The largest ever mercury-arc rectifiers, built by English Electric, were rated at 150 kV, 1800 A and were used until 2004 at the Nelson River DC Transmission System high-voltage DC-power-transmission project. The valves for the Inter-Island and Kingsnorth projects used four anode columns in parallel, while those of the Nelson River project used six anode columns in parallel in order to obtain the necessary current rating.[12] The Inter-Island link was the last HVDC transmission scheme in operation using mercury arc valves.