An overvoltage protection device (20, 30, 40, 50, 60, 70, 80, 90, 100) for a medical device, comprising: a first line (11, 11A, 11B) configured to connect between first and second terminals; wherein the first line (11, 11A, 11B) is a first patient line of the medical device, the first terminal communicates with an internal circuit of the medical device and the second terminal comprises a patient interface; a current limiting device (25, 25A, 25B) connected in the first line between the first terminal and second terminal, wherein the current limiting device comprises a first depletion mode metal-oxide-semiconductor field-effect transistor, MOSFET, the current limiting device having an on state and an off state and being configured to: - limit current passing through the current limiting device between the first and second terminals when in the off state, and - switch from the on state to the off state in response to a predetermined biasing voltage; and a second line (23, 23A, 23B) extending off the first line; a biasing voltage generating circuit (22, 22A, 22B, 24, 24A, 24B, 42, 105A, 105B, 110A, 110B) connected in the second line and comprising at least one biasing element (22, 22A, 22B) and at least one additional circuit element (24, 24A, 24B, 42, 105A, 105B, 110A, 110B) connected in series with the biasing voltage generating device, wherein the biasing element comprises a first biasing resistor and wherein the additional circuit element is a transient voltage limiting device; the biasing element and additional circuit element configured to apply a predetermined biasing voltage to the current limiting device in response to a transient overvoltage on the first line, whereby the current limiting device is switched off and limits current flow through the first line, wherein the first depletion mode MOSFET has a drain terminal connected to the second terminal, a source terminal in communication with the internal circuit of the medical device, and a gate terminal, wherein the first biasing resistor is connected to the gate terminal of the first MOSFET, and wherein first biasing resistor is connected i) at one end to the first line at a location between the first MOSFET and the internal circuit of the medical device and ii) in series with the transient voltage limiting device on the second line, wherein a junction between the first biasing resistor and the transient voltage limiting device is connected via a third line to the gate terminal of the first MOSFET. a first line (11, 11A, 11B) configured to connect between first and second terminals; wherein the first line (11, 11A, 11B) is a first patient line of the medical device, the first terminal communicates with an internal circuit of the medical device and the second terminal comprises a patient interface; a current limiting device (25, 25A, 25B) connected in the first line between the first terminal and second terminal, wherein the current limiting device comprises a first depletion mode metal-oxide-semiconductor field-effect transistor, MOSFET, the current limiting device having an on state and an off state and being configured to: - limit current passing through the current limiting device between the first and second terminals when in the off state, and - switch from the on state to the off state in response to a predetermined biasing voltage; and - limit current passing through the current limiting device between the first and second terminals when in the off state, and - switch from the on state to the off state in response to a predetermined biasing voltage; and a second line (23, 23A, 23B) extending off the first line; a biasing voltage generating circuit (22, 22A, 22B, 24, 24A, 24B, 42, 105A, 105B, 110A, 110B) connected in the second line and comprising at least one biasing element (22, 22A, 22B) and at least one additional circuit element (24, 24A, 24B, 42, 105A, 105B, 110A, 110B) connected in series with the biasing voltage generating device, wherein the biasing element comprises a first biasing resistor and wherein the additional circuit element is a transient voltage limiting device; the biasing element and additional circuit element configured to apply a predetermined biasing voltage to the current limiting device in response to a transient overvoltage on the first line, whereby the current limiting device is switched off and limits current flow through the first line, wherein the first depletion mode MOSFET has a drain terminal connected to the second terminal, a source terminal in communication with the internal circuit of the medical device, and a gate terminal, wherein the first biasing resistor is connected to the gate terminal of the first MOSFET, and wherein first biasing resistor is connected i) at one end to the first line at a location between the first MOSFET and the internal circuit of the medical device and ii) in series with the transient voltage limiting device on the second line, wherein a junction between the first biasing resistor and the transient voltage limiting device is connected via a third line to the gate terminal of the first MOSFET.