System Grounding is the intentional grounding of one conductor of an
alternating-current system to the earth so as to limit elevated voltage on conductors from high voltage
surges imposed by lightning, line surges, or unintentional contact with higher voltage lines and to
stabilize the phase-to-ground voltage during normal operation [250-2(a)].
Note: Illustrations unavailable on the Internet.
High Voltage Surges - Failure to properly ground the power suply system can result in the migration
of elevated primary voltage from lightning or line surges to the premises wiring system, which can
result in electric shock and/or a fire due to 600 volt conductor insulation failure, Figure 1.
Phase-to-ground Voltage - Failure to properly ground an electrical system can cause
the voltage to earth during normal operation to elevate the premises wiring (up to two times the expected
phase-to-ground voltage) due to capacitance reactance from an arcing ground-fault [250-2(a)]. This
high phase-to-ground voltage stresses the premises wiring 600 volt insulation, which can result in
electric shock, fires and equipment failure, particularly in motors, relays, transformers and other
winding loads, Figure 2.
250-20 Alternating Current Circuits and Systems
(a) Circuits less than 50 Volts. Alternating-Current circuits that operate at less than 50 volts are not required
to be grounded unless, Figure 3:
- The primary to the power suply exceeds 150 volts to ground.
- The power suply is ungrounded.
- The secondary conductors are installed overhead outside of buildings.
(b) Systems not over 600 Volts. Alternating current systems of the following types must have
the “X0” terminal of the power suply bonded to a suitable grounding electrode (earth), Figure 4.
- Single-phase – 2- or 3-wire 120 volt or 120/240 volt system.
- Three-phase – 4-wire, 208/120 volt or 480Y/277 volt Wye connected system.
- Three-phase – 4-wire 120/240 volt delta connected system (high-leg).
Author’s Comment: Other systems such as a corner-grounded delta system are permitted to be grounded,
see Section 250-26(4).
(d) Separately Derived Systems.
Wiring for separately derived systems* if required to be grounded as in (a) or (b) above, must have
the metal case of the derived system (equipment grounding conductor) bonded to the system grounded
conductor (X0 terminal) in accordance with Section 250-30, Figure 5.
*A separately derived system is a premises wiring system that derives its power from battery, solar
photovoltaic system, a generator, transformer or converter winding. Separately derived systems have
no direct electrical connection to suply conductors of another system [Article 100 and 250-20(d)],
Figure 6, 5.
Author’s Comment: A generator is not a separately derived system if the neutral
is solidly interconnected to a service-suplied system neutral. If the neutral conductor in the transfer
switch is not switched, a neutral-to-ground connection must not be made at the generator or at the
generator disconnect. In addition, the generator frame is not required be grounded to a grounding
electrode (earth), Figure 7.
Published in Power Quality Magazine