Resistance welding in large-scale manufacturing is carried out with a significant number of disturbances, the cumulative effect of which may exceed the capabilities of modern control equipment. Most resistance welding control systems used in industry to compensate for existing disturbances provide welding current phase control depending on the measured parameters characterizing the process of welded joint formation. The efficiency of such controllers is largely determined by the accuracy of measuring and setting the phase control parameters, which include the opening and conduction angles of welding thyristors. The paper shows that when switching on a contact machine, a phase shift of the mains voltage occurs in the load mode relative to the mains voltage in the idle mode. Using a simplified electric equivalent circuit of a contact welding machine, the paper describes the nature of the phase shift of the mains voltage. Circuit active resistance and inductance are selected as parasitic parameters of the mains. The authors simulated the electrical processes in the contact machine according to the three-loop equivalent circuit. The study shows the influence of mains parasitic parameters on the phase regulation stability, the features of the obtained current and voltage oscillograms. Depending on the mains and contact welding machine parameters, the phase shift magnitude ranges from fractions to units of an electrical degree. With welding current parametric stabilization by the mains voltage, the influence of mains parasitic parameters can be neglected. When the regulator operates in the mode of maintaining the secondary current numerical value, a decrease in the generated current relative to the specified one is observed. The authors proposed and tested a technique for determining the parasitic parameters of the supply mains based on the results of a short circuit test.