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.
73, Jerry, K0CQ
On 3/23/2010 11:38 PM, Ken Brown wrote:
> I'm sure there are more than one kind of surge protector. The ones I am
> familiar with cause more blown fuses, not less. That is intentional, and
> it protects the equipment. The ones I am familiar with are MOVs. Metal
> Oxide Varistors. These have breakdown voltages that should be selected
> to about 2X to 3X the nominal AC mains voltage. They should shunt the AC
> mains voltage after the fuses. When a higher than normal voltage spike
> comes along, the MOV conducts, limiting the voltage, drawing a bunch of
> current and blowing the fuse. Since the MOV breaks down before other
> more expensive things in the equipment do, they cause more blown fuses,
> not fewer. That is a good thing because the excessive current that blows
> the fuse is going into the MOV, not into more expensive and more
> difficult to change parts in your gear.
>
> Of course, you could put the MOVs in a separate "Surge Protector" box
> instead of in the equipment you are trying to protect. In this case the
> external "Surge Protector" will blow it's own fuse or circuit breaker if
> it has one. If it does not have a fuse or circuit breaker, it may still
> be better than no surge protector at all. I think they are better with
> fuses.
>
> Most of my experience with MOVs has been in GE Mastr II repeater power
> supplies on mountain tops. Without the MOVs various parts in the power
> supply or the radio shelf would need replacing after lightning storms.
> After MOVs were installed in all of the Mastr II power supplies, only
> fuses and MOVs ever needed replacing after lightning storms. It was easy
> to replace the fuses and MOVs, because we knew exactly where they were
> located in the power supply. There was no troubleshooting required. Just
> change them. The MOVs do not always need to be replaced, but when the
> equipment is on a remote mountain top, providing public safety
> communications, it is better to change them than to hope they weren't
> damaged. A hand full of MOVs and fuses is much cheaper than the fuel to
> drive back up the mountain. Change 'em all. Much more efficient than
> having to figure out what was blown up in the radios as we had to do
> before we started using the MOVs. Most lighting hits were to the AC
> power grid and not to the antennas. Never had a direct hit on an
> antenna. I'm sure the MOVs would not have been much help in that case.
>
> Blown fuses are not such a bad thing. When a fuse has blown, it has done
> its job, as intended (except in the case of poor quality defective
> fuses) and has probably saved you a lot of trouble and money. Buy
> quality fuses in quantities, not individually, and they will save you money.
>
> DE N6KB
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