Where an effectively grounded metal structural member of a building or an effectively grounded metal water pipe within 5 feet (1.5 meters) of the building's entrance point are available, Section 250-50 of the NEC requires that they be bonded together to become the grounding electrode system. Where these aren't available, some other electrode specified in Sections 250-50 and 250-52 must be used.
A 6-inch sprinkler main would be a good electrode, but Section 8-3.5 of NFPA 24 prohibits using it as such. Sprinkler systems with a dielectric isolator in the main sprinkler supply wouldn't satisfy the requirements for a grounding electrode, either, although the AEC doesn't prohibit bonding the interior piping to the electrical system. Section 250-104(C) of the NEC requires that piping that may become energized be bonded to the service equipment or grounding electrode system.
In electric fire pump installations, the equipment grounding conductor is a wire or metal raceway. The metal sprinkler piping is connected to the pump's metal case, which is bolted to the electric motor so that the sprinkler piping is in electrical contact with the grounding system. Installing a bonding jumper between the electrical grounding system and the sprinkler piping only allows the connection to decrease the voltage differential under a ground-fault condition, making it safer.
The intentional bonding of all the utilities in a building creates an equipotential ground plane that minimizes the voltage differential between the different systems under both normal and abnormal operating conditions. The result is an environment safer from the hazards of electrocution and fire.
