Earthing System

You should check your local codes and standards, but general substation design practice would say:

1. Yes, bond all equipment to ground grid.

2. Yes, same as 1.

3. If you have any wires connecting the substation and plant (control, instrumentation, metering), then there should be a common bonded connection between their ground grids to reduce possible potential differences. Otherwise a voltage referenced to ground in the substation may not be the same referenced to the plant ground, and could be dangerous. Same goes for between the main and distribution substations.

As for substation earth resistance, if you have separate substations, how do you combine their resistances? If they are close enough to each other, you bond them together so they are effectively one grid. If they are not bonded, then you have to treat them as separate grids, and measure their resistance separately.

As for an acceptable grid resistance value, that depends on whether you have any local codes that specify a maximum resistance. Otherwise it is dependent on what is your maximum allowable ground potential rise ("GPR") in the event of a ground fault, which is often determined by what equipment (such as telecommunications or sensitive electronics) is capable of withstanding.

As the project is for Indian site, I suggest you refer to IS: 3043 for all your requirements. As far as your queries are concerned, I think you have to design main earthing network for the entire site and connect each and every equipment to it. Nothing basically is to be left unearthed. The network is also shown in the above code.

BB Raina

I wish to quote from IS 3043:1987, Clause 20.1:

"Where the neutral points of two electrically separate electricity systems are connected to a common earth electrode system at a site, there is a coupling of the systems in the event of an earth fault occurring on either system by virtue of the rise of earth potential due to the' passage of the fault current through the earth, electrode system. Similarly, if non-current carrying metalwork is bonded to the same earth electrode as the neutral point of the supply the metalwork will experience the same rise of earth potential. If complete separation of electrical systems were required, it would be essential that the neutral points of each system and its associated metalwork be separately earthed. If such a method were adopted, each earthing system would require insulation from other earthing systems to withstand the maximum rise of earth potential occurring in any system by virtue of lightning currents or power system fault currents. Insulation to this level is rarely practicable.

The choice of using a common earth or separate earths for the system of different voltages at a transforming point affect:

a) the probability of breakdown occurring in a transformer between the higher and lower voltage sides due to lighting or other surges;

and

b) the safety of consumers or their property supplied by any low voltage system distributed from the station against arise of potential of the earthed neutral by a high voltage system earth fault at the station.

The former risk is reduced by use of a common earth system, and the latter danger only arises if the resistance of the earth electrode system is not sufficiently low to limit the rise of earth potential to a safe value. There is advantage in using a common earth where the earth electrode resistance, including the parallel resistance of any bonded metalwork, etc, to earth is 1 Q or less, as is usual at power stations, large outdoor substations or substations supplying a network of cables whose sheaths have a low impedance to earth."

Going by the above, you must link both HV & LV earthing Systems.

Also, whatever the earthing system in your plant premises MUST be inter-connected only. No earthing shall be kept isolated.

Dear Members,

In IS 3043, for pipe electrode they have shown 100mm dia CI pipe of 2.5 metre length, but lot of Indian Govt departments like CPWD, PWD, etc they have shown pipe electrode drawing it is GI (Galvanished Iron made of a 50mm dia, 2.85meter length.

If there is any formula releating these two? If available please share.

Thanks

Thanks to all of you for your valuable comments, it is really helpful.

One query,If the earthing system is TN-S, what ever you are suggesting is fine,in IS 3043 the earthing system recommended for industrial premises is TT in which case we should not join the earthing grid at substation and earthing grid of plant. Any suggestions.

"Peter"- As you have mentioned "If you have any wires connecting the substation and plant (control, instrumentation, metering), then there should be a common bonded connection between their ground grids to reduce possible potential differences". We have SCADA cables and power cables going from the distribution substation to plant. In this case as I understand, shall the armor of the cables (Multi core) be connected at both the ends? and shall it serve as the bonding of two systems or should we put separate earth strips connecting both the earthing systems?

As mentioned by you "electricalexpert" that no earthing should be left isolated, as per my experiance (and as also some of the posts at CR4 suggests) that electrical system earthing and instrumentation earthing system are kept separate.