[Amps] MOVs

[Jim Thomson]

Date: Thu, 25 Nov 2010 13:08:03 -0500
From: Roger <sub1 at 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, and not the
peak rating.  MOV's are nothing more than hundreds of grains in series.
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.  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. 


##  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.    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. 

later.... Jim   VE7RF   





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73

Roger (K8RI)