Protective device type, for supply system

[biblio]

Scenario:
1908 large three-storey Midlands town house, re-wired over twenty years ago. I could not know if the supply system would be generically classed as TN-S, TN-C-S, TT or other. There is a short length of approx 2 inch diam steel pipe from the original solid floor under the stairs containing two hefty lead-coated cables (from the street about twenty feet away) supplying a serious-looking black box, details: 100 A, Type IIb, Series 5, 415V, BS1361. Four middle-weight pvc-coated cables appear from the top of this black box to the meter, details: S309.2, AC Watthour meter, 166+ revs/Kw, 1PHZW, Class 2. There is alongside a night-rate meter also. Finally there is the CU, details: Wylex, 8-way, fitted with six re-wirable fuses (it might well take the cartridge type too), 60amp total load warning label, Cat No 804IVY. There are two spare slots, three 5amp lighting circuits, two 30amp ring circuits, and one 30amp cooker slot (not used). We also have some plain candles too...(okay, stop laughing now...I have recently acquired a copy of the OSG!)
I can see the CU is surely now due for an upgrade, complete with MCBs etc, which I shall get a professional to install. The two existing ring circuits are vertically wired, and not horizontally according to floor level, so for starters I am making a new and separate ring for the third floor myself.

Question:
In this context what are my options for a CU, and with which type of protective devices please? It probably ought to have scope for say twelve circuits eg. I need to know because whatever I do now has to be compatible with this new CU. Cable lengths and voltage drop are all relative to the supply and protective device. I need to be sure of the actual limitations. I have a Screwfix, and a BDC catalogue which might illustrate some indicative products available.
I am ignorant about the protective device types as described generically in OSG, Table 7.1 Ring circuits. page 42. 30 - 32amp. (top of table, column one) the three BS refs. and the three cb types, (all shown grouped together in column three).
I intend to install 2.5mm sq pvc cables as in OSG, Sec 7.1.(iii) (a). page 40. And group 2.5mm sq pvc cables as in OSG, Sec 7.2.1 (i). page 41.

 

[biblio]

In short, is my particular supply a TT system?
There are two stranded-copper earth cables (no larger than 10mm sq cross-sectional area, sheathed in green/yellow) bonded together to one of the two lead-coated cables from the steel pipe.

 

[ryanj]

There are two stranded-copper earth cables (no larger than 10mm sq cross-sectional area, sheathed in green/yellow) bonded together to one of the two lead-coated cables from the steel pipe.

According to that, I'd say you had a TN-C-S supply, where the earth is bonded to the neutral.

A photo could help though.

 

[biblio]

I hope this sketch will suffice? (Click on the thumbnail to enlarge scale)

http://uk.pg.photos.yahoo.com/ph/[email protected]/album?.dir=/92ee

 

[Lectrician]

Looks like TN-S to me.

 

[securespark]

Ryan- how can you say it is PME, when there is no evidence of N/E link?

Normally these supplies have an earth wire coming out of rh/s of cut out where the N and E terms have been linked by the REC.

I can't say what it is. I'd like to agree with Lec, but have seen installs before where the bonding cables have been attached to the incoming cable, with no proper earth supplied to the CU.

Where do those two 10mm earth tails go?

Why are they only attached to one incoming cable?

And if the method of attaching them is a clamp, that is very wrong. The REC should attend and fit a proper earth terminal, with an earth strap sweated onto the lead sheath.

 

[ryanj]

bonded together to one of the two lead-coated cables

Securespark, I thought it was actually bonded to the conductor, not the sheath.

But after seeing the diagram, it looks more like a dodgy TN-S.

 

[securespark]

Yeah, I think it stinketh, too.

Biblio - gonna call the REC?

 

[biblio]

ryanj: yes, the two 10mm earth cables are only clamped to the outside of the lead-like sheathing.
securespark: one of the earth tails is attached to a protruding tab on the night metering timer. The other 'might' be attached to the back of the meter, and/or if not then it could go to the CU (?). I suspect both earths are connected to the same incoming cable because the clamp was already there when the night timer was added later (presumably by the supplier).

