Did we ever agree if an RCD for the whole installation was allowed?

[Kalico]

I remember this being discussed some time ago, but not sure if we reached a conclusion.

The situation where an RCD is used to protect all circuits in a consumer unit, rather than just sockets etc in a split-load arrangement.

I have to replace an old Wylex fuse-board that has only 5 circuits:-

1x30A Ring main (it's only a studio flat) 1x30A Fan heaters 1x30A Shower 1x15A Not sure yet as not labelled 1x5A lights

My understanding is that the shower circuit should have an RCD fitted, although this is not clear from the OSG.

The problem is that the space available for a replacement CU rules out the usual split-load units, though I guess I could make one up (though this can be very expensive compared to the kits available).

Anyway, I'll wait to hear your thoughts.

Rob

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[Ed Sirett]

AIUI you don't absolutely need to have an RCD if you can show the circuit

• MCB + size of Earth wire would meet the required disconection time of

0.2s. However it is simpler to fit an RCD. This could be done in a separate unit outside of the main CU. If none of the sockets could reasonably be expected to power outdoor equipment then non RCD is needed for the Ring Main either. Given that the space heating is electric is the hob/cooker gas or only 1 or 2 plates (

[Andrew Gabriel]

I don't have the regs or On-Site guide on me, but I think a single RCD protecting a whole installation has to be at least

100mA, which means it's unsuitable to provide protection against electrocution for appliances used outdoors and in some cases in a room containing a bath or shower.

Immersion heater?

First we need to know what earthing system your installation uses.

There are two reasons for using RCDs:

1) Protection against electrocution, which is required for sockets which might be used to power outdoor appliances, and some cases of appliances in show/bathrooms (not the shower though). RCD protection in this case must be no more than 30mA. It is a bad idea to have lighting on a 30mA RCD shared with anything else (and they don't normally merit a dedicated one).

2) Protection against high earth fault loop impedance, which normally applies to TT systems (own earth rod). In this case, the protection is normally a minimum of 100mA.

So the shower would only possibly require the 2) catagory, in which case all of your installation would anyway.

If you don't need protection in the 2) catagory, then a CU with no RCD protection and an RCBO for the ring main would be one way. RCBO's are combined MCB and RCD in one module, and are available for many CU's in same width module as an MCB (check before you buy though as for some CU's they are double width modules, or not available at all).

[Owain]

"Kalico" wrote | I remember this being discussed some time ago, but not sure | if we reached a conclusion. | The situation where an RCD is used to protect all circuits | in a consumer unit, rather than just sockets etc in a split- | load arrangement.

I have suggested that a whole-house RCD contravenes the following regs:

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 *]

The above taken in conjunction with Peter Parry's comments on the number of deaths through falls possibly linked to sudden loss of light on staircases suggests to me that a whole-house RCD is not only in breach of the Regs, but incompetent verging on negligent (unless there are other provisions eg emergency lighting).

You could fit an emergency lighting unit which would help mitigate the risks arising through loss of discrimination, if you decide you need an RCD.

| I have to replace an old Wylex fuse-board that has only 5 circuits:- | 1x30A Ring main (it's only a studio flat) | 1x30A Fan heaters

These would usually be on 15A radials. If its a radial circuit at 30A it should be in 4mm cable not 2.5mm (which is for 30A ring or 20A radial circuits)

| 1x30A Shower

If rewiring, I would suggest provisioning a new circuit at 45A. You could use an RCBO instead of an MCB - most are two-module, but it would save space compared to a 3-module main switch plus 3-module split-load RCD.

| 1x15A Not sure yet as not labelled

As Ed Sirett says, probably immersion heater

| 1x5A lights

| My understanding is that the shower circuit should have an RCD fitted, | although this is not clear from the OSG.

As Ed says.

Owain

[Christian McArdle]

You can get single width RCBOs which replace MCBs and provide RCD protection for that circuit only. Personally, I would like one on the socket circuit as well, although I would also prefer the fridge/freezer on a non-RCD circuit, which would require another circuit (or some careful calculation and possible MCB upsizing on another circuit so that it can share).

The 15A is probably the water heater.

Christian.

[Pete C]

Hi,

The usual objections are freezers defrosting and falling to your death down dark stairs, but with a studio flat there may not be space for a freezer and falling down stairs is unlikely in any case.

