Variac Tripping Outlet GFCI

[MrScrOggs]

I bought an old variac to use for a VTTC and when I plug the variac into the outlet in my garage the GFCI trips every time without fail. Is the variac broken, or am I just doing something wrong?

not sure why picture is upside down

[TMaxElectronics]

I'd guess some loose wire or something like that is shorting the live to earth. You can measure the resistance between the earth connection and live to get an estimate for what it is. A dead short would point to some loose wire (and probably pop the main breaker too) but something in the kOhm range is more likely to be something else. I've had rust from liquid damage cause leakage to earth and pop the RCD for example.

But you will probably have to open it to have a look at what is going on inside to cause the damage.

[klugesmith]

I have an almost identical variac.  See where yours has a 2-slot receptacle?  I bet the 3-prong plug, on unusually short cord, is not the original.  Make sure there is no connection in the variac between the ground prong and neutral prong.

[MrScrOggs]

Here's some photos, it looks like there is a a solid wire shorting neutral and ground on purpose...?

[klugesmith]

Whoever put that plug on didn't understand electric wiring rules.
Now the return current from variac will be resistively divided between the white and green wires in wall.
Of course the GFCI will trip immediately. 

Get rid of that jumper wire in the plug.  And if it's a two-wire cord, get rid of the ground prong so nobody thinks the variac enclosure is grounded.  Note: These changes will increase the electric shock hazard when variac is plugged into a non-GFCI outlet.

I was not the first responder here to say where you should look.

You have a very good old variac.  Here's a thread about replacing the cord on one like it:
https://highvoltageforum.net/index.php?topic=1266.0

[MrScrOggs]

Interesting, your variac is rated at 10 amps but mine is only for 7.5. Did you ground the chasse to ground with the new cable?

[imp]

Most variacs have numerous bare connections for individual taps. Occasionally dust collection on and about those areas accumulates moisture which can conduct current to grounded areas, thus affecting gfi protection. Not the case here, but good to know.