Getting head around RCBOs and split CU's

[flameport]

is one wise to purchase a CU with plenty of spare ways

A small empty box is almost the same price as a large empty box. Unless there isn't enough space to physically fit a larger enclosure, it would be insane not to.

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.

None of the editions specify any particular arrangement of devices, things like '18th edition consumer unit' are terms invented by the marketing departments of various manufacturers.
However:

• 314 Division Of Installation requires that 'every installation shall be divided into circuits as necessary ... ' for various reasons, and shoving half of the installation onto a single RCD is in no way complying with that. No one ever wanted the upstairs lighting to fail because someone shoved a knife into the toaster, or their home office to be disconnected because rain got into an outside light.
• Much modern equipment has a certain amount of residual / leakage / protective conductor current in normal operation, and the maximum permitted is 30% of the RCD rating, which is only 9mA for a 30mA RCD. As a single filter in an appliance could result in 1-2mA, it's very easy to be way over the 9mA when multiple circuits are combined onto a single device. This will result in the RCD tripping in normal operation.
• Adding another circuit to a dual RCD consumer unit is likely to compound that problem further.
• The inverter for a solar panel installation cannot be connected to a circuit which is supplied from an RCD which covers multiple circuits, as disconnection times will not be met.
• Certain types of equipment require certain types of RCD - with a dual RCD consumer unit, there is no option to install non-RCD circuits or other types of RCD as required. Not possible to install certain types of RCD downstream of others either.
• When AFDDs are mandatory for some/all circuits, installation of those is just a matter of replacing an RCBO with an AFDD. With a dual RCD consumer unit, the only option is a new consumer unit.

All of them have a test button - the Wylex reduced height have the test button at the bottom edge, concealed by the trip lever in the picture.
Whether they have a functional earth connection depends on the internal design of the electronics - some do, some do not. Most of the modern ones do not.
Single pole RCBOs are suitable for use on TN systems. For TT, single pole with switched neutral is required - or double pole which are generally twice as wide.
Some manufacturers make both types, others only one, as there is nothing wrong with using those with a switched neutral on a TN system even though that isn't required.

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.

The problem isn't one of tripping, but the exact opposite. Type AC RCDs only work properly for AC resistive loads such as an immersion heater or incandescent lighting. With other loads connected (which includes 99% of electronic equipment, LED lighting, inverter drive motors and much more), Type AC may not work properly or at all.

Surge protection is required for the vast majority of installations. It is not optional, or something the homeowner can choose not to have.

 

[ericmark]

Unless you need type B or F then the RCBO route seems the best, the problem is there is a type B and curve B and the word type is used when they mean curve so I collected 14 type B RCBO's written on the box type B only after I had got a scheme member electrician to fit them noted not type B but type AC and curve B. There was nothing on the paperwork to say type AC either on the box or inside the box, it was only latter I noted the very small

on the device, seems made so small so no one notices it until too late.

Type A

are adequate for most domestic installations, the type F

may be required if using powerline adaptors which I personally think should be banned anyway as they cause so much interference, I can't think of why a type B

would be required in domestic, I have used DC injection with large crushing plants and the like, but not with domestic, unless looking at a fault with solar panels? But solar panels and electric car chargers tend to be connected through an independent unit not the main consumer unit, so really not required to use better than type A

anyway. The only exception is powerline adaptors which is the only device I can think of used in a normal house hold that could need better than type A.

So I totally agree with what @flameport has said.

As to powerline technology it seems that is also installed without the users knowledge, I was unaware until very recently that I have powerline adaptors built in to my Sky Q boxes, what I don't know is if the amount of high frequency AC injected using these devices can actually stop the RCBO from working, I would think likely not, but I didn't think a faulty diode in a switch mode power supply would effect the RCBO either until the John Ward demo.

I will change two of the fourteen RCBO's for type A on the ring final supplies, although the lighting could be affected I consider unlikely I would ever be in a situation where I even need a RCD on lights anyway, so will take a chance. But I will warn others to actually take the RCBO out of the packet and check the logo, and not trust the writing on the outside.

 

[AndyPRK]

Having watched a demo video of DC, apparently whether the test button works or not will indicate whether they will trip in a real life AC fault.
So turn on your SkyQ boxes and presumably stream room to room and then press the test button.

What brand RCBOs are they ?

 

[ericmark]

Brand is Fuse Box, but the button test is no good, I have a 10 mA RCD socket, it was originally fitted in a workshop where the whole workshop was fed from a 100 mA RCD, pressing the test button on the 10 mA RCD socket (MK) would always trip the 100 mA main RCD so if a 10 mA button has 100 mA then a 30 mA button could have 300 mA the button is only to check the mechanical release, need an RCD tester to check the tripping currents.

I can't buy better than type A RCBO's to fit my box, I have two on order to replace the type AC for socket outlets in main house ring finals, I would have never bought a powerline adaptor if I had known what I was getting, as a radio ham it is a big no no for me, however I have not noticed any interference, so not too worried about that, and I will in fullness of time fit a hard wired LAN, and once that is done and the Sky Q is plugged into it, then it seems there is a way to disable the powerline function.

As to how good powerline is when it has to go through two RCBO's I don't know? But plug my PC into the Sky Q satellite box and my internet down load speed doubles.

