Fluke 1625 FE 1625 Users Manual - Page 87

The Potential Gradient Area

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Earthing Resistance Principle of Operation The Potential Gradient Area Around every earth electrode a so called potential gradient area develops during the flow of an electric current (see picture below). U US UE E 40 60 m edw058.eps If the voltage between the earth electrode and a probe with a distance "a" from the earth electrode is measured, the value increases less with increasing distance. Once the voltage does not increase, the probe is levelled to earth potential FE that is, outside the potential gradient area. It is the soil resistivity that mainly affects the diameter of the potential gradient area. This means the diameters in soils with a bad conductivity are correspondingly wide (30 ... 60 m), soils with a good conductivity correspondingly narrow (10 ... 15 m). Determining the probe- and auxiliary earth electrode resistance provides information about the size of a possible potential gradient area. High resistances lead to correspondingly large gradient areas and vice versa. In this context it has to be taken into account that soils with a good conductivity and correspondingly small potential gradient areas result in a relatively steep voltage shape and therefore in a relatively high step voltage. If necessary, such systems have to undergo a potential check. 79

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Earthing Resistance
Principle of Operation
79
The Potential Gradient Area
Around every earth electrode a so called potential gradient area develops dur-
ing the flow of an electric current (see picture below).
U
U
S
U
E
E
40
60 m
edw058.eps
If the voltage between the earth electrode and a probe with a distance "a" from
the earth electrode is measured, the value increases less with increasing dis-
tance. Once the voltage does not increase, the probe is levelled to earth poten-
tial F
E
that is, outside the potential gradient area.
It is the soil resistivity that mainly affects the diameter of the potential gradient
area. This means the diameters in soils with a bad conductivity are correspond-
ingly wide (30 ... 60 m), soils with a good conductivity correspondingly nar-
row (10 ... 15 m).
Determining the probe- and auxiliary earth electrode resistance provides in-
formation about the size of a possible potential gradient area. High resistances
lead to correspondingly large gradient areas and vice versa. In this context it
has to be taken into account that soils with a good conductivity and corre-
spondingly small potential gradient areas result in a relatively steep voltage
shape and therefore in a relatively high step voltage. If necessary, such systems
have to undergo a potential check.