Skeleton CU replacenemt. Earthing question TT or TNS

[dboxguy]

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

 

[DPG]

I may be being thick, but what's a mantel? (apart from what you get in a gas lamp). Daz

 

[dboxguy]

Metal Box that the fuseboard is mounted into in older installations. Sometimes called a clifton i beleve.

 

[Engineer54]

It's what they call the semi flush metal cabinets the old councils used to fit into all there properties!! lol!!

This is a classic example of where a couple of photo's can speak a thousand words OP!!! any chance of you posting a couple or so ??

 

[GLENNSPARK]

i`d put the new main bonding straight into the earth bar on the new C/U....as you will have RCD protection with this new C/U....why the supplementary bonding?...

 

[telectrix]

is the wiring from the old CU in steel conduit? if so the conduit is probably used as cpc's and will have to be connected to earth. even if there are separate cpc conductors, the steel will still have to be earthed.

 

[dboxguy]

Op ?????

 

[telectrix]

op = original poster. in this case, you.

 

[rich.250]

Pfc??
Pssc??

That would be the biggest clue without pics or asking dno

 

[dboxguy]

pfc 1.1 kA
pssc 1.3 kA
from what i remeber.. dont have the test sheet with me.

Here is the original CU

 

[rich.250]

If you think about it as l-n loop and l-e loop.
They are not that different.
Meaning the resistances are similar too.
Obviously tncs would be near enough identical as its the same conductor.

Tns could be more or less as sometimes the sheath is better than a core.
Although my thinking would be tns as its unlikely that a TT would have a similar resistance.
Have you found a electrode anywhere? Or a connection to supply cable?

 

[Edd]

You say its a flat? In most installs like your discribing you will hav steel conduit bolted to the mantel unit with red and black tails. these in most circumstances are fed from a ryfield board at the supply incomer for the block, and would in old propertys be TNS. best look at the downstairs incoming again..

Edit.
Re the rest of your post.
I normally crimp and bolt a 16mm earth to the mantel unit and bring that and all earths including bonding forward to the new skelenton board this then means all earths are easy to get to at a later date..

oh my spelling sucks bad!!!!

 

[dboxguy]

Hi all thanks for all your help.
Here are some other pictures. The first 2 are of the kit under the stairs in the ground floor that supplies all the flats.

The last shows the inside of the mantel showing the mess that was there when i removed the CU also it shows the earth block bolted to the back with the old earth conductors etc

Dont have any pictures of the suppliers fuse etc but that and another box sits in the bottom of the metal box under the meter shown here. I cant see any incoming cable just the red and black tail coming from the main fuse.


Thanks again. The spelling can be as bad as you like if the info is so good

 

[Engineer54]

Well your certainly right about the wiring being a complete mess in that Mantel box!! It really does need a complete rewire this flat, it's full of braided VIR cables, that don't look to be in very good nick. So not just a CU change me thinks!!! Can't quite make it out but are those rubber grommets coming into the top of the mantel box?? If it is, then you will need to pull a separate CPC at least to the first conduit box, but further investigation and testing will need to be undertaken to check the continuity of the conduit installation...

Not quite sure why you have 2 supply meters, one at the main intake and another at the mantel box??
Can't see any earthing cable at the main cut-out, but that doesn't mean there isn't one. Take the trunking cover off and check what you have coming from/out of that DNO cut-out. The earthing conductor may well be taken from the top of the cast iron service/cut-out box. It could also be a converted TNC-S, ...but all will be revealed after removing the trunking cover....

 

[Edd]

As E54 Says, that trunking lids gota come off, and picture 3 does show a mess of VIR's Not sure how far you planed to take this job, but your looking at a rewire! then virs may be sound at the CU area but I bet they break apart at the light fittings... trip trip trip

 

[dboxguy]

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]

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]

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]

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!!

 

[dboxguy]

Thanks for that . Looks like i have an interesting weekend ahead. Good jod its 4 days long lol