Down to earth - can you identify tonight's mystery switch gear?

[John Rumm]

One of the jobs in need of doing shortly is the replacement of my Wylex rewireable fuse consumer unit. Nothing actually wrong with it, but it only has six ways and I am going to be needing eight in the near future, so it seems like a good time to bring it up to date.

This brought about an investigation of the earthing setup. Never having had need to look that closely at it in the past, I had not done a detailed study of what was there, and had probably made a few assumptions! A quick check over tonight raises some interesting points...

First assumption - Being a TT System (overhead power delivery) it will have RCD protection. Having looked close up at said "RCD" I am now not so sure. Have a look at the following (out of focus) picture:-

that likely to be an RCD? Or is it more likely, as I now suspect, an old style ELCB?

The words at the top read "Test this protective switch often by pressing the yellow button", the words above the switch simply show "500V 50~", & "60 AMP". It's made by Crabtree. Nothing else that I can see written on it - no mention of a trip current. That feed through earth wire from the CU looks a bit suspect as well!

The Henley block you see in the photo splits the tails from the meter, one set go to the item in the photo - the second set go to a much newer MK 30mA RCD that ultimately feeds the out buildings.

The second point of interest is the actual earth itself - So far the only earth connection I can find is to the incoming (steel/iron) gas main. Nothing that I have yet found seems to connect to the water main, and as for an earth spike - if there is one, it is well hidden. (don't even bother asking about suplimentary or equipotential bonding!) Not sure how common this was/is on 50's properties? While the gas main might be a resonable earth (not tested it yet I do know that there is no plastic in it for a good 20m minimum) not sure I like the idea of relying on only that.

(the outbuildings do have their own earth spike adjacent to them - I have not yet verified if this is connected back to the house earth as well - although it is certainly not isolated from the SWA armour on the feeder cable).

Hence an upgrade to all things earth seems like a good plan.

How many earth rods, and to what depth would be recommened for a heavy clay soil?

[Jb]

This is without doubt an old voltage trip and needs scrapping.

Many properties of this age were earthed to a rod using steel wire and over the years lost the connection to rust so it may well have disappeared by now. Asking how many earth rods is a bit like asking how long is a piece of string. You need a meter to check (not a multimeter) the final earth resistance. Don't make the common mistake of banging in rods close to each other thinking more is better; getting them too close actually increases the resistance. In clay you probably only need one but you buy a 'proper' earth rod they will screw together to achieve a greater depth should you need to.

[John Rumm]

Thought as much...

I could not even find evidence of a wire inside - the only obvious one running downward from the install was the one to the gas main. If there was a rod is it also possible it got burried when the previous owners laid the concrete drive which continues up the side of the house beside where the meter cupboard is located.

My plan was to measure the fault loop impeadance of what is there first to see what the starting point is, and then add at least one proper earth rod and carry on unitl I get a decent low enough value. Finally add the cross bonding to the water main.

[Christian McArdle]

Make sure the earth loop impedence is OK with just the earth rod connected. This means that when the gas supplier changes all their pipes to plastic without telling you, you won't get fried.

Christian.

[Andrew Gabriel]

The right hand earth wire coming out of the Voltage Operated ELCB should go to an earth rod, and nothing else.

There's no point with the ELCB in the circuit -- its coil will be of the order 100-300 ohms most likely.

If you add more earth rods to such a setup, they should be spaced well away from the existing one, and all connected to the main earthing terminal, not the existing earth rod. Same goes for cross bonding. The right hand earth terminal must only go to an earth rod, which should be outside the resistance area of any other earth rods or equivalent resistance area around bonded services. (The resistance area is the area around an earth rod where the ground changes potential due to current flow in the earth rod.) The ELCB works by monitoring the voltage between your earth conductors and its earth rod, and tripping before it reaches 50V.

You should add an RCD nowadays, but you can leave the ELCB there harmlessly if you lower the overall earth impedance as above, and it provides you with a means to isolate the CU.

[N. Thornton]

A little explanation is in order. V-ELCBs are widely misundertood, and are potentially dangerous. They work by /disconnecting/ all the house earth wiring from earth, and measuring the V difference between real earth and house earth wiring. Once this reaches 50v it should trip.

