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18 key terms defined in NEC system grounding requirements

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18 key terms defined in NEC system grounding requirements
18 key terms defined in NEC system grounding requirements (on photo: Rack Ground Bus bar – RGB located within a cabinet or rack is properly bonded back to the Master Ground Bus bar – MGB; via r56audits.com)

System Grounding Arrangement

The topic of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements.

The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories:

  1. Utility Service – The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer.
  2. Generator – The system grounding is determined by the stator winding configuration.
  3. Transformer – The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration.
  4. Static Power Converter – For devices such as rectifiers and inverters, the system grounding is determined by the grounding of the output stage of the converter.
Categories 1 to 4 fall under the NEC definition for a “separately-derived system”. The recognition of a separately-derived system is important when applying NEC requirements to system grounding. The National Electrical Code does place constraints on system grounding.

As a starting point, 18 key terms from the NEC need to be defined:


1. Ground

A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some body that serves in place of the earth.

Ground definition
Ground definition (photo credit: ibiblio.org)

2. Grounded

Connected to earth or to some body that serves in place of the earth.


3. Effectively Grounded

Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.


4. Grounded Conductor

A system or circuit conductor that is intentionally grounded.

Grounding simple scheme
Grounding simple scheme (photo credit: diy.stackexchange.com)

5. Solidly Grounded

Connected to ground without inserting any resistor or impedance device.


6. Grounding Conductor

A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.


7. Equipment Grounding Conductor

The conductor used to connect the non-current-carrying metal parts of equipment, raceways and other enclosures to the system grounded conductor, grounding electrode conductor, or both, at the service equipment or at the source of a separately-derived system.

Equipment grounding conductor
Equipment grounding conductor

8. Main Bonding Jumper

The connection between the grounded circuit conductor and the equipment grounding conductor at the service.

Service equipment enclosures are connected to the neutral by a main bonding jumper
Service equipment enclosures are connected to the neutral by a main bonding jumper (photo credit: jade1.com)

9. System Bonding Jumper

The connection between the grounded circuit conductor and the equipment grounding conductor at a separately-derived system.


10. Grounding Electrode

The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system.


11. Grounding Electrode Conductor

The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system.

Earthing electrode
Earthing electrode

12. Ground Fault

An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.


13. Ground Fault Current Path

An electrically conductive path from the point of a ground fault on a wiring system through normally non–current-carrying conductors, equipment, or the earth to the electrical supply source.


14. Effective Ground-Fault Current Path

An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high-impedance grounded systems.


15. Ground-Fault Circuit Interrupter

GFCI - Ground fault circuit interrupter
GFCI – Ground fault circuit interrupter (photo credit: education.nachi.org)

A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device.


16. FPN

Class A ground-fault circuit interrupters trip when the current to ground has a value in the range of 4 mA to 6 mA. More information can be found on UL 943, Standard for Ground-Fault Circuit Interrupters.


17. Ground Fault Protection of Equipment

A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit.

This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.

18. Qualified Person

One who has the skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.

With these terms defined, several of the major components of the grounding system can be illustrated by Figure 1 and labeling the components.

NEC system grounding terms illustration
Figure 1 – NEC system grounding terms illustration

Reference: System Grounding – Bill Brown, P.E., Square D Engineering Services

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on