Grounded or Ungrounded Systems

DOE-HDBK-1092-2004

NEC 250.4

Figure 4-1. Circuit and system grounding consists of earth grounding the electrical

system at the supply transformer and the line side of the service equipment. Equipment

grounding and bonding is accomplished by connecting all metal enclosures and

raceways together with the grounding conductors.

Electrical systems can be operated grounded or ungrounded, depending on the condition of

their use. Electrical systems are grounded to protect circuits, equipment, and conductor

enclosures from dangerous voltages and personnel from electrical shock.

4.5

GROUNDED OR UNGROUNDED SYSTEMS

Ungrounded systems may provide greater continuity of operations in the event of a fault.

However, the second fault will most likely be more catastrophic than a grounded system fault.

Whenever ungrounded systems are used in a facility, the maintenance personnel should

receive training in how to detect and troubleshoot the first fault on an ungrounded system.

"Grounded" means that the connection to ground between the service panel and earth has been

made. Ungrounded electrical systems are used where the designer does not want the

overcurrent protection device to clear in the event of a ground fault.

Ground detectors can be installed per NEC to sound an alarm or send a message to alert

personnel that a first fault has occurred on one of the phase conductors. Ground detectors will

detect the presence of leakage current or developing fault current conditions while the system is

still energized and operating. By warning of the need to take corrective action before a problem

occurs, safe conditions can usually be maintained until an orderly shutdown is implemented.

4-3