Why TN-C-S?

Gardner · Dec 22, 2015

Why are UK DNOs TN-C-Sing everything? Why does that hold more advantage then TT?

https://www.youtube.com/watch?v=JRHyqouJPzE

Gardner · Dec 23, 2015

davesparks said:
How are you defining the 'incomer' as its a bit ambiguous.

That is still irrelevant to the point, your op was regarding the advantage to the DNO of converting their networks to TNCS supplies universally. My answer being the cost advantage as TNCS is cheaper for them.

I still dont see how TNC is cheaper then TT, both bring 4 conductors into a structure. Now, if you mean TN-S, then yes, that would be 5 conductors.

davesparks · Dec 23, 2015

Gardner said:
I still dont see how TNC is cheaper then TT, both bring 4 conductors into a structure. Now, if you mean TN-S, then yes, that would be 5 conductors.

But to bury the cables they need an earthed metallic armour or sheath etc. It cannot be connected to neutral as that is a live conductor.

TT or TNCS are likely the same cost for an overhead install, but would not be permitted in new developments as it looks ugly.

It is cheaper to use one standardised method for new installations rather than install different types of supply to different installations.

Gardner · Dec 23, 2015

davesparks said:
But to bury the cables they need an earthed metallic armour or sheath etc. It cannot be connected to neutral as that is a live conductor.

Then how does a 3 core cable work, like the one you mentioned?

TT or TNCS are likely the same cost for an overhead install, but would not be permitted in new developments as it looks ugly.

It is cheaper to use one standardised method for new installations rather than install different types of supply to different installations.

I know, but why not standardize TT? I still maintain both cables cost the same.

davesparks · Dec 23, 2015

Gardner said:
Then how does a 3 core cable work, like the one you mentioned?

I know, but why not standardize TT? I still maintain both cables cost the same.

3 cores with an outer concentric conductor as the CNE

A 3core CNE cable will be cheaper than a 4core armoured cable, it uses less material, weighs less so cheaper to transport and install etc etc

Why would you want to standardise to TT? What would be the advantage?

Gardner · Dec 23, 2015

davesparks said:
3 cores with an outer concentric conductor as the CNE

A 3core CNE cable will be cheaper than a 4core armoured cable, it uses less material, weighs less so cheaper to transport and install etc etc

Why would you want to standardise to TT? What would be the advantage?

Why cant you use CNE for a TT feed? Its done all the time, is it not?

davesparks · Dec 24, 2015

Gardner said:
Why cant you use CNE for a TT feed? Its done all the time, is it not?

No it's not done all the time, where have you seen this arrangement installed?
TT supplies are almost always overhead so either the old bare seperate conductors or ABC, the only bit that may be CNE is the service drops if it's a short distance.

You can use CNE cable if the distributors side of it is a TNC distribution as the outer conductor will be earthed, but then they wouldn't be installing TT system would they? The distribution earth would then be present at the cutout in the form of the CNE and so they make it available to the customer.

Gardner · Dec 24, 2015

davesparks said:
No it's not done all the time, where have you seen this arrangement installed?
TT supplies are almost always overhead so either the old bare seperate conductors or ABC, the only bit that may be CNE is the service drops if it's a short distance.

You can use CNE cable if the distributors side of it is a TNC distribution as the outer conductor will be earthed, but then they wouldn't be installing TT system would they? The distribution earth would then be present at the cutout in the form of the CNE and so they make it available to the customer.

You can just not connect the CNE to the MET and have a TT system.

But my point is, older TT supplies are being converted to TN-C-S, so why do so?

Gardner · Dec 24, 2015

Sorry for the muppet reference, but here is a TT supply via underground cable:

Conversion of TT Supply to TN-C-S or TN-S

davesparks · Dec 24, 2015

Gardner said:
Sorry for the muppet reference, but here is a TT supply via underground cable:

Conversion of TT Supply to TN-C-S or TN-S

If it is a paper/lead cable as described then it is more likely to be a TNS in which the earth hasn't been connected to the sheath than a TT

davesparks · Dec 24, 2015

Gardner said:
You can just not connect the CNE to the MET and have a TT system.

But my point is, older TT supplies are being converted to TN-C-S, so why do so?

Yes, but what would be the point?

Presumably to standardise the system, why don't you ring them up and ask them?

davesparks · Dec 24, 2015

Also see paragraph 4 of regulation 24 of the ESQCR:2002

Gardner · Dec 24, 2015

davesparks said:
Yes, but what would be the point?

Presumably to standardise the system, why don't you ring them up and ask them?

For the same reasons farms and caravans use TT.

However, it appears DNOs prefer TN-C-S since I hear of many older TT services going the way of TN-C-S.

davesparks · Dec 24, 2015

Gardner said:
For the same reasons farms and caravans use TT.

However, it appears DNOs prefer TN-C-S since I hear of many older TT services going the way of TN-C-S.

They don't necessarily use TT, they can use TNS earthing. And it's only required for certain parts of farms.
Caravans don't use it because ESQCR forbids it.

Exactly what do you mean by that? Do you mean the DNO are replacing the distributing mains and service cables or just that the installations are gradually being connected to the TNCS services?

In the village where I live the old bare TT overheads were replaced with ABC as a TNCS distribution over 30 years ago but still not all installations are connected to the TNCS Earth connection. Any house which still has an installation which pre-dates the replacement of the overheads will likely still have its TT earthing setup.

Gardner · Dec 24, 2015

davesparks said:
They don't necessarily use TT, they can use TNS earthing. And it's only required for certain parts of farms.
Caravans don't use it because ESQCR forbids it.

Correct, however, not all DNOs can provide TN-S, and technically the way DNO networks are cobbled together, true TN-S is not something to be found.

Exactly what do you mean by that? Do you mean the DNO are replacing the distributing mains and service cables or just that the installations are gradually being connected to the TNCS services?

New services are migrating toward TN-C-S, and existing TT ones are being converted to TN-C-S in some places.

In the village where I live the old bare TT overheads were replaced with ABC as a TNCS distribution over 30 years ago but still not all installations are connected to the TNCS Earth connection. Any house which still has an installation which pre-dates the replacement of the overheads will likely still have its TT earthing setup.

But what happens when the service is upgraded or changed, does it remain TT? (genuinely curious here)