In an article in EC&M written by Mike, he poses a question and answer but I don?t understand the answer even after reading the actual answer. Can someone explain:
Q. When a building has more than one service (as allowed by Sec. 230.2), is there a requirement to ground these multiple services to a common grounding electrode? (Answer based on 2011 NEC.)
A. Where an AC system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58]. Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.
Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn?t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.
Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.
So if a building has two services and each service enters their own switchgear, do both switchgears need to be listed as service equipment with both having connections to a common grounding electrode or no?
Q. When a building has more than one service (as allowed by Sec. 230.2), is there a requirement to ground these multiple services to a common grounding electrode? (Answer based on 2011 NEC.)
A. Where an AC system is connected to a grounding electrode in or at a building or structure, the same grounding electrode must be used. If separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58]. Two or more grounding electrodes that are bonded together will be considered as a single grounding electrode system in this sense.
Metal parts of the electrical installation are grounded (connected to the earth) to reduce induced voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building/structure. Grounding electrical equipment doesn?t serve the purpose of providing a low-impedance fault current path to open the circuit overcurrent device in the event of a ground fault.
Caution: Potentially dangerous objectionable neutral current flows on the metal parts when multiple service disconnecting means are connected to the same electrode. This is because neutral current from each service can return to the electric utility via the common grounding electrode and its conductors. This is especially a problem if a service neutral conductor is opened.
So if a building has two services and each service enters their own switchgear, do both switchgears need to be listed as service equipment with both having connections to a common grounding electrode or no?