METREL MI 3298, Instruction Manual With Exercises

Page: 51 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
51
2.3.1.2 Equilateral placement
Figure 31-2: Equilateral placement
Measurement 1:
Distance “d1” from tested earthing electrode E/ES to current probe H and voltage probe
S to E/ES (H) should be at least: d1 = 5•a
Probes E/ES, H and S should form an equilateral triangle.
Measurement 2
Position of voltage probe S’ is contrary side regarding to H, at same distances “d1”
between test probes. Probes E/ES, H and S’ should form an equilateral triangle.
The difference between both measurements shall not exceed 10%. If a difference in
excess of 10% occurs, distance d1 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°).
MI 3290 gives you the status about performed measurement in case of present
disturbances:

Consider warnings presented on the display when starting the measurement!

Take care about
the “noise” warning presented on the LCD in case, that high
noise currents and voltages in earth could influence the measurement results

When measuring at high frequencies use the guard terminal and shielded cable
for test probe H.
2.3.1.3 Test probe resistances
Test probes should have a low resistance to earth. In case that the test probe
resistance is too high (usually because of dry soil) the H and S probes can significantly
influence to the measurement result. High resistance of H probe means that most of the
voltage drop is concentrated at the current probe and the measured voltage drop of the