Improving an old electrical installation

Another "daughter's new flat" problem.
At present, the electrical system consists of an incoming three phase company fusebox (one phase unconnected, one for the upstairs flat, one for hers), thence via a Chilton voltage operated earth trip to an isolator (fused? dunno, must check), then by a hefty T&E cable to her CU some 7m away. The external size of the T&E is approximately 21.5mm x 10.5mm, so I take it to be 16mm^2.
There is an earth connection on the side of the company fuse, with no obvious external connection to a Neutral, and no warning notice, so I assume it's a TN-S system. This is connected to a couple of 16mm^2 earth wires going to the upstairs flat, plus 6mm^2 bonding to the incoming gas (certainly) and water (I think).
There is a 4mm^2 green wire from the output end (bottom) of the earth trip to the isolator, and one from the top of the earth trip disappearing under the floor. This may or may not be wire which appears at the consumer unit and is connected to its earth block, but it's not a fat lot of use at present because it's merely resting in the hole in the trip - not clamped at all.
The CU is an old Wylex with wired fuses, and has the novel feature of missing bases to three of its fuse ways, giving interesting scope for shocks, and I am minded to replace it with a new split-load unit asap.
As a first step, I propose to provide an earth cable from the isolator (and thus I hope from the Earth of the big T&E) to the Company terminal, thus rendering the VO earth trip redundant as an earth trip. Does this sound sensible? 10mm^2 OK for this length? (18") or, even easier, as I already have some, 2 parallel 6mm^2?
Please don't mention Part P. We'll perhaps worry about that later.
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Kevin Poole
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Autolycus wrote:

sounds a positively dangerous idea. The ELCB is there for a good reason, and on no account should it be disabled until you have RCD coverage of all circuits.
ELCBs were fitted when local earth rods were used, ie TT systems, and are needed to cut the supply in case of an earth fault that fuses will not clear. Without the ELCB operational, any such fault may leave all earthed items live indefinitely, and consume large amounts of electricity.
About the overall plan, you havent told us whats wrong, other than bare connections on the fusebox. If thats the only problem (if), just fit new replacement parts on the fusebox and youre done.
If you want the plus of RCD protection, a new split CU would make sense, and is not expensive. But realise you will have problems.
1. The RCD in it will likely trip, leaving you with no socket power and the need to trace and rectify the problem. It might be fine, but often something needs sorting out.
2. Get type C breakers for the lights or you may suffer repeated nuisance trips, the usual type Bs sometimes cause lots of trouble.
3. And I would definitely recommend a separate RCBO for each socket circuit rather than one single RCD for the lot. The single RCD may work ok, but it is a recipe with real potential for nowt but trouble. Will cost you an extra 100 for 3x RCBOs.
I'm not real clear what earth system youve got there, ELCBs are normally found on TT systems, but you have a supplier earth... I dont know.
ELCBs require 2 different earths to operate. They trip when they detect 50v difference between the 2. Presumably one is from the supplier, and the other a local earth rod. If one is not connected, the ELCB wont work, and in a system that needs an ELCB this is a real risk.
Hopefully someone else can clear up the things Im unclear about.
NT
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On 16 Jun, snipped-for-privacy@meeow.co.uk wrote:

Elcbs came in two variants, one current operated and the other voltage operated. You describe just the voltage operated one. The current operated one IIRC (looks up 14th edition) were used for earth loop impedences up to 40 ohms, and worked similarly to an RCD.
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An RCD _is_ a current operated ELCB.
Voltage operated ELCB's and RCD's are easy to tell apart. The Voltage operated ELCB has to have two separate earth terminals. An RCD will have zero or one earth terminal (depending how it works internally).
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The OP said it was a voltage operated version<g>
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On 16 Jun 2005 06:55:28 -0700, snipped-for-privacy@meeow.co.uk wrote:

I'd do this on day one. It's just easier to get the work done if the CU is done and dusted before you have to work on anything else.

I'd certainly recommend Type C for workshops (big motors and welders) but I can't say I've ever seen the need on a lighting circuit.
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After I replaced the cartridge fused Wylex board with a new CU in my old house, the lighting circuit MCB's would more often than not trip when a bulb blew. It was a bit of a PITA. I was going to try Type C MCB's , but moved house instead.
I certainly would think about them when I replace the CU in the new house.
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On Thu, 16 Jun 2005 14:16:37 +0100, "Autolycus"

Is it then splitting into 2 single phase supplies with 2 separate meters after the fuse unit?

That kind of thing would have been typical for a TT earthing system to have.

Certainly sounds about right by the dimensions, although it may possibly be the old imperial equivalent.

The lack of a warning notice doesn't mean it's not TN-C-S,[PME]. From the description "Connection on the side of the company fuse" it sounds very like a PME system, _unless_ it's a metal sealing chamber, which is certainly possible on a 3ph service. If it's plastic, is the connection coming at right angles out of the neutral side perchance?
Most TN-S systems I have seen use an earth clamp on the incoming cable if the sealing chamber is of the plastic variety. Is this overhead or underground fed? If the feed to the building is overhead, there's a 90% chance that it was an old TT system and is now PME. Try hunting around outside the building for an old [possibly disused] earth rod.

