A preheating power supply, comprising: power conversion circuitry including first and second output power connectors (230, 228) of the preheating power supply wherein the first output power connector is connectable to a second output power connector of a different power supply, wherein both the first and second output power connectors (230, 228) are connectable to a first contact tip, and wherein the power conversion circuitry is configured to output welding-type power to the first contact tip via the first output power connector and the second output power connector; wherein said preheating power supply establishes a preheat current path via the first contact tip configured to return to the preheating power supply via the second output power connector of the different power supply; being characterised in that: the preheating power supply comprises a bypass path prevention circuit comprising one or more switching elements and a controller, and in that: the controller controls the bypass path prevention circuit to: enable the one or more switching elements to conduct current when less than a threshold weld current is detected as flowing between the first and second output power connectors (228, 230); and conversely when more than a threshold weld current is flowing between the first and second output power connectors (228, 230). power conversion circuitry including first and second output power connectors (230, 228) of the preheating power supply wherein the first output power connector is connectable to a second output power connector of a different power supply, wherein both the first and second output power connectors (230, 228) are connectable to a first contact tip, and wherein the power conversion circuitry is configured to output welding-type power to the first contact tip via the first output power connector and the second output power connector; wherein said preheating power supply establishes a preheat current path via the first contact tip configured to return to the preheating power supply via the second output power connector of the different power supply; being characterised in that: the preheating power supply comprises a bypass path prevention circuit comprising one or more switching elements and a controller, and in that: the controller controls the bypass path prevention circuit to: enable the one or more switching elements to conduct current when less than a threshold weld current is detected as flowing between the first and second output power connectors (228, 230); and conversely when more than a threshold weld current is flowing between the first and second output power connectors (228, 230). the preheating power supply comprises a bypass path prevention circuit comprising one or more switching elements and a controller, and in that: the controller controls the bypass path prevention circuit to: enable the one or more switching elements to conduct current when less than a threshold weld current is detected as flowing between the first and second output power connectors (228, 230); and conversely when more than a threshold weld current is flowing between the first and second output power connectors (228, 230). enable the one or more switching elements to conduct current when less than a threshold weld current is detected as flowing between the first and second output power connectors (228, 230); and conversely when more than a threshold weld current is flowing between the first and second output power connectors (228, 230).