Ensuring that a rotating drum or impeller is correctly earthed may be difficult as it is not always possible to rely on the connection made from its shaft to the body of the machine, owing to the design of bearings, etc.  A popular method of guaranteeing earth continuity is to use an earth monitoring module to test the earth connection to the drum or impeller via a pair of carbon brushes or a slip ring, acting on the shaft.

Such modules may also be used to test the earth connection to key items of fixed plants, such as large storage vessels for flammable liquids.

NFPA 77, when discussing the static dissipation path through bearings (in this case, in rail car wheel assemblies) states that: resistance to ground ... might not be low enough to prevent the accumulation of static electric charge (7.8).

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