Electric Power Generation, Transmission, and Distribution eTool
The placement of protective ground leads will be affected by factors such as work site conditions, type of construction, and the nature of the work to be done. The protective grounding system, which includes conductor grounds and worker bonding, must be engineered to protect workers from hazardous voltages that can be created by line reenergizing, lightning, or induced oltage.
If more than one crew is working independently on the same deenergized line or circuit, each crew must have protective grounds installed to create an equipotential zone at each work location. The types of grounding methods used are described below.
Single-point grounding is the preferred method because it generally yields the lowest potential difference in the work zone and because it usually requires less grounding equipment and effort to install. Single-point grounding involves installing one set of grounds at the work location between the conductors, the system neutral (if one exists), and the ground (which may be a tower or pole ground) to create a bonded work zone that will remain at a near identical state of electrical potential—hence the term "equipotential zone."
An alternative to single-point grounding, bracket grounding, involves installing grounds at two locations, one on each side of the work location, typically some distance apart and away from the work location. With bracket grounding, grounds are installed within a section or two from the work site (for example, at the next pole or tower), allowing workers to be "working between grounds" or "bracketed by grounds." With improperly installed bracket grounding, it is possible that the potential across a worker working within the bracket could rise to a hazardous voltage level at the work location if the line becomes energized. For bracket grounding to be effective, detailed engineering analysis is necessary. The analysis should consider circuit and system design parameters and other engineering factors, including the existence of static lines or overhead ground wires, circuit impedances (including the resistance to ground at poles and towers and of any grounding grid), and the potential exposure of crew members to step and touch potentials under fault conditions.
The decision to use bracket grounding or single-point grounding should be based on an evaluation of the potential hazards to the workers, work site conditions, type of construction, and the nature of the work to be done.