TN-S or TN-C-S?

[pcboffinuk]

Sigh, why are the jobs you do for relatives (at no cost) always turn out to be tricky!

Having sorted out the broken ring the kitchen fitters left, I'm now looking to replace the CU, and was checking for size, length of cables etc. today, when I noticed this little issue.

I had previously assumed that it was TN-S, as there is a main eathing clamp on the incoming cable (braided strap) (along with about 4 other equipotential bonding clamps ).

However, looking closer today, the braided strap from this clamp leads to a small henley type box beside the main incoming fuse enclosure, marked with an earth, and a cable leads from that inside the enclosure.

So my question is, is this a TN-S or a TN-C-S setup?

A pic will help I hope:

The 'main' earth point to the CU is currently on a clamp on the incoming cable, just below the connection for the braided strap you can see in the picture.

From what I can see, this appears to be a TN-C-S, with the earth coming out to a henley box, then going to the clamp on the incoming cable, then via the sheath to another clamp, then to the CU?!

Is it possibly a conversion from TN-S to TN-C-S that someone has muffed?

Or if it is TN-S is there a reason why an earth wire would be taken inside the main fuse enclosure? The only cables exiting that are the Phase and Neutral meter tails.....

Answers on a postcard please...

Cheers

Gavin

 

[plugwash]

it does sound like a former TN-S that has been modified. Maybe it was botched as you say or maybe they discovered the armor was left unearthed due to an underground fault and they used the neutral to provide an earth to the armor as well as the installation.

what are the P-N and P-E loop impedances, if they are very very close then its almost certain there is a N-E link there.

assuming the test above confirms the presence of a N-E link i would reduce the number of clamps on the incoming cable to the one that is connected to that henly block and then connect the main earth and bonding to that henly via a proper earth block.

 

[JohnD]

The braid looks as if it is quite new. I wonder if the DNO fitter looped it back to the clamps (and the existing ?main bonding connectors?) in the absence of a nice big earth block?

 

[pcboffinuk]

Thanks - those replies make sense, though whether it was the DNO or the builders who muffed it up I don't know.

It was originally two flats, converted back to a house when my Mum moved in and refurbished, so presumably the DNO came out to remove one meter and did this.

The braid is pretty new, and I assume the DNO did that, though why I don't know - perhaps as you say the armour wasn't earthed, or perhaps the main bonding clamps were in place and they made the best of a bad job.

Getting a new earth block for the main bonding sounds sensible, has to be better than the mess of clamps round the armour at the moment.

Will check the loop impedances and then hopefully proceed as if a TN-C-S.

Thanks

Gavin

p.s. Am I just unlucky so far, or are 90% of the houses in the UK wired like this?!

 

[securespark]

What is the grey cable leaving the earth henley block?

If the braid comes off the sheath, then into a henley then onto the CU, that is TN-S. It is only TN-C-S if the neutral and earth are bonded together.

One way to try and check is to do a Ze.

If it is TN-S, the max allowed is 0.8. on PME, 0.35.

The reading you get may be a clue: if you get above 0.35, it is fairly certain it is TN-S.

 

[pcboffinuk]

What is the grey cable leaving the earth henley block?

The 'grey' cable is actually black I think - it runs from the DNO cutout, to the henley block, then down to the cable armour via the braid.

The only other cables leaving the DNO cutout are Phase and Neutral connectors - the main earth for the CU comes from a clamped connection on the cable armour, just below where the braid is clamped.

So I am going for the world's first TN-S-C-S system

It seems as if the Earth comes from the Neutral, through the henley block to the cable armour, through the armour to another clamp and through an earth wire to the CU..

One way to try and check is to do a Ze.
The reading you get may be a clue: if you get above 0.35, it is fairly certain it is TN-S.

I don't have the figures to hand, but from memory a direct test Zs on the kitchen ring was of the order of .5, which would suggest that there is a N-E link, but I will take my meter round there later today and have a proper go.

Electrics - what fun!

Gavin

 

[pcboffinuk]

Ok, I managed to run some tests yesterday.

P- N Loop Impedance at the CU supply was 0.13 Ohm, and P-E connecting directly to the braided earth strap in the pic was 0.17, so I'm taking that as proof that there is a N-E link, although the difference is a little larger than I would expect?

However, that along with a measured Zs of 0.26 on the kitchen ring, I'm taking as proof that its a TN-C-S, albeit it a botched one.

