Omicron CPC 100, Reference Manual

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CPC 100 V1.41
7 - 14
RGround
A good substation grounding system is crucial to protect people from injury and
damage of equipment. International standards such as DIN VDE 0101/
CENELEC HD637S1, IEEE Std 80-2000 or IEEE Std 81-1983 give guidelines
how to measure such impedances.
Smaller grounding systems with a diameter of 100 m (300 ft) or smaller can be
tested with the
RGround test card and CPC 100 alone, for large systems the
CP CU1 coupling unit and the Sequencer test card is a better choice. Note that
no other grounding system must be close by.
The current-voltage method as called in CENELEC HD637S1 or fall of potential
method as called in IEEE standards is a good solution to measure the ground
impedance of a substation. The current is fed via a long cable into a remote
ground. This ground can be any ground from a simple test probe to another large
grounding system. The distance between this probe and the grounding system
under test shall be at least 10 times the diameter of the grounding system, 15 to
20 times is better. Then measure the voltages with a second test probe at
various distances around the substation. If possible, choose the measurement
points in a 90º angle (bird’s-eye view) relative to the current path. In any case,
avoid measuring close (< 60º) to the current path.
Measurement data at a large distance (typically three times the length of the
substation) from the substation allow the calculation of the overall substation
ground impedance Z
ground
as defined in VDE 0101. Measurements at different
distances are recommended. The measurement points should all show similar
results. If the points are set too close to the substation or close to other
grounding systems or over buried pipes, the results obtained are not stable.
Figure 8:
Theoretical resistance
characteristic of an
earth electrode
linear range of earth resistance
Distance
100
200
300
400
500
600
Earth resistance m
Ω
0