Keysight 34450A Guia Del Usuario página 99

Measurement Tutorial 3
Figure 3-2 Noise caused by ground loops
The best way to eliminate ground loops is to isolate the multimeter from earth by
not grounding the input terminals. If the multimeter must be earth-referenced,
connect it and the device-under-test to the same common ground point. Also
connect the multimeter and the device-under-test to the same electrical outlet
whenever possible.
Keysight 34450A User's Guide 99