Working Under a 12.47 kV Line
A crew is removing a failed culvert with a track excavator on a county road. A 12.47 kV distribution line crosses the work area. At the pole, the conductor is 22 ft above grade; at midspan over the excavation, sag brings it to 18 ft 6 in. The excavator boom in the travel position reaches 11 ft 6 in above grade, and the operator plans to raise the bucket to dump into a truck parked on the shoulder. The road stays open with a flagger. The track path runs 4 ft from the edge of a 5 ft deep excavation left by removing the old headwall. A 4-inch gas service is marked in the shoulder.
1. Can this machine work under this line? Show the numbers, and identify what changes the answer.
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Clearance requirement For lines up to 50 kV, the minimum approach distance is 10 ft for the equipment (plus the load, plus any part of the machine at any point in its arc).
Available room at the point of work 18.5 ft (midspan) − 10 ft = 8.5 ft of allowable equipment height.
The boom in travel is already 11 ft 6 in - 3 ft over the allowable envelope, and that is with nothing raised. Swinging a loaded bucket to a truck raises the boom and puts the machine, the bucket, and the truck bed all closer to the conductor.
Conclusion No. Not with the line energized. The pole height of 22 ft is irrelevant; the conductor is lowest at midspan over the exact spot the machine must work.
What changes the answer
- Utility de-energizes and visibly grounds the line, or installs insulating cover/hold-offs (utility's call, not the contractor's).
- Reroute the work - approach the culvert from outside the midspan.
- Substitute a lower-profile machine, which does not solve the dumping arc.
Also: measure or have the utility verify clearance at the work point. Sag changes with temperature and load, so a cool morning measurement is not good for a hot afternoon.
2. Who should be watching what, and what are the signal and traffic-control requirements?
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Two different jobs The flagger controls the public. That person is watching traffic and cannot also be the spotter. A dedicated spotter is required whose only job is the line, the swing, and the edge - one person cannot track cars and a bucket at the same time.
Signal rules
- Agree on hand signals before the lift and confirm the operator can see the signal person at all times.
- The operator stops immediately if visual contact is lost, or if anyone gives a stop signal. Anyone on the crew can stop the job; only the signal person can direct a move.
- Hand signals only - no shouting over the engine, no radio unless it is dedicated and tested.
Traffic control The work zone must keep the public out of the swing radius, not just out of the trench. The truck should be positioned so the operator is not swinging over live traffic.
Gas service The marked 4-inch service in the shoulder is inside the area the tracks may cross. It should be potholed and flagged, and the outriggers/tracks kept off it.
3. The operator wants to track within 4 ft of a 5 ft deep excavation edge. Evaluate that and state the control.
Read the worked answer
What the rule covers Spoil and equipment must be kept at least 2 ft from the edge. But 4 ft satisfies the 2 ft rule and is still wrong here - the 2 ft figure is a minimum for a spoil pile, not a design load for a tracked excavator of this weight.
The real issue The machine is a heavy concentrated surcharge on a 5 ft wall in fill that has already been disturbed by removing a culvert headwall. The 4 ft setback is less than the depth of the cut. In this situation the wall has no surcharge capacity to spare, and the fill next to an old headwall is the least reliable ground on the site.
Control
- Competent person inspects before the machine goes anywhere near the edge.
- Move the track path back so the setback is at least equal to the depth (5 ft), and farther if the fill is loose or wet.
- If the machine must work close, install a shield or shoring designed for the surcharge, or approach from the opposite side.
- Watch the ground between the tracks and the edge for tension cracks; if they open, back off and re-inspect.
The judgment call The law says 2 ft. The soil says more, and the soil always wins.