I'm sure there are fatalities due to falling down in the dark when the RCD trips, but there are also fatalities due to faulty wiring and/or householders putting screws or nails in the wrong place, so it's a balance of risks.

I wonder if it's permitted to put a 6/16A RCD garage consumer unit in as well, for lights and fridge/freezer, then the rest of the property could be on a normal RCD protected consumer unit.

I'd be very suprised if a whole house RCD is forbidden for _any_ installation.

Personally I'd fit a whole house RCD in this case unless expressly forbidden or there is a risk of defrosting a freezer with >£100 worth of food, then I'd add a separate unit for it as above if that was permitted.

cheers, Pete.

[Kalico]

Well, thanks to all for the advice.

Seems that the 15A circuit will supply the fan heating and the three

30A supply the ring main, shower and cooker.

The shower is run in 6mm and I am reluctant to replace the cable so will replace the shower with a 8.5KW which is within limits.

Regarding the new CU, I have ordered a small 9way Wylex split load CU which should do the job 'properly' and I can put the shower and sockets on the RCD.

Given it is a tiny flat on the first floor I am minded to think it unlikely that the sockets will supply kit to be used outdoors but I like to see an RCD on sockets anyway, freezer or not.

As an aside, I have some rented houses that have a 100mA RCD on the whole of the CU. Must have been a common way to re-wire them when they were done approximately 10 years ago. Should I now re-wire them split load. I hear the arguements about falling down stairs but think it more likely a tenant will put a nail through a wire or similar.

A good example was the tenant who went to replace a light rose/pendant with a new fitting his wife had bought at B&Q. Of course, he made no notes of what was connected where and couldn't understand why there were 'so many wires' coming through the hole in the ceiling when the new fitting only had a two way bit of chocolate block!

Let me know if you have any more thoughts and once again, thanks for the help.

Rob

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[Shabs]

Talking about MCBs, i have just had a split load cu installed and was going to use the split load to power my outdoor circuits in my outbuilding. i plan to have a 6a mcb on the rcd side taking power to

3 lights, and a 32a mcb taking power to 4 sockets on a ring main. Is this the correct way to do it? as i have read that a 16a mcb should be used for outdoor socket circuits.

I ask this question as i have just read above that a rcd protected mcb should not be used for outdoor lighting.

[Christian McArdle]

It is fine to do so, but you may not wish to share the RCD with another circuit, as leakage may get quite high in wet weather. Personally, I wouldn't want any exterior circuits to share an RCD (i.e. on a split load) with any circuits internal to the building.

I've used RCBOs for this, but you could just use an MCB off the non-RCD side and fit an additional RCD just for that circuit in a separate box (or an entire consumer unit for exterior electrics, if there is a lot of it).

Christian.

[Pete C]

If better protection is required, a faster 30mA RBCO on the ring main and possibly cooker circuit may help, as they're more vulnerable to stray nails and dodgy appliances.

Maybe others can comment on whether a 100mA RCD will normally trip when a light bulb blows.

cheers, Pete.

[The Natural Philosopher]

No, but a 30mA one may.

I have yet to fully sort my setup, but a 30mA RCD was unuseable on the house - all new wiring, no faults. Just lots of leakage from various things. Mainly eletronics of which I have a LOT.

100mA has been utterly reliable - only tripped when washing machine motor coils shorted to ground. And in the odd thunderstorm.

Still looking for slender RCBO's to fit teh MCB positions where needed.

[Buxnot]

Our installation is a strange one. It is a 1950s house that was completely rewired about 12 years ago.

The wylex consumer unit contains a 100mA RCD which doubles as the main switch. Downstream from that are 2 lighting circuits (not upstairs and down but left-half-of-house and right-half-of-house - god knows why it's done this way).

Also downstream from the 100mA RCD is a 30mA RCD. Downstream from that are all the electric circuits including water heater (always off as we only use gas) cooker, socket rings (again left-half-of-house and right-half-of-house).

We're on a PME system.

I can only assume that the circuits have been wired left-half/right-half to allow working on circuits on the same floor while still getting power from an extension cable in another room. All highly dubious to me, I just turn off the whole 30mA RCD. At least we can still get lights with that turned off....

[Kalico]

Not in my experience. In fact, the 100mA trips have never tripped unless they 'should', ie when there has been the type of fault that an RCD is designed for.

