Erecting a 30 m Mast Beside an 11 kV Line
A 30 m hoist must be erected on a tight CBD site. The practical position puts the mast 3.2 m from a live 11 kV overhead line along the boundary and 2.5 m from the neighbouring building's windows. The mast needs four ties; the scaffold contractor offers to tie into its scaffold at 9 m and 18 m. The public footpath stays open behind a 1.2 m coned corridor at the base. Wind at the planned erection time is 14 m/s gusting 19 m/s against a manufacturer's erection limit of 12 m/s. The mobile crane is booked for a four-hour window, and the next available slot is nine days away.
1. Deal with the overhead line. What has to be settled before the mast goes up, and why doesn't 3.2 m settle it?
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3.2 m is a measured clearance, not a safe clearance. Safe approach distances are set by the network operator and the state's electrical safety rules, and for 11 kV plant and equipment the limit is commonly around 3 m. At 3.2 m you have roughly 200 mm of margin - and during erection you have:
- crane slew and boom deflection under load;
- mast sections swinging on the hook with a tagline that can slip;
- plumb adjustment of the mast, which moves the top of a 30 m section a long way for a small shim at the base;
- wind deflection of a partly erected, untied mast.
Before erection: obtain written advice from the network operator on the applicable distances; apply to have the line de-energised/isolated for the duration, or obtain the operator's written approval of a documented safe system with an authorised spotter; brief the crane crew and mark the limit physically (goalpost/height barrier or survey-marked slew limit); check the crane's radius and counterweight tail-swing against the line during the four-hour window. If isolation and an approved safe system are both unavailable, relocate the hoist - that is the decision, not a discussion in the cab.
2. Can the scaffold be used as the tie structure? Explain what ties do and what would be required.
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Not on the scaffold contractor's word.
What ties do: they hold the mast against wind and out-of-balance loads and transfer tension and compression into the building structure. The mast is a slender column; the ties are what stop it becoming a falling object. Tie loads are design loads, not 'a bit of pull'.
Why a scaffold is a poor default: a scaffold is designed for wind and its own platforms and live loads, and its ties to the building are designed for that. Adding hoist tie reactions means you are changing its structural design - the scaffold may be simultaneously loaded by trades, shotcrete or stored material, and the connection points are rarely at structural columns.
What would be needed: the engineer to design the tie loads and the connection, certify the scaffold and its ties for the combined load, and the manufacturer's manual tie spacing to be followed (first tie at the specified distance above the base, then at the specified intervals) - not the scaffold's lift heights. In practice, tying to the building's structural frame or slab edges with engineer-designed anchors is the cleaner answer. Also confirm the neighbouring building's windows at 2.5 m: no tie into it, and consider the cage/landing clearance and glass breakage.
3. The wind is over the limit and the crane slot is now or in nine days. What is your call, and what do you have to manage as a result?
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The call: do not start. The erection limit exists because during erection the mast is at or near its maximum free-standing height and the mast sections and cage act as sails. 14 m/s gusting 19 m/s is over 12 m/s before a single section is flown, and a four-hour window in a gusting breeze will not reliably get you to the first tie.
The worst outcome is a part-erected, untied mast left standing overnight because the window closed - that is a far bigger hazard than a delay.
Consequences to manage:
- nine days of programme and a re-booked crane - give the PM the number early so alternatives can be found;
- if the base frame and first section are already anchored, leave them safe, barricaded and documented, or don't start at all;
- book a longer window sized to reach and fix the first tie in one shift;
- write a wind-hold rule into the SWMS with a site anemometer reading at mast height or the nearest reliable source, and a nominated person with authority to stop;
- re-brief the crane crew and riggers; check the footpath corridor and coning are still valid on the new date;
- keep the powerline isolation booking or approval aligned with the new date - don't let it lapse.