33 KV Earthed Cable Megger Test

5 kV megger can be used to test 33 kV cable and 1000 Volts megger can be used for 6.6 kV cable.

...perhaps resulting in a <...33 kV cable...> that would pass at <...5 kV...> and fail at 17kV, and a <...6.6 kV cable...> that would pass at <...1000 Volts...> and fail at 4.2kV.

Which doesn't sound like a good idea.

More like 'half-a-day-out-with-the-Undertaker' instead.

It depends on the test and what standard your are testing to.

Try the Megger website, I believe they have some good downloadable literature which covers the basics.