For large sized earth systems, for example those formed by multiple earth electrodes on a wide area, some particular precautions are necessary.
First of all, the earth tester used shall have the highest test current as possible to ensure good accuracy on low resistance measurements that are typical for large earthing systems.
The driven auxiliary earth electrode C should be placed as far from the earthing system as practical, with this distance being at least equal to the estimated diagonal of the earthing system.
The auxiliary earth electrode P is then driven in at a number of points roughly on a straight line between the earthing system and C. The ensuing resistance readings should be logged for each of the points and then drawn on a curve of resistance vs. distance.

The correct earth resistance value is usually obtained from the flat part of curve. See the below figure.

• To obtain an ideal earth resistance curve at least 10 readings should be taken at equally spaced intervals.
• True resistance will be obtained where the curve flattens out (typically around the 62% of distance D).
• This method gives a correct value of earth resistance if the earth resistivity and the soil conditions at the flat point do not vary at other points (assuming there are no other measuring errors).
• Localized reading deviations can be caused by buried metallic objects such as pipes or inhomogeneous soil around construction sites.
• If the curve obtained does not show a flat point, the measurements should be retaken placing the auxiliary earth electrode C at a further distance.
• If the expected earth resistance value is very low, say, lower than 1 or 2 ohms, it is recommended to use earth testers with higher test current since the higher test current creates a greater voltage drop that is more measurable.
• The earthing system under test has to be temporarily disconnected from the main installation. To avoid possible risk of electric shock while disconnecting the MEC (Main Earth Conductor), switch off the power supply prior to implementing the temporary measures. The power supply should only be restored after the temporary measures have been removed.