The live/earth and live/live thing dates back to the olden days when the windup megger was used to test the circuit "poles to earth" then if lamps were out it would be tested "between poles"
Incidentally its important to note that its live/earth... not live/cpc. Testing insulation to the cpc in isolation is not strictly compliant. The CPC should be in the earth bar and the test carried out to earth, that way you pick up faults to other "earthy" things
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