MOV & Surge Protection Questions


Have been thinking about this a bit, and realize I'm probably looking at it the wrong way.

The other night we had a moderate lighning storm come thru. Momentary loss of power; perhaps a few seconds.

The only damage was to a newly installed furnace's circuit board which got fried. It obtains it power in the normal way, via a hard wired, dedicated,

110 V branch back to a dedicated circuit breaker on the main panel.

The surprising thing is that nothing else was damaged; even the PC worked fine afterwards.

So, my questions are -

Let's assume that there is only only one cheap extension cord outlet strip in the house that has MOV's for protection, and that the PC was plugged into it.

Let's also assume that it was connected to the L1 side of the 110V coming from the Service Box , as well as perhaps a dozen or so other circuit breakers and branch circuits. (the 220 V service coming in being split into two 110 V sections, L1 & L2)

It would seem to me that the MOV's in this strip, assuming they work(ed) really well would protect anything upstream just as well as anything plugged in downstream (like the PC's) as all they do is clamp the line to gnd. Upstream or downstream should make no difference. Is this correct ?

By the same reasoning, I could argue that All the branches on L1 would be equally protected by this single MOV strip, as all of L1 would get clamped. I guess there would by a few nsec difference in propagation times for the surge of the different branches, but it's hard to believe this would effect anything.

Are all branches coming off L1 equally protected by a single strip on just one of the branch circuits, do you think.

Obviously the MOV strip it didn't protect the furnace circuit which was on L1, so I am probably wrong about this.

Any thoughts on this would be most appreciated.

Thanks, Bob

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In short, No. The power strip with the MOV may not even adequately protect the device that is plugged into the power strip. Some of the cheaper strips give you, "one time protection", Once the MOV blows the strip continues to deliver power. Consider using quality surge protection.

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Look for a connected equipment guarantee.

There are devices designed to protect the entire house or service

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No, I do not work for APC J

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In a perfect world - yes that would be correct. However in the real world no it isn't. The problem is that even a straight wire has some inductance, capacitance and resistance. In the example you gave the inductance in the extension lead could act to isolate the protection device from anything "upstream".

The best advice is to fit protection and conditioning devices close to the input to the building (to protect the whole building against mains transients coming in from the power co.) AND another lot close to sensitive equipment to protect you from yourself (eg the noise produced by the janitors vacuum cleaner or power tools!).

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Mike Berger


The ac outlets in your house have millinery's worth of inductance back to the pole transformer. When a high dv/dt rise in voltage occurs at the pole transformer the large capacitance of the mov as well as its energy clamping ability limit the voltage at the mov. every inch closer to the pole transformer from the mov sees more voltage.








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