But do I have a TN-S system perhaps, dodgy or otherwise in need of inspection?

 

[ban-all-sheds]

ryanj: yes, the two 10mm earth cables are only clamped to the outside of the lead-like sheathing.

So they are connected to the correct place, but not necessarily by the best method.

securespark: one of the earth tails is attached to a protruding tab on the night metering timer. The other 'might' be attached to the back of the meter, and/or if not then it could go to the CU (?). I suspect both earths are connected to the same incoming cable because the clamp was already there when the night timer was added later (presumably by the supplier).

The earth cable should/usually/often/ideally goes to an earth block, similar to this:

http://www.tlc-direct.co.uk/Images/Products/size_2/TLEB4.JPG

and then from there you take the earth conection for the CU, and the main equipotential bonding. CUs can have bonding terminals inside them, and you can use those, but it's more of a tight squeeze.

but do I have a TN-S system perhaps, dodgy or otherwise in need of inspection?

You need to know how good that connection to the sheath is. The options I can think of are:

1) Inspection by a competent electrician

2) Inspection by the DNO - it is their responsibility, but some of them are remarkably reluctant to discharge that responsibility

3) Use a low-range ohmmeter to measure the resistance between the earth bar of the CU and the sheath (using a sharp probe to scratch/dig a little into the lead to get a good connection).

 

[biblio]

ban: you say, 'they are connected to the right place...' might this observation imply you think I have a TN-S system, as opposed to a TT or a TN supply system? (this is like trying to find who my father was... I want to identify my supply system as it is probably something I cannot change; and in order to select a new CU with the correct type of protective devices...as outlined in my original question - don't miss the scenario bit).
What is the main equipotential bonding (sorry) in this context please?
Who are the DNO?
You say, 'You need to know how good that connection to the sheath is' - is this something I could do myself with a multimeter eg? What ohms (?) reading would be good/bad news?
Thanks, biblio

 

[securespark]

The NIC says using an earth clamp on a cable is a VERY BAD IDEA, and I agree. As Ban says, the Authority should provide you with a proper earth terminal. Ring them and ask them to check it out.

 

[biblio]

securespark: if an appropriate authority provided a proper earth terminal could you say now with any certainty exactly what type of supply I would have then? (see previous link to diagram)

 

[securespark]

Why is it so important that you know?

The REC will tell you.

BUT, if you want my opinion, as previously stated, I lean towards Lectrician's diagnosis.

 

[ban-all-sheds]

ban: you say, 'they are connected to the right place...' might this observation imply you think I have a TN-S system, as opposed to a TT or a TN supply system? (this is like trying to find who my father was... I want to identify my supply system as it is probably something I cannot change; and in order to select a new CU with the correct type of protective devices...as outlined in my original question - don't miss the scenario bit).

Looks like TN-S, Not TT anyway, which is the only supply type which affects the CU.

What is the main equipotential bonding (sorry) in this context please?

The earth connections to the incoming gas/water/oil/beer service pipes.

Who are the DNO?

Don't know who yours is - it depends where you live. Distribution Network Operator, used to be (and by some still is) called the REC - Regional Electricity Company. They are the people who own the local bit of the electricity network, the cables into your house, the service fuse etc, and they are the people whose legal responsibility it is to provide a good earth. They are not necessarily the same people from whom you buy your electricity. Another one of the Grantham Gargoyle's madcap schemes....

You say, 'You need to know how good that connection to the sheath is' - is this something I could do myself with a multimeter eg? What ohms (?) reading would be good/bad news?
Thanks, biblio

I doubt that an ordinary multimeter would read low enough - you need to be able to measure down to a resolution of 10 milliohms. As a guess I'd say that anything more than 0.01 or 0.02 ohms would be too high a reading, but that's only a guess - I don't think that there is any expectation of any significant resistance in that part of the earth path.