Exporting TNC-S Earth

[haptism]

This subject has been discussed alot online and has more than a few angles; but can someone tell me if this sounds right:-

TNC-S system's are not favoured (by the DNO'S) to be exported outside the equipotential zone (EPZ), because:-

1. (Consider here the TNCS has been exported outside the EPZ). If there's a break in the supply neutral (also earth), which coincides with a fault in an outbuilding (a short to metal casing) then there exists a potential (risk) to true earth.

2. If a similar fault happened indoors (within the EPZ), the potential to true earth would not be present ??

If the 2nd example is correct, then does that mean all the time you standing in the house (within the EPZ) there can be no possibility of a fault current travelling through your person (via the flooring) to earth. Bear in mind that all bonding would be rendered useless as there is no path for it due to the broken neutral; so would that mean any live extraneous parts would remain potentially static until the neutral were reconnected.

The problem im having with the second example is; any extraneous parts directly connected directly to earth within the EPZ (structural steels for example), would have just as much capacity to carry fault currents to earth as it would be outside the EPZ.

Discuss

Cheers

 

[Engineer54]

For the Upteenth time you are ''NOT'' exporting anything, you are only extending the equipotential zone of the original installation. As for the rest of your post, it has been done to death here, try reading the many past threads covering EXACTLY the same subject matter. I just can't be bothered to explain all over again!!

 

[Deleted member 9648]

As above....cant be bothered to go into the whole scenario again. It's interesting though that those who have a hissy fit about a shed on a TNCS supply never apply the same logic to outside installations other than outbuildings. Nobody ever comes on here saying they are installing a line of outside lights up the drive and is it ok to 'export the earth',or a socket for a pond pump to name but two examples. But as soon as a shed is mentioned they come over all unnecessary.

 

[Rockingit]

As soon as a shed is mentioned they come over all unnecessary.

And there you have it: a Radio 4 worthy sound bite.

 

[Engineer54]

As above....cant be bothered to go into the whole scenario again. It's interesting though that those who have a hissy fit about a shed on a TNCS supply never apply the same logic to outside installations other than outbuildings. Nobody ever comes on here saying they are installing a line of outside lights up the drive and is it ok to 'export the earth',or a socket for a pond pump to name but two examples. But as soon as a shed is mentioned they come over all unnecessary.

Yep, nobody seem's too bothered about installing an outside socket on the wall of the house supplied by TNC-S and wondering if that socket should be connected to a earth rod or not!!!

 

[Silly Sausage]

..................... they come over all unnecessary.

:smilielol5:

 

[Deleted member 26818]

Strangely enough, most DNOs require an earth rod for street lighting, before they will allow connection to their TN-C-S earthing point.

Nothing wrong with using the term 'exporting the earth'.
It suggests that the earthing for one EPZ is being used somewhere else, not connected to the original EPZ.
Whereas extending suggests that the EPZ is being made larger to encompass not only the new point, but the area in between.

 

[haptism]

OK, thats wrapped that one up then. Thanks

 

[Lucien Nunes]

No it hasn't.

Bear in mind that all bonding would be rendered useless as there is no path for it due to the broken neutral

This assumption is wrong. Consider what an equipotential actually is and does, and hence the difference between bonding and earthing.

 

[Deleted member 26818]

One thing that needs to be remembered, is that current will flow along any and every available path.
As such under normal operating conditions, there is current flowing through your person whenever you provide a path between an exposed-conductive part and true earth.
Because of the relatively high resistance of your body in relation to other paths, the current is negligible.
Yes if a break were to occur in the suplier's neutral more current would flow through your body, but because there are so many other paths available for the current to flow, again the current flowing through your body would be negligible.

There is a possibility that as more and more of the other services become supplied via plastic, that alternative paths will be reduced to such an extent that the current flow through your body could become dangerous.

 

[Rockingit]

Yes, I've thought about that one, too. Modern timber frames with no gas, plastic water pipes and underfloor heating. If it all goes wrong, where's it gonna go?!

 

[SPARTYKUS]

Bring back lead water pipes.....

 

[shanky887614]

Bring back lead water pipes.....

copper would be better, nice easy job.

who wants to use this plastic kak

 

[haptism]

No it hasn't.



This assumption is wrong. Consider what an equipotential actually is and does, and hence the difference between bonding and earthing.

So, the euipotential bonding ensures that all exposed conductive parts will remain at the same voltage in the event of a fault; therefore ensuring (within the Equipotential zone) that a person cannot form a potential difference (get a jolt).

However, cannot it be that the act of just standing on a concrete (or wooden, or whatever) floor within the EZ, that the floor itself could provide an earth (PD). Does that mean that within a EZ, the floor is bonded too (i expect re bar of concrete flooring should be). What about wooden floor, or stone floor, are such materials considered to be of a high enough resistance as to not provide a path for fault currents (or PD within a EZ) ?

Surely metalwork going through the floor would make the PD greater

 

[Engineer54]

Nothing wrong with using the term 'exporting the earth'.
It suggests that the earthing for one EPZ is being used somewhere else, not connected to the original EPZ.
Whereas extending suggests that the EPZ is being made larger to encompass not only the new point, but the area in between.

There is , because you aren't exporting anything, you are merely extending an existing EPZ. If that term needs to be expressed more clearly to describe the actual area to which the EPZ is being extended too, so be it, but nothing is being exported in the true sense of the word!!

 

[Engineer54]

So, the euipotential bonding ensures that all exposed conductive parts will remain at the same voltage in the event of a fault; therefore ensuring (within the Equipotential zone) that a person cannot form a potential difference (get a jolt).

However, cannot it be that the act of just standing on a concrete (or wooden, or whatever) floor within the EZ, that the floor itself could provide an earth (PD). Does that mean that within a EZ, the floor is bonded too (i expect re bar of concrete flooring should be). What about wooden floor, or stone floor, are such materials considered to be of a high enough resistance as to not provide a path for fault currents (or PD within a EZ) ?

Surely metalwork going through the floor would make the PD greater

As a matter of interest, do you know the difference between Earthing and Bonding and the reasoning behind the differences??

 

[Gavin A]

However, cannot it be that the act of just standing on a concrete (or wooden, or whatever) floor within the EZ, that the floor itself could provide an earth (PD). Does that mean that within a EZ, the floor is bonded too (i expect re bar of concrete flooring should be). What about wooden floor, or stone floor, are such materials considered to be of a high enough resistance as to not provide a path for fault currents (or PD within a EZ) ?

Surely metalwork going through the floor would make the PD greater

A concrete / wooden floor may well be a extraneous part, but they're not exactly very conductive.