LEM SATURN 100 plus Manuel D'utilisation page 26

Measuring
earth/ground connections is sufficient. If the measured value changes, keep
repositioning the probe until the measured value remains unchanged.
If it is impossible to set an earth/ground rod, the probe measuring lead may be
connected to the earthed/grounded neutral wire (N-wire). In this case the
measurement result includes the resistance of the transformer system earth/ground
connection (see section 3.8 Measurement without Probe).
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