Thoughts on fitting RCBOs

[js.b1]

I asked MK about this long ago, after he wibbled for a bit, I pointed out diversity must surely apply in domestic installs.

GCH fails:

- 14+ RCBO in a row, 11x 32A with 3x 6A at the end.

- Even if #2 & #3 are feeding 32A continuous you only have 16A left on an 80A fuse.

- RCBO heating is thus localised with nearby breakers running cooler.

E7 NSH is the heaviest domestic load:

- NSH are a fixed heating load, so fuse rating is the limit.

- Even 80A is spread over 6x 16A RCBO, each feeding a 13A 3.4kW NSH.

- RCBO heating above IIRC 40oC will reduce their actual trip rating, but not below 13A in 7hrs.

For domestic it should be quite ok, even on a 100A supply.

Whereas multiple 50A RCBO instant water heaters in a commercial environment may create a problem whereby the ambient is higher (nearby DIN transformer, timer motor) and the RCBO do not get time to cool down due to duty cycle.

An increased number of RCBO may actually reduce local heating - because loads may be better distributed across more RCBO re multiple final circuits.

[Bill Wright]

You're only doing it once, and when it's done you'll be pleased with it.

Bill

[Stephen]

The Denmans Curve CUs are all white plastic cases and there is a flip down door on the CU.

For me this has two important advantages.

The first is that I can stick Dymo labels on the inside of the door giving a text description of what each RCBO is.

Also, previous CU's looked ugly so I ended up boxing it in with an access door.

Not needed with the Denmans Curve unit. Just close the door on the RCBO's and its actually easy on the eye so no need to box it in. Even the main switch is covered by the flip down lid.

If you want pictures let me know and I'll put some online.

[Bill]

Right,

1st hurdle crossed, RCBOs and main switch purchased, well sort of anyway. I went into the wholesaler that I use for work and ordered them, I thought that I made it obvious that they were for me and not the company. The fact that I haggled the guy down on cost should have given it away as well!! Last week our storeman asked had I been buying some strange electrical bits as he had just had a delivery with just "Bill" as the order number. The so and so's had billed them to the company, luckily we are quite small and have a very understanding accounts department!

Any way, as I recall someone commenting on, my first problem is going to be space, if it had been a new install I would probably have managed it, but as it is it's going to be a very tight squeeze. So I think the Andrew Gabriel method of having a separate box, as per famous cu1.jpg photo, is called for, except that I will end up having 2 boxes, one above and one below the CU. This is beginning to sound messy, but there are cables coming from above and below the CU and not enough slack to pull enough through to gain any extra length. I'm fortunate that the CU is mounted on a board with a few inches gap behind it and can easily move the board forward to give more space without it looking out of place. This will also allow me to put the 2 boxes further back than the CU and then board over them.

Yes I could crimp leads and use one box of terminals, I use crimps frequently for lower voltages and I'm quite happy with them, but for some reason just don't fancy the idea here.

Of course the short days and long dark nights are just the right time of year to do this, I'm not known for my forward planning :-)

So, the one technical question that I have. I would imagine that 4mm singles should be adequate to link the DIN terminals to the RCBOs, certainly OK for lighting, but should I be thinking of 6mm for some of the others?

All comments most welcome, thanks in advance.

[John Rumm]

Sometimes its a case of needing to carefully identify the wires already there, disconnect everything and start again neatly dressing (and extending if required) the wires...

Adding a RCBO to an already full box can be difficult. (when I replaced my CUs in similar circumstance - I did not spend that much time making it particularly pretty, which I came to regret later when adding an RCBO. Fortunately it went in, but not in the position I would have chosen normally).

You could also use Wago terminals - but to be fair crimps are smaller and slimmer when space is important.

Head torch, spare batts, and battery power tools that are *charged* ;-)

IME RCBOs will come with the required fly lead to connect back to the neutral bus bar.

[spuorgelgoog]

And a thermos of tea for when you're four hours into the job, no power, and another four hours to go before getting power.

