Split load vs full RCBO Consumer Unit

iep

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I have a 12 way split load consumer unit that I plan to have installed in our house. Simple unit, 6 MCBs on the non-RCD side and 6 on the (63A) RCD side.

Having done a bit of reading though I gather that there are several disadvantages to this design. For example, if I put all three ring mains on the RCD side then any apliance in the house can trip all other appliances. This is obvioulsy especially bad for the freezer.

I have also read that lighting cirucits on the RCD side are considered a bad idea as you can be left in the dark during a fire etc:

http://wiki.diyfaq.org.uk/index.php?title=House_Wiring_for_Beginners

In addition, the dedicated radials for the immersion and the cooker should apparently not be on any RCD as they naturally leak a little as they get older.

When I take all this info togeher it looks like a single non RCD box with the correct mixture of RCBOs and MCBs might be a better choice? In that case I would have:

Ring1: 32A RCBO
Ring2: 32A RCBO
Ring3: 32A RCBO
Radial1: 16A RCBO
Radial2: 16A RCBO
Immersion: 16A MCB
Cooker: 40A MCB
Lighting1: 6A MCB/RCBO?
Lighting2: 6A MCB/RCBO?
Lighting3: 6A MCB/RCBO?

Obviously this setup (if it is correct) would not nwork with the split load box I have. I guess I could take out the 63A RCD and replace it with a second 80A switch instead?

Cheers,

iep
 
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Given that you have only a little knowledge I suggest you seek the services of a competant Electrician as a CU change would unfortunately be above your competance level.
 
I would do some more reading.....

With the 'new' 17th edition version of the regs, in a typical domestic dwelling, all circuits will require some form of 30mA RCD protection.....

Your 'split load' consumer unit is out of date when compared to the new 'dual split load' consumer units now being sold in their place. Two RCD's, with the circuits split across them both, mixing the lights and sockets between the two to avoid as far as possible any danger or inconvenience. You can also get 'dual split load' consumer units with additional non RCD protected ways for any circuits that you *may* have, or for use with RCBO's.

RCD's are required for all your domestic 13amp socket outlets intended for normal use. (all of yours in your house, with the exception of sockets that have been installed for a set purpose such as a fridge etc....but see below)

RCD's are also now required for any circuits which have their cables buriedless than 2" deep in a wall. Thats all your typical circuits in a house. If you want a non RCD protected socket for a fridge etc, you need to either run the cable on the surface or in trunking, use SWA, MI or another type of similar cable, or use earthed steel conduit.

Do some googling....
 
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Reactions: iep
Good advice thanks. The problem with Googling is that there is a lot of out of date info out there so it can be difficult to work out what does and does not meet current regs.

The site below for example states that RCDs should not be used on several types of circuit:

http://wiki.diyfaq.org.uk/index.php?title=House_Wiring_for_Beginners

Out of curiosity, with a huge number of posts out there describing issues with immersion heaters and cookers on RCDs, are people juts having to put up with the problem or do the modern CUs somehow avoid this issue?

Either way, I still prefer the idea of proper seperation of circuits. I'm guessing that a full RCBO single unit is an acceptable solution given the new regs?

Ricicle. The whole point of this forum is to share knowledge where you have it and learn more about the topics that you don't fully understand. That's why I asked the question.

iep
 
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If you can afford a CU with RCBO's on every circuit, that would be by far the best way to achieve compliance. The dual split load boards are desgined to save cost.
 
Thanks Lectrician, that makes sense to me.

I'm a bit belt and braces with this kind of job so I'd rather save up for the RCBO solution and do the best that is possible.

On the other points:

1. If it is absolutely necessary to use RCDs on all circuits, how are pople avoiding the issue of current leakage within cooker and immersion heater elements? I guess the elements just have to be replaced more often?

2. Is it possible to replace the RCD in my older split load consumer unit with a second DP 100A switch? This would allow me to use both sides of the unit with RCBOs I know a new CU chassis is only £40 but that's two RCBOs.

3. While I'm here, I know that newer 2.5mm twin and earth cable includes a 1.5mm earth (compared to the older cable that used 1mm earth). My house will almost definitely have the older cable so I need to understand whether this means a full rewire of the house is required in order to use a newer CU. I have asked before but nobody has voluynteered any info.

Cheers,

iep
 
3) - It depends on the thermal constraints, which requires calculating. A rule of thumb is that if you have 3036 rewireable fuses protecting the circuits, the circuit wont comply, if you have 1361's or MCB's etc, you should be fine.

2) - Assuming the board is a variable split, you should have a full size busbar which you cut down as required. Just ditch the RCD and bridge the two neutral bars together. Out of interest, what make board? RCBO's are priced so differently by brand.

