Abstract: An apparatus for arc welding of a pair of spaced steel plates from one side only using a grooved back-up plate which gives a good weld bead shape on the under side, gives high linear welding speeds, and avoids contamination of the deposited weld metal by the metal from the back-up plate. A pair of consumable electrodes connected in electrical series between the output terminals of the power source and with one connected by an independent connection to the workpiece are advanced into the weld gap on lines of movement which intersect at a point above the lower surfaces of the plates so that an arc is maintained between the electrodes spaced from the base of the back-up plate groove. A separately energized electrode trails the first two electrodes and deposits a weld bead on the first deposited weld metal while it is still hot.