A large power pylon has come down.
Photo: Supplied / Top Energy

A full investigation into how a Northland power pylon fell over causing widespread electricity outages has confirmed insufficient supervision of an inexperience worker resulted in too many bolts being removed.

There have been calls for compensation following the June outage which one economist predicted cost the Northland economy $60m.

Initial reports found too many nuts were removed from bolts during routine maintenance by Omexom on the transmission tower and its baseplate near Glorit, north of Auckland.

An independent investigation, released Thursday morning, concluded that was the case.

“Standard practice, taught to its civil works field staff, was to remove all nuts from only one tower foundation leg at a time,” it said.

However, on 20 June, all of the nuts were removed from three legs at the same time.

The outage as it happened.
Photo: Supplied / Top Energy

“The team member who removed the nuts from the foundation legs was not adequately trained to perform the task and was not adequately supervised while performing the task,” the investigation, by Daniel Twigg, found.

The report pointed out that neither Transpower’s specifications, nor Omexom’s written procedures, specified how many nuts were to be removed from the baseplate at the same time.

Evidence suggested nuts were removed from more than one leg at four other baseplate refurbishments carried out by Omexom in June, it said.

Engineering consultancy firm Beca Limited found the removal of the nuts from legs A and B “which were intended to resist tension forces on the tower, compromised the stability of the tower, causing it to rupture and ultimately caused the tower to fall”.

The review found no issue with efforts to restore power after the pylon fell. Power was restored to most areas by late-afternoon on 20 June.

Shops across Northland were closed for business.
Photo: RNZ / Peter de Graaf

The report recommended Transpower specify methodology regarding nut removal and update training materials, hold refresher courses, require greater supervision on baseplate refurbishment sites and encourage service providers to have their staff complete the revised training.

In a statement, Transpower said it accepted the investigation’s findings and recommendations and was already moving to address them.

“We can’t undo what happened, but we can put in place improved measures to prevent anything like this happening again,” Transpower acting chief executive John Clarke said.

“Our specifications require that nothing is done that could compromise tower stability. We don’t prescribe how many nuts should be removed and in what order, as service providers have the flexibility to determine their own processes that achieve the task safely and effectively.”

All baseplate refurbishment work was stopped within 24 hours of the pylon falling and would not be resumed until the recommendations were in place, Clarke said.

Transpower said Omexom’s incident review was still underway, as was the Electricity Authority review requested by the Minister.

Clarke said it was waiting for Omexom’s own review to better understand how an inexperienced worked was not properly supervised.

This incident has been “highly stressful” for those involved at Omexom, he said.

“We want our people and our service providers to know they have our support in undertaking the tens of thousands of hours of work each month that it takes to maintain our country’s national electricity grid.”