Why in TN-S System PE Cables have to be provided in addition to the phase conductors for each loads (like socket, light fixture, Motor etc.)?
Don't bonding the enclosure of the loads (socket, fixtures, Motors etc.) to the local earth electrode or grid makes it sufficient and the fault current takes the route of enclosure --> earth electrode / grid --> return via mother earth to the source?
Earthing to a local electrode is a TT system - the resistance of the soil around the electrode is often in region of tens to hundreds of Ohms - with the result that earth fault currents are often in the region of 1 to 10A - not enough to blow even a 5A fuse or trip 6A MCB quickly. Thus typically RCDs are needed for automatic disconnection of earth faults in TT systems.
In TN systems including TN-S, local earthing is provided by solid metallic connections back to the supply's Earthed point point - thus making Earth fault currents much larger - hundreds or thousands of amps - in this way ordinary fuses and circuit breakers can reliably open quickly on Earth faults without the complexity, cost or reliability issues of having to rely completely on RCDs and local electrodes.
- Andy.
As per Andy's answer what you describe is done, it is just not called TNS, it is TT.
The name TNS is reserved for the case where there is a dedicated metallic connection back to base all the way.
Now TT is common, especially in places where the earth connection may be hard to maintain, and a break may be un-noticed, such as overhead feeds and large rambling places like farms.
However, TT has its problems, the electrodes have to be maintained, and in some soil types that free drain - gravels and sands can be fun, getting a good connection can be problematic - if the resistance between your electrode and the one at the substation is hundred ohms or more (and that is quite common in some places ) then it is not going to blow even a small fuse if there is a fault. Then additional safety measures must be taken, usually RCDs. For the same reason during a fault the earth electrode and every earthed item in the installation rises to a high voltage until the fault current is interrupted.
In contrast a well built TNS system has a large fault current but low touch potentials, there are cases when that is a good thing, and others when it is not.
Mike.
I should have detailed more. By Local earth electrode / grid, I meant the all the earthing connections of the plant are interconnected including the plant transformers neutral. I think in that case, it becomes TN-C in the plant premises. But I do not understand specifying of additional PE conductor in this case
Your question isn't very clear. Let's consider a normal TN-S set-up, then tell us exactly if and how the plant diverges from this, or whether you find a standard TN-S set-up strange.
TN-S: at the transformer, N and the PC are bonded together and are also earthed. N and PC run from the transformer as separate connections to every electrical device in the building.
Nick seems to be asking why TN-S is not TN-C. The nomenclature is: T is Earth, N neutral, -C is common, -S is separated. The trouble with TN-C is that neutral voltage drops lead to various parts of the exposed conductive parts being at various different potentials. If everything were connected together with many extra (supplementary bonding) cables this would not be a great problem, but ensuring that they were all in place would. It is satisfactory for supply cables, to say houses, because both combined neutral and Earth cables would not be simultaneously accessible. This would not be the case if TN-C were allowed inside a single area without much additional bonding.
If the earth path is metallic, and only carries fault currents be it through cables or conduits or trunking, it is TNS
You can use the conduit or whatever as the earth path but you must arrange it so accidental disconnection of the earth is very unlikely, and it can carry the full prospective current, if you cannot be sure of that, then a copper earth path is in order, either all the way or just to bypass the bits where we are less sure of reliable connection. Painted metal and clip on lids are our enemies here, and anything that may vibrate loose.
If the earth path also carries load currents as well as just faults, then it is TNC.
If the fault current path involves terra-firma, then it is TT.
Many systems are a mix of TNC and TNS - it sort of depends how the distribution is done.
Mike
I'm looking at the question differently.
If we distribute the Neutral as a live conductor (as we do now in a TN-S or TN-C_S system), could we not provide PE via direct bonding to an “earth reference plane” which is connected to the PE distributed to each switchboard and distribution board?
The answer to the question phrased in that way is, that it could be done, but there's the potential issue of earth fault currents causing movement of the live conductors if the cpc is not “in the same wiring system or its immediate proximity”. (Regulation 543.5.1).
Having said that, I would have thought in many locations where an earth reference grid or plane is in use (e.g. data centre and similar installations with earthing to BS EN 50310), most of the earth fault current flows back through the common bonding network (CBN) rather than the cpc, which I guess somewhat deflates the importance of Regulation 543.5.1 in this type of installation?
The only issue to address for 543.5.1 in terms of , is to ensure that metallic armour, or containment containing insulated (but not sheathed) cables is suitable for use as a CPC for the largest fault current of a circuit contained within it, or is suitable backed up by a parallel cpc.
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