Rebar Pick Near a 25 kV Line With a Radius Increase
A 60 m jib tower crane is positioned 16 m from a 25 kV overhead line. The conductors are about 11 m above ground. The crane is to unload a 2.3 t bundle of rebar from a truck at a nominal 52 m radius. The chart shows 2.5 t at 52 m and 2.2 t at 54 m before deducting rigging. The slings, shackles and hook block below the hook weigh 120 kg. The lift plan says the actual hook radius increases to 53.5 m when the jib is loaded. The load must be slewed over the line to reach the storage area. A utility spotter is available, but the line has not been isolated. The operator is asked to make the pick before the morning traffic arrives.
1. Calculate the all-up load and check the chart at the actual radius. What is the result?
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The all-up load below the hook is 2.3 t plus 0.12 t of rigging, or 2.42 t. The nominal 52 m radius is not the radius that should be used, because the lift plan states that the loaded jib increases the hook radius to 53.5 m. Interpolating between 2.5 t at 52 m and 2.2 t at 54 m gives a chart value of about 2.275 t at 53.5 m, before deducting rigging. Even if the chart value were treated as a gross figure, 2.42 t exceeds 2.275 t. If the chart requires deduction of rigging, the available load is even lower. The pick is therefore over capacity at the actual radius, and the nominal 52 m check would have hidden the overload. The correct response is to stop and replan: reduce the load, break the bundle, reposition the crane, change the jib configuration if permitted, or use a different lifting point that gives a shorter actual radius. The radius increase caused by jib deflection must be included in every load-chart check, not treated as a tolerance.
2. What electrical and planning controls are required before any lift that slews over the line?
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The overhead line is the controlling hazard and must be dealt with before load-chart issues are reconsidered. The line owner or utility should be asked whether the line can be isolated and grounded for the lift. If it cannot, the site must establish the jurisdiction's minimum approach distance for a 25 kV line, using the utility's requirements and the site's electrical safety plan. The crane, load and rigging must be kept outside that distance at all times, including during slew, trolley movement and any uncontrolled movement. Engineering controls such as a physical barrier, goal posts, a slew limit or a designated no-go zone should be used rather than relying only on a spotter. The lift plan should show the line, the exclusion zone, the planned path and the abort position. The load should not be slewed over the line unless the line is isolated and grounded or the clearance is proven by the utility and the competent person. The morning traffic schedule is not a reason to shorten those controls.
3. How would you coordinate the lift with the signaler and the utility spotter?
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I would hold a pre-lift briefing with the signaler, the utility spotter and the rigging crew, using one agreed radio channel and agreed signals. The signaler controls the crane; the utility spotter has authority to stop the lift if the load, jib or rope approaches the exclusion zone. The sequence should be stated: lift clear of the truck, stop and confirm the load is stable, slew only on the signaler's command, keep the load below the line's exclusion height if the line remains live, and land at the storage area without passing over the line. The operator should confirm the actual radius and load on the chart before lifting and should stop if the load weight, radius or wind changes. If radio contact is lost, the operator stops and holds position. The abort plan should identify a safe landing area that does not require crossing the line. After the lift, the operator should record the line status, the controls used and any deviation from the plan.