On 3/24/2010 1:08 AM, Ken Brown wrote:
>
>> 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?
When the power connector and RFI filter are a module you can't get the
MOV on the line side of the RFI filter nor do you have the RFI filter
protected from burn down by a fuse on the line side unless you do it
British style with a fuse in the line plug. Such plugs have been
available in the US on occasion, but are uncommon and may not be
available today. There may have been some RFI filters with fuse holders
on the line side, but they aren't common.
Yup, you need fuse protection on the line side of most any MOV so when
the MOV gives up from being hit too many times and goes into permanent
avalanche that it gets disconnected. Some aftermarket MOV protected
devices have the fuse only on the MOV with a light across the fuse to
show when the fuse opened, that you still have a working load, but no
more MOV protection.
These days the outlet strips with MOV all say on the package or in the
user instructions that they don't work if they are fed by an extension
cord or the load is on an extension cord and so in the real world their
guarantee of equipment replacement isn't worth the paper its written on.
One day a few years ago, I heard a bang out side. Went out to look and
there was the MOV from the 7200 volt overhead in pieces on the ground.
Fortunately it was on another phase than the house so I didn't see the
power dip as it took too much energy and then blew up.
73, Jerry, K0CQ
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