Owain

[tony sayer]

JOOI what's the part P situation regarding this nowadays changing a Consumer unit?..

[ARW]

It's notifiable.

[Bill]

In message , John Rumm writes

Identifying and labelling is one of the things that I need to do, there are 2 very large bundles at the moment, 1 heading upwards and 1 downwards,.

I had a look and I might, only might, manage to squeeze 1 in! So a definite tidy up is needed.

I can actually cheat a bit here, I have an annexe that will still have power, so an extension lead and lamp could be used. Still not as good as real daylight though. Also if I give up for the night I can adjourn to the annexe!!

Where the feed from the meter comes into the main house I put a Henley block that then goes to 2 100A fused switches, 1 for the main house and

1 for the annexe. I must have been forward planning! :-)

They do, I was asking in regard to the tails from the separate DIN terminals, if I went down that route.

[Bill]

Too right :-)

[John Rumm]

Ah, yes I see what you mean. In that situation then either use the same or greater CSA as used for the circuit wires you are extending. In the case of a ring, you ought to either extend both ends, or bring them in on a thicker single wire which is man enough for the nominal current for the protective device in question.

So arguably with a ring with a B32 MCB that has two 2.5mm^2 ends, you would mathematically arrive at 5mm^2 (hence 6mm^2 in practice), however a 4mm^2 would be acceptable since we know that it can take the full 32A load.

[chris French]

And a plan for what happens when you hit some kid of problem and your wife comes home, it's dark and the power still isn't on.

[Adam Funk]

Does that apply outside the CU too, though? Would it be acceptable to run a 4 mm^2 cable from a CU up to a junction box upstairs (let's say it's a good one with Wago lever blocks in it) & then make a ring of sockets with 2.5 mm^2 starting & ending at the junction box?

[John Rumm]

Its not a standard circuit as such, although its close to being a trunk and branch radial in some respects. Its one of those things that you can demonstrate will work, but at the same time has the potential to cause confusion later. Its not ideal since you then can't adequately test the circuit from the CU since you don't have access to both ends.

[Adam Funk]

Good points (as always), thanks.

[Bill]

In message , chris French writes

Emigration

[ARW]

As John said, it's not a standard circuit. Appendix 15 of the 17th edition is only a serving suggestion and not a reg.

The junction box in this case should be accessible for testing purposes.

[Adam Funk]

I only mentioned the details of the box to avoid the idea of using a round lighting junction box with various cable sizes jammed into the screw terminals. ;-)

Dont worry: I'm not planning to do this --- I was just curious. In practical terms, I doubt that running a 4 mm^2 T&E cable from A to B would be significantly easier than running two parallel 2.5 mm^2 cables the same way.

[Andy Wade]

Don't forget that 4 mm^2 T&E only has a 1.5 mm^2 earth core, so you need to take care to ensure that the disconnection time and CPC thermal protection requirements are met in the event of a line-earth fault. Probably OK on a PME supply and with MCB protection, but full calculations advised if it's TN-S and fuses.

Also this ring-on-a-stick idea won't meet the high-integrity earthing requirements (where applicable).

[Bill]

In message , Bill writes

I think that I can now answer that. I am crazy.

OK I'm not the best electrician in the world and I certainly don't do it for a living, but from start to finish, well almost finished, took nigh on 12 hours today. Fitting 2 boxes with DIN mount terminals above and below the CU to accommodate cables coming from above and below. Identifying each cable, disconnecting each from the CU and then removing the CU from the wall and striping it and rebuilding. Then connecting each circuit to the DIN mount terminals and then jumpering these into the CU.

I even tested out the rings for continuity and all was OK, something I had been meaning to do for a while.

A plus point is having a separately fed annexe, some where to run a lead light from and more importantly, somewhere to boil the kettle!

I did look at just fitting the RCBOs into the CU and not bothering with the terminals, HA!! not a chance, it is cramped even with just one single L&N from each RCBO Never again!

Any way, thanks to all for their advice a while back, it was much appreciated.