Figure 4-2. A grounded system is equipped with a grounded (neutral) conductor routed between the supply transformer and the service equipment

DOE-HDBK-1092-98

NEC Section 250-50 (a)

OSHA Section 29 CFR 1910.304 (f) (3)

Figure 4-2. A grounded system is equipped with a grounded (neutral) conductor routed between the supply

transformer and the service equipment.

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, building steel, etc. 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 continuity 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

4-5