[Amps] MOVs (Vic K2VCO)

[Manfred Mornhinweg]

Vic,

> I have a power supply that needs to work here in the US and in
> Israel, where power is supplied as one phase of 240V plus a safety
> ground. I am not sure if one of the 240V lines is grounded as well.

Yes, normally this power system is three phases, 240V per phase, with 
grounded neutral, and each home gets just one of those phases and the 
neutral, while a grounding circuit is made locally (grounded at the house).

So in principle there should be zero voltage between the ground and the 
neutral, but in practice due to the resistance of the wiring and the 
grounds, there can be a few volts.

> The power supply doesn't require 120V for anything, so there's no 
> neutral.

There is a neutral, it's one side of the 240V line.

> What is the correct way and ratings for MOVs to protect against line
> transients?

Several arrangements are possible. What I would do is connecting a 260V 
MOV from phase to neutral. If you want some additional protection and 
the device connects through a polarized plug, or is wired directly, you 
can add a low voltage MOV (20V would be fine) between neutral and ground.

Any transient will be absorbed by the first MOV, which needs to be large 
enough to handle any transients you would expect, while the second MOV 
would act if for any reason the neutral detaches from ground potential. 
  This would then cause a fault current from neutral to ground, 
triggering the GFI, which is hopefully present.

In equipment that connects through a non-polarized plug, the 260V MOV 
across the 240V line is still fine, but additional protection is 
possible only in the form of additional 260V MOVs from each side to 
ground. There would be no good protection against floating neutral.

The 260V rating refers to the maximum RMS voltage the MOV can resist 
while conducting negligible current. Beware of different MOV ratings. 
260V RMS is 368V peak, so that MOV needs to have an actual trigger 
point, where it starts conducting, above 368V. And hopefully not too 
much above.

Manfred


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