Loose/corroded terminals, excessive length of conductors or insufficient CSA of conductors.
Tighten/replace terminals, increase CSA of conductors or install supplementary bonding.
One way to reduce impedance is to use extraneous conductive parts as a conductor. eg. steelwork of a building - obviously this should be bonded but you can also connect a 'bond' for your electrical appliance to the steelwork as well.
Also if your using metal conduit/trunking then use that as an earth conductor as well.
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