Formula to calculate min insulation resistance?

insulation resistance = (2 * operating voltage +1000V)

at operating temperature

if you define what you mean by minimum insulation resistance maybe i could help

132,000V on the towers is usually happy with 6' of air

some insulation is far better than air

dry air 500v/mill

thermoplastics 6000v/mill

your power drill has 1/8" - 1/4" of plastic on the cord

and perhaps .001" on the wire in the coils

the cord gets rougher handling

anything practical insulation resistance (2 * operating voltage +1000V)

witch means a 1500Vdc megger is ok for 120V appliances

should be safe to handle if its more than 10megohms

the 1500V will bridge any pinholes or airgaps

NEVER use a vom the 9v battery wont find a pinhole or an air gap

ceramic on heaters and toasters has a way of letting metals diffuse in it lowering the resistance over time also it usually lowers with temperature

so the resistance reading will be a lot lower while its hot