A friend of mine has a TIG welder, which uses a high frequency signal to make the start of the weld much smoother. He's noticed that the welder seems to be tripping his GFIs (works fine on non-GFI circuits). So, that got me thinking, is it possible to build a low pass filter for something like this? It would need to let the mains current through (60 Hz), but reject anything above say 1Khz (not really sure about that since I don't have the welder datasheet at the moment, but I figure that's a good starting number for argument's sake). The only real issue I see is that the welder runs on 240v at 100 amp (max). That suggests to me that you would have to use some seriously large resistors, even in an active configuration. So, what's the best way to approach this problem? Is there any way to avoid having to bite the bullet and buy several, ultra expensive, 100w (or even 1kW) resistors?