All questions and answers are based on the 2005 NEC.
Q. A new exception was added to 250.50 in the 2005 NEC, which states that concrete-encased electrodes are not required for existing buildings or structures where the conductive steel reinforcing bars aren't accessible without disturbing the concrete. Why was this exception added to the 2005 NEC, and what does this all mean?
A. Changes to 250.50 were made to clarify that where any of the following electrodes “are present” they must be bonded together to create the grounding electrode system.
Underground metal water pipe [250.52(A)(1)]
Metal frame of the building or structure [250.52(A)(2)]
Concrete-encased foundation or footer steel [250.52(A)(3)]
Ground ring [250.52(A)(4)]
Ground rod [250.52(A)(5)]
Grounding plate [250.52(A)(6)]
The intent of the change to 250.50 (wording changed from “if available” to “are present”) and the addition of the exception was meant to require the use of concrete-encased foundation or footer steel as part of the building or structure grounding electrode system in new construction (if they are present), since they are considered “present” before they pour concrete.
For example, if a new building contains rebar in the footing or foundation, it must be used as a grounding electrode. This electrode would serve as the supplement water pipe electrode that is required by 250.53(D)(2).
Note: Ground rods will no longer be necessary for most new construction, since virtually all new construction uses concrete encased reinforcing bars or rods in the foundation. Since they exist, they must be bonded to and used as part of the grounding electrode system. (Figure above)
Q. Must I bond the lightning protection system to the building grounding electrode system? This seems dangerous to me.
A. Where a lightning protection system is installed, it must be bonded to the building or structure grounding electrode system as per 250.106. In addition, the grounding electrode for the lightning protection system cannot be used for the building or structure grounding electrode [250.60].
Note: The bonding of the lightning protection system to the buildings or structures electrical system, via the grounding electrode system, is intended to prevent lightning “side flash” or arcing between metal parts. Minimizing the difference of potential between the lightning protection system and the electrical system reduces the likelihood of a fire.
See NFPA 780, Standard for the Installation of Lightning Protection Systems for additional details on grounding and bonding requirements for lightning protection.
Q. What is the correct method for conduit sealing of low-voltage and power-energy cables installed in a Class 1 hazardous (classified) location? My inspector wants the cable jacket removed within the sealing fitting and the conductors spread apart before the compound is poured. This is an almost impossible task.
A. Where a cable seal is required [501.15(A) and (B)], it must be installed in accordance with 501.15(C). However, the removal of shielding material or the separation of the twisted pairs isn't required within the cable seal fitting [501.15(D)(2) Ex. and 501.15(E)(1) Ex. 2].