Silver Maple Over a 12.47 kV Primary
A two-person tree crew is asked to remove a storm-damaged limb from a silver maple beside a driveway. The 55-ft bucket truck is parked on a 6% slope, and the right outrigger pad sits over a septic tank lid 18 in below grade. The target limb is about 10 in in diameter and overhangs a 12.47 kV primary conductor. At its closest, foliage is 7 ft from the conductor; the conductor is 24 ft above grade at the pole and sags to 21 ft mid-span. Wind is 15 mph with 22 mph gusts. The platform is rated 350 lb. Two arborists, two chainsaws, ropes, and rigging blocks are on the platform. The client wants the work completed before a forecast storm. The utility has not been contacted, and the truck's boom was last inspected 14 months ago.
1. What is your go/no-go decision, and what hazards must be controlled before any platform entry? Show how you prioritize them.
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Go/no-go: no-go until the utility, ground conditions, and load are resolved. The controlling hazards are electrical (7 ft to 12.47 kV), ground stability (outrigger over a septic tank on a 6% slope), and platform overload. Prioritize electrical and ground failure because either can cause a fatal event without warning. Steps: (1) stop and barricade the drop zone; (2) call the utility for a line-clearance order or cover-up; (3) verify the septic tank is not a void and use engineered pads or cribbing rated for the outrigger load, or reposition the truck to level ground; (4) confirm the boom inspection is current; (5) complete a written risk assessment and job briefing. Do not treat the 22 mph gusts as acceptable without checking the manufacturer's wind rating and sail area. If the utility cannot respond, reschedule. The client's storm deadline does not override the minimum approach distance or ground capacity.
2. Calculate and apply the minimum approach distance. Is the planned position acceptable? What specific controls would make it acceptable, and what would not?
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Minimum approach distance for tree work near lines of 50 kV or less is 10 ft, measured from the energized conductor to the closest part of the platform, worker, tool, or limb, not just the boom tip. The 7-ft foliage clearance already violates the minimum approach distance. Repositioning to the opposite side only helps if the entire platform and the limb's travel path stay at least 10 ft away; a 10-in limb can move several feet when cut. Controls that work: de-energize and visibly ground the line with utility confirmation, or install approved insulating line cover or hose rated for 12.47 kV. A ground spotter with a radio and a stop-work signal adds defense but does not create distance. Controls that do not work alone: a fiberglass saw, an insulated bucket, warning signs, or relying on operator skill. If cover is used, maintain the cover and re-check distance as the limb loads and shifts. Account for conductor sag at 21 ft mid-span and wind sway.
3. Evaluate the platform load and rigging plan. What limits apply, and how should the limb be controlled?
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Platform capacity is 350 lb. Two workers at, say, 180 lb each equal 360 lb before saws, ropes, and blocks, so the planned two-person load is over the rating. Even one 180-lb worker plus two saws at 20 lb each, rope at 10 lb, and blocks at 15 lb is about 245 lb, leaving little margin for dynamic forces. Reduce to one worker and minimal gear, or use a larger platform. Do not attach rigging blocks or lowering lines to the platform unless the manufacturer approves it; shock loads from a caught limb can tip or overload the boom. Rig from the tree with a separate lowering line and a ground anchor, and keep the platform out of the limb's fall path. Verify the load chart for the boom position, not just the platform placard.