I can understand the need to check withstand voltage on the 240c. The newest addition to the procedure changes that test into a hipot, probably for safety reasons. If you do the math though, the procedure is looking for a current in the nano amps. Is this overkill? And can I continue to use the old procedure, maybe with a limited cal of 660v and still be following the "letter of the law"? :-D
The wires from the resistor bank pass through a rubber grommet. The rubber grommet slides in a metal slot that is attached to the case. This rubber grommet cracks and breaks when it gets old. This causes leakage current which the 660 V test looks for.
Good thoughts about the Clarostats. We use a number of them, and sometimes as a load for some hipot checks. As it is a high power decade, that stands to reason that you want it to "withstand" higher voltages.