Earth Resistance
'Earth Resistance' are measured using frequency near to the standard supply frequency either 50Hz or 60Hz. Usually, the tester are coming with capability to adjust the frequency in order to prevent noise from influence the reading while test conducted. This device is easy to find and some of the brand such as Fluke and Megger.
Generally, the normal requirement for the earth resistance of grounding system should be below than 10 ohm for the lightning protection system (BS 6651 and IEC 62305-3) and may less for special purpose such as for transformer, generator and sensitive equipment i.e. instrument and telecommunication.
Earth Impedance
'Earth Impedance' are measured using frequency near to the lightning frequency which is in MHz in order to measure the actual impedance (resistance + inductance) in the earthing network system. However, this device are not usually sell by manufacturer and may not have supporting certificate for calibration.
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