The motor is fully earthed,
Maxxy said:
With the method you described, it would be too dangerous
Why? If the motor is fully earthed then there will be no problem. If the motor is not earthed then it will rise to mains potential, but your tester will show this.
If you like, for piece of mind you could carry out an ELV continuity test between the earth at the motor and its source before proceding with the EFLI test.