Grounded or Ungrounded Systems

DOE-HDBK-1092-98

4.5 GROUNDED OR UNGROUNDED SYSTEMS

Ungrounded systems may provide greater continuity of operations in the event of a ground 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 ground on an ungrounded system.

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

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

enclosures from dangerous voltages and personnel from electrical shock. See NEC Sections 110-9,

110-10, 230-65, 250-1, and 250-2 that list the requirements to provide this protection.

"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 Section 250-5(b) FPN to sound an alarm or send a message

to alert personnel that a ground 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.

4.5.1 GROUNDED SYSTEMS

Grounded systems are equipped with a grounded conductor that is required per NEC Section 250-

23(b) to be run to each service disconnecting means. The grounded conductor can be used as a

current-carrying conductor to accommodate all neutral related loads. It can also be used as an

equipment grounding conductor to clear ground faults per NEC Section 250-61(a).

A network of equipment grounding conductors is routed from the service equipment enclosure to all

metal enclosures throughout the electrical system. The equipment grounding conductor carries fault

currents from the point of the fault to the grounded bus in the service equipment where it is transferred

to the grounded conductor. The grounded conductor carries the fault current back to the source and

returns over the faulted phase and trips open the overcurrent protection device.

Note: A system is considered grounded if the supplying source such as a transformer, generator, etc.,

is grounded, in addition to the grounding means on the supply side of the service equipment

disconnecting device per NEC Sections 250-23(a) or 250-26 for seperately derived systems.

The neutral of any grounded system serves two main purposes: (1) it permits the utilization of line-

to-neutral voltage and thus will serve as a current-carrying conductor to carry any unbalanced

current, and (2) it plays a vital role in providing a low-impedance path for the flow of fault currents

to facilitate the operation of the overcurrent devices in the circuit. (See Figure 4-2.) Consideration

should be given to the sizing of the neutral conductor for certain loads due to the presence of harmonic

currents (See NEC Sections 210-4 and 310-10).

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