RCD wiring

[Johan]

My recent question about problems with my boiler got very knowledgeable replies (problem now sorted). So now I have another question (google doesn't get me a clear answer):

I have bought a Wylex 63A 30mA RCD fom Screwfix (wylex shower unit, product code 90026) to use for my new electric shower. It is not clear to me how to wire this thing. It sits in its own box, and I imagined I would put it close to the consumer unit, in the 10mm cable coming from the CU and going to the shower.

The RCD has 4 connectors, one marked L at the top left hand side, the others unmarked. The enclosed leaflet says 'The incoming cables to the RCD may be taken either to the top or the bottom set of terminals, dependent entirely on the ease of wiring the device'.

That could be read to mean that only 2 out of 4 terminals are used (that must surely be wrong). As it has 4 terminals, I guess the live in and out go to the one marked 'L' and the one below that, the neutrals go to the other 2. What happens to the earth? Do I connect that using a connector block?

Hope all this makes sense,

Johan

[John Stumbles]

It could do but that's not how it's supposed to be installed: it should be wired as a mini Consumer Unit with its own feed from the meter in parallel with that to the main CU. You'd use a Henley block to split the meter tails, and remove the main incoming fuse to isolate the supply while working unless you hav a meter with an isolating switch.

Yes, that's wrong

Yup, incoming L & N to top, outgoing to bottom - or vice versa. Incoming from meter via Henley block Outgoing N to load Outgoing L from RCD to MCB, from there to load

Yes, there should be a 4 way nickel-plated brass terminal block near meter which you connect in to. If Earths are all connected to sheath clamp on incoming cable I'd reconnect to a terminal block (you can get 8-way ones which are handy since sod's law says you'll need 5 ways if you have a 4-way block). Again reconnect with incoming power off. You should really check earth loop resistance (and trip time on your RCD) after doing all this but you need sparkies' test kit for this.

[Johan]

But then it wouldn't be protected by an MCB. Is what you describe not what is supposed to be done with a MCB/RCD unit? What I bought is only an RCD.

I intend to get it all checked over by a sparkie when I have finished all the wiring.

[bigcat]

I dont mean to be funny, but if your knowledge is so lacking, what on earth are you thinking wiring your shower up.

You might dislike me or take offense, but then again you might survive to be able to take offence!

Why not just run the 10mm and let the spark connect it all up etc, including equipotential bonding. You probably dont even know what that is.

NT

[John Stumbles]

Sorry, I didn't (CBA to) look up the product you gave the code for so when you said 'shower unit' I just assumed you meant an RCD and an MCB in one box like this

there a particular reason to spend £13 more on just the RCD rather than the combined unit? :-)

Do you have a sparks lined up for this? If so could you get him to look over the installation and bits & bobs you've got and agree what bits he'll do and what he's happy to have you do?

[Johan]

What makes you think that my intention to get things checked over is anything to do with lack of knowledge (nothing wrong with asking the question, but I don't see your logic)? I have read the IEE on-site guide and lots of stuff on the web, and think I know what the regulations say. The shower wiring is part of a lot of other wiring to do with an extension we had built. I want to get an electrician to check things like the earthing (which I don't have the equipment for) and to get a certificate. Overly careful? Waste of money? Worth it for my peace of mind, I think.

Anyway, as there is no legal way to switch off the supply before the consumer unit, I decided to make sure the shower had its own RCD (CU only has an RCD for the garage/workshop, no space for another one). Never used one of these, and want to make absolutely sure I use it as intended. Perhaps my initial post wasn't phrased very well.

Absolutely!

Thankfully I do. Then again, all the new plumbing is in speedfit (Yes, I have read all the 'dicussions' about that on uk.d-i-y as well ;-)

[Andy Wade]

Most of your original article actually.

That's a non-sequitur. What has the inability (or otherwise) to isolate the supply to the CU go to do with the need for the shower to have its own RCD?

Point 1: there is a legal way to isolate the supply to the CU and that's to ask via your supplier to have the main fuse withdrawn.

Point 2: why do you think the shower circuit needs to be RCD protected?

Point 3: what device will be providing overcurrent protection in your proposed wiring scheme?

