METREL MI 3298, Instruction Manual With Exercises

Page: 50 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
50

Distance “d1” from tested earthing electrode system E/ES to current probe H
shall be: d1 ≥ 5•a1

Distance “d2” from tested earthing electrode system E/ES to potential probe S
shall be:
Measurement 1:

Distance from earthing electrode E/ES to voltage probe S shall be:
d2 = 0.62•d1
Measurement 2:

Distance from earthing electrode E/ES to voltage probe S’’ shall be:
d2 = 0.52•d1
Measurement 3:

Distance from earthing electrode E/ES to voltage probe S’ shall be:
d2 = 0.72•d1
In case of properly selected “d1” the result of measurements 2 and 3 are symmetrical
around the result of measurement 1. The difference between (measurement 2-
measurement 1) and (measurement 3 - measurement 2) must be lower than 10 %.
Higher differences or non-symmetric results mean that the voltage funnels influence the
measurement and the distance d1 should be increased.
Notes:

Initial uncertainty of measured resistance to earth depends on distance between
electrodes d1 and size of earthing electrode a.
d1/a1 Uncertainty [%]
5 10
10 5
50 1
Table 10-2: Measurement result uncertainty vs d1/a1


It is recommended to repeat the measurement at different placements of test
probes.

The test probes shall also be placed in the opposite direction from tested
electrode (180° or at least 90°). The final result is an average of two or more
partial results.

According to the standard IEC 60364-
6 the distances between S’-S
(measurement 2) and S’’-S (measurement 3) shall be 6 m.

Metrel recommends unrolling the whole cable from the cable reel to avoid
electromagnetic interference. Cable should not form loops and should not be
placed close to each other or parallel to other metal conductors.