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Thread: Earthing advice needed

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    Gold Member IMHO's Avatar
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    Earthing advice needed

    I have a property on a hill that is VERY prone to lightning strikes. I now eventually erected a lightning pole to try and sort out the problem. 27m high and R16K later. Readings I get with an earthing meter spikes to 4ohm and then settles at about 0.1 ohm. The electrician used a meter with 4 wires, three connected to short earthing spikes that he hit into the ground about 1m away around the pole and the 4th connected to the base of the lightning pole.

    I am showing a picture trying to explain what I did. I used +- 10mm bare copper cable. It goes down into a big hole that I dug into the very rocky earth. The hole is 1.5m deep, 6 m long and 1.5m wide. The cable then runs the length of the hole, 6m, and turns back for another 3m. There is 3 of these cables. Then I took a 4th one, laid it into the hole the same way, but instead of turning back for 3m, it goes up and out of the hole at the far end. The hole was then filled up with soil and compacted with paving over the top. The picture shows the cables neatly spaced, not touching but in fact they are all over the show crossing and touching each other. Only in hind sight I realized that it probably would have been better to space them apart and not touching. Maybe it does not matter and maybe it is better the way I did it?

    My idea was to use this 4th cable and lengthen it to some electronic equipment, cameras and IT stuff, 30 meters away, as an earthing point for the lightning protectors connected to the equipment. My problem is that I can not get a decent spike into the ground deep enough to give a decent earthing point to the lightning protectors.

    But now I am getting cold feet and I worry that I am actually taking lightning strikes directly to my equipment. I have not done the extension of 30 meters yet, as I am scared and do not want to waste money. If it will work, what do I use for the 30m extension, bare copper cable as well, or will something cheaper suffice?

    I am also thinking of running an earthing wire from the base of the lightning pole underneath the paving, to the point some 30m away. My logic tells me the lightning strike will follow the route to ground in the hole and not follow the earthing wire, but does it make any difference? At the end of the day it is just a different route and I do not think lightning is going to make an exception and just follow all possible routes?

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    Gold Member IMHO's Avatar
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    Can Andy come out and play?
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    I used to erect lightning masts, don't connect any electronic equipment to that earth wire, in fact make sure that it is buried as lightning could be attracted to the end of your cable instead.
    Apparently there is nothing that cannot happen today.

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    Gold Member IMHO's Avatar
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    Yes, did bury it underneath the paving, but not deep though? Could you get electrocuted if standing on that spot while lightning strikes, say barefoot? I would say it is about 8-10 cm soil over it and then the paving brick.
    ~Expenses will eat you alive! - My first Boss~

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    Diamond Member AndyD's Avatar
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    Sorry I'm late, I see you've got lots of stuff going on here and some of it isn't really related to some of the other things.

    Lightning protection isn't my core business but I'll answer what I can so starting at the bottom and working upwards;

    On ground where people and animals can walk we worry about what's know as 'step voltages' and 'touch voltages'. On a side note concerning animals, is it just the photo or does you dog have some accident damage, it looks like its rear chassis is out of allignment? Anyway, a step voltage is very much what it sounds like, it's the voltage difference that could develope across the ground in the distance of a step. With people, if you stand on two points that are of different potential (at different voltages) it causes current flow through your legs and pelvis mostly but with animals the same current would flow through their front legs, through their body and out through their back legs. Because their body (including their heart) is part of the current path the step voltage that's fatal to animals is considerably lower than the step voltage that would be fatal to a person. In areas where there's livestock we'd want to ensure that the maximum step voltage that might occur is lower.

    Relating this to your photo in the OP, installing an earth mat or earth grid under the paving could possibly cause step voltages to occur in the vicinity during a lightning strike, it probably wouldn't have been my first choice as a location. Predicting step voltages under lightning conditions requires special test equipment, if the installer did the required ground tests and a thorough risk assessment then the location shouldn't be a concern.

    Touch voltages are kinda similar to step voltages in so much as they also determine whether sufficient current would flow through a person (or less likely an animal) that is subjected to them. Touch voltages occur across a person when they're standing on a surface that's at a different voltage than something they're touching. With people, touch voltages are more likely to kill than step voltages because the resulting current flow has a higher tendancy to travel through the persons heart and cause death by defibrilation. There's a common misconception that you can just whip out your multimeter and accurately measure a touch voltage with respect to earth...but you can't. Any accurate test of a touch voltage must simulate the event of someone getting a shock so there needs to be a capacitive and resistive network in the test circuit to represent the impedance of the poor soul who's getting the shock. Most digital testers are not capable of simulating this and their extremely high input impedance means the test result will be irrelevant. Again, if proper testing was done to ensure the mast you've erected won't develope lethal touch voltage then its easily accessible location would be fine.
    Last edited by AndyD; 27-Mar-14 at 11:17 PM.
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    Gold Member IMHO's Avatar
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    haha. Im happy now that I know you are aware of the tread Andy. I know you will come back when you have time.

