Fluke 1625Earth/Ground Manual do Utilizador descarregar pdf (Página 87)

Earthing Resistance

Principle of Operation

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).

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

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.

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Fluke 1625Earth/Ground Manual do Utilizador Página 87