Abstract: Operation control system for an electrothermal furnace having one or more electrode and a transformer unit for each electrode for supplying heating current to its associated electrode. The or each transformer unit includes a main transformer having a primary winding for receiving current from a mains and two secondary windings, and an additional transformer connected to derive an adjustable voltage from one of the second secondary windings. The adjustable voltage and the voltage across the other secondary winding of the main transformer are combined to supply the heating current to the electrode. The control system includes a switch and a current control unit connected in the transformer unit for giving the additional voltage a selected polarity relative to the voltage across the other secondary winding of the main transformer and for controlling the magnitude of the additional voltage.