electrode. They are similar to structures used for lines with voltages of 10–30 kV, but normally carry only one or two conductors. AC transmission towers may be converted to full or mixed HVDC use, to increase power transmission levels at a lower cost than building a new transmission line. Railway traction line towers Towers used for single-phase AC railway traction lines are similar in construction to those towers used for 110 kV three-phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). These are usually arranged on one level, whereby each circuit occupies one half of the cross arm. For four traction circuits, the arrangement of the conductors is in two levels and for six electric circuits, the arrangement of the conductors is in three levels. Towers for different types of currents AC circuits of different frequency and phase-count, or AC and DC circuits, may be installed on the same tower. Usually all circuits of such lines have voltages of 50 kV and more. However, there are some lines of this type for lower voltages. For example, towers used by both railway traction power circuits and the general three-phase AC grid. Two very short sections of line carry both AC and DC power circuits. One set of such towers is near the terminal of HVDC Volgograd-Donbass on Volga Hydroelectric Power Station. The other are two towers south of Stenkullen, which carry one circuit of HVDC Konti-Skan and one circuit of the three-phase AC line Stenkullen-Holmbakullen. Towers carrying AC circuits and DC electrode lines exist in a section of the powerline between Adalph Static Inverter Plant and Brookston the pylons carry the electrode line of HVDC Square Butte. The electrode line of HVDC CU at the converter station at Coal Creek Station uses on a short section the towers of two AC lines as support. The metallic return of HVDC Vancouver Island is partly installed on poles that carry also AC-circuits. The overhead section of the electrode line of Pacific DC Intertie from Sylmar Converter Station to the grounding electrode in the Pacific Ocean near Will Rogers State Beach is also installed on AC pylons. It runs from Sylmar East Converter Station to Southern California Edison Malibu Substation, where the overhead