Fluid warmer and method of operating a fluid warmer

1. A fluid warmer for temperature control of a medical fluid guided in a fluid tube, the warmer comprising:

a resistance heating element for alternating voltage, said resistance heating element having a first and a second electrical supply connection conductor, wherein said first electrical supply connection conductor has a first switch and said second electrical supply connection conductor has a second switch;

a heat transfer element which is thermally coupled to said resistance heating element;

a functional grounding conductor, which is electrically conductively connected to said heat transfer element, said functional grounding conductor having a third switch;

a sensor for determining a present magnitude of a leakage current via said functional grounding conductor; and

a control device, said control device being structured to simultaneously switch said first, said second and said third switches into an open switching state within a defined time interval on occurrence of a leakage current that is greater than or equal to a defined maximum threshold current magnitude.

2. The fluid warmer of claim 1 , wherein different threshold current magnitudes are defined for an operating condition of the fluid warmer with an activated resistance heating element and for an operating condition with a deactivated resistance heating element.

3. The fluid warmer of claim 1 , wherein the maximum threshold current magnitude corresponds to a sum of a current magnitude of leakage current for a maximum heating power of the resistance heating element and a defined differential current magnitude.

4. The fluid warmer of claim 1 , wherein the maximum threshold current magnitude corresponds to a differential current magnitude or to a sum of a differential current magnitude and a current magnitude of leakage current with a closed switch of a supply connection conductor connected to a line conductor of the alternating voltage source and with an open switch of a supply connection conductor connected to a neutral conductor of the alternating voltage source.

5. The fluid warmer of claim 3 , wherein said differential current is between 1 microampere and no more than 50 microamperes.

6. The fluid warmer of claim 4 , wherein said differential current is between 1 microampere and no more than 50 microamperes.

7. The fluid warmer of claim 1 , wherein said first, second and third switches can be put into open switching states thereof within a defined time interval on occurrence of a leakage current which is less than or equal to a defined minimum threshold current magnitude.

8. The fluid warmer of claim 7 , wherein different threshold current magnitudes are defined for an operating condition of the fluid warmer with an activated resistance heating element and for an operating condition with a deactivated resistance heating element.

9. The fluid warmer of claim 1 , wherein said defined time interval within which switches can be put into said open switching state by said control device is less than one period or less than one half-period of the alternating voltage used to power the resistance heating element.

10. The fluid warmer of claim 1 , wherein said first, second and third switches are constituted as a bidirectional triode thyristor, as a metal-oxide semiconductor field-effect transistor, as a relay or as switches which are duplicated.

11. The fluid warmer of claim 1 , wherein a supply connection conductor that is conductively connected to a line conductor of the alternating voltage source can be identified by said control device and a switch of this supply connection conductor can be controlled during operation of the fluid warmer by said control device to activate or deactivate said resistance heating element.

12. The fluid warmer of claim 1 , further comprising an electrical or electronic compensation device for compensation of the leakage current induced by said resistance heating element.

13. The fluid warmer of claim 12 , wherein said compensation device comprises an LC resonant circuit with an inductor and a capacitor.

14. The fluid warmer of claim 13 , wherein said inductor of said LC resonant circuit is constituted as a secondary side of a transformer, whose primary side is connected to said first and said second supply connection conductors of said resistance heating element.

15. The fluid warmer of claim 1 , wherein said heat transfer element has a holding chamber for a bag-shaped vessel of the fluid tube.

16. The fluid warmer of claim 1 , wherein two or more resistance heating elements are provided.

17. The fluid warmer of claim 1 , wherein a power output totals at least 0.15 KW or more than 0.2 KW.

18. A method for operating a fluid warmer for temperature control of a fluid flowing in a fluid tube, the fluid warmer comprising:

a resistance heating element having a first and a second electrical supply connection conductor for connecting the resistance heating element to an alternating voltage source, the first electrical supply connection conductor having a first switch and the second electrical supply connection conductor having a second switch;

a heat transfer element, which is thermally coupled to the resistance heating element for transferring heat to the fluid flowing in a vessel of the fluid tube;

a functional grounding conductor connected to the heat transfer element for functional grounding of the heat transfer element, the functional grounding conductor having a third switch;

a sensor for sensing a leakage current via the functional grounding conductor; and

a control device for controlling the first, second and third switches;

the method comprising the steps of:

a) defining a maximum threshold current magnitude for a leakage current via the functional grounding conductor;

b) comparing a leakage current measured with the sensor with the defined maximum threshold current magnitude; and

c) putting the first, second and third switches into open switching states thereof within a defined time interval on occurrence of a leakage current, which is greater than or equal to the defined maximum threshold current magnitude.

19. The method of claim 18 , further comprising the steps of:

d) comparing the leakage current measured with the sensor with a defined minimum threshold current magnitude; and

e) putting the first, second and third switches into open switching states within the defined time interval on occurrence of a leakage current which is less than or equal to the minimum threshold current magnitude.