Question about TN-S and TN-C-S

[DDAS]

I have just been looking at the difference between TN-S and TN-C-S.
If the supplier uses the neutral as the earth with the TN-C-S system then why isnt this used with all installations instead of using a separate earth with TN-S or an earth rod with TT ? After all neutral goes to ground anyway doesn't it? Is something done at the sub station end? or is there a very technical reason for this?

 

[plugwash]

TN-C-S is the most common system for new installations now mainly because its relatively cheap but it does have a couple of problems

1: the danger of broken neutrals. This can be mostly mitigated by multiple earth rods and concentric cables but is still considered good enough reason to ban it for caravan supplies.

2: currents flowing in the earth system even under non-fault conditions. because of this things like petrol stations and boats aren't allowed to be TN-C-S (with petrol stations the problem is currents flowing through underground tanks and possiblly creating sparks, with bots the problem is corrosion caused by currents through the boats into the water).

 

[pdcelec]

A TN-C-S system (PME) has the conbined neutral and earth up to your intake then splits. The system has many earthing points along its route, so that a return path to the supply is maintained in the event of a broken neutral, whereas the TN-S is earthed at the supply transformer with a solid earth. With a TNS, if there is a long distance between your intake and the transformer the "true" earth potential (at your property) may well be different from the supplied earth. A potential could exist between the suppliers earth and the real earth at the intake. The problem with a TNC-S is that if the supply neutral is lost (broken) (so the earth is also lost), a voltage would exist within the installation on the system with no means of returning to earth (except gas and water pipes, if the installation is bonded). A train of thought exists that PME installations should also have an earth rod installed (since a lot of new installations have plastic water pipes) to alleviate this problem.

 

[plugwash]

but unless that earth rod has a very low impedance you are still going to end up with significant potential between the earthing system and the general body of earth if your neutral currents are flowing through it. especilly on a single phase install where the neutral has to carry the full live current rather than just the imblance

i'm personally of the opinion that TN-C-S is a bad idea period but the recs managed to convince the government to accept thier cost saving plan.

 

[Chris.J]

Slightly OT.
I've always wondered how good the usual earth rods are, don't they corrode after a few years?
Would not, say, a six inch square sheet of stainless steel buried two foot down be better?

 

[DDAS]

Thanks folkes

I think I know what your talkin about

 

[Steve]

Slightly OT.
I've always wondered how good the usual earth rods are, don't they corrode after a few years?
Would not, say, a six inch square sheet of stainless steel buried two foot down be better?

copper takes a long time to corrode - think about water pipes . .