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Joined: Aug 2005
Posts: 10

I'm a retired electrical engineer, not in the trade, but will take the liberty of posting this question here, as I thought it might be of enough general interest to others. Also, with a hope that all the experts would chime in, as I'm really curious about this.

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 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 on L1.
(the 220 V service coming in being split into two 110 V sections, which I'll call
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 being essentially in parallel would get clamped.
I guess there would by a few nsec difference in propagation times for the surge on 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 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.


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Joined: Jun 2004
Posts: 56
There are too many variables at work in lightning strikes to make many hard and fast rules about the effectiveness of MOVs or other protection devices.

If the strike was near enough to induce the fry-up of a circuit board in your house, then you were very lucky that the MOV was sufficient to protect your PC. A MOV will protect against small surges, but larger surges will vapourise the MOV without mercy and probably the equipment too.

I have had a bit to do with designing lighting protection for telecoms installations located up on mountain tops on New Zealand's South Island west coast, where lightning is a constant hazard. This protection typically consists of two very meaty high-energy gaseous surge arrestors linked by a decoupling coil to give two-stage protection. When these are hit by a strong enough strike, they sometimes succeed in protecting the equipment only by getting comprehensively fried themselves.

For best results the surge arrestor(s) should be installed with a little physical distance between them and the equipment to be protected as possible.

Mark aka Paulus
Joined: Feb 2005
Posts: 693
Bob, to add to Paul,s response, the impedance of the house wiring also matters. The best protection is a combination of whole-house protection at the main panel cascaded with point-of-use protection near the sensitive equipment.

Larry Fine
Fine Electric Co.
Joined: Aug 2005
Posts: 1
Junior Member

I am a new member and I came upon this post. I also have the same situation recently. One of our telecom site was hit by lightning. The site already had 2 TVSS units installed, one at the main entrance and one at the point of use(ERICO Brand), where a rectifier unit is installed. Our system ground reading is 4.2 ohms, a value which I presume is still good.

When lightning struck the area, the surge entered our systems through the power lines. The commercial power was cutoff, all other telecom sites in the area have several equipment damages, with several circuit breakers tripping. Only the rectifier I have mentioned above malfunctioned in our building. Upon inspection of the rectifier, no fried components were visible and no burnt smell can be detected. There is however a small regulator circuit board with 11 small components. The two zeners were damaged and the IC transisors broke. The rectifier transformer windings are okay but the rectifier does not have a voltage output.

From my research, lightning induced surges generate a very high voltage magnitude which, if not handled properly, could fry any equipment it comes into contact with. However, there is also one component which is not mentioned always, the frequency. A lightning induced surge also bring about an induction of a very high frequency ranging from 2-50 Mhz.

From my deductions, from the viewpoint of the voltage, I believe that the TVSS was able to suppress the very high magnitude of voltage surge. This can be seen by the normal operation of other equipment connected to our power system and the fact that no fried and burnt equipment can be seen in the malfunctioned rectifier. Or I may be Wrong?

From the frequency standpoint, this is where I have my doubts. Can you enlighten me on the possible effects of high frequency entering an equipment through the power lines? What extend will damage to the equipment be?


Joined: Feb 2005
Posts: 693
Erick, all wiring entering the premisis must be protected, not just the power wiring; phone, cable, even underground wiring. Even well-pump wiring has been known to bring lightning currents into a building.

Larry Fine
Fine Electric Co.
Joined: Nov 2002
Posts: 134
The best you can do is try to minimize yourself from'll never eliminate the hazard.

I support a distributed control system at our factory. The only things that seem to help are fiber cabling and surge protection directly on the network. If you use MODBUS, PROFIBUS, Ethernet, or any other open or proprietary network you need to have that network protected. Protecting the power just isn't enough. You need to make stop it everywhere.

You want to keep it out of your networks....and if it gets into your network you want to contain the damage.

I've had great success with Leviton and Phoenix Contact surge protection devices for communications related hardware.


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