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I can't advise on the modification to connect the variac to earth ground. O ne would have to know the design of the variac to accomplish this safely, i f indeed it is a good idea at all. IMHO, I would not modify the autotransf ormer. One need to be aware of the 'upstream' power distribution network. In the states, residential customers are fed by three wire split phase dis tribution transformer that sits on a power pole or power utility box. The 1
20 VAC power is between ground and one half of the phase winding.I would also place the isolation xfmr after the variac. Variac (short for variable autotransformer) is an interesting device in that there is no isol ated secondary winding. A portion of the primary winding serves as a secon dary. There are taps along the primary winding that allow for stepping up or down the output voltage. One can use a variac for: reforming power supply capacitors in equipment th at has been sitting idle for a number of years, testing the tolerance of el ectronic equipment/designs to under/over voltage conditions, gradually incr easing the voltage to equipment to avoid 'shock' of the components, dimming incandescent light bulbs, and testing dropout of electronic devices under low voltage conditions. We used variacs to lower the supply voltage to comp uters to prove that the machines we designed would function down to at leas t 80% of line voltage, and when a threshold was hit, the machine would grac efully shutdown in x milliseconds. It was a requirement of the system.
Isolation transformer are used a lot in servicing certain type of electroni c equipment (e.g. tube TVs, amplifier, radio transmitters, etc.). It remov es the direct path to ground of the electrical power, significantly reducin g the chance of death. Ask any service tech or engineer who has seen their scope probe or DMM probe almost vaporize when the accidentally probe a cir cuit that effectively causes a path to ground through the probe.
good luck J