Ungrounded Systems

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

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