This means that assuming it is a v-elcb, none of your house wiring will be earthed. V-elcbs must never be used with an electric shower.

With a v-elcb there is no current path from earth wiring to earth, other than via the high impedance coil, so the impedance of your real earth is immaterial. Bonding to a gas pipe is more than sufficient for such an install as long as it is wired through the v-elcb.

It is important to test v-elcbs frequently as a failure leaves the householder in real danger. Realise all earthed items are connected together, and none are connected to earth.

The old earth connection may be ineffective with no v-elcb in circuit, as it may be of high impedance and rely only on the v-elcb for protection.

v-elcbs also dont trip on /all/ types of earth fault, so in fact a v-elcb install can occasionally allow dangerous Vs to be acessible even when not faulty. And due to the earth disconnection inherent with v-elcbs, an earth fault anywhere in the house can leave the house earth wiring ready to bite.

Theyre best replaced with more modern methods of protection. Their only advantage is I've very rarely seen one nuisance trip. But despite their clear risks I've yet to see them fry anyone. A bit of a bite yes, but thats all so far.

Regards, NT

[John Rumm]

It currently runs to an earth terminal block, from there is a connection to the gas main, plus one other (as yet undefined) connection that runs up to somewhere else in the building.

Good point - I am certainly planning to ditch the ELCB anyway (space is a bit limited in the cupboard) so that is not to much of a problem to replace it with an RCD first.

Keeping away from the existing one should be easy ;-) (since it's not there). Keeping away from the other services is not quite as simple since they both run down the side of the property. I also want to avoid banging an earth rod through any drains if I can help it.

I was thinking of placing the earth rod toward the front of the property just beyond the last drain gully. That way I can be reasonably sure of being clear of the drains (and other services). It also means I can run a 10mm^2 earth wire inside the building to level with the rod and exit through the wall right by it (about a 6m cable run it total).

The distance from the rod to the gas main (where it enters the property) would then be approx 4m, and about 6m to the water main. The underground pipes would however still pass within a couple of meters of the earth rod in this situation. The other alternative would be to try and find a route out the back of the house to the back garden - this is a little more awkward however since there is a substantial patio area to cross.

Where do they normally site earth rods on new builds these days?

Looking through the TLC catalogue shows MK RCDs at twice to three times the price of the Contactum, Hager, Volex ones etc. Any one got any preference for brand? Are the MK ones worth the extra?

[John Rumm]

Not much chance of that - I just put in a 35kW combi ;-)

Although I have not measured the impedence yet, I expect the gas main in this case is relatively decent earth since it is fully metalic, and passes through lots of heavy clay soil (mostly under concrete so it is unlikely to ever dry out).

Probably less of an issue with no supplimentary equipotential bonding as was common at the time the ELCB would have been instaled. Also for that matter, the number of earthed appliances in use at the time would have been far fewer as well.

I shall demote the old connection to cross bonding only (might upgrade the wire size while I am at it).

Must admit in the ten+ years we have been here I have never had a trip from either the ELCB or the RCD feeding the outbuildings (and yes they do self test ok). Other than a mains halogen taking out a lighting circuit fuse, I have never had any other fuses blow either (although that could be related to the crummy earthing!)

[Christian McArdle]

Make sure you replace it with a 100mA time delayed RCD. Your socket circuits/outside electrics etc. should have additional 30mA instant acting types. I would definitely continue to use a separate 30mA RCD for the workshop compared to the house electrics.

Christian.

[John Rumm]

I could do that with RCBOs in place of the MCBs in the new consumer unit for the relevant final rings. I chose a consumer unit that would accept "tall" single width RCBOs in addition to the more common double width ones - so the only downside would be the extra cost.

(There is not enough width available in the cupboard to use a split load CU)

I assume the main advantage of going for the discriminated RCD chain like that is reducing the likelihood of nuisance trips? Compared with the alternative (i.e. having a standard 30mA type AC RCD feeding the whole CU), do you think the discriminated system is worth the extra expense?

Yup was not planning on changing that (it is a far more recent addition to the house electrics anyway and seems to be well implemented).

