MEGABRAS MTD-20KWe, Руководство пользователя

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16
The appropriate compliance of this condition must be checked by carrying
out three successive measurements of GDS resistances, keeping the
current rod position but moving the voltage rod about 2 meters between
measurements (points L, M, y N). If the three measurements show the
same result (within the earth meter specified error) the measurement should
be considered correct. Otherwise, the distance to the current rod should be
significantly increased and the process, repeated.
Potential plateau
R
A
L M N
B
fig. 05
Voltage Rod E2
measurements
points
Current Rod E3
influence area
Ground
diffusion
resistance
Current
Rod E3
Ground diffusion
resistance influence
area
Generally, GDS dimensions are greater than those of the auxiliary current
rod, thus its area of influence diameter is also greater. So, in order to
comply with the required condition, the probe must be set up closer to the
current rod than to the GDS. A distance of 62% is usually adopted as a first
try (62% rule). It should be noted that when measuring GDS resistances in
large systems (such as a large grid that underlies a substation) distances of
hundreds of meters are required to fulfil the condition. Technical literature
describes alternative methods that make it possible to reduce those
distances with valid results, such the “slope method” developed by Dr. G.
Tagg in 1970.
All these considerations refer to physical aspects, essential to the
measurement process, thus they are applied to all earth meters, and do not
depend on the manufacturer or the technology used.