On these factors the resistance of earthing system are depends-
- Shape and material of electrode or electrodes used.
- Depth in the soil at which the electrodes are buried.
- Specific resistance of soil surrounding and in the neighborhood of electrodes. The specific resistance of the soil varies from one type of soil to another type of soil. Means its not constant.
- The specific resistance of soil varies between wide limits and very much depended open its moisture contain.
Method of measuring earth resistance:
Full of potential method:
- The CTK for measurement of earth resistance with fall of
potentialmethod can be drawn as follows:
- A current is passed through earth electrode an auxiliary electrode B (which usually iron spike) increase in
a insertedin earthat a distance away from the earth electrode. Between E and B a second auxiliary electrode A is inserted in earth. The potential difference V, between A and Eis measured for a given current I. The flow of ground current is shown further.
- The lines of the first electrode current diverge for that the second electrode current converge. As a result, the density is much greater in the vicinity of the electrodes than at a distance from them. The potential distribution between the electrodes is shown further.
- It is apparent from this curve (figure-b) that the potential rises in the proximity of electrodes E and B and is constant along with middle section. The resistance of earth therefore is:
RE = V/I or VEA/I.
- The position of electrodes B and E is fixed and the position of electrode A is changed and resistance is measured for various positions of electrode A. A graph is plotted between earth resistance against the distance between electrode A and E. This is shown in figure as follows:
- From the above graph, it is clear that the measured value of earth resistance depends upon the position of the auxiliary electrode A. The earth resistance rises rapidly initially.
- When the distance between earth electrode E and auxiliary electrode A is increased, it then becomes constant and when the auxiliary electrode A approaches the auxiliary electrode B, the resistance rises again.
- The placing of electrodes is thus very important and serious error may be cause by incorrect placing of the electrodes. The correct value of resistance of earth, R., is when the auxiliary electrode A is at such a distance that the resistance lies on the flat part of curve.
- The spacing between the auxiliary electrodes A and the earth electrode E, should be large so as to get proper results. The distance may be few hundred meters in case the earth resistance is low.
Also Read- Measurement of resistance