Re: MOV lightning arrestor in place of safety gap...MCOV?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Aaron,

I posted this response to your question Friday around 9 AM
CDT, but the Tesla list appeared to be down most of Friday
and Saturday and I never saw my post appear on the Tesla
list. So, I'm going to repost it here:

Aaron,

I've had experience using these in the place of a safety gap before.
I originally also asked this list about the feasability of this idea and
I believe it was fellow list member Jim Lux? that stated that this
would indeed work, assuming that I had the right voltage rating
of the arrester(s) combo. Anyway, I originally used a 12 kV rated
arrester on my 10 kvA, 14.4 kV pig and it worked and didn't
start "clipping" the output of the pig until I ran the input variac
to the pig over about 85-90 on the 0-100 scale, with 100 being
280 volts input. It was easy to tell when this started happening
as the output sparks of the coil would start "sputtering" instead
of the constant roar. I now have a single 18 kV rated arrester
(15.6 kV MCOV) that I will be using with my 14.4 kV, 15 kVA
pig, so I don't think that I should be experincing much clipping
with this unit, even with 280 volt input into the pig. BTW, this
18 kV unit is one of the newer types with the butyl rubber ex-
terior ridges as opposed to ceramic and I think I also payed
more for the shipping of this item than I payed for the item itself
off of ebay. I think I only ended up with < $12 in it, total. I
was able to test it for functionality with an x-ray transformer
being slowly ramped up with a variac and it does start to clip
the voltage around 20 to 25 kV.

The guts of these things are interesting and appear to be several
seriesed cylinders of some type of refractory (almost like cement)
semiconductor material which are pressed against each other in linear arrange-
ment with a strong spring at one end of the arrester housing. There
is usually also some ceramic ring spacers and a couple of bronze
disc between each of the refractory semiconductor cylinders
to act as a controlled spark gap between each cylinder, I assume.

I think that your idea should work fine if you series enough of them
that their rated peak voltage equals or slightly exceeds the output voltage
rating of your pig. In your case, 4 or 5 of the 3 kV rated ones in series
would probably work just fine :^))

David Rieben

PS-Overvolting these arresters does NOT harm them as that's exactly
what they're designed to do on the utility primary line. Once the line
voltage
drops back below their cutoff voltage, they act just like an open circuit
once again.

----- Original Message ----- From: "David Rieben" <DRIEBEN@xxxxxxxxxxxxxxx>
To: "Tesla list" <tesla@xxxxxxxxxx>
Cc: "David Rieben" <DRIEBEN@xxxxxxxxxxxxxxx>
Sent: Friday, April 29, 2005 9:29 AM
Subject: Re: MOV lightning arrestor in place of safety gap...MCOV?


> Aaron,
>
> I've had experience using these in the place of a safety gap before.
> I originally also asked this list about the feasability of this idea and
> I believe it was fellow list member Jim Lux? that stated that this
> would indeed work, assuming that I had the right voltage rating
> of the arrester(s) combo. Anyway, I originally used a 12 kV rated
> arrester on my 10 kvA, 14.4 kV pig and it worked and didn't
> start "clipping" the output of the pig until I ran the input variac
> to the pig over about 85-90 on the 0-100 scale, with 100 being
> 280 volts input. It was easy to tell when this started happening
> as the output sparks of the coil would start "sputtering" instead
> of the constant roar. I now have a single 18 kV rated arrester
> (15.6 kV MCOV) that I will be using with my 14.4 kV, 15 kVA
> pig, so I don't think that I should be experincing much clipping
> with this unit, even with 280 volt input into the pig. BTW, this
> 18 kV unit is one of the newer types with the butyl rubber ex-
> terior ridges as opposed to ceramic and i think I alos payed
> more for the shipping of this item than I payed for the item itself
> off of ebay. I think I only ended up with < $12 in it, total. I
> was able to test it for functionality with an x-ray transformer
> being slowly ramped up with a variac.
>
> The guts of these things are interesting and appear to be several
> seriesed cylinders of some type of refractory semiconductor
> material which are pressed against each other in linear arrange-
> ment with a strong spring at one end of the arrester housing. There
> is usually also some ceramic ring spacers and a couple of bronze
> disc between each of the refractory semiconductor cylinders
> to act as a controlled spark gap between each cylinder, I assume.
>
> I think that your idea should work fine if you series enough of them
> that their rated voltage equals or slightly exceeds the output voltage
> rating of your pig. In your case, 5 of the 3 kV rated ones in series
> would probably work just fine :^))
>
> David Rieben
>
> PS-Overvolting these arresters does NOT harm them as that's exactly
> what they're designed to do on the utility primary line. Once the line voltage
> drops back below their cutoff voltage, they act just like an open circuit
> once again.
>
> ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
> To: <tesla@xxxxxxxxxx>
> Sent: Friday, April 29, 2005 8:44 AM
> Subject: MOV lightning arrestor in place of safety gap...MCOV?
>
>
> > Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> > Hi all,
> >
> > I recently picked up a baker's dozen of 3kV MOV
> > distribution lightning arrestors on eBay for little
> > more than a dollar each.  Seemed like a steal; these
> > things are each the size of a soda can!  Ok, on to
> > practical matters:  How do I use them?  My intent was
> > to use a few of these in series in place of (or in
> > addition to) a safety gap for protecting my 15kVA
> > 14.7kV pig.  Or as part of the world's largest Terry
> > Filter :D  Trouble is, I can't decide how many to put
> > in series.  Should I be going by the arrestor rating
> > (3kV), or the MCOV rating (2.55kV)?  Or maybe neither?
> >  I'm afraid I don't quite get why there have to be
> > multiple ratings on these things, but then I'm not a
> > power systems engineer.  Those folks are a little
> > strange (no offense! :D)  Maybe I should just put a
> > bunch of 'em across the output of my pig then rev the
> > variac up and down and keep removing arrestors until I
> > start seeing big currents on the input of the pig?
> > Hmmm...
> >
> > I've heard of a few people using these in place of
> > safety gaps, but I haven't seen a lot of details.
> > Seemed like an interesting project, though.
> >
> > Recommendations?
> >
> > 73,
> > Aaron, N70E