Getting head around RCBOs and split CU's

[frank999]

Hi,

Trying to get my head around RCBO's in a CU, and the different types.

I am looking at Wylex gear, what is the difference between a standard height RCBO and a 'miniature RCBO', apart from obviously the height.

Thanks

 

[flameport]

what is the difference between a standard height RCBO and a 'miniature RCBO', apart from obviously the height.

It's just the height.

There are many different RCBOs, including those which have switched neutrals or not, and various types of RCD component in them, but those are unrelated to the physical size of them.

Split load and dual RCD consumer units should not be considered for any new installation.

 

[AndyPRK]

Wylex rcbos have been type A for probably a Decade. (Good type)

If the miniature ones have an led light on the front they probably include arc detection which is fairly new.

 

[ericmark]

TT supply really needs a switched neutral, and the type depends on what equipment you have, in general with a TN supply the RCD is an addition, but with a TT supply the RCD is the main device which auto disconnects the supply in the event of a fault.

So with a TN it does not matter too much, but with a TT I am also not sure what we can or can't use safely, in general I like the idea of a RCBO I don't want a fault in the shower stopping me using the cooker. But best type it seems is type A and finding them that switch the neutral is not so easy, so if you want powerline adaptors, electric cars, and solar panels you may have no option.

So technically the powerline adaptor puts a high frequency AC current on the supply so need type F, I am not convinced, I don't think in real terms you would have any problems using type A, but I can't find anything which really says what you can use.

 

[Taylortwocities]

Hi,

Trying to get my head around RCBO's in a CU, and the different types.

I am looking at Wylex gear, what is the difference between a standard height RCBO and a 'miniature RCBO', apart from obviously the height.

Thanks

Aren't these questions that you should be asking the registered electrician who will be installing, certifying and NOTIFYING the replacement consumer unit?

 

[ericmark]

Aren't these questions that you should be asking the registered electrician who will be installing, certifying and NOTIFYING the replacement consumer unit?

I think it is right for the user to take an interest, and considering I made a mistake even when I have worked with RCD protection for at least 30 years, why would you think the person notifying would know what type is best?

So I think there are more than four types, but the AC, A, F, and B are most common. The major problem is the word type, with a RCD and I include RCBO when saying RCD type refers or at least should refer to how well it works with pulsed DC, high frequency, and DC all of which the electrician in real terms has no idea as to what is required.

As to if there is a real problem with using the wrong type I really don't know, but in theory at least if using a powerline adaptor you should use type F which I have not seen as a single width RCBO.

So be it twin RCD's, twin RCD's with some RCBO's or all RCBO's although the electrician may have a preference, it is the user who looses a freezer full of food if they trip while on holiday not the electrician, and the user decides if he wants solar panels on roof, and electric car, inverter fridge/freezer or washing machine, not the electrician.

So if you look at cheap like the fuse box range, you can get a all RCBO consumer unit for around £200 populated, but if looking at Wylex looking at £180 before you buy the MCB's and RCBO's MCB not so bad at £3 each but RCBO at £28 compared with £10 to £15 for Fuse Box depending on type, that's a £180 difference in the price of 14 RCBO's without considering cost of board.

In the main cost of a consumer unit change is the electrician not the parts, so I can see the idea of do it once and do it right, when I found all the RCBO's clearly marked type B on the boxes were in fact type AC I was some what upset. I am changing the ones supplying sockets, but leaving the rest.

But had I made the same error with some one else's house do you really think I would return to them and say sorry I fitted wrong type, I want you to pay £15 each for the correct type? Fact that when I phoned my supplier he said he only has type B in stock which were actually type AC does not help, look at this advert it clearly says B type, but look at picture and it shows the logo for type A, yes curve B but not type B.

Try doing a google for a type B RCBO and see how many are curve B not type B. This is typical

the one to right is type AC if you look at the picture and curve B, and on left type A and curve B.

 

[Gasguru]

And you'll now find some boiler manufacturers are covering themselves by specifying either the RCBO is a must or is recommended to be a particular type...and despite the fact that they are nearly all using similar switched mode power supplies on their pcbs, pumps and fans they each have their own preferences The unintended consequence of trying to save a tenner a year on the boilers electrical energy.

 

[AndyPRK]

Why is it an unintended consequence of trying to save a tenner a year ?

 

[frank999]

Thank you for your replies, very helpful.

I will be involving my spark in the design, but before we sit down I want to understand as best I can what he might spec in the box before it is purchased.
PV and EV charging points maybe along the line later - and understand they will need separate protection, is one wise to purchase a CU with plenty of spare ways - if Regs change and later require/recommend additional protective devices.

Split load and dual RCD consumer units should not be considered for any new installation.
Has this been ruled out in the 18 Edition ? I have not read anything to this affect.

