Electrical Installation Earthing

Depends on the location it's installed and the type of installation - is it domestic/commercial/industrial/on-street? Has it been confirmed with the DNO that it's definitely a TN-S system and doesn't convert to a PME at any point back to the origin? If it's all confirmed to be 100% good, it's generally fine to connect onto the DNO earth and continue the installation as normal and keeping in line with 7671 regs et al.


You can always ask the DNO if you're unsure.

Is it yours ? - if you have a private generator or private substation, and the wiring is entirely under your control,   it may be practical.

If not, and you do not know the route and type of cable from the substation to the property, you need to consult the network owner. The whole point of TT, is that it is what you do when there is no earth supplied, and for some good reason the neutral may not be used, either for reasons of voltage offset, or the security of the cable, and how great is risk of loss of neutral while live remains connected, such as with overhead single cables.

Yet when you report this kind of thing to the DNO, they shrug and say it's fine

No one is going to rush to replace the millions of earth clamps on DNO cables that earth the customers installations and the DNO won’t generally respond unless the cable is hot and smoking.

What are the factors need to be considered when converting an existing TT earthing system to a TN-S earthing system? 

 

PEFC will almost certainly be higher than under TT - if higher than PSCC then it would be wise to check the breaking capacity of protective devices (although probably not necessary for a domestic installation with a UK style CU with its 16kA rating).


Small possibility that some reduced c.p.c.s might be undersized, given the larger PEFC - especially where circuit breakers are used rather than fuses.


Isolation and disconnection times are more onerous for TT than TN-S, so no problems there.


Consideration might be given to the removal of some RCDs if they're no longer necessary (e.g. ≥100mA units on submains) to reduce maintenance overheads and increase reliability.


   - Andy.