Before connecting the differential start relay to the main transformer, it is necessary to
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Before connecting the differential start relay to the main transformer, use a DC voltmeter to verify that there is no voltage between the two ends of the connector with respect to ground, and then make the connection； Can it be put into use if there is no voltage difference at both ends of the connected section when measured directly? Why is it necessary to measure that there is no voltage with respect to the ground? ?
I usually start the device as soon as it shows no activity; I need to be more careful in the future