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Old 09-20-2007, 01:54 PM   #1
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Surge Protectors


We protect our stereo's and related equipment. Is it time to consider using these to protect the circuit boards controling the heating and air conditioning equipment?

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Old 09-20-2007, 03:21 PM   #2
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Surge Protectors


Depends on where the lightning comes in.

I have a lot of electronics, including home automation and live in an area that is vulnerable to lightning hits. I use panel surge supressors mounted right to the AC power panel, Leviton 51120's. They require dedicated breakers at the panel, but they can dump a sizable hit right to the entrance panel ground rod. These devices are very effective at controlling hits that come in from the power pole.

I am a bit of a contrarian on the use of those strip type surge supressors. You have to follow what happens with the excess current and where it goes. Typically they use MOV's or thermistors. These devices become low resistance once they reach a trigger voltage. During a hit, those devices dump the energy to the ground WIRE. This wire has resistance and it is surprisingly large for fast rise pulses like lightning (100 to 1000+ times the DC resistance). Every device that is plugged into that feed sees this voltage rise on the ground wire which exacerbates the problem. Those ground wires are made for personal safety, and are ineffective at dissipating surges.

TV and cable lines should be arrested using coaxial arrestors built for that service right where they enter the house. Same thing for telephone lines, especially if they get connected to modems. You need to protect every wire that comes into the house.

In any case, you should have ONE grounding point for all these devices. In my area it is code to have a ground rod near the power panel. I recommend tying arrestors to this ground with as large a conductor as possible and as direct as possible to reduce the AC impedance.

If you have multiple ground rods, a strong local hit causes large voltage gradient through the soil and the ground rods are no longer at the same potential. This can cause "circulating currents" in the neutrals and really mess things up.

BTW, nothing is effective with a direct hit!

Sorry to be so verbose.
Good luck!

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Old 09-20-2007, 04:31 PM   #3
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Surge Protectors


I have a lightning arrestor on top of the circuit breaker panel and I think its just grounded to the panel. The panel is grounded by an in ground rod.
My outside tv antenna and cable are grounded to the cold water pipe at the back of the house. But this ground runs all the way via the cold water pipe to the front of the house where the cold water pipe enters. I'll take your hint and run heavy wire from the rear cold water pipe, direct to the grounding rod.

The thought of surge protectors came to me as I just had some work performed by my hvac man. The thermostat is the best that the company offers and I could imagine its replacement cost as $150.00 or more. The circuit boards in the furnace and condensor may be simular in price.

I was not thinking of those plug in strip type suppressors, but something that can be installed and approved by the HVAC industry.

Some years ago, I found shards of chimney liner all over the rear end of my flat roof. So I must have had a direct hit on the chimney. But I do not recall having any damage to anything that requires electricity to run. Is that what you mean by "depends where lightning hits"?
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Old 09-22-2007, 08:38 AM   #4
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Surge Protectors


Yes. The idea is to get the surge as early as possible before it gets into the wiring. I had a close hit to my well (650') a few years ago, and it blew the pump controller right off the wall! Since the pump surge went into the panel, and the panel had the Leviton guard on it, it did not find it's way to other devices. As far as I could tell, that hit did not cause any other damage.

For example, if I had a hit on my outside FM antenna, it would get into the receiver via the cable, then to a surge arrestor connected to the ground pin on the radio. Then it travels to every other devices on the line until it found earth. The voltage would rise until one device broke down. If that happened to be my HDTV, I might have a catastrophe on my hands. However, if the strike was shunted to ground before it got into the house, the effect would be lessened. That is no guarantee of protection, but it is the best you can practically do.

The panel guard protects the AC incoming feeds. It is the hardest to do because it is the lowest impedance and goes everywhere. Antennas, telco lines and others (I have remote sprinkler valves with subsurface wiring that makes a great lightning collector), all have to be evaluated to keep lightning out of the house. if you do that, then the sensitive stuff, like the electronic furnace controller, thermostats etc. would be as protected as possible.
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Old 09-22-2007, 09:07 AM   #5
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Surge Protectors


Lightning damage can come from indirect hits, like your chimney example. It does not have to actually strike the house wires.

The current follows a path that is not alway obvious, sometimes bizarre (Google "Ball lightning"). The current "induces" a voltage in nearby wires, the same effect as an antenna or a transformer. The induced currents then flow into the devices they are connected to. If the induced voltage is high enough it can damage things. So it is possible that a chimney hit could cause damage in the furnace controller for example, even if it was not directly hit. But it might not, like in your case. It is never a sure bet.

The lightning might flow just under the soil for many yards before it dissipates. That is how underground power lines, like mine get hit too.

It all depends on the current path and what is nearby.
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Old 09-22-2007, 07:09 PM   #6
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Thanks,
The next time my hvac man comes, I'll ask him if he has come across customers with suspected surge damage on their equipment.

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