Rob

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[Martin Angove]

The usual cause for installing a 100mA RCD to protect everything, followed by a 30mA for sockets is where the earth is provided locally - i.e. by an earth rod or similar. In these "series" setups the 100mA should be time-delayed in order to allow the 30mA to have a chance to trip first.

If, as you say, you have a PME system then the 100mA RCD is not usually required. Be cautious though - in my area the local distribution company upgraded everyone years ago to be PME *capable* but no more - most installations are still TT. This could be confusing for anyone who doesn't know about it because the distribution company replaced the "cutout" (the bit their street cable goes into which includes the fuse before the meter), and most of these cutouts have labels on them saying "warning, this is a TN-C-S or PME system" (or something similar).

The fact is that unless there is an earth wire going from the main earth terminal into the neutral block of this cutout, then the setup is still TT. Getting your earth into this terminal varies from area to area. Around here Western Power will install a short tail and a brown earth terminal for nothing, whereas in Derby (forgot the name of the company) the charge is 70 quid.

There's nothing at all dubious about having a left/right split versus an up/down split. All the regulations require is that installations are split into a sensible number of sensibly protected circuits. Just because up/down is *normal* doesn't mean it's the only way, so long as the labelling is clear.

Lighting is usually done up/down because it is much easier to do it that way - especially in an unboarded attic it is the work of a couple of hours to run cables over every room to lighting points. Sockets are a little more difficult as they usually come up from below (upstairs).

Some houses have everything together. The usual way to do it in terraces around here is 30A ring main for all sockets (up and down), 30A cooker,

15A immersion (if installed) and 5A lights (up and down). When replacing a consumer unit it is often quite easy to split the lights as there are usually two cables from the same 5A fuse, and one often goes straight up into the attic.

When I rewired our house (1920s council semi) it got a lot more complicated than that; I have three ring circuits; "east", "west" and "kitchen", one 30A radial for the utility room and external power and an upstairs and downstairs lighting circuit (and a couple of other minor things).

The lights are not RCD protected, the three rings are on a common RCD and the 30A radial is an RCBO. Part of the reason for the east/west split was the way we renovated the house. By doing it this way we still had power upstairs and down in the part of the house we weren't working on. There was no reason from a loading point of view to split east/west or indeed at all, given that the kitchen has its own circuit.

Hwyl!

M.

[Christian McArdle]

Indeed, for lighting circuits, the safest solution is to randomly assign light fittings to one of two circuits. This way, if the MCB trips, it is more likely that a nearby light is on and working, so you are less likely to fall. In the event that the fault can't be cleared, the house is then more usable, without a complete floor out of commission.

Christian.

[Martin Angove]

Arranging the wiring for this can be fun though, and labelling is vital; most people would for example expect all 1mm2 cables in the loft to be the same lighting circuit.

There are also issues where you, for example, have two or more fittings in the same room and wish to switch them from the same location - there is then power from two separate circuits behind the -gang light switch. Mind you, this can also be the case for a hall/landing 2-way arrangement in a "conventionally" wired (i.e. up/down) house.

It's a good idea in theory though!

Hwyl!

M.

[Andy Wade]

It's probably worth adding that if you split a lighting circuit like that you must be very careful to ensure that the two circuits created are electrically separate. In particular you need to know how hall/landing two-way circuits are wired so that you don't end up with a live (phase) feed coming from circuit A and the corresponding neutral return going to circuit B. A good test before connecting the circuits to the new CU is to strap the L & N of each circuit together and measure the insulation resistance between the two with your 'megger', with all lamps in place and all lights switched on.

If the circuit being split is wired in then modern twin/triple-and-earth way the problem is unlikely to occur, although it could if somewhat non-standard methods have been used. Where the wiring is singles in conduit the issue is highly likely to occur and due diligence is required.

[Martin Angove]

Good point well made, and one I have very recently met. Practically all the downstairs lights were switched from a 4-gang switch in the hall (totally impractical, but it had obviously been a cheap rewire :-) and the wiring was (basically) live looped to the switches (reduced the number of cables required back to the central junction box). The live into this 4-gang was the "downstairs" circuit, and one of the switches fed the landing light which, as you have guessed, used the N return of the upstairs circuit.

Ho hum. Some people don't make it easy, do they?

Hwyl!

M.