In the 2002 NEC, Section 250.52(B) stated, “where available, the metal frame of the building or structure, where effectively grounded, must be used as a grounding electrode.” Determining whether the metal frame of the building or structure was effectively grounded was often difficult. The metal was effectively grounded if the structure's metal columns connected to bolts that were connected to the footing rebar within the concrete, and the concrete had contact with the earth directly. If the metal-support structure was connected to bolts inserted into the concrete, but not connected to rebar in the footings, then the metal structure was not effectively grounded, and could not be used as a grounding electrode.
In the 2005 NEC, Section 250.52(A)(2) was changed to provide four different methods of ensuring the metal frame of a building or structure could be used as an earth connection and, therefore, a grounding electrode. This change was meant to clarify what building or structure metal frames qualified as a grounding electrode.
The first method of ensuring earth connection to the metal structure is where 10 feet or more of a single structural metal member is in direct contact with the earth, or the structural metal is encased in concrete that is in direct contact with the earth. This appears to be the most effective of the four methods.
The second method is to bond the structural metal frame to one or more of the other grounding electrodes, as permitted by 250.52(A)(1) for metal underground water pipe, by Section 250.52(A)(3) for concrete-encased electrodes, or by Section 250.52(A)(4) for a ground ring.
The third and fourth methods involve bonding the structural metal frame to rod, pipe or plate electrodes, as defined in Sections 250.52(A)(5) or 250.52(A)(6), respectively, or to use any other approved means acceptable to the authority having jurisdiction for establishing a connection to earth.
The problem with establishing an effective connection to earth for the metal structure seems to occur in the second method provided in Section 250.52(A)(2). If the metal of the building is not connected to rebar within the concrete footing of the building or structure and the only other grounding electrode present at the building or structure is 10 feet or more of metal underground water pipe, then a bonding connection to the metal water pipe provides the sole connection to earth for the metal structure.
However, Section 250.53(D)(2) requires a metal underground water pipe to have a supplemental electrode installed and connected to the metal water pipe. Over a period of years, deterioration of the metal underground water pipe may occur. Since the metal water piping may be replaced with a nonmetallic water-piping system at some future time, the NEC requires the metal water pipe electrode be supplemented by an additional electrode of any of the types specified in Sections 250.52(A)(2) through 250.52(A)(7).
This is where the problem occurs within the 2005 NEC. This supplemental electrode for the underground metal water-piping system can be the metal frame of the building or structure as indicated in Section 250.52(A)(2). Since Section 250.52(A)(2)(2) permits the underground metal water-piping system to bond the building metal, if the metal underground pipe is replaced with nonmetallic water pipe, the entire grounding electrode system would be lost.
The easy answer would be to require an additional electrode to be installed, such as a driven ground rod, pipe or plate electrode where these electrodes have a resistance not in excess of 25 ohms. Where the ground rod, pipe or plate electrode exceeds 25 ohms to ground, then an additional electrode, as described in 250.52(A)(3) through 250.52(A)(7), must be installed but using building steel where connected to building water in accordance with 250.52(A)(2)(2) should not be permitted.
This may be an important enough issue to warrant a TIA (Tentative Interim Amendment) to the 2005 NEC. A TIA is a temporary fix for a major problem and thus would be an interim fix between the 2005 NEC and the 2008 NEC. At the very least, conscientious electricians and contractors should install supplemental electrodes to ensure the electrical system has proper connections to a grounding electrode. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at firstname.lastname@example.org.