Basis for safety and power quality
Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.
During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.
Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.
Careful consideration should be given to installing a grounding system that exceeds the minimum NEC requirements for improved safety and power quality.
Recommended practices for grounding
- Equipment Grounding Conductors
- Isolated Grounding System
- Branch–Circuit Grounding
- Ground Resistance
- Ground Rods
- Ground Ring
- Grounding Electrode System
- Lightning Protection System
- Surge Protection Devices (SPD) (formerly called TVSS)
1. Equipment Grounding Conductors
The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.
Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.
The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.
2. Isolated Grounding System
As permitted by NEC 250.146(D) and NEC 408.40 Exception, consider installing an isolated grounding system to provide a clean signal reference for the proper operation of sensitive electronic equipment.
Isolated grounding system for branch circuits (photo credit: iaeimagazine.org)
Isolated grounding is a technique that attempts to reduce the chances of “noise” entering the sensitive equipment through the equipment grounding conductor. The grounding pin is not electrically connected to the device yoke, and, so, not connected to the metallic outlet box. It is therefore “isolated” from the green wire ground.
A separate conductor, green with a yellow stripe, is run to the panelboard with the rest of the circuit conductors, but it is usually not connected to the metallic enclosure. Instead it is insulated from the enclosure, and run all the way through to the ground bus of the service equipment or the ground connection of a separately derived system. Isolated grounding systems sometimes eliminate ground loop circulating currents.
Note that the NEC prefers the term isolated ground, while the IEEE prefers the term insulated ground.
3. Branch-Circuit Grounding
Replace branch circuits that do not contain an equipment ground with branch circuits with an equipment ground. Sensitive electronic equipment, such as computers and computer-controlled equipment, require the reference to ground provided by an equipment grounding conductor for proper operation and for protection from static electricity and power surges.
Failure to utilize an equipment grounding conductor may cause current flow through low-voltage control or communication circuits, which are susceptible to malfunction and damage, or the earth.
Surge Protection Devices (SPDs) must have connection to an equipment grounding conductor.
4. Ground Resistance
Measure the resistance of the grounding electrode system to ground.
Take reasonable measures to ensure that the resistance to ground is 25 ohms or less for typical loads. In many industrial cases, particularly where electronic loads are present, there are requirements which need values as low as 5 ohms or less many times as low as 1 ohm.
For these special cases, establish a maintenance program for sensitive electronic loads to measure ground resistance semi-annually, initially, using a ground resistance meter. Ground resistance should be measured at least annually thereafter.
When conducting these measurements, appropriate safety precautions should be taken to reduce the risk of electrical shock.
Record the results for future reference. Investigate significant changes in ground resistance measurements compared with historical data, and correct deficiencies with the grounding system. Consult an electrical design professional for recommendations to reduce ground resistance where required.
5. Ground Rods
The NEC permits ground rods to be spaced as little as 6 feet apart, but spheres-of-influence of the rods verlar.
Recommended practice is to space multiple ground rods a minimum of twice the length of the rod apart. Install deep-driven or chemically-enhanced ground rods in mountainous or rocky terrain, and where soil conditions are poor. Detailed design of grounding systems are beyond the scope of this document.
6. Ground Ring
In some cases, it may be advisable to install a copper ground ring, supplemented by driven ground rods, for new commercial and industrial construction in addition to metal water piping, structural building steel, and concrete-encased electrodes, as required by Code.
Grounding rings provide a convenient place to bond multiple electrodes of a grounding system, such as multiple Ufer grounds, lightning down-conductors, multiple vertical electrodes, etc.
Install ground rings completely around buildings and structures and below the frost line in a trench offset a few feet from the footprint of the building or structure. Where low, ground impedance is essential, supplement the ground ring with driven ground rods in a triplex configuration at each corner of the building or structure, and at the mid-point of each side.
The NEC-minimum conductor size for a ground ring is 2 AWG, but sizes as large as 500 kcmil are more frequently used. The larger the conductor and the longer the conductor, the more surface area is in contact with the earth, and the lower the resistance to earth.
7. Grounding Electrode System
Bond all grounding electrodes that are present, including metal underground water piping, structural building steel, concrete-encased electrodes, pipe and rod electrodes, plate electrodes, and the ground ring and all underground metal piping systems that cross the ground ring, to the grounding electrode system.
Bond the grounding electrodes of separate buildings in a campus environment together to create one grounding electrode system.
Bond all electrical systems, such as power, cable television, satellite television, and telephone systems, to the grounding electrode system. Bond outdoor metallic structures, such as antennas, radio towers, etc. to the grounding electrode system. Bond lightning protection down-conductors to the grounding electrode system.
8. Lightning Protection System
Copper lightning protection systems may be superior to other metals in both corrosion and maintenance factors. NFPA 780 (Standard for the Installation of Lightning Protection Systems) should be considered as a minimum design standard.
A lightning protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.
9. Surge Protection Devices (SPD) (formerly called TVSS)
The use of surge protection devices is highly recommended. Consult IEEE Standard 1100 (The Emerald Book) for design considerations. A surge protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.
Generally, a surge protection device should not be installed downstream from an uninterruptible power supply (UPS). Consult manufacturers’ guidelines.
Recommended Practices for Designing and Installing Copper Building Wire Systems – Copper Development Association Inc.