Simple link that blacked out half of Auckland

A failed D-shackle caused Monday's huge blackout.

A simple metal shackle has been isolated as the cause of Auckland's chaotic power blackout, prompting Transpower to launch a nationwide inspection of similar equipment.

It was revealed yesterday that a D-shackle - a D-shaped item used to fasten wire to other equipment - failed and subsequently caused Otahuhu substation's earthing wire to snap.

In the resulting chain of events, much of Auckland lost power for several hours on Monday in an embarrassing outage which has put the Government under pressure.

Transpower is one of several parties now working on urgent investigations into the blackout, and the national grid operator last night confirmed its discovery that a D-shackle was the main culprit.

The piece of equipment is similar to what is used at the end of a chain to connect a trailer to a towing vehicle.

Spokesman Chris Roberts was unable to say if maintenance was a factor in the shackle's failure.

"All those details will be fully revealed in due course," he said.

Energy Minister David Parker has demanded a report from Transpower by next Friday.

Transpower has also called in independent engineering experts Sinclair Knight Merz to review of incident, including an assessment of the role maintenance may have played.

The national grid operator is inspecting equipment similar to that which failed at Otahuhu across the country.

Transpower said it was also looking into the amount of time taken to restore power to Auckland and whether there were any lessons that could be learned.

Electrical engineering experts spoken to by the Herald last night speculated that the shackle's failure could have been caused by corrosion, physical wear and tear, or a manufacturing or design fault.

One engineer said he understood the failed shackle held the earth wire which snapped and triggered Monday's blackout. He said he had been told the shackles were "old hardware" and were gradually being replaced in the system.

Transpower is not intending to release information about the shackle failure until it reports to the Government next week.

Acting Energy Minister Trevor Mallard alluded to the new information during continued questioning from the Opposition in Parliament.

The Government is under pressure to provide answers about the blackout and has faced intense questioning from Opposition MPs over the past three days.

Despite being absent from question time yesterday, Prime Minister Helen Clark was in the spotlight over her statement on Wednesday that "under the current Government, Transpower has been investing, on average, around $300 million a year in its system".

The Energy Minister's office confirmed yesterday that $300 million applies only to spending during one year of Labour's 6 1/2 years in Government - the figure was much smaller in earlier years.

National MP Nick Smith seized on the statement as an example of how "desperate" the Government had become to avoid responsibility for the blackout.

But Labour MPs again strongly defended the party's record on infrastructure spending, and highlighted that national grid investment had increased steeply under its watch.

Mr Mallard said Transpower's expenditure was expected to increase further in coming years.

The Electricity Commission is also investigating the blackout. The regulator's relationship with Transpower has also been in the spotlight, as the national grid operator seeks approval for a major project to upgrade Auckland's power supply by building a new line in from the Waikato.