In a given electrical system, a number of earth pits are installed at various places for different applications like transformer neutral, equipment body, lightning arrestors, electronic equipment etc.
The practice adopted earlier was to keep all these earth pits electrically separated. i.e., they were not interconnected solidly with a permanent connection. Various theories and reasons were to put-forth to justify this practice.
To have clarity on the correct approach to this subject, we have looked at various national and international standards of repute like IS3043, BS7430, IEC61000-5-2, IEC62305-3, and IEEE142-2007.
It is to be noted that all the above standards have clearly advocated the need to interconnect all earth systems to achieve equipotential.
1) Interconnection of transformer neutral with the general earth grid.
Transformer neutral should be solidly interconnected to the general earth grid as per guidelines are given in Section-5 Page-53 & 54 Clause No. 22.1.3 and 22.3.1 of IS3043. The said standard has permitted and advocated the interlinking of the neutral earth pit with the general body earth. The standard goes on to mention that the earlier practice of separate/isolated earth for the transformer neutral needs to be revised in favor of an interlinked system to AVOID UNNECESSARY REDUNDANCY AND EXPENDITURE.
The design methodology, measurement of resistance and acceptance criteria of the earth grid while using such an interconnected earth system is clearly defined in the above-mentioned Clauses of IS3043. The relevant extract of the IS3043 is enclosed.
2) Interconnection of lightning arrestor earth with general earth grid.
The Standards IEC62305-3, BS7430 have clearly recommended that the lightning protection system earth should solidly bond to the general earth grid. This will be essential in order to avoid dangerous potential rays that will occur between two unconnected earth systems in case of a lightning current.
The relevant extract of IEC-62305-3 & BS7430 is enclosed.
3) Inter-connection of electronic equipment earthing with general earth grid.
This has and continues to be a major area of misunderstanding/misconception. Most electronic equipment manufacturers demand a dedicated earth pit for terminating their equipment. The term dedicated earth is grossly misunderstood and often construed as an isolated earth.
In this connection, we make reference to the guidelines given in standards IEEE142-2007, IEC-1100—2005 and IEC-6100-5-2.
The above-said standards specifically deal with the subject of grounding electronic equipment. While it is true that the electronic equipment should have a dedicated earth pit/earth terminal to which it should be connected, this earth should be connected to the general earth grid and not kept isolated.
The specific recommendations of various standards with regard to this subject are detailed below:
IEEE142-2007: This standard under the Section of Electronic Equipment Grounding has discussed the subject of separate/isolated grounds vide Clause No. 5.5.3. It is specifically mentioned in the above-cited Clause in Page – 193 that a possible lack of understanding of the function and operation of the neutral conductor and the equipment ground system by electronic equipment manufacturers led to erroneous installation requirements using isolated earthlings for electronic equipment.
The problems of such erroneous installation practices are highlighted in Clause – 126.96.36.199 Page – 194 of the said standard. In this Clause, it is clearly mentioned that analysis of various catastrophic incidents involving isolated grounding practice indicated that the separation of grounds was responsible for very large voltages being impressed on electronic equipment components under thunderstorm conditions. These voltages occur whether or not electronic equipment was in operation.
The single point grounding system as described in Clause 5.5.4 Page – 197 of the standard is the recommended method of grounding sensitive grounding of electronic equipment as mentioned in the standard itself.
IEC-61000-5-2: This standard vide Clause 5.3.1 – Page 29, 31 and 33 has clearly mentioned that the use of independent “ISOLATED" earth electrodes for computer and electronic systems is not recommended. The acceptable methods of electronic equipment are highlighted on Page 33 & 35 wherein it is clear that the electronic earth pit/system should be solidly connected to the general earth grid under the soil.
IEEE-1100-2003: This standard Clause- 188.8.131.52 Page 130 has deliberated the subject of isolated grounding and single point grounding system. It clearly mentions that the electronic equipment should not use isolated grounds and discourages the use of isolated grounds. The benefits of connecting the dedicated electronic earthing to the general earth grid are detailed in Pages 130, 131, 132 & 133 of the said standard.
Conclusion: It is clear from the above that all the earth systems in a given electrical installation ie., neutral earth, lightning protection system earth and electronic earth should have their own dedicated earth pit/earth terminations. However, there should never be isolated from the general earth grid. These dedicated earth pits should be connected to the general earth grid to achieve equipotential thereby avoiding catastrophic failures.