    As far as my dog goes, no, she is fine. She is coming toward me and probably just started a sprint.
    ~Expenses will eat you alive! - My first Boss~

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    Diamond Member AndyD's Avatar
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    Maybe it was loss of traction during harsh acceleration that was causing the chassis flex, glad to hear she's fine though because I have a cat with accident damage which I doubt will ever be the same again

    Quote Originally Posted by IMHO View Post
    I have a property on a hill that is VERY prone to lightning strikes. I now eventually erected a lightning pole to try and sort out the problem. 27m high and R16K later. Readings I get with an earthing meter spikes to 4ohm and then settles at about 0.1 ohm. The electrician used a meter with 4 wires, three connected to short earthing spikes that he hit into the ground about 1m away around the pole and the 4th connected to the base of the lightning pole.
    If you did what was effectively an Ra test with an earth impedance tester and got a reading that settled at 0.1 ohm then I'd be very happy, in fact I'd be ecstatic...however once the euphoria had worn off I'd be wondering why the impedance is so low because it sounds too good to be true.

    To put it in perspective I had an installation in the middle of nowhere that required a sub-ohm earth impedance and it took me the best part of a week with nearly a dozen guys and a large plant hire deposit to install an network of earth spikes over an area of several hundred square meters with large diameter rods that were driven 16-25 meters into the ground and interconnected with 95mm wires which were doubled up. The result was an Ra of around 0.71ohms on the day and it settled to 0.48ohms over the next six months. I'm highly sceptical about you getting a 0.1 ohm Ra by just burying a few pieces of copper at a meter or so depth unless you're living on salt flats and it's the middle of the monsoon season and even then I'd still demand a second opinion with a different test instrument.
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by IMHO View Post
    I am showing a picture trying to explain what I did. I used +- 10mm bare copper cable. It goes down into a big hole that I dug into the very rocky earth. The hole is 1.5m deep, 6 m long and 1.5m wide. The cable then runs the length of the hole, 6m, and turns back for another 3m. There is 3 of these cables. Then I took a 4th one, laid it into the hole the same way, but instead of turning back for 3m, it goes up and out of the hole at the far end. The hole was then filled up with soil and compacted with paving over the top. The picture shows the cables neatly spaced, not touching but in fact they are all over the show crossing and touching each other. Only in hind sight I realized that it probably would have been better to space them apart and not touching. Maybe it does not matter and maybe it is better the way I did it?
    The '+- 10mm bare copper cable' you used, was it 10mm diameter or was it 10mm square CSA (cross-sectional surface area)?

    I doubt it would have made much difference if you'd laid them out neatly.

    Quote Originally Posted by IMHO View Post
    My idea was to use this 4th cable and lengthen it to some electronic equipment, cameras and IT stuff, 30 meters away, as an earthing point for the lightning protectors connected to the equipment. My problem is that I can not get a decent spike into the ground deep enough to give a decent earthing point to the lightning protectors.
    Is the electronic equipment in the main house or is it in some kind of remote building?
    When you say 'lightning protectors' do you mean surge arrestors?
    Just out of interest do you know what the earthing arrangement is for the main house, it should be written on a CoC if you have one somewhere. Do you have an earth spike that belongs to the electrical supply anywhere on the property or even out of the front of the property if there's a meter box out there?


    Quote Originally Posted by IMHO View Post
    But now I am getting cold feet and I worry that I am actually taking lightning strikes directly to my equipment. I have not done the extension of 30 meters yet, as I am scared and do not want to waste money. If it will work, what do I use for the 30m extension, bare copper cable as well, or will something cheaper suffice?
    To be honest your plan sounds a bit sketchy. Have you had an electrician advise you about the lightning protection or did you just Google it and erect the 27 meter mast?

    Obviously I'm not familiar with your property or your electrical supply so I can't give you specific advice but usually when we install lightning protection we're looking at what's effectively two installations, one that protects the buildings from actual lightning strikes and one that protects electronic equipment from the surges caused by strikes that occur in the area. If a lightning strike hits an overhead supply line that's going to a building for example, there's no way to protect against the damage this would cause. The only protection in this case would actually be prevention of the strike hitting the line in the first place.

    Here's some light bedtime reading for you.

    **Edit** Sorry it's a 14.2 Meg PDF so I can't attach it. If you PM me with a gmail address I'll mail it to you.
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