"This particular clause is commonly referred to as the "foil
test", and simply stated, requires that tin foil (or a similar flexible conductor) be wrapped around the
exterior of the insulated sample and connected to a power frequency (hipot) test set. The conductor is
connected to ground, and the exterior foil energized at full rated line-line voltage (4.76kV or 8.25kV or
15kV or 38kV, according to the switchgear rating) for one minute. If the insulation successfully withstands this test, it meets the requirements for use in medium voltage switchgear. "
so evidently the insulation is stressed using a 'tubular conductor' of foil or a wrapped conductive ribbon, and the voltage gradient within the insulation is between this foil and the conductor.
If a needle point was substituted for the foil the insulation would probably fail because it's easier for a spark to jump between a needle and the side of a wire than between a tube with a wire down the middle.
I guess the one minute is to allow for dielectric heating, which I guess reduces the breakdown voltage.
