On 11/26/2010 12:53 AM, Jim Thomson wrote:
> Date: Thu, 25 Nov 2010 13:08:03 -0500
> From: Roger<sub1@rogerhalstead.com>
> Subject: Re: [Amps] MOVs
>
> On 11/25/2010 12:53 PM, Ken Brown wrote:
> The original post for this thread has not shown up here yet.
>>> A similar thing happened with garden variety 130 v rated MOV's....
>>> too many house fires. Modern MOV's for 120/240 v use, will have a 3rd
>>> lead sticking out [offset to one side]
>>> and an internal fuse built into em.
>>>
>>>
> A 130V MOV on the 120V AC line would get hot in a hurry 1,414 * 120 =
> 169.7V so the thing would be conducting a high percentage of the time.
> You'd want at least a 180V MOV.
>
> ### Not a chance Roger. That 130 v rating IS the RMS rating,
Not according to the manual. GMOVs according to the manual are rated at
breakdown voltage and not RMS. They may have changed due to people not
being smart enough to figure out the difference between RMS and PEAK,
but mine still states peak and to multiply by 1.414 to get the operating
voltge.
> and not the
> peak rating. MOV's are nothing more than hundreds of grains in series.
Actually thousands of of tiny Metal Oxide Diodes randomy packed
together. The thickness sets the voltage and the volume the rating in
Joules.
> Every time a spike/surge/transient occurs, the series grains conduct, and
> short out the transients and spikes. Trbl is, a grain here and there will
> short
> out permanently.
Yup!
> The RMS rating of the mov will gradually decrease, until
> it either gets too low... and the normal 120 vac shorts all the remaining
> grains,
> or the V rating is on the hairy edge, and the next big transient takes it out.
Or they are on the ragged edge conducting a small current all the time.
They don't usually stay that way long though<:-))
> ## On most of these cheap power bars, they used 130 v rated, low joule movs.
> That's where most of the house fires came from, the pwr bars would be
> sitting
> behind desks, on the carpet, covered in dust bunnies, papers, junk, and
> other flamable stuff.
Mine's cat hair. They seem to collect it, but at least the new, high
volume, low velocity computer fans aren't as bad as the little high
speed ones.
> The newer variety, have an internal fuse and a 3rd lead.
> 3rd lead is offset to one of the other 2 x leads, so they can only be
> installed in the
> circuit board one way, and not reversed. That red led you see on the newer
> style pwr bars is wired ultimately to the ouput of the built in fuse.
> Internal fuse
> blowsd open, red led goes out, and that tells you, you no longer have any
> protection from tranisents.
>
> ## The bigger the joule rating, the longer they will last. The big Joslyn
> types, that
> are wired to your main 200A panel, usually have a 20A slow blow fuse in
> series with each
> big MOV. Each MOV is wired from the 120 v hot buss in panel.... to
> NEUTRAL. The neutral
> is bonded to the metal grnd anyway on a main panel. [ neutral not bonded to
> metal grnd of box in
> any sub panel]. Wired in the above config, you have a protection from
> transients on each of the
> 120 vac hot legs....and also across the pair. Again, if u re-draw it,
> you have 2 x 130-150v rated move's
> in series, with their CT bonded to neutral/grnd. So it then doesn't
> matter whether the transient comes down
> either sdie of the line..or down both sides simultaneously.
>
> ## On the Josyln commercial units, a simple neon is wired between output of
> fuse and neutral. [output
> of fuse also feeds the input of the mov]. A 4pdt mech relay using a 120 vac
> coil is also used. The 120 vac coil is
> also wired between ouput of fuse... and neutral. [one relay per mov] Both
> relays are alway's operated.
> If either / both fuse blow open, one/both neons goes out, and relay drops
> out, [which brings in an alarm if you want].
> That way, as long as they are fused, you won't have exploding mov's. The
> entire assy is contained in a metal box,
> with hinged lid, and normally mounted directly below the main panel. The
> idea here is, the entire home is protected
> from tranisents/spikes/surge...coming in off the street..whether lightning
> induced, or typ power co glitch's.
>
>
>
>
>
>
>> I don't understand how this can be a useful device. I thought that MOVs
>> were supposed to draw lots of current, instead of letting it got
>> elsewhere causing damage to more expensive and sensitive electronics.
> That is correct, but an under sized, under rated MOV could cause a fire
> by itself.
>
> ## You can get Mov's in any joule size from dime size one's..to huge
> things, puck sized. You can also series mov's to increase their V
> rating's.
> [they don't have to have identical V ratings either]. You can also parallel
> em
> for a higher joule rating, or do both.
>
>
>
>
>> I
>> had always thought of MOVs as sort of "sacrificial" devices, intended to
>> blow up under the worst power surges,
> They are.
>> and mounted inside a metal
>> cabinet so that when they do blow up nothing else is damaged.
> Unfortunately many omitted this part.
>> I would
>> never mount an MOV where it could set fire to something else.
>>
> Many did.<:-)) MOV's are, or used to be, rated by their breakdown
> voltage. So people were purchasing 130V MOVs for 120 V AC lines
> forgetting, or not knowing, the peak voltage is 1.414 times the RMS.
>> DE N6KB
> ## see above. Most catalogs will provide both the RMS ratings, the peak
> rating's, and also the continuous DC v ratings. Dahl used these huge
> 277 vac rated bolt down movs on his commercial transient suppressor
> assemblies..wired straight across the 240 line. He also used 3 x paralleled
> 130 v
> rated movs, from each 120 v hot leg to grnd. No fuses were included.
Some where around here I think I might still have one of those 50,000
joule MOVs and they are big. They are mounted on their own aluminum
back plate. IIRC they are about 2.5 X 3.5 X about 1" thick.
73
Roger (K8RI)
> later.... Jim VE7RF
>
>
>
>
>
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> 73
>
> Roger (K8RI)
>
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