Skeleton CU, Mantel and earthing question

[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

 

[sparkytim]

If this is a metal Conduit installation then how is this connected to a means of earthing at the supply intake position?

That will determine what your earthing system is.

 

[Manator]

This type of skeleton boards were a very common method of installation in the late 1960,s early 1970,s for most council owned flats. The whole installation was done in steel conduit which was used as the cpc for all the circuits. Surprisingly continuity can still be very good, but sometimes there are breakdowns in continuity in joints concealed in concrete floors and ceilings. As this installation is over 40 years old it will now require more frequent testing as it has come to its end of life expectancy.

Your Ze reading I assume was taken from your DB, if so what you have recorded is the Zs for that DB and not the Ze.

The reading is low and could be both TN earthing arrangements. The earthing can be verified by testing the neutral, or by visual inspection at the meter.

 

[Evans Electric]

Dboxy , what is the main feed ? Is there a conduit with the meter tails in it , the conduit being the earth ,bushed into the metal enclosure, possibly.?

I'm thinking TNS rather than TT .

 

[dboxguy]

Hi all, Thanks for the info. Here are some pics I took that might help View attachment 3301View attachment 3302View attachment 3303View attachment 3304View attachment 3305View attachment 3306

The CU, The meter, The supply kit under the stairs on the ground floor of the block of flats and then the mess behind the CU when i removed it

 

[Manator]

I think you have more problems than you think, it is a 1960,s install on a three phase supply TNS. Most of the cables I can see have passed their best by date. If I was to do any testing I would be interested in the IR values to evaluate deterioration of insulation, and the continuity of earthing within the whole installation. It still may be safe, but would need far more frequent testing to ensure its suitability.

 

[batty]

I am unable to see the attachments.

 

[alpha spark]

Same here can't see attachment

 

[Manator]

I hope this works

 

[alpha spark]

No sorry I'm on iPhone seems that only pics I can see are through photo bucket

 

[batty]

Wow there are some old looking cables there are they rubber.

 

[sparkytim]

A supply of that type and age is most likely to be TNS.

Can you see a bare earth coming from the Lead sheath incoming cable?

 

[dboxguy]

Hi all,

IR was fine >299 megohms on all circuit. Cable condition better than I expected. 3 of the 6 circuits needed i have replaced now anyways.

I measured the Z from the meter side Line of the main breaker(switched off) to the earthing block at the back of the metal box with nothing else connected in to the block . that is all the main and supplementary bonding was disconnected. If the metal box is the earth? which is what i am trying to confirm here then i would say that this reading is the Ze?

There is no incoming cables to the box that i can see as that must happen in the wall via some metal conduit . There is just the red and black tails going to the meter from the main fuse. The main fuse is bolted to the bottom of the box along with another black box thing (bakalite or ceramic i am not sure). I will try to get down to the property tonight and take a picture of that part

I am still stuck on the suppliers earthing. Is it ok to use the steel box? Can I simply connect the eathbar on the new CU to the existing earth block or do I need to actually see and connect to whatever is coming out of the service head which i think must be in the stuff in the ground floor in the pics i posted.

Thanks again guys

 

[Evans Electric]

Again Dboxy, I'd guess the earth is the incoming conduit , you only appear to have sub-circuit earths and a couple of 6mm bonds. In which case the enclosure is your earth.

At the main position I think you'll find its a TNS , earth will from the cable sheath via the metal cut-out and the trunking , metal isolators and outgoing conduits will be bushed directly to it .

Typical 1960,s job.

 

[dboxguy]

Thanks Evans,

Sounds like what i kinda suspected. I am going to try to get a better look at the stuff on the ground floor and hopefully I should see the sheild 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 eath cable from the supply or get a specific one run to this installation. The regs says something about "suppliers conduits and equipment being a valid earth from supply" will need to look it up tonight.

Thanks to all you guys for helping out a newb .

 

[SLIPSHOD & SLAPDASH]

I assume like all Mantel units i have seen the supply fuse and meter is in the unit, so that means your supply is a TNS. Doesn't matter to you whats before that and a Mantel unit should not be used for a TT supply anyhow.

 

[steptoe]

'60s install worked on in the '70s and thats probably the last time anyone done anything remotely useful to it.

 

[kelvfoozinda]

Its tns, fed from lead sheathed pilc cable. Are the risers in the tube wires in Vir cable (rubber with cloth outer), as down here (Southampton) we're in the process of replacing these risers where we find them with new split concentric? Either way at the moment its just mainframe earth through the conduit probably with a bit of cable between the trunking and head downstairs.