BG Split load board 18th edition compliant?

BG are stating that these are 18th edition compliant. I'm doubtful if they meet regulations 536.4.3.2 & 536.4.202 - overload protection of switches and RCCBs.

the RCDs are rated at 63A. There is no way of configuring the supplied MCBs that doesn't exceed 63A.

I guess if the main fuse was 60A it might be compliant but then would there not be an issue with the potential load of the installation being > 60A causing the DNO fuse to blow?

It will be the installer that makes it non compliant. If it suits your purpose and has suitable upstream protection then it is fine.

dougalthedog said:

6 Way Metal Dual Split Consumer Unit c/w 2 RCDs (Type A) c/w 6 MCBs | BG (CFUDP16606A-01)

BG are stating that these are 18th edition compliant. I'm doubtful if they meet regulations 536.4.3.2 & 536.4.202 - overload protection of switches and RCCBs.

the RCDs are rated at 63A. There is no way of configuring the supplied MCBs that doesn't exceed 63A.

I guess if the main fuse was 60A it might be compliant but then would there not be an issue with the potential load of the installation being > 60A causing the DNO fuse to blow?

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Left RCD
32A MCB - ground ring
20A MCB - cooker
6A MCB - first floor lights

Right RCD
32A MCB - first floor sockets
6A - Ground lights
6A - Combi boiler.

The above is realistic for a small house. The CU you have mentioned is likely to be suitable for many small houses or flats. I personally just fit RCBO boards now, but I would fit the one you have mentioned if for some (unlikely) reason it suited the job more than an RCBO board.

With the above example, I wouldn't fit the 6 way dual RCD board you have mentioned, I would fit a 10 way and have spare ways, but the point is that each side of the board would not reach the rated current of the 63A RCD (even without applying diversity)

Some houses I have come across (not many) only have 3 circuits. Sockets, lights, cooker.

Agreed the above would be ok but the board comes with 2x 6A, 1x 16A,2 x 32A, 1 x 40A MCBs. I can't see any way of combining these that isn't over 63A. Is it ok to apply diversity to the RCD rating?

There's no requirement to use every MCB supplied with a populated board.

Further to your original point; BG haven't made any claim in the link you've posted - the claim is made by TLC and is a bit more specific than you suggest "All metal consumer units comply with 18th Edition Wiring Regulations BS7671".

Ultimately it is up to the designer/installer to ensure any installation is compliant.

Agreed the link is to TLC but BG make the same claim on their website. They even have a 10 way board with 63A RCDs.

Other threads on here seem to confirm that diversity should not be applied to the RCD rating.

dougalthedog said:

Agreed the link is to TLC but BG make the same claim on their website. They even have a 10 way board with 63A RCDs.

Other threads on here seem to confirm that diversity should not be applied to the RCD rating.

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Why don't you email BG? I'd be genuinely interested in the response.

They will just say 'with diversity it is fine', and I'm sure they would be right. However, what would be interesting is for them to show the diversity calculations, so I would specifically be asking for these.

Let us know how you get on.

Omission of overload protection because of the characteristics of the load.
The rated current (In) of the sum of the downstream MCBs does not exceed the rated current of the RCCB when installed within the consumer unit (Inc).
In of the downstream MCBs is taken to be Ib for this specific calculation as it errs on the safe
side.
For individual circuits with load currents that are unlikely to be increased, cannot be overloaded and where spare way(s) cannot introduce a total sum exceeding the original calculated value, the installation designer may decide to use the design current of the circuit instead of the rated current of the OCPD.
It must be noted, that omission of overload protection is on the basis that the RCCB is not likely to carry overload current and therefore its rated current can be lower than the upstream overload protective device rated current e.g. the house service fuse-link.
The electrical installation designer should be confident, that the characteristics of the load without diversity i.e. the total connected load, is unlikely to cause an overload.

Consideration should always be given to variables such as spare ways, increased future loads and upgrading the service fuse-link Regulations 433.3.1, 536.4.3.2 and 536.4.202 refer.

