Where an ac system is connected to a grounding electrode in or at a building or structure, the same electrode shall be used to ground conductor enclosures and equipment in or on that building or structure. Where separate services, feeders, or branch circuits supply a building and are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used and the requirement of (1) is met.
(1) An audible or visible alarm shall be installed at each service to indicate the grounded conductor brought to the service has opened.
Exception No. 1: In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure that only qualified persons service the equipment.
Exception No. 2: Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision.
Substantiation: Upon the opening of the grounded (neutral) conductor at one service, potentially dangerous current will flow between the common electrode to both services. Any person who comes in contact with exposed metal parts at the service equipment or grounding electrode system could be exposed to lethal current.