[Johan]

I've clearly got myself into a mess here. I've been far too inaccurate in what I wrote. And of course it is possible I have misunderstood something fundamental here. Let me try to clarify again:

- My CU has one MCB/RCD for the garage, no split load or other RCD provision; no more space for an MCB/RCD that takes up the space of 2 MCBs. I suppose I could have got a RCBO that takes one module/way.

- Non-sequitur indeed. I was partly replying to someone else's post who mentioned using a Henley block after the meter/before the CU and an MCB/RCD unit for the shower. That would have required to get the supply switched off (at least I realised that ;-) Obviously one can ask the supplier. Wanted to avoid the hassle.

- The MCB in the CU does the overcurrent protection, surely? My idea was to go CU (MCB) -> RCD -> shower. I must admit that I bought the RCD as part of a larger order from screwfix without being 100% sure what the connections would be like. Having considered the separate MCD/RCD unit and the option of putting an RCBO in the CU, I assumed that the availability of a separate RCD unit implied this was another way of dealing with this. Hence my original question.

So to summarise again: I would like my shower to be protected by an RCD, I have considered the options, bought a separate RCD and would love to know how these things are wired in.

[Johan]

See my other replies.

Not yet

[bigcat]

finished

I dont think that. Its your assorted basic errors that leave me wanting to stay away from your shower.

I'm doubtful. Take your reason for installing an RCD.

pull the supply fuse.

a necessity, but for entirely different reasons.

usually easy to get round compliantly, eg by turning 2 lighting circuits into one.

NT

[Dave Plowman (News)]

Sounds pretty clear to me.

Well, yes, you are wrong. An RCD needs both line and neutral to work unlike an MCB which is just in the line.

No. Feed to one pair of terminals - either top or bottom - and load to the other pair. If there's also an MCB, the load line should go to that with a link between the MCB and RCD. Although I'd have expected this to be pre-wired. I'd also expect an earth bus bar.

Something like this:-

Feed L N | | | | | | ------------ | L | | | | RCD | ------------ | | L| |N | | -------- | | | | | MCB | | | | | -------- | | | | | L N to shower

[Johan]

?? what errors?

That's ok (being doubtful). But I haven't given a reason, other than 'I want my shower to have one'.

Not legal if I were to do it. As I said elsewhere, didn't want the hassle of asking the supplier.

Are you not confusing a reply from someone with my original posting? I never gave a real reason.

Only two lighting circuits (as usual) and combining them would make the cable run too long.

Why not forget about whether other solutions would have been possible? Now that I have the RCD, I might as well use it (unless there is a _clear_ reason not to). Question remains, how?

Johan

[Andy Wade]

If you mean a combined MCB and RCD for one circuit, that's called an RCBO.

Oh yes, that's OK. It wasn't clear to me before how you were intending to feed the RCD. Some people wrongly think that an RCD provides overcurrent protection too and I just wanted to eliminate the possibility that you were going to wire straight off the meter tails via the RCD to the shower, giving no overcurrent protection (other than the supplier's fuse).

But you haven't said why. There's not usually any need to use an RCD for a shower - unless the installation is TT-earthed of course, but I don't think that applies here. What type of earthing do you have?

The important things when wiring a shower circuit are these:

(a) not to use an undersized cable - you mentioned 10mm^2m so that's probably OK - refer to Table 7 in the OSG for maximum circuit lengths;

(b) not to use an under-rated fuse or MCB; (c) to ensure that all connections are sound, with all terminal screws done up really tight;

(d) to ensure that the circuit protective (earth) conductor is also soundly connected and to verify this by doing a loop test at the shower appliance's supply terminals with any supplementary bonding temporarily disconnected - see Appendix 2 of the OSG for the maximum permissible measured Zs value;

(e) to ensure that any and all supplementary bonding required in the bath/shower room is in place, properly sized and soundly connected.

Of these the most important, in terms of risk of being electrocuted in the shower, is (e), closely followed by (d). Use of an RCD is not in any way a substitute for doing (a) to (e) properly.

L & N in side-by-side at the top or bottom and out at the bottom or top, observing any polarity markings on the device. Earth straight to the earth bar in the CU. Some electronic RCDs require a separate earth connection (single wire).

