This International Standard deals with the safety of electric storage water heaters for household and similar purposes and intended for heating water below boiling temperature, their rated voltage being not more than 250 volt for single-phase appliances and 480 volt for other appliances. However immersion heater units intended to be retrofitted in a heat exchange closed water heater are prohibited unless the immersion heater unit has been tested with the tank models and brands listed in the instructions of the immersion heater unit; the tank models and brands list the models of the immersion heater units that can be retrofitted.