and remains de-energized until the work is completed. The same precautions apply to new lines when construction has progressed to the point where they can be energized from any source.
The installation of protective grounds and short- circuiting leads at the work site protects against the hazards of static charges on the line, induced voltage, and accidental energizing of the line.
When a de-energized line and an energized line parallel each other, the de-energized line may pick up a static charge from the energized line because of proximity of the lines. The amount of this static voltage "picked up" on the de-energized line depends on the length of the parallel, weather conditions, and many other variable factors. However, it could be hazardous; and precautions must be taken to protect against it by grounding the line at the location where the work is to be completed. Grounding will drain any static voltage and protect the workman from this potential hazard.
When a de-energized line parallels an energized line-carrying load. the de-energized line may have a voltage induced on it in the same manner as the secondary of a transformer. If the de-energized line is grounded at a location remote from where the work is being done, this induced voltage will be present at the work location. Grounding the line at the work location will eliminate danger from induced voltage.
Grounding and short circuiting protect against the hazard of the line becoming energized from either accidental closing ofthe line or accidental contact with an energized line that crosses or is adjacent to the de- energized line.
The procedures established to control the operation of equipment in an electrical system practically prevent the accidental energizing of a transmission or distribution line. Hold-off tagging procedures have proven to be very effective. If a circuit should be inadvertently energized, the grounds and short circuits on the line will cause the protective relays to initiate tripping of the circuit breaker at the source end of the energized line in a fraction of a second and de-energize the hot line. During this short interval of time, the grounds and short circuits on the line being worked on will protect the workmen (fig. 4-38). If it is not grounded, it is not dead!
Figure 4-38. - Grounding cluster installation.
Electric distribution circuits have been installed underground for many years. The conventional underground systems employ the use of some, if not all, of the following: conduits encased in concrete, manholes, ducts and trenches, direct burial cable and riser/potheads, underground power cables, and underground communication cables. After it has been determined that the load density is high enough to justify the expenses associated with an underground system, the system must be designed; and then construction may begin.
Manholes, handholes, and vaults will be designed to sustain all expected loads that may be imposed on the structure. The horizontal or vertical design loads will consist of dead load, live load, equipment load, impact, load due to water table or frost, and any other load expected to be imposed on or occur adjacent to theContinue Reading