Replies

1. 
   jim_l | Mar 12, 2002 04:32pm | #1

   *
   I'm not qualified to answer, but I had a similar problem and I laid 3 eight foot ground rods horizontally in different directions as deep as I could.
   I'll also be interested to hear from the electricians reading this board.
   jim

1. 
   Jeff_Clarke_ | Mar 12, 2002 05:03pm | #2

   *... I suspect they'll tell you that ground rods aren't good for much. Grouting into rock doesn't help since it won't necessarily increase moisture (decreasing resistance). There is a carbon-based backfill material that decreases resistance in difficult (rocky) soils, though.Jeff

1. 
   George_W._Carpenter | Mar 12, 2002 08:31pm | #3

   *Jeff, currently (ha ha) facing the same problem.The revision to the NEC has three solutions.I'm doing all three, in preparation for the new regulations.1) A ground rod, 8 foot, driven.2) A 20 foot minimum of correct sized ground wire (100A = #6) placed horizontally, in my case under the footings, and bonded to the ground rod.3) Bonding to the copper piped water supply at the point of entry to the residence, then bonding that point to the ground rod/ground wire/panel.GWC

1. 
   xJohn_Sprung | Mar 12, 2002 11:12pm | #4

   *For tough situations like this where you're up against rock everywhere, the NEC does allow laying copper pipe in trenches to create horizontal ground rods. A really good electrician will have the special test equipment to measure the resistance of your grounding arrangements. If you find one in your area, he may have some experience with similar problem lots. It could be about $100 to get a test and some good advice.-- J.S.

1. 
   splintergroupie_ | Mar 12, 2002 11:29pm | #5

   *My electrical inspector recommended i ground to the rebar in the footing. This sounded like a great way to get a lot of conductor in contact with a lot of dirt, and quickly.

2. 
   Wayne_Law | Mar 12, 2002 11:44pm | #6

   *Like the others said, you are probably not grounded properly now. You have to get the conductors in contact with soil that is damp year round. So, as you are looking for a place to run horizontal ground, pick the lowest, wettest spot you can. I don't know your climate, but some suggestions are: put at least a portion of the conductors in the area near your downspouts (assuming you get rain in your area), run the conductors some distance from the house to a low & wet area, or use one of the ground-enhancing chemicals made especially for this purpose. These are poured in the trench when the conductor is installed.Grounding to the rebar is generally not done because the concrete is usually a poorer conductor than if you burried a long enough conductor in the dirt. Also, if you get a lightning strike you could shatter the concrete.It's worth the trouble to get a good ground. For equipment such as electronic surge suppression to work well the ground had to be sound.

3. 
   Jeff_Clarke_ | Mar 13, 2002 12:31am | #7

   *Grounding to the rebar (bonding the rebar to the building ground) is an NEC requirement in the new code.Jeff

4. 
   IanDGilham_ | Mar 13, 2002 01:05am | #8

   *I lived for a time in an area of shingle sub-soil, where an effective ground was very difficult and the code requirement there was for a buried 'mat' of copper wire.

5. 
   splintergroupie_ | Mar 13, 2002 01:23am | #9

   *http://www.pupman.com/listarchives/1995/july/msg00016.htmlSince you are building your house, you have an opportunity now to install a *Ufer ground*. Mr. Ufer developed this technique during WWII for army ammo bunkers. The NEC approves its use for commercial and home grounding systems. In essence, a Ufer ground is just rebar in concrete. When the builder is preparing to pour your slab, make sure all the rebar in the slab is bonded together, either cadwelded or mechanically clamped, before the pour. And make sure to leave a convienent attachment point exposed. A rule of thumb for a Ufer ground is that it takes about 20 feet of 0.5 inch rebar to absorb 8,000 amperes of surge. More is better. The rebar should be embedded in at least 4 inches of concrete.The way a Ufer ground works is through two paths. First it forms a large capacitance to Earth. This is an excellent RF coupling. Second, concrete's ions generally are more conductive than native soils, so you have a large number of virtual resistors in parallel connected to Earth that offer a lower resistance than would a smaller collection of driven rods. Earth is actually a lousy conductor. Most currents are dissipated through Earth by capacitive coupling and arcing from soil grain to soil grain. Concrete is a better conductor since the grains are tightly bound together.Ufer grounds and ground rods.

