fuse blew from fatigue, I replaced the
fuse and tried to power-up.

As it turns out, the fault was easy to find because the MOV inside
the unit promptly self destructed by catching fire and sending
a small plume of smoke up to the ceiling.  Luckily for me, I powered
it up by way of plugging it (cautiously) into an extension cord and
when I heard the plug go POP I imediately stopped trying to insert the
plug any further.

The new fuse did not blow (in the time I had it powered) but the MOV
is cracked-open and burned.  It is still in a low resistance state.
I'm glad that Williams had the foresight to mount these puppies in a
"blast-safe area" that is framed-in by metal plates and a Corcom line
filter.

I'd hate to think of one of these failures in a plastic box!

Gary
(hplabs,allegra,ihnp4)oliveb!oliven!gnome