Transformers – All transformers (except autotransformers) are considered
a separately derived system because the primary supply does not have any direct electrical connection
to the secondary.
Other Systems - Generator, converter winding, UPS systems, or power from solar photovoltaic system
are only considered to be a separately derived system, when the grounded (neutral) conductor in the
transfer switch is switched, Figure 1.
Note: Graphics not provided on the internet.
Bonding Provides Low Impedance Path to Clear Phase-to-Ground Faults – Section 250-30(a)(1)
The low impedance path necessary to clear a phase-to-ground fault for separately
derived system is created when the metal parts of the separately derived system (equipment grounding
conductor) is bonded to the system grounded conductor. This neutral-to-ground bond must be made at
(1) The source of a separately derived system or
(2) At the separately derived system disconnecting means, Figure 2.
DANGER: Failure to provide a low impedance ground-fault path (no neutral-to-ground
bond) for the separately derived system can create a condition where a phase-to-ground fault cannot
be removed. The result is that all metal parts of the electrical system, as well as the building structure
will remain energized with dangerous line voltage if a phase-to-ground fault occurs, Figure
CAUTION: The neutral-to-ground connection at the secondary of a transformer cannot be made at more
than one location. To do so would create multiple neutral-to-ground connections, which produces a
condition where neutral current has multiple return paths to the grounded (neutral) conductor of power
supply. This condition (multiple neutral return paths) can create a fire and/or shock hazard, as well
as violating NEC Sections 250-6 and 250-142(a).
Author’s Comment: Multiple neutral-to-ground connections can also cause power quality problems
from elevated ground voltage and electromagnetic interference from net current; this was covered in
previous issues of PQ magazine, Figure 4.
Grounding Stabilize System Phase-to-Ground Voltage –Section 250-30(a)(2)
A grounding electrode conductor must connect the system grounded (neutral) conductor to a suitable
grounding electrode. The termination of the grounding electrode conductor must be at the same point
on the separately derived system where the neutral-to-ground connection is made as required in Section
250-30(a)(1), Figure 5.
Section 250-30(a)(3) identifies that grounding electrode conductor must terminate to a grounding electrode
that is located as close as practicable, and preferably in the same area of the neutral-to-ground
termination of the derived system. The grounding electrode shall be the nearest one of the following:
(a) Effectively grounded metal member of the building structure.
(b) Effectively grounded metal water pipe, within 5 feet from the point of entrance into
(c) Where the above two electrodes are not available, then any of the following
can be used:
- Concrete-encased electrode [250-50(c)]
- No. 2 ground ring [250-50(d)]
- Made electrode, ground rod [250-52]
FPN: Interior metal water piping in the area served by a separately
derived system must be bonded to the grounded (neutral) conductor at the separately derived system
DANGER: A separately derived system must
have the system grounded to the earth to stabilize the phase-to-ground voltage under normal operation.
According to the IEEE Std. 242-1986 (Buff Book), “if a ground fault is intermittent or allowed to
continue, the [ungrounded] system could be subjected to possible severe overvoltage to ground, which
can be as high as six or eight times phase voltage. This can puncture insulation and result in additional
ground faults. These overvoltage are caused by repetitive charging of the system capacitance, or by
resonance between the system capacitance and the inductances of equipment in the system."
The IEEE Green Book also states that “field experience and theoretical studies have shown that arcing,
restriking, or vibrating ground faults on ungrounded systems can, under certain conditions, produce
surge voltages as high as six times normal. Neutral grounding is effective in reducing transient voltage
buildup from such intermittent ground faults by reducing neutral displacement from ground potential
and reducing destructive effectiveness of any high-frequency voltage oscillations following each arc
initiation or restrike,” Figure 6.
Author’s Comment: Elevated
phase-to-ground voltage of an improperly grounded Wye system is beyond the scope of this column, but
if you want more information on this subject go to http://www.mikeholt.com/Newsletters/highvolt.htm.
CAUTION: Systems (generators, UPS systems, etc.) where a grounded (neutral) conductors
is not switched within a transfer switch is not considered a separately derived system. Under this
condition, a neutral-to-ground connection shall not be made at the new system, and a grounding electrode
is not required, Figure 7.