Rooftop Unit Lift Near an Overhead Distribution Line
A 1,200 lb rooftop make-up air unit is being replaced on a 4-storey building. The crane is set up in the parking lot with its boom over a 600 V three-phase overhead distribution line; the utility has not been contacted. The rigging plan uses a two-leg bridle of 3/8 in. 6x19 wire rope slings, each leg at 60° from horizontal, with forged eye hooks. The unit has no engineered lifting points; the rigger plans to choke the slings around the 16-gauge sheet-metal casing. The roof has a 4 ft parapet, and the unit must land 8 ft back from the roof edge. Wind is 35 km/h gusting to 50. A worker will guide the load with a tag line from the roof.
1. Calculate the sling leg tension and assess the rigging arrangement.
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Each leg tension is T = (W/2) / sin 60° = 600 / 0.866 = 693 lb. The tension itself may be within the capacity of 3/8 in. 6x19 wire rope, but the capacity must be read from the sling tag and reduced for the choke and any angle derating. The bigger problem is choking 16-gauge casing: it will crush, the load will shift, and the hook can slip. The unit has no engineered lifting points, so the plan needs a spreader beam or manufacturer-approved lifting points. The rigging plan is therefore not acceptable even if the arithmetic is correct.
2. Identify the non-rigging hazards and describe the controls you would require before the lift.
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The crane must not operate over the 600 V line without contacting the utility and either de-energizing the line or maintaining the regulated limit of approach. Voltage class does not make the line safe to work near. Mark the boundary, use a dedicated spotter, and stop the lift if the boom or load can approach the line. Wind at 50 km/h may exceed the crane manufacturer's load-chart limit; confirm the limit and postpone if necessary. The roof worker needs fall protection and must stay out of the drop zone; the tag line should be handled from a safe position, not from the roof edge. The 4 ft parapet and 8 ft landing distance affect the required boom radius and crane capacity, so the operator must verify capacity at the actual radius.
3. Outline the corrected lift plan.
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Contact the utility first and establish the line clearance or de-energization. Use a spreader beam or engineered lifting points with shackles, and select slings rated for at least 693 lb per leg plus the choke and angle derating; inspect slings, hooks, and hardware. Calculate crane capacity at the actual radius, including the parapet and the 8 ft setback. Assign a signalperson and a spotter, barricade the drop zone, and require fall protection for roof workers. Use tag lines from outside the drop zone and suspend the lift if gusts exceed the crane's wind limit. Do not let a worker guide a 1,200 lb load by hand from a roof edge.