1) - Nothing you can do. Well used elements will only cause issues when they are failing. An element that is rarely used can absorbe some moisture and cause issues. Usually these damp elements dry themself out when not on an RCD protected circuit...
 
how are pople avoiding the issue of current leakage within cooker and immersion heater elements?
They don't. If elements have sufficient leakage to trip an RCD, the element is faulty and must be replaced.

Just because elements not connected to an RCD will still work even when fairly significant faults have occurred, this does not mean they should be used in this condition, or that they are safe to use.

Correctly installed elements for immersion heaters, ovens and other items should last for many years before replacement is required.

Problems occur with RCDs tripping all the time where incompetent persons change the CU, believing it to only consist of changing over a few wires. They won't have tested the entire installation to identify and then repair any faults, and they won't have tested any items such as cookers, immersion heaters or other fixed appliances either.
 
Well if you use RCBO's then one device is able to leak a max of 30mA (or in practice 15mA) without problem.

Where as if you have 6 circuits on one RCD, then the total off all the circuits can only leak a max of 30mA (or in practice 15mA), which will give you little headroom and where you may start to get false trips.

Don't bin your new CU though, just buy RCBO's for the non RCD side.

Mix light circuits over both sides, so you will always have some lights working if the RCD trips.

You could also run a decicated socket for the freezer and just use a 16A MCB for that, if you are particularly concerned.
 
Thanks to all for some very useful info.

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3) - It depends on the thermal constraints, which requires calculating. A rule of thumb is that if you have 3036 rewireable fuses protecting the circuits, the circuit wont comply, if you have 1361's or MCB's etc, you should be fine.
**********

My own take on the 1mm to 1.5mm change in the size of the earth wire was that a 1mm wire cannot cope with much more than 13A when burried in insulating material. As a result, it can fail before the fuse. Is that about right?

Since all our wiring has been run in open spaces within the loft (above insulation) and down through large cavities between walls, the 1mm wire can carry closer to 15-16A so I guess we will be okay with the 1mm earth wire provided I have MCBs or RCBOs?

As it happens, we do have 3036 rewirable fuses. So, either we do already have 1.5mm earthed cabling (I doubt it but will check), or our current C.U. is not super safe.


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2) - Assuming the board is a variable split, you should have a full size busbar which you cut down as required. Just ditch the RCD and bridge the two neutral bars together. Out of interest, what make board? RCBO's are priced so differently by brand.
**********

The board is not variable split as it came pre-populated with MCBs and an RCD (6 MCBs on the RCD, six not). Howevr, perhaps I can buy a new busbar for it? It's a wylex board that was on special offer in screwfix about 3 years back. I bought it for a flat that I owned but was later told that the existing C.U. was okay after all.


**********
1) - Nothing you can do. Well used elements will only cause issues when they are failing. An element that is rarely used can absorbe some moisture and cause issues. Usually these damp elements dry themself out when not on an RCD protected circuit...
**********

I actually noticed last night that our immersion heater doesn't work at all anyway, so looks like it needs a new element now. Likewise, I just replaced the elemnt in our cooker so neither of those should be a problem for a couple of years.

Ideal world, I would run a suitably protected dedicated feed to the fridge freezer from an MCB but it is a real pain to get wiring through from our C.U. to the kitchen so I will have to continue running it on the kitchen ring and hope that it doesn't become a problem.

Again, thanks to all.

iep
 
i have similar CU as previous user, and the right hand side is protected by RCD 30ma and the left hand side not... right side is 2 ring mains, shower supply 2 spurs for permanent sockets internet, and fridge, its the left side non rcd protected side i would like to have this rcd protected too... it only has 3 6amp lighting circuits the other two ways are spare.

could i fit a 20amp rcbo straight off the incoming switch, via the busbar, then cut the rest of the non rcd bus bar off, and feed the output side of the rcbo to the three 6 amp cicruits ? thereby giving me my three 6amp lighting circuits protected by the 20 amp rcbo... just thought it may have been cheaper than fitting an additional 60 amp RCd to provide me with the protected circuits ?
 
You would have been better off starting a new thread, but anyway, the answer would be no because if you loose one lighting circuit due to an earth fault, you'd lose them all wiring the circuits as you describe.

If the supplementary bonding is up to scratch in the bathroom then I'd leave it well alone if it were my house.
 
You would have been better off starting a new thread, but anyway, the answer would be no because if you loose one lighting circuit due to an earth fault, you'd lose them all wiring the circuits as you describe.

If the supplementary bonding is up to scratch in the bathroom then I'd leave it well alone if it were my house.

thanks....

forgive my ignorance but would that not be the same if i had a split load board with dual rcds?

Or are the lighting circuits split on each side of the board to avoid this happening ?
 
Yes the circuits usually are split so upstairs lights on one side downstairs lights on the other.
Usually the socket circuits are arranged the other way round so you still have SOME power in parts of the house if there is a fault.
 

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