ADInstruments PowerLab 30 Series Owner's Manual page 33

The earthing stud can also be used where a suitable ground connection is not
provided with the mains supply by connecting the stud to an earthed metal
infra-structure, such as a metal stake driven into the ground, or metal water
piping. This may also be required in laboratories where safety standards
require additional grounding protection when equipment is connected to
human subjects. Always observe the relevant safety standards and
instructions.
Note that magnetically-induced interference in the recorded signal can be
reduced by minimizing the loop area of signal cables, for example by twisting
them together, or by moving power supplies away from sensitive equipment.
This can reduce the inductive pick-up of mains frequency fields. Please
consult a good text for further discussion of noise reduction.
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Appendix A Technical Aspects