Fluid warmer for tempering of a medical fluid guided in a fluid line, comprising: a first and a second resistance heating element for alternating voltage, a heat exchanger which is thermally coupled to the two resistance heating elements and comprises a holder for the fluid line (12, 12a), characterized in that - each resistance heating element has a first and a second electrical line connector (24a, 24b), respectively, for connection of the resistance heating element to an alternating voltage source, - each line connector (24a, 24b), respectively, of the two resistance heating elements is provided with a drivable first switch; - a respectively inversely phased alternating voltage is applied to each resistance heating element during the tempering mode; or - respectively in-phase alternating voltages are applied to the resistance heating elements during the tempering mode and a compensation alternating voltage, which is substantially inversely phased with respect to the alternating voltage, is applied to the heat exchanger using a compensation circuit; and that there are moreover provided: a voltage measurement device for determining a voltage between the heat exchanger and a neutral conductor of the alternating voltage source; and a control device for synchronized transfer of the switches into their open switching state in dependence on the voltage. a first and a second resistance heating element for alternating voltage, a heat exchanger which is thermally coupled to the two resistance heating elements and comprises a holder for the fluid line (12, 12a), characterized in that - each resistance heating element has a first and a second electrical line connector (24a, 24b), respectively, for connection of the resistance heating element to an alternating voltage source, - each line connector (24a, 24b), respectively, of the two resistance heating elements is provided with a drivable first switch; - a respectively inversely phased alternating voltage is applied to each resistance heating element during the tempering mode; or - respectively in-phase alternating voltages are applied to the resistance heating elements during the tempering mode and a compensation alternating voltage, which is substantially inversely phased with respect to the alternating voltage, is applied to the heat exchanger using a compensation circuit; - each resistance heating element has a first and a second electrical line connector (24a, 24b), respectively, for connection of the resistance heating element to an alternating voltage source, - each line connector (24a, 24b), respectively, of the two resistance heating elements is provided with a drivable first switch; - a respectively inversely phased alternating voltage is applied to each resistance heating element during the tempering mode; or - respectively in-phase alternating voltages are applied to the resistance heating elements during the tempering mode and a compensation alternating voltage, which is substantially inversely phased with respect to the alternating voltage, is applied to the heat exchanger using a compensation circuit; and that there are moreover provided: a voltage measurement device for determining a voltage between the heat exchanger and a neutral conductor of the alternating voltage source; and a control device for synchronized transfer of the switches into their open switching state in dependence on the voltage.