Skeleton CU replacenemt. Earthing question TT or TNS

dboxguy · Apr 3, 2012

Hi All,
Very useful forums it great to see so many helpful people. Was wondering if anyone can help me with this as I have been searching for an answer and cant seem to find one.
I am replacing the old Wylex Skeleton CU in my flat with a new Dual RDC replacement. This is mounted as you might expect in a metal box mantel that seems to be fairly common. There is a Earth block bolted inside to the back of the mantel were the original main and supplementary bonding are attached(what is of it that is) i have run new bonding too.
I am not sure how to determine if this is a TT or an TNS system? I guess that the mantel is the earth terminal. I have measured the Ze from the earthing block with nothing connected and it came out at 0.19. It nice and low so i figure its unlikely to be a TT and its way to old and installation to be a TNCS. I guess the earth is connected via the metal conduits to the mantel from the distribution gear under the stairs on the ground floor? I have looked at this but there is nothing obvious and I can’t see any of the cables as there are all in conduit that is sealed
How can i confirm if this is a TNS system?
Should I connect the new main and supplemental bonding to the earth block in the mantel or to the earth bar in the CU?
Do I need to run an Earth from the earth block to the CU? The old one just earthed through the bolts connecting it to the mantel.
Any help would be greatly appreciated
Cheers

dboxguy · Apr 4, 2012

Thanks guys.

Sounds like kinda what I suspected. I am going to try to get a better look at the stuff on the ground floor and take what covers off i can and hopefully I should see the cable shield earthing arrangement there attached to the the conduit that leads to the metal box in the flat.
I guess if I have continuity from there to the metal box in the flat then I am all good and do not need to see an actual earth cable from the supply or get a specific one run to this installation. From what i remember the regs says something about "suppliers conduits and equipment being a valid earth from supply" will need to look it up tonight.

After sorting through the mess the cable condition were better than I expected. IR was fine >299 megohms on all circuit. But i am rewiring 4 of the 6 circuits including the lights as there was no CPC

. A lot more work that I was originally expecting but thats how it goes sometimes

Thanks again guys

for helping out a newb .

Engineer54 · Apr 4, 2012

Just because the mantel box has a good earth, doesn't mean that the final circuit conduits will have. I can see CPC's emanating from some of those entries into the mantel box. Now i don't know if there is pipe clamps that are earthing these conduits outside of the box, as i'm pretty sure the entry into the mantel was through rubber grommets!! As i said before, subject to continuity tests on this conduit, (even better additional EFI tests) you may only need to run in a CPC (min 2.5mm) to the first conduit box on the lighting circuit conduit(s) etc...

Any chance of a photo of the main supply to the mantel box??

dboxguy · Apr 4, 2012

Hi E54,
I think you have the wrong end of the stick with your last post. I am not asking about CPC to the final circuts. I am not expecting to use conduits as the protection for the final circuits. All the final circuts will have there own CPC in there twin and earth cables and all of these will be connected to the earth bar in the CU.
I do not know how the electrical supply company Earth is connected or supplied to the mantel.
I will take picture of where the main fuse is mounted in the bottom of the box but there is no indication of a cable entering the box as its enclosed in the wall.

Cheers

Engineer54 · Apr 4, 2012

Be aware then, that wiring T&E in older conduit can be a real pig. Any old T&E in those conduits would have been much easier to install, as they were stranded cables and far more flexible when going round bends etc than the crap modern solid core T&E...

In short, ...Rather you than me!! lol!!