Boom Contact With an Overhead Line on a Tight Site
A 32 m four-section boom is positioned in a residential street to place concrete for a first-floor slab. The only practical setup puts the boom's working arc within 4.5 m of a 22 kV overhead line running parallel to the kerb. The line is not insulated and the span sags about 1.2 m at mid-span. The spotter is a labourer who has been on site for three days and has not received a site induction on electrical hazards. The pump operator plans to work the boom at full extension to reach the far corner of the slab, which is 29 m from the truck. Wind is calm. The concrete is a 32 MPa mix delivered at 150 mm slump, and the pour is expected to take five hours.
1. Assess whether this setup can proceed and identify the controls required before any boom movement.
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The setup as described cannot proceed. The boom arc is within 4.5 m of an uninsulated 22 kV line, and the sagging span means the minimum clearance point is not where the line appears highest. The relevant clearance is the approach distance required by the jurisdiction's electrical safety rules and the machine manufacturer's manual, whichever is more stringent; for high-voltage lines this is typically several metres and increases with voltage. A 4.5 m horizontal separation is insufficient once you account for sag, boom deflection under load, wind-induced sway and operator error.
Controls before any movement: obtain the line owner's advice on voltage and sag, and request de-energisation or isolation with written confirmation if the work cannot otherwise achieve clearance. If isolation is not possible, establish a physical barrier or goalpost limit that prevents the boom entering the exclusion zone, not just a painted line. Brief every person on the exclusion zone and the spotter's authority to stop the boom.
The spotter is not competent for this role: three days on site and no electrical induction means they cannot judge approach distances or recognise when the boom is drifting. Replace or train them before work starts.
2. The operator wants full extension to reach the far corner. Explain the interaction between radius, boom deflection and clearance.
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Reaching 29 m from the truck at full extension puts the boom tip near its maximum radius, which has two effects on clearance. First, as the boom extends and the sections deflect under their own weight and the weight of concrete in the line, the tip drops and the mid-span of the boom sags. The deflection is not uniform and is greatest toward the tip, so the part of the boom closest to the line may not be the part the operator is watching.
Second, at maximum radius the machine is closest to its stability limit, so the outrigger reactions are highest and any ground movement is amplified. Operating at full extension also reduces the operator's margin for correcting a drift toward the line, because there is less spare reach to retract.
The correct approach is to reduce the radius: reposition the truck so the far corner is reachable at a shorter radius, use a longer boom or a placing line, or pour the far corner in a separate setup. If the radius must remain, the clearance calculation must use the deflected boom position under load, not the unloaded geometry, and must include the sag of the line at the point of closest approach.
3. During the pour the spotter reports the boom 'looks close' to the line. What is your immediate sequence of actions?
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Treat the report as real until proven otherwise. Immediate sequence: stop pumping and stop all boom movement; instruct the operator to hold position and not to retract or swing until told, because an uncontrolled movement could close the gap; clear personnel from the area beneath and around the boom; and have the spotter, from a safe position, describe what they see and where.
Then assess: is the boom actually within the exclusion zone, and is there any sign of arcing, tracking or contact? If there is any doubt, keep the area clear and contact the line owner before any movement. Only the operator, following a planned and briefed retraction path that increases clearance, should move the boom, and only once the path is confirmed clear.
Afterwards, investigate why the drift occurred: was it deflection under load, wind, ground settlement, or an error in the original clearance calculation? Record the event, retrain or replace the spotter if their competence was inadequate, and do not resume until the setup is re-planned with verified clearance.