Earthing in an Industrial Plant

Should not be below 1ohm means that it can be 1 millions ohm?

Why you need to defend?

What is the scope of work in your contract?

What does the customer really want? Try to give them that. If cannot, negotiate at the appropriate level.

What do you mean by Data earthing for CCR and TX neutral resistance?.

I find it surprising the system is TT in an industrial situation. What voltage is the system?

But, the Japanese do it, even in industries. Some years back, I did have a tough time convincing the Japanese Clients to shift to TN-S system for their Automobile Plant in Chennai (India) - for which they had originally proposed TT and that was objected to by the Regulating Authorities.

Now, coming to the OP's question, i am surprised that someone has to "defend" a higher earth resistance. And, you call 500mA ELCB very senistive. In our country, India, we do try to maintain the earth resistance below 1 Ohm, though not mandated anywhere in the Standards, we never had any problems whatsoever, even witha 30mA ELCBs. So, you CANNOT defend that the earh resistance should be more than 1 Ohm. The earth resistance should be as low as possible. That is what the Standards world over say. Please do not defend a higher earth resistance.

Or what is your apprehension in having a lesser than 1 Ohm earth resitance?

Request to explain in details what will happen in electrical system or safety point of view if the earth resistance is high( more than one ohm)

Well! Let us for a while assume that the earth resistance is more than 1 Ohm. But, during an earth fault in the system, if the earth fault current is so high as to cause the short circuit protective device to isolate the supply to the faulted portion much before it can cause harm to a human, what is the issue?

This is what many Standards across the world mean when they insist that the earth resistance shoul be "low enough". As far as I can remember, no Standard on earth (pun unintended) specfies a certain maximum earth resistance for System Earthing.

EE65,

I think the Japanese just do it especially for the large plants such as glass or cement as the Op's case. You will see what happen when we need to run the large earthing wires around the plant. It would not a good choice.

For smaller plant, they will just apply one earthing system for the whole plant such as TN. I have came through that since 2007. We designed one system for industrial gas plant. It is a very packaged plant and the mother company was from Japan.

Agreed! But, the biggest problem in TT System is the integrity of the earthing system. It requires a very high degree of maintenance discipline - to ensure that the earthing system resistance is kept to a low value at all times - as to offer effective protection - as natural earth serves as the return conductor for fault current in TT Systems. In situations where such discipline can be ensured, TT is the best system, as it avoids the cost & labour of one more conductor to be laid all across the plant.

But, in developing countries, such discipline cannot be ensured and hence they go for TN System, as in this system, irrespective of the resistance of the natural earth, a metallic conductor serves as the protective conductor, to carry the fault current, whose resistance cannot be varied appreciable, even by poorest maintenance practices.

I remember having read a wonderful Cahier Paper (Schneider Electric) on Earthing Practices Worldwide, wherein different earthing practices across the world are analysed.

We are seeing the other conflict right here. Follow to TT or not, I think that if we follow to TT, we still need the bonding system to bond all of the separated earthing together. Your thought?

No! TT is TERRA & TERRA which means that the system neutral is sepeartely "terraed" (connected solidly to the general mass of earth) and the equipment's exposed & extraneous conductive parts are seperately "terraed" (connected to the general mass of earth) and there is NO electrical inter-connection between the two earthing systems. May be, different earth electrodes of the equipment earthing can be linked by an 'equi-potential' bonding conductor, but there should not be any bond to the system earthing. If it so happens, the system becomes TN.

Really? I will never go for TT in my future design.

We have 1* 66kv/11kv main TX which feeds 2*11kv/3.3kv TXs ,there 3.3kv motors and 3.3/415v distribution transformers .The star point of 3.3/415 txs are Earthed and tied together as what the japanese provided as the value for resistance was 5ohms .As for the others the values are what is given

What is the your issue then?

Hi mr hien my senior just informed me the values that were given were for a single rod /measurement for 1 pit. hence the high value,he further informs me they did a line earth loop test and they found the values to be between 0.25 ohms and 0.4ohms in different distribution rooms,the same was done for line neutral loop and the values were below 1 ohm,since the plant has a network of interconnected rods and plates does this values give the true condition of earthing in the plant back to you guys for your valued input.

Still cannot catch up what you want. Your senior informed you the value which is for a rod/measurement for 1 pit. Hence high value. But how high is that?

Tell us more about the method you applied for testing in those different distribution rooms.