Stupid question re wiring MK RCD

[John Anderton]

Hi all,

I've just bought an MK consumer unit containing an RCD in preparation for wiring up the garden shed before the building reg changes kick in. The unit comes with helpful installation instructions like "Tighten up the screws" but not with a wiring diagram except one printed on the RCD itself which helpfully doesn't indicate which intersections of lines are actual connections and which ones are just places where wires pass over each other nor any indication of which terminal is "in" and which is "out". There are also no helpful markings on the RCD like "L in" or even just "L" !

I'm fairly sure I know what I'm doing but before I electrocute myself, could somebody confirm what I'm doing is correct, please ?

ACSII art is:

------- ------- Neutral block -> |o o o o| |o o o o|

[Andy Hall]

Yes this is correct. You should have jumper cables with the unit allowing you to connect 1 to the neutral supply and 3 to the live supply at the main incomer breaker and 2 to the neutral bar on the downstream side of the RCD. Make sure that you remove the link between the two (or more) sections of neutral block or the RCD will not work and will trip out.

[Andy Wade]

Yes.

[John Anderton]

Thanks for that and also thanks to t'other Andy but what do you mean by "Make sure that you remove the link between the two (or more) sections of neutral block or the RCD will not work and will trip out.", please ?

The neutral block is just a solid lump of brass with four terminals in it, one of which has a heavy duty wire connected to terminal 2 on the RCD. I assumed that it was just there to connect the four neutral cables for the two shed ring mains to the neutral out from the RCD ?

Cheers,

John

[Dave Plowman (News)]

IIRC, that's because you don't make any connections direct to the RCD. The feed goes to the main switch, and circuit line to the MCB outlet, neutral to the appropriate buss.

All the other connections are internal, as it were.

If it's a split load one there will be two neutral busses so use the correct ones for protected/unprotected circuits.

[Andy Hall]

Is this a small CU?

If so, then the neutral block would just be for the downstream side.

There are also split CUs which have provision for some MCBs to be between the incoming side and the RCD and some downstream. These have a set of neutral terminal blocks connected with links and a bus bar that you cut. This allows you to position the RCD with different numbers of MCBs on either side. In this configuration, the neutral blocks must be separate and associated with circuits on the appropriate side of the RCD.

[John Anderton]

Yes, it's only intended for a garage so it's only got two MCBs, one for sockets and one for lighting.

Ah, right, that makes sense.

Thanks again,

John

[Andy Hall]

I don't know whether it matters for what you intend to do in the garage, but it could be a good idea to have lighting connected upstream of the RCD. That way if the RCD trips for any reason, the lights are unaffected. If you are working with power tools, this may matter in that they could be still winding down as you are plunged into darkness.

I have various woodworking machinery in my workshop and although most if not all of it has ways to spin down within 10 seconds (saw etc. has electronic braking), I played safe in two ways. The first was to put the main lighting upstream of the RCDs. The second was to make some of the lighting maintained emergency types. These have an internal rechargeable battery which will power the fitting if the power fails, but will still run as normal lighting. Therefore, whether there's just an RCD trip or a complete power failure, I don't get plunged into darkness while doing a tricky operation.

If this matters to you, you could just implement the emergency lighting option and run it downstream of the RCD. The effect would be virtually the same. Just a thought.

[John Anderton]

I hadn't thought of that. Thanks for the suggestion,

Cheers,

John