6. 
   CaseyR_ | Mar 13, 2002 01:26am | #10

   *Grounding to footing rebar is the preferred method of grounding in my county even though, as far as I know, they haven't adopted the new code yet. The rebar needs to be bare or zinc coated and at least 20 feet in length. Copper wire can also be encased in concrete if it is not smaller than No.4 and is at least 20 feet in length. An alternative grounding electrode can also be a "ground ring" of not less than No. 2 copper wire that surrounds the building in direct contact with the earth and is not less than 20 ft in length. Plate grounding electrodes are also described in the code as being at least 2 sq.ft. in direct contact with the earth and of a prescribed thickness depending upon the metal used. The device xJohn mentions is usually referred to as a "megger" (as in megohms). The resistance between two ground rods a set distance apart (20 meters?) is measured. An acceptable ground is not supposed to exceed 25 ohms. If the resistance is greater than 25ohms, then multiple grounding electrodes are to be used.There are other types of grounding systems including electrolytic ground rods in which an electrolyte is used which seeps into the ground to lower the ground resistance. I have no first hand experience with this and I would assume that these are only used in critical industrial situations. As my text book says: "So far, the resistance to ground of the grounding electrode at a residence has not been that big a jproblem. It may become one in the near future as the "Smart House" becomes more and more a reality..." Electrical Grounding, 5th Edition Ronald P. O'Riley, 1999, p55. Of course, I am not a licensed electrician, so discount all of this information accordingly...

7. 
   bungalow_jeff | Mar 13, 2002 06:42am | #11

   *Thanks for the replies. Since this is a 100 year old house, we can eliminate rebar in the foundation as an option. I think the copper ground pipe in a trench is my best option to explore. When we had the service upgraded to 200A the electricians complained when the inspector insisted on replacing the ground connection to the water line. I'm glad they sucked it up and made the connection. The idea popped in my head at work. I am designing a rail tunnel with a traction power substation over the track area. The power guys want to connect ground plates in the substation down to a wire mat under the box, connected to rods drilled to the rock. They were very adamant about grounding down to the rock, which is why I thought maybe a long rod drilled into the rock would work. Mainly, it made me realize that spring gardening can practically kick the rod out of the shallow soil.

8. 
   bake_ | Mar 13, 2002 07:44am | #12

   *We are allowed to use ground plates now, they are approx 2'x 2' galvanized plates you bury 3' deep.cheersbake

9. 
   bungalow_jeff | Mar 13, 2002 03:17pm | #13

   *That's another good option. I'll probably stick with the long pipe since I need to trench along the foundation to add drainage.

10. 
    George_W._Carpenter | Mar 13, 2002 05:18pm | #14

    *Jeff, if you can, go with two 10 ft. sections of copper pipe, bondyour ground wire to both with bronze "bugs". This should provide the 20 ft min. of horizontal conductor, and be relatively corrosion resistant.Avoid copper to steel/iron connections.-gwc

11. 
    bungalow_jeff | Mar 13, 2002 07:54pm | #15

    *Thanks GW,Now I look forward to reading the responses to the lightning rod post by David Lechel.

12. 
    Erich_Muschinske | Mar 14, 2002 08:05am | #16

    *My military electrical installation manuals talk about putting in ground systems on rocky mountaintops. The conditions are thin soil over bedrock. The usual way of getting a good ground for lightning protection is to lay out a ground grid around the building. As a previous post said, the object is to get less than 25 ohms, 10 ohms is the desired maximum. This is usually measured using a Vibraground, via the fall of potential method. An electrician that builds radio sites would probably have the equipment to do the measurement. Also your local phone or power utilities will also have the equipment.

13. 
    Steve_77 | Mar 15, 2002 02:28am | #17

    *Checkout Feb.94 , JLC. (Not a answer to the question , but useful )!

14. 
    4Lorn | Mar 15, 2002 08:31pm | #18

    *An electrical contractor from your local area, especially one that does commercial grounding, should be able to help you out. Grids are good but if your location is elevated and the soil doesn't hold moisture they can be a waste of money. There are chemicals that can be added around the rod or in a hollow, typically copper pipe, ground rod. These chemicals are selected to both absorb water and help with conduction much the way common salt , the original chemical used, would. Unfortunatly salt can be toxic and corrosive in the amounts used. Grounding to the rebar in a slab or footing does much the same thing as concrete tends to absorb and hold water even in relatively dry conditions. But even this doesn't work well in extremely dry locations. In difficult locations and where it is critical to have low grounding resistance. I have seen a well sunk for the purpose of grounding. Of course a steel pipe sunk several hundred feet to get to the ground water level, the mother of most good grounding, doesn't come cheap.If it is just a home your grounding the actual ground resistance might not be important. Lightning protection is largely a matter of relative resistances. The lightning want to go to ground and will take the easiest way. Making sure that your grounding system is substantially better grounded than the stuff you wish to preserve is more important than the actual ground resistance. Greatly simplified; if your metal roof is 40 ohms and the surface of the earth around your house is 30 ohms and your lightning rod and ground rod is 25 ohms the lightning will tend to strike the lightning rod. Talk to a local contractor. The one you want will be familiar with your soil conditions and should be able to give you sound advice and some options at a nominal price.