Re: MOV failure

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

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

Hi Jason,

you actually have a great test there to tell if MOVs are damaged.  With
nothing but the NST connected to the filter and with the NST running at
rated input voltage, the MOVs should stay cool.  With a variac you can try
different settings and find at what input voltage they start to heat up.
Obviously, be sure the systems is totally powered down before touching
anything.

It sounds like your MOVs are damaged and should be replaced.  I would also
double check the safety spark gaps since they should fire before the MOVs
conduct and really do all the work.  Normally the MOVs will only operate if
the safety gaps fail or if there is a streamer hit or transient.

Cheers,

	Terry

At 09:17 AM 8/1/2001 +0100, you wrote:
>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
>>
>>
>