Electrical Grounding Systems:
Use of electricity brings with it an electric shock hazard for humans and animals particularly in case defective electrical apparatus. It is therefore necessary to connect the system to ground at suitable points. In case of fault, sufficient current will flow through and operate the protective system isolating the faulty circuit Hence the connection to ground should be of low resistance.
Soil resistivity is a deterministic factor in evaluating the ground resistance. It is an electro physical property. The soil resistivity depends on type of soil, its moisture content and dissolved salts.
| Type of Soil | Resistivity (Ω m ) |
| Loam, garden soil | 5-50 |
| Clay | 8-50 |
| Sand and gravel | 60- 100 |
| Sand stone | 10-500 |
| Rocks | 200- 10,000 |
Resistance of a Electrical Grounding Point Electrode:
The ground resistance of a hemisphere electrode is sum of resistances of an infinite number of thin hemispherical shells of soil.
R = ∫r r1 ( ρdx) / (2πx2) = ρ / 2π ( 1/r - 1/r1)
where ρ is earth resistivity.
As r1 =∞, R (infinity) = ρ / 2πr
Resistance of driven rods for electrical grounding:
Driven rod is one of the simplest and most economical forms of electrodes.
R = ρ / 2πl ln (4l/d )
If rod is cylindrical with hemispherical end,
R = ρ / 2πl ln (2l/d )
when rod crimes current uniformly along its length,
R = ρ/ 2 π l (ln(8l / d) -1)
The resistance of n rods in parallel is found to exceed (1/n) of that of a single rod because of their mutual screening. The screening coefficient η for n electrodes in parallel is defines as
η = Resistance of one electrode/ n (resistance of n electrodes in parallel)
Electrical Grounding grids:
To obtain a low ground resistance at high voltage sub-stations we used inter connected ground grids. Size of the grid conductors required to avoid fusing under fault current I is
a = I √ [ ( 76 t ) / ( ln[234+ Tm ]/[ 234 + Ta]) ]
where,
a = copper cross section,
t = fault duration (sees),
Tm = Maximum allowable temperature,
Ta = Ambient temperature.
Such a grid not only effectively grounds the equipment but has the advantage of controlling the voltage gradients at the surface of the earth to values safe for human contact.
Neutral Electrical Grounding:
Grounding of the neutral points of generators, transformers, and transmission schemes is an important item in the design of power systems. It has a considerable bearing on the levels of transient and dynamic over-voltages stressing the equipment insulation. It also directly affects the levels of short-circuit currents in the power network and accordingly, the ratings of switchgear needed to cope with them.
The methods of system neutral grounding include resistance and low reactance for effective grounding.
(Examples of power system neutral grounding)
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