The basic objective of good earthing is to ensure the safety of life and property from shock and fire.
Lightning, Surges or Unintentional contact between an energized electric conductor and the metal frame or structure that encloses it or an insulation failure in electrical equipment etc., can cause dangerously high voltages in the electrical distribution system. Under such circumstances, grounding provides an alternative low impedance path and thereby minimizes damages.
A good and an efficient earth ensures that all parts of apparatus other than the live parts shall be at earth potential, i.e. zero at all times.
The resistance can be lowered only by altering the medium adjacent to the electrode. The IEEE80, IS3043 and BS 7430 have all mentioned as under: "To obtain overall low resistance, current density should be as low as possible for the MEDIUM ADJACENT TO THE ELECTRODE which should be so designed to cause the density to decrease rapidly with distance from the electrode." The IEEE80 has also mentioned that the soil around the electrode should be modified to obtain a low earth resistance.
Thus the most important factor for obtaining low resistance value is SOIL TREATMENT i.e. the material around the electrode - which is the backfill material and not the electrode itself.
The type of metal used has no significance on the earth electrode resistance. None of the formulae given in any of the international standards factor in the type of metal in calculation of the earth resistance.
However, it is to be noted that the copper electrode has a higher current handling capability as compared to the GI / steel electrode.
Also in highly corrosive areas, use of GI / Steel electrode may not be appropriate. Copper or copper coated electrodes have a better resistance to corrosion and hence are better suited in such areas.
With the increased degree of sophistication in all equipments, the grounding system needs to be reliable over a long term to ensure safety and proper working of these equipments.
Besides, all industries are facing a shortage of manpower and as such it may not be possible to test the Earth Pits frequently. Also, in a lot of cases the Earth Pits may not be accessible after a few months / years due to various reasons. Further, in a running unit, disconnecting some Earth connections may warrant Shutdown. This coupled with the fact that the equipments are far more sensitive today make a clear case for adopting maintenance free earthing solutions in all scenarios.
While Bentonite clay is effective in certain areas, it cannot be of help in all / dry areas. As per the IEEE 80 "Bentonite has a resistivity of 2.5 Ohm Mtr at 300% moisture. It may not function well in a dry environment because it may shrink away from the electrode increasing the electrode resistance.
When the moisture level in Bentonite goes down below 300% its' resistance will increase exponentially. Hence the use of Bentonite clay will not provide a maintenance free earthing in most areas especially dry areas.
The BS-7430 - 2011 provides a formula vide Clause No.9.5.7 Page 40 which is as below:
The above formula specifically gives a weightage to the resistivity of the in-fill material. The lower the resistivity of the in-fill material, lower will be the overall earth resistance (this fact is mentioned in all standards like IS-3043, BS-7430 and IEEE-80 .The resistivity of our backfill material is less than 0.1 O Mtr and thus , by using our special Backfill, the reduction in resistance between 30- 60% over conventional earthing systems can be obtained for a given site conditions.
As mentioned in all the standards like IEEE & BS etc, the inner shell of soil closest to the electrode comprising the bulk of the electrode ground resistance to remote earth. As per the Table given in IEEE No 9,The first 100 mm around the electrode accounts approximately for 48 to 50% of the total resistance. Hence by having a highly conductive compound, the electrode in a 4 inches (100 mm) dia bore hole, upto 30 to 60% of the total resistance can be controlled. This is the reason why a 4" bore hole filled with our Backfill product would be sufficient to obtain a better resistance value.
Bore sizes beyond this will not be cost effective.
It is impossible to get the resistance of < 1O with individual electrodes in all areas. All standards of repute including IS, BS, IEEE80 etc have categorically state, the resistance of the electrode depends on soil resistivity.
Although, our Jef Ecosafe grounding system will generally last for life, we mention the life of the system as 15 plus years.
As explained earlier in Point No. 13 the earth resistance largely depends on the resistivity of the soil (Mother Earth) at site. This soil resistivity changes
seasonally and will vary through the year depending on the weather and other extraneous conditions. As such, the earth resistance also will vary in tandem with variations in soil resistivity.
Further, it has also been recorded by various agencies like CBIP etc, that the soil resistivity in a given place changes over time. Consequently, the earth pit resistance will also change over the years.
However, it is to be noted that this variation will be generally limited to a certain bandwidth only.
The earth pit can definitely be closed with concrete / tiles etc
However, if any garden / lawn / rain water harvesting facilities are in the vicinity, they will be useful for compensating the surrounding soil with moisture as explained in Point No. 10 above.
We can offer a variety of solutions for rocky areas like counterpoise earthing, radial earthing, deep bore earthing etc. Depending on the site conditions and the requirements / applications, the right solution can be adopted. Our team will be happy to help you in suggesting a right method once all the required inputs are shared with us.
In some UPS systems which have an isolation transformer, a separate neutral is derived on the output side through a dedicated earth pit. No other connections shall be made to this earth pit apart from UPS neutral. However, this earth pit can be connected to the general earth grid underground to achieve equi-potential.