So be it twin RCD's, twin RCD's with some RCBO's or all RCBO's
This is what I refer to when I say Split Load (ie two busbars), have a misunderstood the term split load ?

The main changes for the 18th Edition I read are ... all circuits to be 30mA RCD protected, more accurate selection of RCD type, AFDD's if deemed necessary, same with SPD's, separation of circuits to eliminate tripping to 'high priority circuits', consideration of any potential earth leakage with regards RCD tripping, socket circuits greater than 20amp (up to 32amp) to be RCD covered ...


In an existing CU a Wylex WRS63/2 is installed, this is a AC type, the lights have all been changed to LED, some with dimmers ... and note that this could cause issues for the AC type RCD, an A type is recommended - but so far though there has been no tripping.

Am I right to say the above RCD is known as a Residual Current Circuit-Breaker (RCCB) without protection against overload.

Also installed are several Wylex NHXSBS1B16's, these are B Curve full height RCBO's, I note the test button and additional white wire that I understand connects to earth:

Back to one of my original questions, full height vs Miniature RCBO's, the Wylex NHXS1B16 is also a B Curve, miniature RCBO, but lacks the test button and earth wire.
Although the miniature RCBO provides more space in the CU for dressing cables etc, how do you test it without the test button ? and why no earth wire ?

In trying to understand the differences between types for RCD and RCBO's, the labelling for an RCBO is a little confusing as stated.

So, to try get this clear in my mind - the Type letter for an RCD refers to the 'curve', but an RCBO has both RCD and MCB functions, but I only see the Curve Type in the Wylex catalogue being B or C, no mention of A, D or F type for an RCBO part, is that simply because you cant buy an RCBO with these Type characteristics, only B or C, ... and so to get RCD Types A, D or F, you would need separate MCB and RCD's to provide the right type of coverage.

And lastly 'poles' (not the hard working variety), I note the full height RCBO above is single pole, but the miniature is 2 pole ... is the lack of earth wire and test buttion on the latter something to do with it being double pole ?

 

[AndyPRK]

You are mostly correct.
A lot of the requirements u mention came in under the 17th edition.

Your existing wylex rcbo are type A Rcd

The wylex RCDs with yellow test buttons are type AC (note symbol ~) which don’t trip when they should if a DC fault is present.

The curve B or C is to do with over current protection on MCBs and RCBOs.

A B curve is more sensitive and is the norm for domestic. Eg B32
For 32 amp

the test button on the RCBO is under the lever in the pic.

 

[scousespark]

Eric.

It’s not complicated. We have 4 types of RCD (AC, A, F, B) each working slightly different to each other. We then have 3 types of MCB (B, C, D) where the ‘type’ denotes the ‘curve’ detailed in BS7671. The RCBO has a decal denoting the RCD type and a letter denoting the MCB type.

This is something I would expect any trading electrician to know. In domestic, the majority of RCBOs are type AC or A. Your type AC should give no problem. There are specialist suppliers offering type F or B RCBOs, but they cost hundreds of pounds. I have never seen one in a domestic install, but that doesn’t mean they are not out there. If there was a need for a type F or B, the cheaper option is an RCD and an MCB.

FYI,
The RCBOs you listed are all type A, as far as I can tell expanding the image on my iPad which does blur the image.

 

[AndyPRK]

Rcbo with arc detection

 

[AndyPRK]

Do you have an earth rod ?

A pic if your current Cu is always interesting.

 

[scousespark]

I can’t see us fitting many of them until the price drops.
Much like the early RCBO upgrades, but a lot more expensive.

 

[frank999]

We have 4 types of RCD (AC, A, F, B) each working slightly different to each other. We then have 3 types of MCB (B, C, D) where the ‘type’ denotes the ‘curve’
Thanks for that explanation, I see the types are unique to either RCD or MCB, not used for both ... makes it clear now.

The RCBOs you listed are all type A,
you are right ... your above texts helps understand.

So understanding this a bit better, on the existing CU the single AC type RCD protects 2x lighting and 1x kitchen circuits on MCBs - so protection against commonly used domestic appliances, the A type RCBO's protect habitable rooms such as bedrooms, where there are likely to be chargers, TV's and other equipment that might trip an AC type.

Does the CU needs to be split, with the RCD and MCB's on one busbar, and the RCBO's on a separate busbar.

What is the significance of these being 1 or 2 pole ?

the test button on the RCBO is under the lever in the pic.

Rcbo with arc detection
I note that in Germany these are standard ... considering the hotchpotch of electrics sometimes found in our aging housing stock - surprised that this wasn't specified before ... the price would lower once adopted, though currently your average CU would be quite expensive in comparison.