[Johan]

Indeed. Is there a difference between RCBOs and what toolstation calls a 'MCB/RCD Combined' (part number 58556) in what they do? I can see that the toolstation one only takes up the space of 1 module, while RCBOs usually take the space of two. Just asking out of curiosity, might as well learn a little more now......

Thanks for making sure. I should have been clearer.

Bigcat, please take note ;-) I have an (irrational) aversion of electric showers as they bring electricity and water in very close proximity. An RCD would give me (if I get everything else about the installation right :-) ) a little more peace of mind.

TN-S (via the sheath of the incoming cable).

should be ok, about 6 meters, but I realise this depends on the kW of the shower.

not bought the shower yet, will make sure this is ok.

this is where I need an electrician to come and check

diagrams in IEE ons-site guide are very clear for this, in my opinion, so should get that right.

thanks, that sounds reassuringly how I assumed it would work, and tells me how to deal with the earth connection.

Johan

[Johan]

Hmmmm. I initially thought it was clear, then started to worry that it _could_ mean what I describe in the next sentence of my original message. Exactly because electrical installations _can_ be dangerous if not installed properly (as others have pointed out), I decided to ask first.

It would have been very odd, but better safe than sorry.

That's what my woeful description says, but clearly not in a helpful way.

Just to be clear: in my case the MCB would be on the feed side of the RCD.

Johan

[Dave Plowman (News)]

So does a kettle. Coffee maker. Immersion heater. Etc.

[Stefek Zaba]

No problem there.

I was about to ask whether your RCD came with an enclosure, but since you provided the Screwfix partnum I can see that it does - so that's OK too. As the estimable Mr Stumbles noted, S-fix also carry a more-for-less RCD+MCB-in-one-enclosure - Volex rather'n Wylex, reputable enough. With the combined part, you'd use a Henley block to feed your existing CU and the shower unit in parallel from the main fuse; with your RCD-only, you need to take the feed from a suitable (typicalluy

40A) MCB in the main CU.

And as Andy Wade mentioned, for the higher currents your electric shower pulls, you really do want to do up the connections tighter than you might think (and it's a good idea to recheck them after a month or so, as the strands settle/deform). We're not talking gorrilla-level

6ft-scaff-pole-on-the-end-of-ratchet torques, but "well firm with ordinary screwdriver"...

HTH - Stefek

[Dave Plowman (News)]

IMHO, all permanent wiring connectors should be done up until they 'groan'

- using a screwdriver with a blade the same width as the screw. I've yet to break a screw or strip a thread, but I've come across lots of problems on installations where they simply haven't been tightened enough.

Of course if you're a gorilla you might need to be careful. ;-)

[Mark Carver]

Almost every socket in our kitchen has had loose terminals. This came to light about a week after moving in. My wife had the iron plugged in to a socket. She called me because she could hear 'sparking' from behind the socket. I tapped it, there was indeed a cracking sound from behind.

I killed the breaker and removed the socket, as I undid the fixing screws the entire socket virtually fell away from the wall ! The two T&E L and N tails were so short (probably because the wall had been tiled as part of a kitchen re-build) that it would have been impossible to have screwed the terminals down. Thankfully the Earth tails were at least still twisted together and connected. The L and N terminals were black with arcing. The sparky must have simply positioned all the wires in the 'correct' place, opened up the terminals and pushed the whole socket home. It must have taken ages to get that 'right', far longer than it took me to extend the tails, replace the socket, and do the job properly.

I checked the other sockets, some had loose terminals, some had cross threaded fixing screws, so I had to drill those out and start again. An FCU had been fitted on the wall behind the back of a kitchen cabinet, but the hole in the back of the cabinet is too small to allow removal of the FCU.

Hopeless. The plumbing in the kitchen is better, except you need to be a contortionist (and have a crystal ball) to find and operate the main stop c*ck.

[bigcat]

You do need an RCD, why is another matter.

10mm2 is OTT unless you've found an unusually heavy duty shower, and have the supply capability to match.

dont miss anything out on your equi bonding then! BTW if you dont bond the plughole on a PME system you're creating a problem.

If you had an electric immersion tank plus mixer shower, your leccy would not just come close to the water, but would be running right through it as you shower.

NT