MEGABRAS MTD-20KWe, Руководство пользователя

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15
2.12. Influence of grounding resistance of the
auxiliary rods
Whenever the probe grounding resistance is lower than 1,000
Ω
, the error
caused is not significant (less than 5%). The current rod may have a
resistance of up to 5,000
Ω
, without affecting measurements and without
causing significant errors.
2.13. Special considerations about grounding
resistance measurements
By employing the Fall of Potential method, normally used to measure
Grounding Diffusion System (GDS) resistance, two rods are used as
auxiliary electrodes. In
figure 01 , D
1
is the distance between GDS E
1
and
the E
3
current electrode , whereas D
2
is the distance between GDS and the
E
2
probe
.
The current generated by the equipment flows through GDS and
the current electrode, and the voltage between GDS and the E
2
probe is
measured. The R value is obtained as the quotient between the voltage and
the current.
In figure 05 the potential profile has been graphically shown relating the
GDS in the area between this and the current electrode, assuming that the
distance between those points is enough for its corresponding “influence
areas” not to be overlapped. An “influence area” is the area close to each
electrode where a significant potential gradient is observed. Out of that
area, the potential is constant (potential plateau between points A and B -
Fig 05).
In order to obtain a valid measurement of the GDS resistances, it is
necessary to move the auxiliary current rod away enough to comply with the
“no overlap condition” of the areas of influence, and the voltage rod (probe)
must be set up in the potential plateau area. The criteria commonly
accepted is to consider that the radius of each influence area is three to five
times bigger than the electrode dimension.