MI 3290 Earth Analyser
Appendix C
–Functionality and placing of test probes
150
Measurement 2
Distance from earthing electrode to voltage probe S (S’):
d2, contrary side regarding to H
The first measurement is to be done at the S and H probes placed at a distance of d2.
Connections E, probes H and S should form a equilateral triangle.
For the second measurement the S probe should be placed at the same distance d2 on the
contrary side regarding to the H probe. Connections E, probes H and S should again form a
equilateral triangle. The difference between both measurements shall not exceed 10%. If a
difference in excess of 10% occurs, distance d2 should be proportionally increased and both
measurements repeated. A simple solution is only to exchange test probes S and H (can be
done at the instrument side). The final result is an average of two or more partial results.
It is advisable for the measurement to be repeated at different placements of test probes. The
test probes shall be placed in the opposite direction from tested electrode
(180° or at least 90°).
Test probe resistances
In general test probes should have a low resistance to earth. In case the resistance is high
(usually because of dry soil) the H and S probes can significantly influence the measurement
result. A high resistance of H probe means that most of the test voltage drop is concentrated at
the current probe and the measured voltage drop of the tested earth electrode is small. A high
resistance of S probe can form a voltage divider with the internal impedance of the test
instrument resulting in a lower test result. Test probe resistance can be reduced by:
Watering in the vicinity of probes with normal or salty water.
Depleting electrodes under dried surface.
Increasing test probe size or paralleling of probes.
METREL test equipment displays appropriate warnings in this case, according to IEC 61557-5.
All METREL Earth testers measure accurate at probe resistances far beyond the limits in IEC
61557-5.
Figure C.6:
Different measured voltage drops at low and high probe resistance
