Why TN-C-S?

[Gardner]

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


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

 

[davesparks]

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

TT needs 4 cores for the 4 live conductors (3x L + N) which would all need to be surrounded by an earthed metal covering or armour.
TNCS does not have a seperate neutral conductor so needs one less core.

 

[Guest77]

Hows so? TT requires the same amount of wires.

Really?

 

[davesparks]

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.

Yes they do have something which will clear the fault, usually fuses or ACCBs, which clear the fault in the required time. Bearing in mind they have their own standards to work to with regards operating time for such protection.

As for Australia I couldn't possibly comment, and don't see how it is relevant

 

[Gardner]

TT needs 4 cores for the 4 live conductors (3x L + N) which would all need to be surrounded by an earthed metal covering or armour.
TNCS does not have a seperate neutral conductor so needs one less core.

Wait, why would TT need a dedicated neutral conductor? If the armor on a TN-C-S supply serves as a neutral, then why cant it for TT?

 

[Gardner]

Really?

Yup.

TT= L1,L2,L3, N

TN-C-S= L1,L2,L3, PEN

4 conductors for both.

 

[Gardner]

Yes they do have something which will clear the fault, usually fuses or ACCBs, which clear the fault in the required time. Bearing in mind they have their own standards to work to with regards operating time for such protection.

As for Australia I couldn't possibly comment, and don't see how it is relevant

If the conductors do not have fuse protection, an outer armor does nothing.

 

[ImpededLoop]

Wait, why would TT need a dedicated neutral conductor? If the armor on a TN-C-S supply serves as a neutral, then why cant it for TT?

As the neutral is referenced to earth it also acts as a cpc. If you applied this to TT it would become TNCS!

 

[davesparks]

Wait, why would TT need a dedicated neutral conductor? If the armor on a TN-C-S supply serves as a neutral, then why cant it for TT?

Because the neutral in a TNCS is a CNE (combined neutral and earth) so is one conductor serving both functions with strict requirements for the number and impedance of N-E connections made via earth electrodes along the distributing main.
In all other systems N and E are seperate conductors and are not permitted to be connected after the N-E link at the substation/transformer.

 

[davesparks]

If the conductors do not have fuse protection, an outer armor does nothing.

But they do, that's the point

 

[Gardner]

Because the neutral in a TNCS is a CNE (combined neutral and earth) so is one conductor serving both functions with strict requirements for the number and impedance of N-E connections made via earth electrodes along the distributing main.
In all other systems N and E are seperate conductors and are not permitted to be connected after the N-E link at the substation/transformer.

No connection to earth occurs in TT either at the customer's premises. A PME incomer will happily serve a TT supply.

 

[davesparks]

No connection to earth occurs in TT either at the customer's premises. A PME incomer will happily serve a TT supply.

Yes it would, and does in some places, but is irrelevant to the point as it would still be the same TNC distribution side as a TNCS supply not a TT distribution

 

[Gardner]

Yes it would, and does in some places, but is irrelevant to the point as it would still be the same TNC distribution side as a TNCS supply not a TT distribution

Ok, maybe I did not clarify. By TN-C-S I meant the incomer which is pure TN-C. A pure TN-C incomer can go either TN-C-S or TT.

 

[davesparks]

Ok, maybe I did not clarify. By TN-C-S I meant the incomer which is pure TN-C. A pure TN-C incomer can go either TN-C-S or TT.

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.

 

[Deleted member 26818]

This is a video of an 11kva cable strike.
I think they must have something to clear faults?
https://m.youtube.com/watch?feature=youtu.be&v=bpX3VHKAKak

 

[Gardner]

This is a video of an 11kva cable strike.
I think they must have something to clear faults?
https://m.youtube.com/watch?feature=youtu.be&v=bpX3VHKAKak

Normally they have fused cutouts at the riser. LV is another animal.