Separately Derived Systems
[Transformers, Generators, etc.] (4-10-2K)

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By Mike Holt, for Power Quality magazine.

The purpose and objective of bonding separately derived* systems is to insure that the electrical system is safe from electric shock and fires from ground-faults. In addition, separately derived systems are grounded to stabilize the phase-to-ground voltage during normal operation [250-2].

* A separately derived system derives its power from a transformer, generator, converter winding, batteries (UPS), or from a solar photovoltaic system. The wiring (including the grounded (neutral) conductor) of separately derived systems does not have any direct electrical connection to the supply conductors [Article 100 and 250-20(d)]. 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 either:

(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 3.

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 the building.

(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 [250-104(a)(4)].

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 this link.

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.