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

davesparks · Dec 22, 2015

Because it's cheaper for them

NDG Elecs · Dec 22, 2015

Is it definitely cheaper than TT? I have no idea and have never give it much thought TBH. It was discussed briefly on the C&G2330 but can't remember much from back then!

badwolf · Dec 22, 2015

I recently spoke with a dno engineer who was investigating a tns with a 10 ohm Zs, he seemed to think tncs was lazy and that tns being too costly to maintain, that they would no longer supply an Earth and spec a tt. Nice chap he had to dig up the road until they found the cable, because they had no idea where the joint was, even though they had fixed it once 8 odd years ago!

NDG Elecs · Dec 23, 2015

badwolf said:
I recently spoke with a dno engineer who was investigating a tns with a 10 ohm Zs, he seemed to think tncs was lazy and that tns being too costly to maintain, that they would no longer supply an Earth and spec a tt. Nice chap he had to dig up the road until they found the cable, because they had no idea where the joint was, even though they had fixed it once 8 odd years ago!

Nice to see they keep all their maps up to date eh! I can understand why TNS would be more costly to upkeep, but can't quite grasp why so with TNCS. TNCS is excellent for cct Zs's and as long as the neutral doesn't get lost then is a good arrangement. Lazy possibly, but TT lazier!

davesparks · Dec 23, 2015

It is cheaper to follow one standard system than three different ones.
Very few new distributing mains are going to be installed overhead, new housing estates etc will have underground distributions.
So the one system they select will have to be suitable for underground use.
To install TT underground would be more expensive as you would need an earthed armour around four cores whereas TNCS needs only three cores with a combined earth/neutral outer conductor.

Gardner · Dec 23, 2015

davesparks said:
Because it's cheaper for them

Hows so? TT requires the same amount of wires.

Gardner · Dec 23, 2015

davesparks said:
It is cheaper to follow one standard system than three different ones.
Very few new distributing mains are going to be installed overhead, new housing estates etc will have underground distributions.
So the one system they select will have to be suitable for underground use.
To install TT underground would be more expensive as you would need an earthed armour around four cores whereas TNCS needs only three cores with a combined earth/neutral outer conductor.

I disagree, why would the armor have to be earthed for TT? The same cable that does TN-C-S can do a TT supply.

ImpededLoop · Dec 23, 2015

Gardner said:
I disagree, why would the armor have to be earthed for TT? The same cable that does TN-C-S can do a TT supply.

An underground cable should normally have an earthed armour. This wouldn't be the case with TT.

joehat · Dec 23, 2015

Btw the OPD will still operate, even though the Rcd won't??

Trevle · Dec 23, 2015

Went to change a CCU a few months ago and got a Ze of 13 ohms on a Tns system. Called UKPN, they came round an hour later. The guy was umming and arring about having to dig up the road, then he said 'do you want me to see if you're eligible for tncs?' I said okay then. He just did an impedance test between L and N at the service head which came up at 0.13. I gave him a bit of 16mm G/Y and an earth block for him to screw onto the back board and that was it. He even renewed the main fuse and gave me a few meter seals. Last thing he said was 'just make sure the main bonding complies'.

Deleted member 26818 · Dec 23, 2015

Cheapest option, is to run one single cable, and use the general mass of the earth as the neutral.
Which I believe is the method used in remote areas in some African countries?

davesparks · Dec 23, 2015

Gardner said:
I disagree, why would the armor have to be earthed for TT? The same cable that does TN-C-S can do a TT supply.

Seriously? It's one of the most basic principles we follow for safely burying cables, surround it with an earthed metal covering so that any conductive object penetrating it becomes earthed before hitting any live conductor

HandySparks · Dec 23, 2015

joehat said:
Btw the OPD will still operate, even though the Rcd won't??

Is this a question?

Gardner · Dec 23, 2015

davesparks said:
Seriously? It's one of the most basic principles we follow for safely burying cables, surround it with an earthed metal covering so that any conductive object penetrating it becomes earthed before hitting any live conductor

Yes, seriously, though I am partly at fault for misinterpreting what you said as 4 cores meaning no cover:

To install TT underground would be more expensive as you would need an earthed armour around four cores whereas TNCS needs only three cores with a combined earth/neutral outer conductor.

However Im still trying to wrap my mind around why TT needs 4 core and TN-C-S only needs 3 cores. Both TT and TN-C-S will do fine with 3 phases plus a neutral.

Also, since we touched the subject, if something does pierce the cable, do the DNOs have something to clear the fault or at least fast enough? Im aware of places like Australia where an LV cable fault results in major fireworks as the secondaries are unfused with the primaries being fused at 2x the trafo's primary full load current.