apparatus for controlled dielectric heating a plurality of thermoplastic preforms for blow molding is provided . the apparatus controls the energization of a plurality of electrode pairs by a radio frequency generator . the generator is equipped with a load current limit detector and the electrode energization is enabled selectively for a predetermined time or until a predetermined load current magnitude is reached . the choice of control is determined by the anticipated load as reflected by the number of preforms within the heating station . the generator is further provided with overload detecting means indicative of the occurrence of electrical arcing within the heating station and the apparatus has further means responsive to the arc detector for selectively momentarily or indefinitely deenergizing the electrodes .