On Wednesday, November 7, 2018 at 3:39:07 AM UTC-5, GS wrote:
Ivan Vegvary wrote:
Purchased used Variac. Good action, very smooth from 0 to 140V AC.
Question: Input and output are both 2 prong, no ground. Should I modify
it to three prong with ground?
BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac? Please
advise as to best procedure. Thanks. Main use of Variac will be to power
up ancient tube equipment.
Thank you!

I would place ISO XFMR after variac. The transformer is not 1:1 and
designed to be 1:1 under full load. Depends on current either way you can
end up say 160 volts no load.

Greg
I can't advise on the modification to connect the variac to earth ground. One would have to know the design of the variac to accomplish this safely, if indeed it is a good idea at all. IMHO, I would not modify the autotransformer. One need to be aware of the 'upstream' power distribution network. In the states, residential customers are fed by three wire split phase distribution transformer that sits on a power pole or power utility box. The 120 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 isolated secondary winding. A portion of the primary winding serves as a secondary. 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 that has been sitting idle for a number of years, testing the tolerance of electronic equipment/designs to under/over voltage conditions, gradually increasing 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 computers to prove that the machines we designed would function down to at least 80% of line voltage, and when a threshold was hit, the machine would gracefully shutdown in x milliseconds. It was a requirement of the system.

Isolation transformer are used a lot in servicing certain type of electronic equipment (e.g. tube TVs, amplifier, radio transmitters, etc.). It removes the direct path to ground of the electrical power, significantly reducing 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 circuit that effectively causes a path to ground through the probe.

good luck
J