Lighting arrestor to dampen the ground voltage differential

December 26, 2007 6:24 PM

Using a lighting arrestor to dampen the ground voltage differential between two building.

Ok so my mother moved down to mexico a few years ago. The place she rents does not have a phone but the neighbor does. So she got DSL at the neighbors place and then we ran a phone line over to my mothers place to hook up the DSL modem. This all works great.

Now the neighbor has bought a computer and wants internet access. So I need to get a connect back over to her house. (yes I could move the DSL modem to the neighbors but the house is poorly grounded and they burn out cordless phones all the time so I do not expect the DSL modem would fair very well. I have grounded by mother place so it is not as much on an issue for the DSL to stay are here place.

I have two plans one long term and one short term. In the long term I will place two external wireless modems down there to connect the buildings. They have to be external because all the buildings have 12 to 15 inch thick concrete and masonry walls that I wireless signal will not make it thru. Also I will have to cut down a few trees probably to get a clear shot.

Now the short term solution an the reason for this question. I am thinking I can run long outdoor grade ethernet cable between the buildings and use a lightning arrestor http://metrix.net/cat-5-lightning-arrestor-p-23.html do dissipate the ground voltage differential between that is bound to be between the two building.

Am I crazy to think that this might work?

posted by jumpsuit_boy to computers & internet (6 comments total)

DISCLAIMER: IANAE and IANYE

I'd get a meter first and measure the ground differential in both DC and AC modes. Do it this way:

Run a wire from a ground in the remote house to your mother's house. Don't hook it up to anything, and use the meter to measure the difference in potential in the grounds.

Ideally the difference will be small, and item you called out will work. Understand, however, that surge suppressors in general are designed to filter out small surges.

The item you mentioned is a voltage isolator, and should work, to the limits of the device. I'd call the manufacturer to make sure, though.
posted by pjern at 8:05 PM on December 26, 2007

That lightning arrestor has pretty high voltage limits. I think you could still burn out equipment with limits that high, if you had a DC bias. I think you need to fix the other building's grounding, or at least ground the equipment over there properly (say by driving in a new grounding rod and isolating the equipment's outlets there to it). I'd also consider bonding the two ground systems together with heavy-gauge copper (or running the thing in bonded metal conduit), although I don't know if that's legit, and I'd only do it once the other end had a solid ground.

If you put in a brand new grounding rod at each end of the run (driving it however deep you need for the soil conditions -- consult electrical codes or one of the ARRL books if none exist, but at least 8' or so) and you have a non-negligible current running between them before you connect any equipment, you're deep into consult-an-electrician territory.

But basically, I'd say that fixing the ground problems at the far end is the only way to make this reliable and avoid equipment damage down the road. You could try to find a product that totally isolates both sides from each other, using transformers (though I've never seen isolating transformers for Ethernet, and they may not exist) or optical fiber, but the Lightning Arrestor doesn't seem to be the right product. That's designed to take out big surges, not to eliminate ground leakage problems and low-voltage DC bias.
posted by Kadin2048 at 11:42 PM on December 26, 2007

You're misinformed as to what a lighting arrestor does - it doesn't 'bleed' off excess current / voltage from anywhere; it sits there and does nothing until asked to pass a high voltage, which it then shorts to earth.

In your case, if the potential between the signal earth and the reference/safety/electrical/whatever earth^* is higher than the breakdown voltage of the lighting arrestor, it'll short that - and your signal too - to earth. In other words, while it might clamp the earth potential difference between the two houses to its rated breakdown voltage, it'll also kill the signal.

(^* There's another point - that arrestor needs to be tied to an earth; that's what the big screw in the middle of the case is for. Which earth? The one at the neighbours house? The one at your mothers house? Somewhere in-between? The telecommunications earth (if fitted)?)

Go the wireless route, or go fibre. There are CAT-5 isolation devices available, but the only ones I'm aware of are expensive - they're designed for medical or volatile environment use.

(Personally, as a son of an electrician and with plenty of experience in telecomms power/earthing practices, I'd just bond the two house earths together at their reference points. Note that this is definitely illegal in most places unless you're a licensed electrician, possibly illegal anyway depending on your local electrical distribution practices and codes, and should not be attempted except by trained stunt electricians. IAALTC, but IANYLTC (I Am A Licensed Telecommunications Cabler, but I Am Not Your Licensed Telecommunications Cabler))
posted by Pinback at 12:34 AM on December 27, 2007

I guess I am going to have to go back down there sooner rather than later. Well the weather is sucking in western washington anyways. (ice on the roads mean I can not ride the motorcycle)
Wireless it is then. These guys Metrix make some nice outdoors kits and they have been tested. What ever I set I needs to be pretty rock solid as I am only down there maybe once a year.

Thanks for the feed back. It was and iffy idea anyways.
posted by jumpsuit_boy at 12:43 AM on December 27, 2007

If you want rock solid, use fiber, with a decent fiber-to-twisted-pair converter at each end. Wireless with no tech support is the debbil.
posted by flabdablet at 1:13 AM on December 27, 2007

Seconding Kadin. If you want to connect any copper between the two buildings, fix the grounding on the far building first. Sounds to me like there's a loose connection or two in the house's internal wiring, probably in the neutral circuit, possibly in the main box. This would explain burning out appliances.
posted by exphysicist345 at 3:25 PM on January 2, 2008