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DOE-HDBK-1092-2004
4.5.2
UNGROUNDED SYSTEMS
Ungrounded systems operate without a grounded conductor. In other words, none of the circuit
conductors of the electrical system are intentionally grounded to an earth ground such as a
metal water pipe, or building steel. The same network of equipment grounding conductors is
provided for ungrounded systems as for solidly grounded electrical systems. However,
equipment grounding conductors (EGCs) are used only to locate phase-to-ground faults and
sound some type of alarm. Therefore, a single sustained line-to-ground fault does not result in
an automatic trip of the overcurrent protection device. This is a major benefit if electrical system
reliability is required or if it would result in the shutdown of a continuous process. However, if an
accidental ground fault occurs and is allowed to flow for a substantial time, overvoltages can
develop in the associated phase conductors. Such an overvoltage situation can lead to
conductor insulation damage, and while a ground fault remains on one phase of an ungrounded
system, personnel contacting one of the other phases and ground are subjected to 1.732 times
the voltage they would experience on a solidly neutral grounded system. (See Figure 4-3.)
Figure 4-3. An ungrounded system does not have a grounded (neutral) conductor
routed between the supply transformer and the service equipment because the supply
transformer is not earth grounded.
Note:  All ungrounded systems should be equipped with ground detectors and proper
maintenance applied to avoid, to the extent practical, the overcurrent of a sustained ground fault
on ungrounded systems. If appropriate maintenance is not provided for ungrounded systems, a
grounded system should be installed to ensure that ground faults will be cleared and circuits,
equipment, and personnel are safe.
4.5.3
HIGH-IMPEDANCE GROUNDING
Electrical systems containing three-phase, three-wire loads, as compared to grounded neutral
circuit conductor loads, can be equipped with a high-impedance grounded system. High-
impedance grounded systems shall not be used unless they are provided with ground fault
4-5


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