Ground plate in lieu of ground rod electrode

[American Lightning]

I have a customer with a situation regarding grounding an electrical service for a project. Said project is the renovation of a 19-story bldg, renovation of a 6-story bldg and constructing a new 19-story bldg, all located on the same city block. Each bldg will have an NFPA-780 (Lightning Protection Code) grounding system installed, composed of a 3/0 ground loop with ground plates, and tied to the electrical service ground. We have utilized ground plates because the site contains a layer of impenetrable blue shale rock 24 inches below grade.

The project electrical engineer has designed a (separate) grounding scheme for the electrical service using 4 - 3/4" X 10' ground rods spaced 10 feet apart and interconnected with a 4/0 bare conductor. When discussing the rock issue with the EC, I mentioned that the NEC also permits the use of ground plates in lieu of the ground rods (See 250.52(A)(6) and 250.53(H).) similarly to NFPA-780, and that they should consider doing the same.

NFPA-780, Appendix L, paragraphs L3.4 - L3.7 discusses the rationale for using plates, i.e., highly resistive soil (sand, for instance) lowers the effectiveness of transferring energy to earth. And in the case of rock, "it would be impossible to make a ground connection in the ordinary sense of the term because most kinds of rock are insulating." In summation, NFPA's assessment is that "the more extensive the underground metal available, the more effective the protection" and apparently is why the use of plates is advisable.

Apparently there are some in the electrical field who feel the use of plates in this situation is ineffective even though it is allowed. What I fail to understand is why, considering that the NEC permits their use. Is there separate design/engineering criteria established for using plates in electrical service grounding as opposed to that described in NFPA-780, i.e., high voltage/amperage (lightning) transference as opposed to (electrical) low voltage? If so, I would appreciate some clarification.

 

[charlie tuna]

actually what we have found out that the typical ground rod set up per the code is useless in most cases--- i took two ten foot ground rods ten feet apart and applied a 120 volt power source and it conducted less than one amp. but like mike holt say's we gotta keep these ground rod manufacturers in business............

 

[American Lightning]

Yes sir, we have found the same. Most grounds installed are poor at best, especially in our area which is both arid and rocky. Thanks for your reply.

 

[bphgravity]

There is good evidence that a grounding electrode system of any ground resistance or surge impedance does very little, even if that resistance is very, very low. In fact, the lower the ground resistance of the GES, the more normally operating current will flow through the metal parts of the service equipment and to the earth and back to the source.

I personally feel too much thought and effort is placed on the specific details of the grounding electrode system. The most minimal electrode system will likely suite the needs of nearly ever occupancy type in nearly every region of the nation.

If lightning is of great concern, then a LPS with proper surge protection equipment is by far more effective than any grounding of the electrical system can ever be. In your particular situation, I doubt there will be any discernible difference in power and grounding quality regardless of the electrode type used, rod or plate.