Trains on the 25 kV AC Auckland Metro Rail Network have been experiencing flashover of their pantographs, from the overhead wire to the train’s roof. This is a major safety hazard to passengers and staff, and causes significant disruption to railway operations.

KiwiRail, the rail network operator, commissioned Middleton Group to commence a root cause investigation of this safety & operational hazard. This investigation began with the power quality of the network.


Middleton Group set out to test the hypothesis: “Arcing on the Auckland Metro Rail Network is caused by KiwiRail’s power supply.” .


Middleton Group designed a test circuit to measure the pantograph to rail voltage on the network. Our engineers specified the test equipment, test locations, and testing methodology in accordance with appropriate European and KiwiRail standards to accurately record the voltage.

The raw data was analysed and assessed against European and KiwiRail standards and presented to KiwiRail, as well as broader stakeholders, in the form of a written report and an online presentation.


Middleton Group found no evidence that the KiwiRail power supply is the cause of arcing at rolling stock pantographs. However, several possible causes were identified and recommended for further investigation. The next steps are currently under discussion amongst the broader stakeholders to continue a holistic investigation into the root cause of the pantograph flashover.

Contact us

Dr Stephen Goh

Traction Power Lead | Technical Director – Traction Power, EMC, E&B

Eric Bentsen

Power Engineer