A MOV is far more effective if its located on the line side of any RFI
filtering. That's because the the LC RFI filter acts like an integrator
and slows the rate of rise and fall of the fast spike, but passes nearly
all of the energy at a lower voltage, with a longer duration. And the
lowered voltage makes the internal MOV less effective. So a device with
RFI filter at the power inlet connector isn't protected as well by any
internal MOV.
That makes sense to me. Still I think it is also preferable to have the
MOV on the load side of a fuse. That way, if the surge has enough energy
to blow up the MOV, it will also have blown the fuse. Doesn't have to be
the fuse in the equipment being protected, but it could be if the fuse
is before the RFI filter. Maybe that is not usually the case?
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