![]() There are empirical equations for calculating the resistance for both direct buried and concrete encased earthing rods. Where there is adequate real estate it may be a better option simply to increase the number of directly buried rods while ensuring adequate horizontal separations (separate them by at least their driven length – read our article on the Fundamentals of Earthing Design) to reduce the proximity effect between multiple buried rods. However, it is a costly measure and should be undertaken with some measure of caution. ![]() Therefore concrete encasing of earthing rods can be used to reduce their ground resistance and improve overall earthing system performance. ![]() It has been measured that wet concrete resistivity is almost uniform throughout whereas regular concrete has an equivalent two layer model consists of a bottom layer having a resistivity of 1244 Ohm.m with 195 Ohm.m for the top layer (interestingly the resistivity of concrete increases with depth/thickness).īecause of the relatively low resistivity of concrete it is known that metallic earthing rods which are encased inside concrete will have a lower resistance to ground at both power frequency and at lightning impulse levels than the same rod which is directly buried into the earth. Concrete has a typical electrical resistivity of between 30 Ohm.m to 200 Ohm.m. It is only required when there is steel and the footer has no vapor barrier and is called out for on the plans.Concrete is hygroscopic which means it absorbs water and when concrete is buried, depending on the moisture levels at the time of the surrounding soil, it behaves as a semiconducting medium. If the building plans don’t call for steel in the footer we cannot make them put it in. We just need to train the General Contractors and the Electrical Contractors that if this steel if available it is required to be part of our Grounding Electrode System. Maybe next time they will remember to connect the steel and call for an inspection. So when someone calls me in the future and they ask what to do, my solution will be to get them to dig a trench and lay a 20’ piece of 1/2” rebar and make a connection to it and then encase it in at least 2” on concrete. You could cause cracks in the concrete by hitting and vibrating the steel rebar when chipping into the footer. At that point I feel that the steel is not available and although you may want to chip into a footer to find a piece of steel rebar how do you know if it is 20’ long, and is not above a vapor barrier? I have spent time trying to come up with a solution, because chipping into a slab is probably not the best answer. I get calls all the time about what to do when they miss the steel in the footer. In section 250.50 they talk about the Electrode being present. We are creating another electrode for our Grounding System. and the third and probably the most important is that it cannot be isolated from the Earth by a vapor barrier. The second is that it has to be at least a 1/2” in diameter. The first is that the rod be at least 20’ in length, and it does not have to be one 20’ piece, it can be several shorter pieces connected together to make a 20’ piece. There are three requirements to make a good concrete–encased electrode. What will require it to be made into a concrete-encased electrode. Footer steel, when is it required to be made part of the Grounding Electrode System? We will have to check with the building plans examiner or the building inspector and see if there is going to be steel in the footer and if it will need to be made into a Grounding Electrode? This weeks column is not about a Code change it is about a common problem we all face on a regular basis.
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