Tomorrow I change the CU, so will sort out the earth mess then.

Every time I take a look though I find some new horror! - Just discovered that the kitchen lights are on the upstairs CU, well hidden in a cupboard, and not the downstairs one - presumably because it saved the lazy arse who did it a few feet of cable.

Obviously not good practise, but is a fixed label on the CU warning that they isolate elsewhere acceptable practise (to get through a Part P competent person assessment) if I can't easily get the cable down to the right CU?

Mind you, with the two colours label, the testing label, the rcd label, they are going to ahve to make CUs bigger in the future just to fit all the damn labels on!

Gavin

 

[plugwash]

whats the arrangement with the two CUs at the moment? there should ideally be a single point of isolation for the whole install.

 

[pcboffinuk]

Hi Plugwash - another late bird I see

Currently the only point of isolation is the DNO cut off fuse.

The cable from that is split in a henley block to the lower CU with standard 25mm meter tails, and via 10mm2 cable to the upstairs CU.

I did try to buy a rotary isolator from my local Electrical wholesalers today, but he could only provide a 60A one (The DNO fuse is 100A).

I already know from recent experience of trying to contact my DNO (EDF) to ask if one can be fitted that there is bugger all chance of them doing it, and I can't seem to find anything on tlc that is not expensive or aimed at commercial applications.

What is the 'right' way to wire something like this? a 100A DP DIN mounted isolator switch in a small enclosure?

What do/did the DNO install in the areas where they bothered/still bother to do this?

Gavin

 

[plugwash]

The cable from that is split in a henley block to the lower CU with standard 25mm meter tails, and via 10mm2 cable to the upstairs CU.

seriously dodgy having a long 10mm cable to a CU with no protection but the service fuse.

I did try to buy a rotary isolator from my local Electrical wholesalers today, but he could only provide a 60A one (The DNO fuse is 100A).

normal practice is to use a CU type main switch in a two module box and make damn sure that you drill the holes to line up with the switches terminals.

Since your replacing the downstairs CU anyway can't you just put the upstairs CU on a 50A breaker in the non-rcd side of the downstairs one (assuming there are no showers on the upstairs one), deals with both the issue of a single point of isolation and of protecting the 10mm cable in one go.

 

[pcboffinuk]

seriously dodgy having a long 10mm cable to a CU with no protection but the service fuse.

Just about everything about this install is seriously dodgy, but I will eventually get it all sorted out!

I will fit a 2 module CU to make it a lot easier to fiddle in future.

Since your replacing the downstairs CU anyway can't you just put the upstairs CU on a 50A breaker in the non-rcd side of the downstairs one (assuming there are no showers on the upstairs one), deals with both the issue of a single point of isolation and of protecting the 10mm cable in one go.

Good idea, but there is a shower upstairs and I *think* its electric

It may not be too powerful though and there are only 3 small bedrooms upstairsso I may get away with it - I'll check the figures tomorrow.

Thanks for the suggestion.

Gavin

 

[comms]

I've used a wylex enclosure part number REC2s with a 100A dp main switch as an isolator between meter & cu's, used the wylex enclosure as it's cable entry points are for 25mm tails so you don't have to worry about lining up any drilled holes with the switch terminals correctly, as plugwash say's it's a pain in the arris.

A piccy of the enclosure is on page 33 of wylex catalouge on link.

http://www.electrium.co.uk/download.asp#pos_wylex

 

[JohnD]

I've used a MEM two-module enclosure, fits a 100A DP, this enclosure
is made with a little screw-retained sliding hatch over the "load" side terminals so that it the enclosure could be wire-sealed into place, but you could still access the load side terminals to fit/change the load tails without exposing the supply side. Cunning, but I don't know the p/no.

 

[Adam_151]

I've used a MEM two-module enclosure, fits a 100A DP, this enclosure
is made with a little screw-retained sliding hatch over the "load" side terminals so that it the enclosure could be wire-sealed into place, but you could still access the load side terminals to fit/change the load tails without exposing the supply side. Cunning, but I don't know the p/no.

I beleive thats what the DNO fit round here when you ask for an isolator, they do (or at least did when we had ours fitted) it FOC (central networks btw)

 

[securespark]

use a CU type main switch in a two module box and make damn sure that you drill the holes to line up with the switches terminals.

good advice about drilling, but use a four mod box - there's a lot more room!