A possibly cheaper equivalent if the CU is in otherwise ok condition would be the wylex plug in breakers, they also come with the bases for the carriers thus eliminating this nasty shock hazard.

Cue much sucking in of breath between teeth. I would suggest replacing the VO trip with a modern RCD in an enclosure, to maintain the earth fault protection, sounds essential in an installation of this age

10mm^2 sounds good for that kind of length, assuming due to the 16mm T&E that it's a 60 amp main. If it's a 100 amp main, I would go with 16mm^2 earth even for that distance, I wouldn't bother with the parallel 6mms myself.

Part what? <g>
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wrote:

Yes
I'll have another look at the weekend - it's unfortunate that I'm 135 miles away - but I don't think so.

It's an underground feed, in a city street of 19th century houses, so I'd guess it's never been an overhead supply. No obvious old earth rods outside.

The general consensus seems to be to avoid having all power lost because of one earth fault - another reason to get rid of Mr Chilton's finest?

It will certainly finish up with a modern RCD, but I'd prefer to put it in a split load CU as the present VO trip is in the (shared) entrance hall, whereas the CU is in an understairs cupboard in her flat.
Thanks for the advice, Chipmunk
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On Thu, 16 Jun 2005 15:34:11 +0100, "Autolycus"

Ahh, unlikely to be a converted TT system then, but it still *might* be PME, Which is really immaterial, the Power company have provided an earth terminal, this is A Good Thing (TM)

[snip]
The continuity of lighting is certainly a serious consideration which does point to the use of a split-load board at some point, Possibly with a 100mA RCD as the main and a 30mA feeding the portable appliance circuits, (Expensive though, as most ideal solutions are :-( )
From your overall description, I would be tempted to run a 10mm^2 earth from the main earthing terminal, parallel it with the earth in the T&E, and run it to the CU.
Just as a matter of interest, who pays for the lighting in the communal hallway?
And I know you said not to mention Part P, but being a shared dwelling this could get horrible with liability and so forth, good luck sorting this lot out.
As an aside, voltage operated ELCBs have safety issues with modern cross bonding, such as is often carried out in ignorance of their existence/function, inasmuch that a parallel earth path bypassing the voltage coil will prevent the device _ever_ functioning, which is *NOT* A Good Thing (TM).
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No, this is a (very common) misunderstanding of how they are supposed to work. They monitor the voltage between your building earth conductors and the real ground. They must trip off before it reaches 50V. You are allowed any number of parallel earth paths, and if you have enough of them, chances are the earth resistance will be low enough to stop a potential difference of 50V developing. In this case the Voltage Operated ELCB has no need to trip because enough current will flow to trip the fault current device (fuse or MCB) instead. The Voltage Operated ELCB only needs to trip when the product of earth leakage and earth resistance is high enough that the fault current protective device won't trip. They are not designed nor required to trip in the case of any/all leakage to the earth conductors (and unlike RCD's, they are not designed to protect you from electrocution in any way).
If you can find a copy of the 14th edition Wiring Regs, this is described quite clearly in the section which deals with testing Voltage Operated ELCB's. There's a typical installation wiring diagram in there which shows the parallel earth paths, which although optional, are normally likely to exist.
Now, who's still got a test transformer for a Voltage Operated ELCB? ;-)
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On 16 Jun 2005 16:42:15 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

AIUI, you can indeed have lots of parallel earth paths and cross bonding However what you mustn't do is to connect anything additional to that "sense" earth connection. This is the setup (bypassing the voltage coil) that stops it working.
Is that right ?
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Yes, and also there must be no other [real] ground connections within the earth rods's resistance area, which would have the same effect to a lesser degree if it's from the same installation, or can cause nuisance trips if it's from a different installation and current leaks back in through the earth rod.
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On 16 Jun 2005 19:14:22 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

Ahh that is where my misunderstanding came in, I had assumed from the original article that there was in fact _no_ sense rod. It appeared that the voltage operated ELCB was connected between the case of the isolator and probably the consumer unit. However reading back it appears that one of the wires disappears to *who knows where*, which all emcompassing term might include "long forgotten earth rod that is now buried".
[quote: "and one from the top of the earth trip disappearing under the floor"]
That may well be the hookup to the sense terminal. I can't remember the labelling on those things but the lack of knowledge of them [that I demonstrated too] can cause them to be inadvertently disabled.
If there is now a decent earth available from the power company, (which has yet to be determined) then of course an RCD would be the way to go, with a split load board to prevent plunging the place into darkness in the event of leakage.
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Well, you might be right. There's not enough detail and I couldn't actually form a good picture from the description.
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writes:

Thanks again, to Chipmunk and Andrew especially, and apologies for starting another NT v the rest thread.
I've had another look at the setup. The incoming supply is here:
http://www.mainbeam.f2s.com/flat/incomer.jpg
with a closeup of the earth terminal here:
http://www.mainbeam.f2s.com/flat/earth.jpg
It is indeed next to the incoming neutral. There is also what appears to be a covered flat strip emerging from the incoming cable sheath immediately below the box and entering it just in front of the main cable. One the two 16mm^2 cables is connected to the upstairs flat's installation, and the other goes to an earth block with connections to the equipotential bonding of the incoming gas pipes. A smaller cable disappears under the floor.
So it's looking like a TN C-S setup? The isolator which is adjacent to the incomer and the Chilton VO trip contains a 60A fuse, which gives me some comfort about the 16mm^2 T&E to the CU.
I'm still minded to replace the CU with a split-load unit (Volex from Screwfix or GE from Toolstation - opinions?), and to connect all the green or green/yellow cables at the incomer/meter board together, with a beefed-up cable linking the "earth" of the 16mm T&E directly to the other earths rather than via the trip. Am I right that it does not matter if one of the untraced green cables does indeed go to an earth rod under the floor?
Since the earth trip will then be redundant, would it be better to remove it completely and replace it either with a Henley block or a 100mA time-delayed RCD in an enclosure? I'd prefer to avoid new tails, as even I don't reckon to break meter seals.
To allay concerns others have rightly expressed: I'm buying a copy of the On-Site Guide, and the fact that it's a flat doesn't really come into it: my daughter owns the freehold of the whole property, with the upstairs flat, which has its own meter on a different phase from the incomer, leased.
One for the rogues gallery: the isolator has a bit of length of 0.5mm^2 three-core flex poked into its output terminals to feed the bell transformer
http://www.mainbeam.f2s.com/flat/transformerflex.jpg
Dunno what to do with that - the ring main is yards away.
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On Thu, 23 Jun 2005 13:48:35 +0100,it is alleged that "Autolycus"

I am glad you did start the thread, I learned a few things about VO ELCBs :-)

I would say it's looking like a TN-S setup, if the 'flat strip' you mention is below the box it sounds like an earth clamp onto the [probably lead] sheathing of the supply cable. Fairly standard in urban/suburban areas until fairly recently.
Installing an RCD protected (even part protected/split load) certainly isn't going to _lessen_ the safety.

That one I shall leave to others, I know what I'd do and it would involve buying the aforementioned RCD, but I might be wrong to do it.

Excellent on both counts:-)

Hmm, eek. I take it that will be going byebye fast!
Good luck with the project:-)
On the subject of CUs, right now TLC are selling their Hager panels for very very cheap prices, 18 pounds today for a 63amp/30mA RCD, the 100amp/100mA version would have been 28 pounds.
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Can I just check - this is for ELCBs only isn't it or does this apply to RCBs as well ?
And if so, what do you count as the earth rod's resistance area ? We've got an earth rod (and RCD'ed CU) per outbuilding but some are pretty close to each other.
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There's no longer anything in the regs about it that I recall. (The concept of earth resistance area isn't even mentioned now that voltage operated ELCB's aren't in the regs any more, except for a dangling reference in the index which points to the Part 2 definition which is no longer there;-)
However, it would still be a good thing to do, but not for the sake of the RCD which doesn't care.

The 14th Edition regs may have said how to measure it (sadly I've lost mine). I don't have the exact definition to hand, but ISTR it was the area round the earth rod where a voltage difference was measureable (relative to some more distant point in the ground) as a result of leaking current into the earth rod under test. I can't remember how much current you were required to leak into the rod for the test.
However, as I said, this isn't so critical for RCD's which won't nuisance trip if some portion of someone else's nearby earth leakage is finding its way back up your earth rod due to overlapping earth resistance areas.
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Andrew Gabriel wrote:

ok but lets be clear about something:
1st this problem can only occur if they are incorrectly installed, with the earth rods of 2 houses in close proximity.
2ndly One also has to get near 240v on the foreign earth to get the problematic 50v on your sense earth. This will of course be detected and rectified very quickly, either as a result of the neighbours ELCB or RCD, either of which is universal on earth rod protected systems. Or even if the neighbour has a 1930s install with no protection, they will soon get a shock and get it fixed. Either way its a self fixing problem, even when the thing was installed wrongly.
2nd this is a lot rarer than RCD nuisance trips.
3rd it is usually self fixing, and in the exceptional case that its not, its probably easier to fix than RCD troubles:
ELCB nuisance trip fixing: hammer in new earth rod further along wall, extend earth wire, done. Expect no further problem.
RCD nuisance trip fixing: PAT test all appliances replace an often satisfactory appliance or Trace neutral/earth connections throughout install trace borrowed neutrals throughout install. Insulation test install Rewire any problem ... and know in either case that nuisance trips may occur in future.
ELCBs arent the ultimate in earth leakage protection, but they do have real advnatages over RCDs as well as cons, and are in some respects the better device. And it is better in some cases not be replace them with an RCD.
NT
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