Re: MOV failure

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Terry, Paul, everyone

The reason I ask is because when I run my filter without any load then the
MOVS get very hot - I was wondering if I had damaged them in any way, or to
destroy a MOV does stuff actually have to start burning/melting?
As a little experiment I DC charged my 60nF MMC bank with a rectified 10/85
(deshunted-repotted) and it actually exploded a MOV...

Regards,
Jason
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, August 01, 2001 3:18 AM
Subject: 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
>
>