Rigging, Aerial Devices, and Specialty Equipment
Rigging and aerial devices decide whether the load stays in the air and whether the bucket stays insulated. The exam tests load charts, sling angles, hardware ratings, knots, hand signals, and the inspection and dielectric testing routine for digger derricks and insulated aerials.
Rigging fundamentals
- Working load limit is not breaking strength: common design factors are about 5:1 on wire rope and higher on synthetic slings, per manufacturer and regulator.
- Sling leg tension = load ÷ (number of legs × cosine of the angle from vertical). Tension climbs quickly as legs spread.
- Shackles are loaded through the bow, pins are fully seated, and hooks carry a functioning safety latch and are not side-loaded.
- Knots reduce rope strength; the bowline gives a fixed loop that unties after loading, a clove hitch ties off quickly, and half hitches secure a tag line.
- Keep the centre of gravity below the hook, control rotation with tag lines, and keep everyone out from under the load.
Aerial devices
- Digger derrick capacity falls as radius and boom angle change; read the chart at the actual working radius with outriggers fully deployed on mats or cribbing.
- Insulated buckets are dielectrically tested on a scheduled basis along with the liner and blankets; upper and lower controls must both be familiar, and the lower control is the rescue override.
- Wear the harness attached to the designated anchor, never to a bucket lip, and work to a written rescue plan.
- Ground the truck per utility practice or isolate it; never mix the two philosophies on one setup.
Worked example
A 1,000 kg load hangs from a two-leg bridle with a 60° included angle. Each leg carries 1,000 ÷ (2 × cos 30°) = 577 kg. If the sling's working load limit in that configuration is 500 kg per leg, the lift is over capacity even though the total, 1,154 kg, looks close to the load. Reasoning: the angle multiplier, not the load, dictates leg tension; at 90° included angle each leg would carry 707 kg.
Traps that cost marks
Measuring radius from the wrong point on the boom; shock loading a sling by jerking the load; using a sling with an illegible or missing tag; entering the fall zone; and assuming the bucket insulates when the truck is not set up on its outriggers.