Painting power pylons helps them resist corrosion, but the power transmission entity has a "backlog" of "deferred painting" a 2023 report says. Photo: RNZ / Rebekah Parsons-King
The vast bulk of Transpower's electricity towers are over 35 years old, and few have been put up in the last three decades.
A tower - or pylon - collapsed that serves Northland on Thursday, cutting power to 100,000 properties. The replacement was set to be finished Sunday, Transpower has said.
Transpower crews preparing to install a temporary high voltage tower to shore up the Northland power supply, on a farm in Glorit, in the north of the Auckland Region. Photo: Transpower
The state-owned country-wide power transmission entity Transpower has promised a full investigation, and the government said it was "completely unacceptable".
Transpower's asset management plan in 2023 said corrosion was the big threat to towers, and painting them was the primary remedy.
Unpainted towers lasted as little as 18 years in severe corrosion zones, but up to 120 years in low zones. Painting was crucial but the 2023 plan referred to "deferred tower painting" and a "backlog".
A chart showed about 10 percent of tower protective coatings were in poor or very poor condition.
A chart showing Transpower's pylon 'tower asset health chart' for 2023. Photo: Supplied/ Transpower
Transpower would not say what corrosion zone the fallen tower was in, or if it was painted or not.
"We have made it clear we are not going to speculate on the causes of the tower collapse until we have time to properly investigate," it told RNZ on Friday.
While a 2011 map in a Transpower report suggests the fallen tower is at least in a "severe" corrosion zone, the 2023 plan appears to put assets north of Whangārei on the lower side of a "Condition Assessment (CA) Degradation" zone.
A map of Transpower's corrosion zones, used to help determine wear on power pylons. Photo: Supplied/ Transpower
Only just over 200 of more than 23,000 towers nationwide are in "extreme" corrosion zones (geothermal and exposed coastal areas), but another 5500 are in very severe or severe zones.
Its 2023 report described the interval of structural checks on towers: every four years in high corrosion zones, at eight year intervals "for tower lines that have high public safety criticality" and every 12 years for towers in more benign zones.
Over 19,000 of the towers are between 35 and 75 years old, with not much more than a thousand newer than that, and a couple of thousand are over 75 years old.
Painting them is key: About three-quarters of the towers in the harshest three zones are painted, whereas relatively few of the towers in the other three zones are painted.
"Our approach to maintaining the structural integrity and performance of our steel transmission towers is primarily through our paint programme," the 2023 plan said.
The plan referred to a "backlog of now-due towers" for painting, and deferred painting. "We will start picking up the backlog of 'now-due' towers to be painted."
But "we have a well-established tower painting programme," it said.
A graph showing Transpower asset's age profiles, 2023. Photo: Supplied/ Transpower
The harshest zones needed repainting every 12 years, and "if the tower is in very poor condition, numerous steel members and all bolts need to be replaced before paint can be applied", a 2013 plan said.
Investigations and timelines
Energy Minister Simeon Brown has directed the Electricity Authority to investigate why the Northland power supply pylon collapsed on Thursday.
Transpower said it would also organise an external agency investigation into the incident.
The company's chief executive Alison Andrew said the Northland outage was unacceptable something must have gone "terribly wrong" for the pylon to fall over.
Work to set up a first temporary replacement tower at the Glorit pylon site in Northern Auckland was finished on Sunday afternoon, but more placeholder pylons were needed to ensure full security of supply to Northland.
The region had three high voltage circuits supplying its power, but one of the two main 220kilovolt transmission lines was still stuck under the fallen tower.
Transpower warned there was an increased risk of short power cuts to the region until the repairs were completed, which it hoped could be done by Friday 28 June.
The ongoing maintenance plan
"Painting of towers does have significant benefits in addressing minor structure maintenance tasks such as steel and bolt replacements, earth plate installations," Transpower's 2023 asset management plan said.
Around 2013 Transpower boosted its yearly painting from around 200, to 500 towers a year, and increased the number of corrosion zones as its "understanding" increased.
It was costing over $60,000 to paint each tower in a severe zone back then, and $30-40,000 in a kinder zone.
Painting was "the lowest-cost lifecycle option", but still was the biggest cost in the current management period 2020-25. An alternative strategy - replacing some ageing towers with poles - would be used in 2025-30.
It aimed to paint about 500 more towers - 2700 - in the 2025-30 period than in the current period, focused on "larger, more expensive towers".