[Christian McArdle]

There are 3 basic methods (with variations, of course)

1. Whole system 30mA RCD
2. Split load 100mA time delay+30mA RCD
3. 100mA time delay + RCBOs

The problem with system 1 is that it kills the lights in the event of an earth fault, which could be very dangerous, either due to rotating machinery or fire escape. Some would say that system type 1 is no longer allowed by the regulations, although it isn't very explicit about it, if I recall.

The advantages of system 3 over system 2 are twofold.

Firstly, it reduces the chance of a nuisance trip as there are fewer appliances protected by one device, leading to a lower quiescent residual current that isn't so close to the trip point.

Secondly, it reduces the effect of a nuisance trip, limiting it to the circuit the trip occured on.

Obviously, system 3 can be very expensive if many circuits must be 30mA protected. You would probably err on the side of protecting more circuits on a TT system, as the consequences of a nuisance trip on the non-RCD protected circuits are greater, as they would trip the time delay RCD and kill the lights.

Personally, on a TT system, I'd be tempted to abandon the whole installation RCD and use RCBOs for every circuit without exception and an insulated consumer unit. This provides maximum discrimination, a single box solution and the required RCD protection for every circuit. However, it is very much more expensive than the cheapest permitted solution. I can't remember if the consumer unit internals require RCD protection. If they do, you'd still need the 100mA time delay incomer.

Christian.

[Andrew Gabriel]

That's true, but unfortunately much of what you say afterwards is less so.

The house earth is connected through the ELCB, and it is allowed to have any number of other parallel connections to earth too such as additional earth rods (see ELCB wiring diagram towards back of 14th edition wiring regs, in the testing section IIRC). Providing there's no fault in the system, all the wiring remains earthed. In any event, the system remains bonded.

You misunderstand voltage operated earth leakage circuit breakers.

Why?

No!

All bonding is done to the main earthing terminal (earthing strip in your consumer unit or nearby where all the house earthing conductors are brought together), just as with any type of earthing. You can have zero or more earth rods connected to the main earthing terminal too. You have to have one earth rod connected to the other side of the voltage operated ELCB which must be outside of the resistance area of any other earth rods so it is measuring true earth potential as far as possible, and not affected by earth leakage from other earth rods, yours or someone else's.

The danger is exactly the same as with a failed RCD on a TT system, and an RCD is a far more complex device (I've seen failed RCDs, but not failed VO-ELCB's).

Certainly you shouldn't use an earth rod installed for a VO-ELCB as your earth electrode if you remove the VO-ELCB without testing the earth electrode impedance.

Only if your bonding is also broken.

Actually, their main problem is with nuisance tripping, due to stray current leaking into your earthing system from their ground rod. This could happen if someone too close starts leaking current into the ground nearby, and during lightning storms.

I would not advocate the use of VO-ELCB's nowadays as there are better devices around. Also, you would be hard pressed to find anyone with a test transformer for testing them. However, they performed an important safety role for very many years, and they weren't and didn't suddenly become dangerous just because the RCD was developed and is better. They are now badly misunderstood.

[Owain]

"Christian McArdle" wrote | 1. Whole system 30mA RCD | 2. Split load 100mA time delay+30mA RCD | 3. 100mA time delay + RCBOs | The problem with system 1 is that it kills the lights in the event | of an earth fault, which could be very dangerous, either due to | rotating machinery or fire escape. Some would say that system | type 1 is no longer allowed by the regulations, although it isn't | very explicit about it, if I recall.

| Obviously, system 3 can be very expensive if many circuits must be | 30mA protected. You would probably err on the side of protecting | more circuits on a TT system, as the consequences of a nuisance | trip on the non-RCD protected circuits are greater, as they would | trip the time delay RCD and kill the lights.

An alternative could be to use a split load CU, with *all* the circuits off the non-RCD main switch being RCBOs (say socket rings and lights), with the RCD side being used for items where discrimination is less essential, eg cooker, shower, immersion heater, radials to wall heaters, etc. (There would be no whole-house RCD at all.)

