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RE: MOV failure



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


Hi Paul,

At 09:32 PM 7/31/2001 -0400, you wrote:
>
>Well,... One time I was putting a set of three MOVs on an AC line and
>our crib lady accidentally gave me one rated for 18VAC. (later, she said
>that they *looked* the same) The resulting explosion was loud enough to
>bring most of my fellow employees running to my office to see what I had
>done to myself.
>
>They found me sitting stock still, frozen in the position I was in when
>it went off. I was going through my mental disaster checklist (Thinking,
>so I'm still alive... Seeing so I'm not blind. 1-2-3-4-5-6-7-8-9-10
>fingers... No visible signs of blood... Pain hasn't landed yet....
>must be OK ;)
>
>The MOV, on the other hand, wasn't doing so well. The oxide layer vaporized
>and blown the 2 contact disks apart from each other. The leads, red casing
>and contact disks were still intact.
>
>Paul
>

I have had more than a few 120 VAC MOVs put into 220 VAC equipment by
production.  I have 220VAC 100 Amp three phase with very slow breakers :-))

The exploding MOVs cut about a 2 inch hole in the PC board where they used
to be.  The explosion and crashing wires in the conduit racks is pretty
cool.  I now don't even flinch and when people come running asking "what
was that!", I reply "what was what??" ;-)))

However, in the NST case, a short will draw about 60,000 fewer amps :-))  I
use 2000 amp MOVs with say a 1/4 amp NST source.  So nothing should
explode.  Even a primary cap dumping into them will not do much at all.
There may be more of a concern on the AC line MOVs if an arc could fail one
to short quickly.  It is common to cover MOVs with thick plastic tubing to
contain an explosion.  However, you usually can't get too much current out
of 120VAC with extension cords and such so the bangs are typically "dull"...

Cheers,

	Terry