Hi-Pot Tester
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This is the basic schematic, adapted from others on the web. They are all basically the same with changes made to utilize the transformer and other components one has available. No claims for original design here, just adaptation.
 

The thickness of an insulator has an effect in determining its breakdown voltage, otherwise known as dielectric strength. Specific dielectric strength is usually listed as the number of volts per mil (1/1000 of an inch), or as the number of kilovolts per inch, but in it has been found that the routine relationship between breakdown voltage and material thickness thickness is not linear. An insulator three times as thick has a dielectric strength slightly less than 3 times as much. For rough calculation, using the volts-per-thickness ratings is acceptable.

Material
Kv/inch
inch/Kv
V/mil
Vacuum
20
.05
20
Air
20 to 75
.05 to .013
20 to 75
Porcelain
40 to 200
.025 to .005
40 to 200
Paraffin Wax
200 to 300
.005 to .003
200 to 300
Transformer Oil
400
.0025
400
Bakelite
300 to 550
.003 to .0018
300 to 550
Rubber
450 to 700
.002 to .0014
450 to 700
Shellac
900
.0011
900
Paper (dry)
1250
.0008
1250
Teflon
1500
.00066
1500
Glass
2000 to 3000
.0005 to .00033
2000 to 3000
Mica
5000
.0002
5000