IR Testing of Cables

It has always been common practice by the testing branch of my company that when doing a IR test on a cable, the +ve terminal of the IR
tester (megger) is connected to the screen and the -ve terminal to the cable core conductor. the accepted reasoning (since the 'olden days' it seems) was connected with water absorption during the test....but the exact logic behind this is absent. i'd be grateful if anyone could give an explanation for this method of connection. many thx in advance,
Daniel
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Just to further define my question: It is asserted by the inherited testing branch wisdom that if the testing polarity is reversed (+ve on core conductor and -ve (gnd) on the screen then the cable will absorb moisture and degrade the measurement. Any explanation for this would be gratefully recieved.
Thx
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