Improving an old electrical installation

[Autolycus]

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.

[bigcat]

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=2E 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=2E Get type C breakers for the lights or you may suffer repeated nuisance trips, the usual type Bs sometimes cause lots of trouble. 3=2E 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 =A3100 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

[Chipmunk]

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.

[<me9]

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.

[Autolycus]

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

[Andrew Gabriel]

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).

[Andy Dingley]

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.

[Chipmunk]

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)

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).

[Andrew Gabriel]

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? ;-)

[John]

That will probably be the attachment to your consumer installation earthing system

Which is "probably" attached to an earth electrode rod driven into the ground. The problem with many of the old voltage operated breakers was parallel earth paths causing them to have operating problems.

Well no the T&E conductor ought to be the connection from the breker sensing terminal via the Switchfuse earth terminal to the earth bar in the CSU

Time to call a halt here NOW! the set up sounds distinctly dodgy and you should take immediate steps to rectify the protection before an incident arises.

I would suggest that might be a very sensible course of action. If you have access to an earth loop impedance tester you could check the integrity and suitability of the earth terminal at the side of the suppliers cutout. It "should" be good if it is provided by the supplier but don't just assume it is. Also worth checking is the size of the CPC in your cabling for the various circuits.

No for TN-S or TN-C-S the main Earth conductor should be in 16mm^2, in your case I suggest a seperate 16mm^2 earth "wire" be run from company terminal to CSU Main equipotential bonding should be in 10mm^2. What size "tails" do you have from the meter to the VO circuit breaker? ideally you should change them to 25mm^2

Try to get hold of a copy of the On-Site guide (also the new booklet An electricians Guide to the building regulations). There is enough guidance and example diagramatic info to enable you to sort out your problem.

[John]

The OP said it was a voltage operated version

[bigcat]

No, this would not be clever. The ELCB is there because it needs to be. You can not simply replace an ELCB with a modern RCD and expect all to be well.

Leave the ELCB in place, make sure its connected up properly. If for whatever reason you want RCD cover, achieve this by replacing your fusebox with a split load CU with either one RCD or much preferably RCBOs.

To remove the ELCB may well be dangerous. To replace it with an RCD will cause no end of trouble, and is not regs compliant, for good reason.

For whatever reason, voltage operated ELCBs seem to be widely misunderstood. They offer a satisfactory level of protection against inadequately low earth impedance leading to the live earth problem. They are almost immune to false trips, about the only things that will trip them is a lightning strike or a massive electrical fault in a _very_ close neighbouring property.

ELCBs do not offer the touch trip protection of a modern RCD, which is why RCDs are now chosen instead. But the safety record of ELCB installs is still excellant.

An RCD can not be slotted in to replace an ELCB for 2 reasons. Firstly dated installs are likely to trip an RCD the moment theyre switched on, for a vriety of reasons. Secondly RCDs are in/famous for nuisance trips, and having one install on one RCD is such a cause of trouble and risk it is no longer permitted under the 16th edn regs.

You have options, but removing the ELCB is not part of a good option. Keep ELCB and put your desired RCDs in a new CU if you want to - but wire fuseboxes are quite ok in reality, they do not have any bad safety record, and are still permitted to be fitted as new.

PS those Wylexes look much older than they really are.

NT

[Andy Dingley]

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 ?

[Andrew Gabriel]

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.

[Chipmunk]

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.

[chris French]

In message , Andy Dingley writes

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.

[Andrew Gabriel]

Well, you might be right. There's not enough detail and I couldn't actually form a good picture from the description.

[bigcat]

ELCBs are designed to prevent user electrocution, that is their main aim. They do this by protecting against the earth wiring voltage rising to dagnerous levels. RCDs also offer partial protection against users touching live wiring, ELCBs dont.

Andy D>AIUI, you can indeed have lots of parallel earth paths and cross bonding

Its true you should not connect anything to the sense earth, although even if you do, everything will still work. Its only if you commit both sins at once of connecting to the sense earth plus crossbonding to the other earth that their function is affected. But there is no reason to connect anything to the sensing line.

The OP needs to tighten the connection to make sure it will work if needed.

If you have only one ELCB/RCD covering a whole install, an ELCB is the better option. Why?

1. Although RCDs offer extra protection, this translates to no less deaths in real life. Its possible it may even translate to more, but I dont have the figures to know.
2. Whole install RCDs are a recipe for trouble, they nuisance trip with a vengeance. ELCBs almost never nuisance trip.

NT

[Andrew Gabriel]

This is very incorrect. See my earlier posting for details.

Oh yes they did, and the nuisance tripping was normally caused by something outside your premises, such as something your neighbours were doing, which made it much harder to diagnose, and even if you did, you probably had no control over it.

[Mike]

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.