When measuring the relative phase and phase insulation of the cable, the influence of the ungrounded phase on the measured value is measured
The insulation resistance of grounding and grounding measured without you that said, insulation test is a rigorous work, please follow the operating rules for electric power, otherwise it will cause people and equipment irreparable loss.
If ungrounded, you can't find it if the cable has an interphase short circuit.
When measuring multicore cables, the rest must be grounded.
