electrode cells are buried along an 800 m length of a stony beach. Each electrode cell consists of a high silicon-chromium iron electrode, suspended in a vertical porous concrete cylinder. The cells are surrounded by selected and graded stones and geotextile layers to allow seawater ingress, but prevent the buildup of silt. The electrode to ground resistance is 0.122 Ω. The South Island ground electrode station is located at Bog Roy, 7.6 km from Benmore. It comprises buried electrode arms arranged in a star configuration over a site of approximately 1 km2. Each electrode arm is a 40 mm mild steel rod buried in a coke bed of around 0.26 m2 cross sectional area, in a 1.5 m deep trench. The electrode to ground resistance is 0.35 Ω. A small transmission line carries a twin conductor electrode circuit from the Benmore converter station site to the South Island land electrode at Bog Roy, which in conjunction with the shore electrode in the North Island, allows one pole to operate using earth return when the other pole is out of service. Transmission faults and outages Like all transmission systems, the HVDC Inter-Island link is not immune to failures. The importance of the link means that an unplanned outage can have major implications for the entire New Zealand electricity system, potentially causing nationwide frequency deviation (underfrequency in the receiving island, overfrequency in the other island), electricity shortages in the receiving island, and a spike in wholesale electricity prices. The most catastrophic situation is a simultaneous bipole outage at high transfer when there is low to medium generation in the receiving island—instantaneous reserve generation and load shedding systems in the receiving island would not be able to come online fast enough to prevent the frequency dropping, resulting in cascading failure and outage of the entire receiving island. Planned outages of the link are required occasionally to carry out maintenance that is not possible while the system is live. Maintenance outages are planned well in advance to minimise the effects – they are usually carried out in summer when national electricity demand is at its lowest, and on only one pole at a time, with the other pole remaining in operation providing half of the full two-pole capacity, using the earth electrodes providing a path for return current through the ground. Notable faults and outages on the HVDC Inter-Island link: 1973 – an electrical