How rare is the TN-C Earthing system past the DNO property?

[JohnW2]

As far as I am aware railways use TN-C for powering electric trains.

As do tramways. Running rails are a combined neutral and earth, overhead wire or third rail is the live conductor. ... IIRC operators of "electric guided transportation systems" (railways, tramways etc) have an explicit exception to the "no TNC" rule in the ESQCR.

That seems to make sense, given the impracticality (well, at least, cost) of insulating the running rails from earth - and if they are going to have paths to earth all along their length, I suppose they might just as well be CNEs.

I may need some education. I've never really thought too much about this before but, thinking of 'the past', I'm not sure what was the perceived problem with TN-C (i.e. CNEs) within installations, provided that required bonding was present and adequate - am I missing something? However, I suppose that's become moot - since the advent of RCDs, it's obviously become essential to have N and CPCs separated within the installation.

Kind Regards, John

 

[bernardgreen]

Electrical supply to trains does not rely on the ground for the return circuit,

A return conductor is used and the return current is forced to flow along this conductor by current transformers. These take the current in the over head catenary and use it to force an equal and opposite ( geographical direction ) current in the return conductor. This is normal strung as close as is safe to the over head catenary to reduce the amount of electro-magnetic radiation from the catenary,

Untitled

• bernardgreen
• 8 Feb 2015
• 1

( a student's thesis but the drawing is correct )

This system was not used on the first over head system and stray currents through the ground and the voltage gradients through the ground caused many problems for telecom systems, local electrical supplies and animals on the ground close to the tracks.

 

[JohnW2]

Electrical supply to trains does not rely on the ground for the return circuit,

I think we probably all understand that. I think the discussion (at last, the uncertainty in my mind) is in regard to what would be gained by having a separate 'protective conductor'.

Kind Regards, John

 

[plugwash]

I may need some education. I've never really thought too much about this before but, thinking of 'the past', I'm not sure what was the perceived problem with TN-C (i.e. CNEs) within installations,

Theres a couple of issues with combined neutral and earth.

One is what happens when the CNE breaks. If the CNE core breaks (and the live doesn't) one of two things happens, either the "earth" becomes live (severe electric shock hazard) or the current finds another path potentially causing severe overheating.

Another is that earths inevitablly get cross-connected (signal cables, metal enclosures in contact with each other, deliberate bonding). So in a system based on CNE conductors there is nothing to restrict the return currents to paths designed to take them.

 

[JohnW2]

Theres a couple of issues with combined neutral and earth. ... One is what happens when the CNE breaks. If the CNE core breaks (and the live doesn't) one of two things happens, either the "earth" becomes live (severe electric shock hazard) or the current finds another path potentially causing severe overheating.

How does that differ from the situation in which a (separate) neutral 'breaks' (but the live doesn't)? In the absence of a second fault, I don't see that necessarily presents any immediate hazard,does it?

Another is that earths inevitablly get cross-connected (signal cables, metal enclosures in contact with each other, deliberate bonding). So in a system based on CNE conductors there is nothing to restrict the return currents to paths designed to take them.

That sounds far more credible as an issue, but I need to think about it, because I don't think it's totally straightforward!

Kind Regards, John

 

[RF Lighting]

Theres a couple of issues with combined neutral and earth. ... One is what happens when the CNE breaks. If the CNE core breaks (and the live doesn't) one of two things happens, either the "earth" becomes live (severe electric shock hazard) or the current finds another path potentially causing severe overheating.

How does that differ from the situation in which a (separate) neutral 'breaks' (but the live doesn't)? In the absence of a second fault, I don't see that necessarily presents any immediate hazard,does it?

Yes! If you lose a CNE conductor then everything downstream automatically comes live due to neutral feedback. Being a CNE all the earth becomes live too. It's the same as a TN-C-S supply losing its neutral before it is split at the cutout.

 

[JohnW2]

Yes! If you lose a CNE conductor then everything downstream automatically comes live due to neutral feedback. Being a CNE all the earth becomes live too.

That makes sense - and, as you imply, would be a hazard if there were any earthed things around.

It's the same as a TN-C-S supply losing its neutral before it is split at the cutout.

On reflection, a lost CNE within an installation would, in practice, potentially be a lot 'worse' than that. Loss of a TN-C-S earth prior to the split would presumably leave the entire CPC system and main bonding intact, so the place should remain 'equipotential' (in terms of 'touchable conductors') regardless of the potential of the 'floating' neutral conductors.

Whatever, as I said, since the advent of RCDs, any discussion about within-installation CNEs has become moot.

Kind Regards, John

 

[stillp]

This system was not used on the first over head system and stray currents through the ground and the voltage gradients through the ground caused many problems for telecom systems, local electrical supplies and animals on the ground close to the tracks.

They still do!