The rated current of the RCCB in the related assembly circuit in the consumer
unit (Inc) stated by the assembly manufacturer, must not be less than the rated current of the upstream overcurrent protective device (OCPD). For a domestic installation, this is likely to be the service cut-out fuse-link rated at 63 A, 80 A or 100 A.

Why don't you email BG? I'd be genuinely interested in the response.

They will just say 'with diversity it is fine', and I'm sure they would be right. However, what would be interesting is for them to show the diversity calculations, so I would specifically be asking for these.

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Funny you should say that. I have emailed them and they did say "with diversity it is fine" but gave no details and I have asked them for more info. I will report back when they reply.

dougalthedog said:

Funny you should say that. I have emailed them and they did say "with diversity it is fine" but gave no details and I have asked them for more info. I will report back when they reply.

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The electrical installation designer should be confident, that the characteristics of the load “without diversity” i.e. the total connected load, is unlikely to cause an overload.

dougalthedog said:

BG are stating that these are 18th edition compliant.

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Marketing departments will say anything to sell their wares.

As supplied, in the box, not connected to anything, they are fully compliant.

BG have replied with a link to the instruction sheet showing diversity factors. Applying 0.6 to the supplied MCBs - 40+32+32+16+6+6=131 x 0.6 = 78.6A

If my calculation is correct then this would require upstream protection of 80A which is greater than the RCD rating of 63A so would be non compliant.

dougalthedog said:

BG have replied with a link to the instruction sheet showing diversity factors. Applying 0.6 to the supplied MCBs - 40+32+32+16+6+6=131 x 0.6 = 78.6A

If my calculation is correct then this would require upstream protection of 80A which is greater than the RCD rating of 63A so would be non compliant.


View attachment 117638

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Quick reply!
But the 78.6A is the max demand for the whole system. This current is shared between 2 RCD's. Neither RCD will have 78.6A going through them.
I'm guessing you must see this already as it's really obvious, so you must have something else on your mind?

You could easily split the loads so each side has a max demand of less than 63A.

Side A = 40+32+6 =78A
Now apply diversity from the given table.
78A x 0.8 = 62.4A

You could easily split the loads so each side has a max demand of less than 63A.

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Agreed, however, using the above calc you would need upstream protection of at least 80A to prevent unnecessary / nuiscance tripping.

536.4.202 states:
“However overload protection shall not solely be based on the use of diversity factors of the downstream circuits. To achieve overload protection of RCCBs or switches, the rated current of the OCPD shall be selected according to the manufacturers instructions”.

This is clarified in the BEAMA guidance here:

which makes it clear (Step 3 / method 2 on P3) that if using diversity calcs downstream you must have upstream protection rated <= the RCD rating - ie 63A or less. This conflicts with the above calculated rating of 80A which would suggest the installation would not be compliant.

dougalthedog said:

Agreed, however, using the above calc you would need upstream protection of at least 80A to prevent unnecessary / nuiscance tripping.

536.4.202 states:
“However overload protection shall not solely be based on the use of diversity factors of the downstream circuits. To achieve overload protection of RCCBs or switches, the rated current of the OCPD shall be selected according to the manufacturers instructions”.

This is clarified in the BEAMA guidance here:

BEAMA Technical Bulletin Ed 2 Mar 2019 - Overload Protection of an RCCB or Switch BS EN 61439-3

www.beama.org.uk

which makes it clear (Step 3 / method 2 on P3) that if using diversity calcs downstream you must have upstream protection rated <= the RCD rating - ie 63A or less. This conflicts with the above calculated rating of 80A which would suggest the installation would not be compliant.

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Personally I think the diversity and max demand are crude to the point of absurdity much of the time. I've got big houses with 30+ circuits, 8 or more ring circuits that are "non compliant" according to any max demand calc method - what's the solution - join all the rings into 2 big ones? One house has 4 kitchens! Yet 2 people living in it! And power consumption that gently ticks along...

I put this to the inspecter when he questioned why I hadn't filled out the max demand box, that in a large 4 storey house 5+ rings circuits is a better job than 2 and won't use any more power regardless of what max demand says and he squirmed and shuffled and mumbled a bit with nothing meaningful to offer...