Thank you for letting us know. The experience that you’ve had indicates a build-up of static electricity or an issue with the wiring to which your Glowforge is connected, as @eflyguy mentioned. Please make sure that your wiring is properly grounded and that it meets your local electrical code.
Regarding extension cords, we have performed extensive safety and performance testing with the Glowforge unit plugged into the wall. We haven’t done that with third party electrical products, so unfortunately we can’t recommend any.
Let us know if this helps!