The present invention relates to a method for preparing a polyurethane foam, which comprises reacting an organic polyisocyanate and a polyol in the presence of water as a blowing agent, a cell stabilizer, and an acid-blocked tertiary amino alkyl amide catalyst composition. Then, an inverter main circuit connection switch 10 of a high-voltage battery unit 8 is put into an open state. The guide element has an upper chain channel and a lower chain channel provided with a guide slot for the plow baseplate. The pulse control unit may detect a potential for arcing and then provide for periodic pulsing operation of the shunt.