If you own an irrigation plant remember the need to take extreme care with booms and tall equipment near power lines.
In February 2018, a pivot irrigator in Waimate was being commissioned near a 110 kV Transpower overhead transmission line. The irrigator was moved around 7am by the farmer, who was unaware that part of the irrigator was sitting directly under the line. The load (the current) through the line increased during the day as did the air temperature. These two factors caused the transmission line to sag closer to the irrigator and at around 11.40am it was close enough (300mm) to cause a flashover, trip the circuit, create serious safety hazards for anything connected to the irrigator, and cause a local power outage.
Had anybody been touching the irrigator or the path through which this excess current was felt (which included a switchboard in a farmhouse over 1km away) then they would have been seriously burned or electrocuted.
Fortunately, no one was in the vicinity at the time and no people or animals were hurt, but the incident highlights the critical importance of keeping well clear of transmission lines with mobile equipment.
Transpower’s Senior Property Advisor for the South Island, Mark Batty says while there hasn’t been another incident since this event, he has visited a number of landowners where an irrigator could have caused a problem for the National Grid and the irrigator.
“I think the biggest issue is people think they’ve worked out the calculation of what height the conductor is, but what they don’t account for in their calculations is the fact that the height can vary.”
“This is where contacting us can be really helpful – we can give you some really good information around how much potential sag is in a line span and where you might need to be careful.”
Safe clearance distances
Although it is designed to meet all safety specifications and standards, there are inherent risks associated with high voltage electricity infrastructure. Electricity at high voltages can arc through the air even without direct contact.
As the February incident in Waimate graphically shows, transmission line wires (or conductors) are not always in the same place. During the day they can increase and decrease in height from ground depending on ambient air temperature and current. To make it more complicated, they can also move from side to side quite considerably in strong winds – and the greater distance between supporting towers, the greater this swing can be. Finally, to add further complexity, irrigation plant unlike most mobile plant operates without someone on site to supervise – so any solution must allow for the worst-case scenario.
The New Zealand Electrical Code of Practice 34 (NZECP34) specifies a 4m minimum clearance between all mobile plant, including irrigators, and overhead power lines. It can be hard to judge the distance from a transmission line – so don’t feel you need to make that call yourself. Please contact Transpower for assistance: they and their contractors can provide information on where the conductors will sag to at peak operating conditions (i.e. the worst-case scenario), they can also provide on the ground advice on your irrigator setup and how to avoid any issues no matter what the conditions.
There are also important rules in NZECP34 relating to excavation around the supporting structures of transmission lines (i.e. poles and towers). For example, the distance between any excavation greater than 300 mm deep (e.g. pivot irrigator foundation and pump house) and a transmission tower, must be further than 6 m from the visible tower foundation – unless prior written consent is obtained from Transpower. The rules are quite technical, but Transpower has a helpful information sheet available from its website (www.transpower.co.nz search ‘irrigation’) which provides some good guidance around irrigation and the requirements of NZECP34. You’re welcome to call them and get their advice directly.
Please contact Transpower on 0508 526 369 (0508 Landowner) for more information. There is also more information about activities and development near National Grid lines at www.transpower.co.nz.
How can irrigators be operated safely around power lines?
Risks and practices to consider when installing and operating irrigation systems:
- long irrigation systems, where practical, should be operated at right angles to the transmission line.
- where possible, avoid locating the pivot point of a pivot irrigator within 20 m of a tower and maintain at least the minimum clearance of 4m from the irrigator to the conductors (the wires).
- where layout alternatives are available, maximum separation from transmission lines is recommended.
- during relocating, assembly or disassembly, care needs to be taken so that pipes or long metal parts are carried in a horizontal position, especially around power lines.
- adjust the nozzles so the jets of water do not hit the conductors or the tower steelwork – direct spraying could cause a flashover and can corrode the tower structure.