[Amps] MOVs

[Jim Thomson]

Date: Thu, 23 Jan 2014 22:31:53 -0800
From: Jim Brown <jim at audiosystemsgroup.com>
To: amps at contesting.com
Subject: Re: [Amps] MOVs

On 1/23/2014 7:18 AM, Paul Baldock wrote:
> If the amp is running off 240V where in the USA this is symmetrical 
> about ground and then the MOV is placed between the two phases, it 
> seems to me that there would not be a surge current to ground.

Right.  The problem comes when you take the MOV to the ground.

73, Jim K9YC


##  On whole house protectors, they use a pair of huge, FUSED 
130 V rated movs.... wired from each hot side to the neutral...which of 
course is also grounded.   So you end up with 2 x movs, nose to tail, in series
across the 240 V line..with the junction point bonded to neutral + ground. 

##  telcos use the same deal on 240 v –120v  sites..like cell sites.  In bigger offices,
where 208 +  120 vac  3 phase power is used,  3 x movs are used...and each is wired
from each hot...to neutral.  neutral is grounded  at the service entrance.   The fuse is
between  each hot leg and mov.   On the output side of each fuse is the 120 vac coil
for a 4PDT  relay..wired  from fuse output ..to neutral.  A 120 vac neon is also  wired from
output side of each fuse..to neutral.    In normal operation, the neon is lit..and the 4PDT relay
is operated at all times  24-7.  IF the fuse ever blows open, the neon goes out..and the relay
also drops out..... which brings in an alarm. 

##  In the case of 120 vac ham gear, you might be able to wire a FUSED mov between the 120 vac
hot leg and neutral.   In the case of  240 vac ham gear,   You could wire a SINGLE  FUSED  250 vac rated 
MOV between the 2 x hot legs.   You could also wire 130 v rated movs  from each hot leg to neutral...
then you are still protected from spikes on either  or both sides of the hot legs.   Then  nothing is getting
conducted back to ground...except back at the main 200A panel..where the neutral is bonded to the ground.

##  On paper, it should work.  Ideally the best way would be to use the whole house protector scheme
right at the main 200A  panel.  Then the entire 200A panel is protected from transients  coming down the
drop wire into the home.  It should also kill transients + spikes   from any equipment  on any branch lines...
like electric motors etc.   But then again, the electric motor, etc could be a loooong way from the main panel
movs. 

##  On paper, using  MOVS  at the 200A main panel  AND the equipment, line to neutral  and not line to ground  at the
equipment end...  should work.   IF  movs are not fused..you are asking for trouble.   Real simple to fuse em..plus add
a neon or led on the output side of the fuse to serve as a visual indicator that the fuse is functional..... which of course
also means the mov is functional. 

##  I will defer to Jim Brown as to his thoughts  of using movs wired  from hot to neutral at the equipment end of things. 
Another thought is to wire all ham gear for 240 vac use if possible..including xcvrs etc..but that is not always possible. 

Jim   VE7RF