Indian Earthing System

<...Earthed Neutral is brought to the Main Switchboard neutral bus and is further distributed to the distribution panels....>

Well, that makes it either TN-C or TN-C-S, not TN-S.

So that might be where the confusion lies, perhaps?

http://en.wikipedia.org/wiki/Earthing_system

If an earth loop impedance test fails, possibly because of an isolated neutral, then it would be inappropriate to energise the system on the basis of its being unsafe. That is the reason for carrying out such a test: to prove that there is adequate earthing that will operate the circuit protective devices in the event of a fault appearing.

Perhaps I was little unclear there; there is separate earth conductor running from the Main S/W board to downstream panels. My confusion is about source side interconnection of N and E. I think that is not provided in Indian intallations.

Those installations cannot be seen from here.

Provided the earth loop impedance test goes properly, then it is safe to energise the system with a high level of confidence that the circuit protective devices will work properly when a fault appears, irrespective of the actual configuration of the earthing arrangements. Carrying out the test proves that all is well between the earth conductor and the neutral conductor. Recording the test outcome is a requirement under British Standard 7671, and an inspection, repeat test and report at prescribed intervals is a requirement under the Electricity at Work Regulations in the UK.

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The reason is that Indian Standard Code of Practice for Earthing IS 3043 clause 4 "Statutory Provision for Earthing" makes it mandatory and wants it to complied that way.

Clause 4.2 states" All medium voltage equipment shall be earthed by two separate and distant connection with earth. In case of high and extra high voltages Neutral points shall be earthed by not less than by two separate earthing and distinct connection with earth"

So contractors have to comply with it and cannot deviate.

Let the neutral be earthed separately, no issue, ie the system earth. The question is why it is not( OR is it) linked with the so called ' body earth' / the earth bar of the Main Switch Board(MSB)?

If the MSB earth bar(with no link to System Neutral ) is having its own earth pits, will it be sufficient for the protection to operate?

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Two options:

1. Prove it by earth loop testing, as is required under BS7671, or
2. Introduce a deliberate fault and see if it operates...

<reunsubscribes>

The clause 4.2 quoted does not refer to equipment "body".

If the transformer is "medium voltage equipment" then connecting body to local earth and to neutral earth would provide the "two separate and distinct connections to earth".

So clause 4.2 does not seem to me to explain the emphatic rejection of an interconnection.

The answer would seem to me [without having the codes concerned] to lie in keeping the High voltage and 415/240V earthing systems distinct to minimise interaction.

Dinesh,

Best earthing is one where we have metallic bodies/ frames/ structures etc. at one potential and at earth potential.

Therefore for safety the earthing of Neutral and body shall be interconnected.

A separate earth or earth link in neutral of transformer is provided for testing of transformer (Insulation Resistance or Hi pot test etc.).

You must have seen to further enhance the safety, limit touch and step potentials during fault within safe limits, in High Voltage systems, even we go for earthing mat and all bodies/ frame of all equipment and structure as well as neutral is bonded to the earth mat grid.

Also you might have come across multiple earthing system in Distribution system to avoid any potential difference carried by armoured of cables from remote station (where earth fault has occurred) to other end (receiving or sending end or other consumers). There is a famous B.S. Code of Practice for earthing (I am not recollecting the number), please go through - it is quite informative.

In Indian practice, even I have seen earthing pit filled with charcoal and water added or provision to add water to lower earth resistance - but CP009?? (Number could be wrong) does not permit that for the simple reason, during fault, due to heat generated the water will evaporate increasing earth resistance and leaving void between earth rod and earth.

In Indian practice use GI pipe is very common as earth electrode. We know Zinc and iron both are sacrificial anodes -easily eaten by Copper or other electro positive metals due to galvanic corrosion - but still it is being used. Whereas in Europe or Middle East Cu cladded steel core electrodes are used - which last for ever.

Similarly to control conductivity of soil around earth electrode the bore is filled with Sodium Bentonate clay (which has better conductivity retains natural moisture and does not shrink on loss of moisture), thus superficially increasing dia of earth electrode and reducing earthing resistance.

Hence not disputing the standards, I prefer to use the commonsense to achieve better and safe earthing than blindly following the standard.

If the Neutral point at the transformer, is connected to an earth electrode or grounded, that is sufficient to start with.

Then the body of the transformer can have a separate earthing / grounding system (Electrode or else). This separate earth can be used to distribute it to the load side distribution boards: If this is what you have, and if you have also other multiple earthing rods, at each panel or Distribution board, connected to the bodies of these DBs, then you have a distributed earth conductor to all the equipments connected to the DBs, ==> You have a return path to ground then to the Neutral at the transformer site, where it has been grounded!

No problem with the return path as long as you can test each branch DB side for the earth return path resistance, and if the protection devices do operate by simulating an earth fault. Otherwise, you need to earth the neutral, again, at different points in the distribution system, to insure proper operation of the protective devices. This is the main objective of all the regulations, no matter how they are implemented.

Thanks Lucke. I think you have really understood the intent of my original question.

1. It is most important to realise that with the system described by Lucke, a fault current to earth in a house would have to return through the earth between the two earth systems, in order to complete a circuit between a load earth and the transformer neutral.
2. A good earth system has a resistance of 0.5 ohm. When one considers that a single 3m rod gives a resistance of tens of ohms in soil with standard conductivity, such a system is a lot of work.
3. So going between the two earth systems would result in 0.5 + 0.5 = 1 ohm resistance minimum. To that must be added the resistance of the local wiring.
4. In the UK, to BS7671, socket circuits must be disconnected within 0.4 seconds in the event of an earth fault. The maximum earth fault loop impedance with a 30 amp re-wirable fuse is given in Table 41.2 as 1.09 ohms (nominal 230V).
5. My reckoning is that with the 1 ohm resistance between the earth systems, the 0.09 ohms left only allows a 12 metre run to the socket with 6 sq.mm copper cable.
6. This would not be a very practical system.
7. The TN-S system has the source neutral earthed, neutral and protective earth are connected at the source and separate everywhere else. There is always a copper earth return to the source without the uncertainty of earthing systems, which could be very high if the earth is in dry sand.

