LEM A188506510, Руководство по эксплуатации

Страница: 58 / 66

56
12 Basic APPENDIX
12.1 Earthing resistance
Per definition, the earthing resistance consists of several individual resistances.
1) The resistance of connecting lead to earth electrode
2) The resistance of the actual earth electrode; earthing rod, earthing plate,
earthing strip, mesh earth electrode etc.
3) The dissipation resistance, the resistance between earth electrode and soil
potential
.
As the connecting cable and the resistance of the earth electrode are neglectably
small after correct dimensioning, the earthing resistance dominantly dependends on
the dissipation resistance. This shows that an accurate measurement of the
dissipation resistance is necessary to determine the exact earthing conditions for
protective measures. As the dissipation resistance is not only dependent on the
specific soil resistivity, that is the resistance of the actual soil ( gravel, clay, granite ),
but also significantly depents on the shape of the earth electrode, a metrological
check has to be made even if the position of the earth electrode and the condition of
the soil is well known.
For redimensioning of an earthing system, e.g. a lightning protection, an approximate
calculation, according to the table below, is possible. As a basis for this calculation
the soil resistivity of the spot where the earth electrode is to be installed has to be
known.
Type of soil Soil resitivity
Earthing resistance (
Ω
)
[
ρ
E
]
Earthing rod m depth Earthing strip m
Ω
.m
3 6 10 5 10 20
Moist humus soil, 30 10 5 3 12 6 3
moor soil, swamp
Farming soil,loamy
and clay soils 100 33 17 10 40 20 10
Sandy clay soils 150 50 25 15 60 30 15
Moist sandy soil 300 66 33 20 80 40 20
Dry sandy soil 1000 330 165 100 400 200 100
Concrete 1 : 5* 400 160 80 40
Moist gravel 500 160 80 48 200 100 50
Dry gravel 1000 330 165 100 400 200 100
Stoney soil 30000 1000 500 300 1200 600 300
Rock 107 - - - - - -
* for concrete mixtures 1 : 7 the values have to be increased by 24 %