Why cannot test phase to phase on 3 phase motor?

Motor voltages and amperages can both be tested. The best way to test the continuity of a motor is with a megger. This will check that the motor windings are intact and if there is any voltage leakage to ground or voltage leakage to one of the other two windings in the motor. To check amperages on the motor it will have to be energized. Leave a message in discussion page as to what you are trying to do.

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Because there is no such thing as 'phase to phase'! However, there is such a thing as 'line to line', and this can be tested!