Common grounding electrodes installed for alternating current outdoor substation grounding are concrete-encased electrodes, ground rings and ground rods. Section 250.191 requires a grounding electrode system in accordance with the applicable requirements for grounding electrodes contained in Part III of Article 250. The NEC falls short of providing many specific details and requirements for substation grounding.
IEEE 80, Guide for Safety in AC Substation Grounding, provides specific information about outdoor AC substation grounding. When dealing with substation grounding, typically, system and equipment grounding are usually accomplished. Outdoor AC substations typically include a variety of conductive parts and equipment that must be grounded.
One of the primary objectives in grounding of metal parts at an outdoor substation is to establish an equipotential grid to which all conductive parts are connected. This grid is installed underground outside of a fenced enclosure, typically about 3 feet from the fence and completely encircling the enclosure. There are usually several grounding electrodes, usually ground rods, driven and connected to this ring system. Any underground metal structures, such as piping and framing, should be bonded to this grid. The minimum size of the main grid conductor and any conductive connections from metal enclosures and structures within the fenced area and from the fence should be made using 4/0 AWG copper conductors at a minimum of, and no less than, 25% of the output conductors of the system.
There are different philosophies on fence grounding at outdoor substations. Most designs require the fence to be commonly connected to the grounding grid. Another approach is to isolate the fence from the grid system in the event of a phase-to-ground fault that elevates the potential of all connected metal parts. In this case, the potential of the fence is raised, which can present a shock hazard for people coming in contact with it during an abnormal event.
In many designs, the grid system for a substation is created by laying copper conductors in a checkerboard arrangement at 3-foot intersections to create a mat that is buried in the ground where the substation is constructed. This establishes a convenient point of connection for any conductive metal in the substation. The length of bonding jumper connections is reduced when this construction method is used. The connections for substation grounding grid systems have to be effective and strong, thus exothermic welding processes and irreversible compression connectors are often specified and used. These connectors are evaluated to endure the stresses of a rise and fall of potentials caused by events occurring in normal operation and abnormal events.
The grid system for outdoor substations provides the ability to dissipate lightning strikes effectively to minimize possibilities of damage. Section 250.194 provides specific requirements for grounding and bonding metal fences and structures enclosing or surrounding electrical substations. For multiple reasons, including local ordinances, safety, security and economics, metal fences are often installed around substations.
At the medium- and high-voltage levels, live parts are exposed in substations. Since fences are often accessible to the general public and other personnel, they must be grounded and bonded to limit the rise of potential. This set of provisions in the NEC establishes basic prescriptive requirements for grounding and bonding of metal fences built in and around electrical substations. For situations where step and touch potential considerations indicate that additional grounding and bonding design is required, alternate designs performed under engineering supervision are allowed. Although many substation fences are required by and typically covered by utility regulations and the National Electrical Safety Code, these provisions in Part X of Article 250 provide a basis for installations covered by the NEC .
Where metal fences are located within 16 feet of exposed electrical conductors or equipment, they must be bonded to the grounding electrode system with wire-type bonding jumper(s). Bonding jumpers are required at each fence corner and at maximum intervals of 160 feet along the fence. At substations where bare overhead conductors cross above the fence, bonding jumpers must be installed on each side of the crossing. Any fence gates are required to be bonded to the gate-support post, and each gate-support post must be bonded to the grounding grid and electrode system.
Any gates or openings in the fence must have a bonding connection installed across the opening by a buried bonding jumper. The grounding grid or grounding electrode systems must be located to cover the swing of all gates. Barbed wire strands above the fence also must be bonded to the grounding grid. Where subject to contact by people, all exposed conductive metal structures, including guy wires within 8 feet vertically or 16 feet horizontally of exposed conductors or equipment, must be bonded to the grounding grid and electrode system.
About The Author
JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].