67model

At 0.09 Ohm, and 6 sq mm cable, the length will be more like 30m!

In a domestic installation, the system should be protected with differential / residual c.b. that are sensitive to 30mA in general. As long as these are functioning properly, you have your protection. This is in addition to the fusing system.

In Industrial installations, your recommendation comes into effect specially on circuits not protected by RCDs.

LAA Lucke

For Dinesh kp's clarification, since you did not comment, do you agree with my comments on the problems of earth return through the actual ground. I do not think an LV supply earth shared with a transformer tank is desirable, an HV fault in the tank could kick the LV earth up to a dangerous voltage.

In my BS7671 standard, the Loop volt drop per meter for 6 sq.mm Cu cable is 7.3 millohm/metre (AC). Copper is much the same everywhere, though insulation temperature varies.

This loop resistance is go and return, at the maximum continuous insulation working temperature of 70 'C.

There is only 1.09 - 1.0 ohm = 90 mΩ left, after deducting 1 ohm for the ground path. So 90/7.3 = 12.3 metres.

The 1.09 ohms is, so far as I know, given for semi-enclosed (rewirable) BS3036 fuses in table 41.2 of BS7671 :2011 which was based on 230V supply - formerly the standard supply was 240V. Apparently, electricity suppliers are not hurrying to drop supply voltage on old installations, since it will reduce consumption!

My figures apply to domestic installations still, RCD/RCCBs are an extra measure for sockets (which only became mandatory, 15th edition outdoor use, in the 17th edition).

My thinking was that re-wirable fuses would be closer to the majority situation in India than what applies to new installations in UK.

Could Dinesh kp inform me please.

I do not think there is any suggestion that socket earth conductors should be reduced in size because they have RCD protection allowing an enormous increase of earth loop resistance.

In reply to Dinesh kp post #13 remark on type B MCB, the earth loop resistance permitted is 1.44 ohm c.f. 1.09 rewireable - it is still true that having a ground-ground resistance 0f at least 1 ohm is a huge bite out of the allowable circuit length. I am sure that such buried earthing systems take so much cost that a direct copper earth connection to the LV star point is cheaper.

@67model

<< For Dinesh kp's clarification, since you did not comment, do you agree with my comments on the problems of earth return through the actual ground. >>

Is it really required that the earth fault current should return to the neutral for the protection to operate? To me it looks like that,(and as LALucke commented) if the EF current can get a low resistant path (through the earth pit at the MSB) to complete the circuit, the protection can operate; however the disconnection time , as you mentioned in your post ,should be within the limit.

<< I do not think an LV supply earth shared with a transformer tank is desirable, an HV fault in the tank could kick the LV earth up to a dangerous voltage.>>

But I can see that this is a common practice in US where equipment earth bar is linked to Transformer body. As far as I know in INDIA, Transformer body is separately earthed by 2 isolated earth pits. As mentioned in my first post, It is neither linked to Neutral nor to the MSB earth bar.

<<My thinking was that re-wirable fuses would be closer to the majority situation in India than what applies to new installations in UK >>

Re-wirable fuses are out from the Indian market almost 20 years back. MCBs are widely used.

In reply to Dinesh kp #15,

Return to the neutral is the essential part.

Suppose you had a single phase supply, protected by an RCD, in a plastic aircraft.

The only way the RCD could operate would be if current passed from live to neutral by a path other than the protected conductors.

So the only way to be sure that a leakage to ground [which is a sea of variously conductive materials unavoidably surrounding any electrical system on the ground] will be big enough to operate protection is to connect the neutral to ground with an "earth electrode" of tested resistance. That tested resistance, plus the resistance of any expected fault path, must be less than the maximum resistance to operate the protection device in the given time. One is specifying and testing the part of the path which is controllable.

The real business in any electrical system is to bond any metalwork and other conductors which could carry a current under fault conditions back to the source, so that overcurrent protection can work (RCD is overcurrent outside the proper path). Much of that metalwork is deliberately put around electric equipment. Also, to prevent dangerous voltage between equipments during faults, everything is bonded together and to "earth", which is really a low resistance connection back to the power source. Conduction via earth is unavoidable for remote equipment faults, and in some cases, the only practical way of identifying faults.

On an oil/gas rig, in the sea, the fact that the whole may have a far better connection to "earth" than most things on land has very little to do with protection operation.

I agree with your comment.

The Neutral at the transformer is earthed separately from the transformer body 2 Earth pits). The Building earth pit should be separate from the transformer body pit.

In this way, there will be only one earth path from the building earthing conductor to the Neutral at source. If the distance from the transformer to the building is great, then the neutral can have another earth pit near or at the premises of the building.The building Earth pit can always be separate from the neutral unless the low ohm path could not be achieved (~1 ohm).

@ 67model: Thanks for those points.

I recon that with a type B MCB circuit , we can have longer run for the sockets.So the key point would be to keep the earth resistance at the MSB earthing as low as possibble by using multiple earth rods.

However,I do not think there is any legal stipulation for disconnection time and earth fault impedence tests in Indian standards.

You are wrong! I am from India & I have commissioned a number of systems wherein the Neutral & the Body earth pits of the transformer (and, even generator) were interconnected below the ground using metallic conductors. This is also shown clearly in IS 3043. See Fig (2) & Fig. (3) See the solid link between N & PE at the source.