Checking ground electrode impedance for commercial, industrial and residential buildings:Part2

Technical information supplied by John Wilson on behalf of Comtest for the Fluke Corporation

The second of a two part discussion on earth/ground principles covers selective testing, stakeless testing and 2 pole testing.

Published in:

Electricity+Control, June 2016 (pages 26 – 28)

Enquiries: Email jgpwilson@xsinet.co.za

Summary of Ground Electrode Test methods.

	Advantages	Drawbacks
Fall-of-Potential	Widely accepted When you see the characteristic curve you know you have a good measurement	You have to disconnect the ground The stakes may not be easy to drive There may not be space around the ground electrode to drive the stakes
Selective method	Do not have to disconnect electrode Widely accepted When you see the characteristic curve you know you have a good measurement	The stakes may not be easy to drive There may not be space around the ground electrode to drive the stakes
Stakeless method	Convenience	Assumes a low-impedance parallel path Possible to get very low readings by mistakenly measuring on a hard-wired loop
Two-pole method	Convenience	Impossible to judge on the integrity of the ‘auxiliary electrode’ Cannot be sure if you are outside the area of influence

Take note

The impedance from the grounding electrode to the earth varies depending on the resistivity of the surrounding earth and the structure of the electrode.
Resistivity defines the material’s ability to conduct current.
Since resistivity may decrease with depth, one way to reduce earth impedance is to drive an electrode deeper.