That would provide the advantages of system 3 on power and lighting circuits, which are more likely to suffer high quiescent residual current and also form the greatest danger/inconvenience in the event of a trip, whilst reducing the number of RCBOs required. If the washing machine and dishwasher are wired to radials on the RCD side, a separate kitchen ring might not be required, so most houses would be sufficed by 2 socket rings and 2 lighting radials only requiring 4 RCBOs.

The downside is that most split-loads only have about 60A available through the RCD.

Owain

[John Rumm]

Only 1 or 3 would be viable for me...

In my case, most rotating machinery is in my workshop which is on the end of an external feed that is already connected via the 30mA RCD - so I have lost that one before starting. While not an ideal situation, I can't think of any situation where it would be a major problem should all power get cut - just a case of stand still, stop pushing whatever you are pushing and wait for the noise to stop ;-)

(all non hand tools have NVR switches)

The main nuisance would be loss of lights - I expect that in many case some emergency lighting could be provided for less than the cost of option 3...

I would as a minimum need to protect the downstairs, and kitchen ring circuits. I also plan to install a heated towel rail in my new second story that will have a 100W backup heater in it for summer use. Hence that would really require another 30mA RCBO for that ring.

That or option 3 would certainly be the "nicest" solution... to be honest I am leaning toward option 1 at the moment based on a suck it and see approach. If it runs like the current setup (i.e. no unwanted trips at all) then it can stay that way, if it causes problems, then I can "upgrade" to option 3 at a later date without too much wasted expense (i.e. just the cost of the unused incomer RCD that would get replaced by the time delayed one).

As an aside, anyone know of a good source of time delayed 100mA RCDs (60A rating would probably be enough). The only ones I can find are in RS with their usual prices!

[Christian McArdle]

You could use an MK 6400S. Your main incomer should really be 100A rated.

Christian.

[John Rumm]

Nice idea, but I don't think it will work for me in this case. I currently have 5 circuits, 3 final rings, and two lighting circuits. No radials, no electric heating, showers, immersion etc. I will be adding one extra final ring, and one extra lighting circuit, and a dedicated smoke alarm circuit. So the above suggestion would require 7 RCBOs. The secondary problem would be that a split load CU with enough ways will not fit in the available space. That then turns the job of "swapping" CUs into a much bigger job of relocating and replacing the CU.

I could add the new second floor sockets and lighting to the existing circuits since I would still be within the permitted maximum area and loadings. However I much prefer having the ability to isolate each floor discretely.

Which is all my supply is good for anyway. Having said that I will probably install a 100A RCD to allow for the possibility of future upgrades.

[Christian McArdle]

relocating and replacing the CU.

So basically you've got the choice of spending close around 300 quid on an RCBO solution, or 60 quid on an MCB/whole installation RCD system that doesn't really comply with regs...

Christian.

[Christian McArdle]

Just a thought, does it help if you orientate the consumer unit 90 degrees out?

Christian.

[Owain]

"Owain" wrote | "Christian McArdle" wrote | | 1. Whole system 30mA RCD | | Some would say that system type 1 is no longer allowed by | | the regulations, although it isn't very explicit about it, | | if I recall.

I meant to add in the gap:

130-01-01 Good workmanship and materials shall be used.

130-02-01 All equipment shall be ...installed .. so as to prevent danger as far as is reasonably practicable.

314-01-01 Every installation shall be divided into circuits as necessary to: (i) avoid danger in the event of a fault, and (ii) facilitate safe operation, testing and maintenance.

314-01-02 A separate circuit shall be provided for each part of the installation which needs to be separately controlled for compliance with the Regulations *or otherwise* to prevent danger, so that such circuits remain energised in the event of failure of any other circuit of the installation, and *due account shall be taken of the consequences of the operation of any single protective device*.

from and copyright IEE Wiring Regulations Sixteenth Edition 1991. [* my emphasis *]

Owain

[John Rumm]

You mean install it so that the DIN rail is running verticaly?

Hmmm never seen on done that way round before but, I suppose it could work. The things limiting the available height in the cupboard are at least moveable (unlike the walls which limit the width!) It would still result in having to crimp extensions onto a number of the circuit wire ends though I would expect.