All questions and answers are based on the 2017 NEC.
Underlined text indicates a Code change.
Q. What is the reason stated in the NEC for connecting electrical systems to the earth (grounding)?
A. Electrical power systems are grounded (connected to the earth) to limit the voltage induced by lightning, line surges, or unintentional contact by higher‑voltage lines and stabilize the system voltage to earth under normal operation [Sec. 250.4(A)(1)], as shown in the Figure.
System grounding helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components.
Note 1: To limit imposed voltage, the grounding electrode conductors shouldn’t be any longer than necessary, and unnecessary bends and loops should be avoided.
Note 2: See NFPA 780, Standard for the Installation of Lightning Protection Systems, for grounding and bonding of lightning protection systems.
Q. Which alternating-current systems are required to be grounded by the NEC?
A. Systems operating below 50V aren’t required to be grounded or bonded in accordance with Sec. 250.30 unless the transformer’s primary supply is from [Sec. 250.20(A)]:
(1) A 277V or 480V system.
(2) An ungrounded system.
The following 50V to 1,000V systems must be grounded (connected to the earth) [Sec. 250.20(B)]:
(1) Single-phase systems where the neutral conductor is used as a circuit conductor.
(2) Three-phase, wye-connected systems where the neutral conductor is used as a circuit conductor.
(3) Three-phase, high-leg delta-connected systems where the neutral conductor is used as a circuit conductor.
Q. What are the Code permitted methods for the termination of the grounding electrode conductor (GEC) for a grounded system?
A. Service equipment supplied from a grounded system must have the GEC terminate in accordance with Sec. 250.24(A).
Grounding Location. A GEC must connect the service neutral conductor to the grounding electrode at any accessible location, from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral, up to and including the service disconnect [Sec. 250.24(A)(1)]. Some inspectors require the service neutral conductor to be grounded (connected to the earth) from the meter socket enclosure, while other inspectors insist that it be grounded (connected to the earth) only from the service disconnect. Grounding at either location complies with this rule.
Grounding Termination. When the service neutral conductor is connected to the service disconnect [Sec. 250.24(B)] by a wire or bus bar [Sec. 250.28], the GEC can terminate to either the neutral terminal or the equipment grounding terminal within the service disconnect [Sec. 250.24(C)(4)].
Neutral‑to‑Case Connection. A neutral‑to‑case connection isn’t permitted on the load side of service equipment, except as permitted by Sec. 250.142(B) [Sec. 250.24(A)(5)]. If a neutral‑to‑case connection is made on the load side of service equipment, dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment [Sec. 250.6(A)]. Objectionable neutral current on metal parts of electrical equipment can cause electric shock and even death from ventricular fibrillation, as well as a fire.
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