I am trying to establish a good earth for my old farm house in Spain. Unfortunatly the soil is only 12" deep before I hit solid rock. I have a 24" long drill so I drilled two 10mm holes about a foot apart into the rock, cut my 4 foot, 10mm copper earthing rod in half, sunk the two halfs into the two holes, clamped on my earth wire to each, and covered over with watery cement. The question is, is this a good enough earth. I tried running a 40W light bulb between my earth and live, and although the bulb lights, it is getting 30 volts less than if I wired the bulb across the live and neutral, ie 170V instead of 200V. Anybody give me some help please.
snipped-for-privacy@markXscotford.com (remove the X to reply)
I do NOT KNOW about Spain for grounding. I have run into rock below the surface several times. The National Electric Code allows for you to install the rod horizontally if needed. Cutting the rod in half is not the answer. Depth is the biggest part of a grounding system. Second comes mass. I suggest you replace your 2 foot rods with 2- 4 foot rods horizontally as deep as it is practical, separate them by at least 6 feet in any direction. If you have a planter that gets watered regularly even better. Here in the USA we use 8 or 10 feet long ground rods. I have installed 20 foot chemical ground rods for sensitive installations. Something you probably do not need. Second method is to dig 3 holes as deep as you can and use a 24 inch 1/4 in thick with a #4 brazed on to the steel using silver solder. Separate these plates/holes by at least 10 feet. It is called a counter poise. Depending on the soil conditions this method may not last as long as a copper clad ground rod. It will last over 5 years even in very aggressive soils.
Your description indicates that Spain is 220v or 240v single phase like France or Britton. How do you get your water? Is the piping metal? Do you have gas? Is it metal? If so then make sure your bonding these as well at your service. Just get a pipe grounding clamp and run the #4 bare through the clamps until you have every electrically connected.
Please note I am suggesting #4 because that is what I use. It is the smallest wire I feel safe not installing in conduit. Might be a good time to check with the locals, Good luck, some day maybe I will get to see Spain.
I would suggest drilling holes will of been of little use, but it depends upon the type of substrata. A better way would be to lay the rods horizontally.
If it is permitted and practical, you might try running out several bare copper wires in a star pattern from your earthing connection, then going along with a spade to cut a slot to push them into. Solid copper will be better than stranded for this purpose.
I believe that an effective method of reducing the resistance between earth rods and the rock is to fill the "voids" with bentonite ( AKA Fuller's Earth and cat litter). Bentonite retains moisture and therefore should remain conductive under dry conditions. I presume you would add it dry and then "water" it in. Bentonite is used in drilling muds so if 10mm x 24" is no use then 300mm x 1000m ? ;-)
Indeed, this latter method is the norm for some if not the majority of a.m. radio station tower grounds, except (at least going by the book) they generally run 120 #10 or #12 bare copper radials to a 1/4 wavelenth distance from each tower. Even then the grounding efficiency will measurably vary depending on the season and how dry the soil is.
Rods too close together ( two rods unless well separated will be little better than a single rod of the same length), and too short. As suggested by others, lay out wires under the ground in a star or even a grid.
-- Don Kelly firstname.lastname@example.org remove the urine to answer
Just to add a bit more detail. I have no open ground, the only access I have, is to the front drive which is concreted over (so no natural watering). I can bore holes OK, but breaking up the whole drive, to start laying out star patterns is a tad tricky. I surely cannot be the only soul to have this problem. If I do not get a better suggestion, this weekend I will try another rod, buried at 30 degrees into the ground. Not being a fully fledged electrician, pray tell me what test I do, to know that I have reached the required criteria??
Not my area of expertise, or my country, but I would suggest an 'earth loop impedence tester' be used. Even then you would need to know the local regulations (or code) for the type of installation you have, to be able to determin what would be a satisfactory reading on the above instrument.
Is there perhaps a Spanish electrical engineering newsgroup who might be able to offer proper and accurate local advice on this?
There are specialized testers but few U.S. electricians have them. One of the best grounds is a concrete encased electrode. It is just 20 foot of 1/2" rebar encased in concrete. The rebar can be several pieces wired together with the normal rebar ties. Would there be any chance of something like this working?
You're hardly the first person with this problem. The local professionals will no doubt have a solution. Could be as simple as a longer drill bit ;)
Apart from the risk that you are doing something you aren't allowed to without a license, you need to check what the local regulations have to say. Is this a TT or TN-C(-S) or perhaps even a TN-S service? In the latter two cases, you don't really need the rods at all. In the case of a TT system, you can choose an RCD rating such that the present earth resistance is acceptable. (If you want to convert to a TN system, you need to check with the utility to see if they allow you to use the neutral as earth.)
If you are unsure about what you are doing, call a pro.