 

[frank999]

Thanks Mr Port for your explanation.

and shoving half of the installation onto a single RCD is in no way complying with that
So does the 18th edition require amost all RCBO's now in a CU.
Where there is more than one circuit on an RCD there will always be an inconvenience to the other circuits if one of them trips. I see pre 18 edition CU's with exactly that arrangement, often with just one RCD for all circuits, that seems the norm. I have that configuration at another location, and to my knowledge it has only tripped once in 4 years.
Am I right in suggesting that a dual split CU went some way to dividing up the inconvenience, but now the 18th Edition requires it to be all split, so you can't now have a single RCD that protects both a house lighting circuit and say an outbuilding, as an example, which is not the arrangement I see being sold by CU manufacturers, they still have several MCB's under an RCD in there CU offerings.
FYI - the site is residential.

with a dual RCD consumer unit, there is no option to install non-RCD circuits or other types of RCD as required
Is a Split Load/Dual RCD CU mostly the same thing ... from what you are saying such boxes are quite limiting, and if a CU is to consist of all RCBO's - then multiple busbars in a CU are not required. Do manufactrurers make it easy to convert CU's (where enough ways exist) between single and split types.

Single pole RCBOs are suitable for use on TN systems, For TT, single pole with switched neutral is required.
I see all Wylex RCBOs have switched neutral as standard.

Surge protection is required for the vast majority of installations. It is not optional.
I do not see these being sold as standard in CU's ... if they are not optional then why not ?

The problem isn't one of tripping, but the exact opposite.
Thanks for pointing that out, so where RCD's are used domestically where LED lighting is involved, would it help to have the RCD as a A type rather than AC.

it was only latter I noted the very small

on the device, seems made so small so no one notices it until too late.
Thanks for pointing this out.
The Symbols, although tiny - are very helpful though.

 

[AndyPRK]

I think what happened was :-

16th edition - Sockets were RCD protected (particularly ground floor that outside equipment could be plugged into). Lights and immersion heaters didn't require RCD protection. So half way along the board would be 1 30mA RCD.

17th edition required everything to have RCD protection (unless cables were burred more than 5cm's which they usually aren't, or run in metal sheathed cables)
The dual RCD consumer unit became common. The idea was that sockets downstairs and lights upstairs would be on the same RCD. So even if one RCD was off you would still have some power in a room.
The last amendment 3 of 17th edition required non flammable CU's (basically metal)

18th edition introduced SPD's and ARC protection


I quite like the compromise of taking a 16th installation, and fitting RCBO's to the light circuits adding RCD protection to them
And you can also move over a critical socket circuit or two to the other side and put them on RCBOs (in the event the RCD can't be reset)

You should consider a scenario that the RCD trips and can't be reset. You are in the middle of winter and can't get an electrican out for several days to fix the fault. What circuits must you have power on and what circuits can you live without for a few days. The critical circuits should be on RCBO. e.g kitchen sockets, boiler, ground floor sockets, and cooker. It usually makes sense for lights to be on RCBO.

If you have many circuits RCBO's can be very costly. And this is where Dual (or single) RCDs are an advantage

 

[AndyPRK]

"I see all Wylex RCBOs have switched neutral as standard."

This isn't correct.
All RCBO's have a neutral wire (to sense current imbalance) however it is rare for them to disconnect the neutral.
It states above I believe the single height modern ones switch neutral.

 

[frank999]

Wylex miniature RCBOs bring higher levels of safety to anelectrical installation and its users because they includeswitched neutral as standard and bring cost savings byreducing installation and testing times.

Sorry I misread the catalogue blurb, it is just the miniature RCBO's that have the switched neutral. (ie 2 pole).

The critical circuits should be on RCBO. e.g kitchen sockets, boiler, ground floor sockets, and cooker. It usually makes sense for lights to be on RCBO.
I'm getting a clearer picture of what will be required.

If you have many circuits RCBO's can be very costly. And this is where Dual (or single) RCDs are an advantage
But cost should not over rule Regs ??, and that from my understanding here, you can only really use RCD's now to protect mutliple circuits if they are not high priority/critical circuits. All high priority/critical circuits should be on there own separate RCBO.

 

[JohnW2]

Wylex miniature RCBOs bring higher levels of safety to anelectrical installation and its users because they includeswitched neutral as standard and bring cost savings byreducing installation and testing times.

Maybe I'm missing something but, other than in extraordinarily rare (combinations of) situations, I'm not sure that I can think of any way in which an RCBO which switched neutral would result in any higher level of electrical safety.

The only safety implication of having an RCBO which switches neutral I can think of is the oft-discussed theoretical issue of people dropping pans of boiling oil, falling off ladders etc. as a result of being 'plunged into darkness'. If a N-E fault arises in a non-lighting circuit, then having that circuit protected by a DP (i.e. switching neutral) RCBO (or dedicated RCD) would prevent that fault affecting other circuits (including lighting ones) - but I think that is really scraping the barrel for 'safety issues'. As I often say, although others' experiences obviously differ, in my house it is far far more common for us to be 'plunged into darkness' (well, it would be, if we didn't have emergency lighting!) by a brief power cut than by any protective device being tripped.

Kind Regards, John

 

[ericmark]

My understanding is with TT the neutral needs switching, with TN it does not, however when I first read the 17th I considered it considered an RCD as a circuit, and the "314.1 Every installation shall be divided into circuits, as necessary, to: (iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation" only makes sense if you say the RCD is forming a circuit, however I was told many times on this forum I was wrong, and a RCD did not form a circuit as it does not measure over current, going over 30 mA it seems was not going over current?

I was worried that at some point in the future some one would say that has never complied with the regulations so you should get the electricians back to correct it, however although we report can it with an EICR it would only get a code C3 at worse, so it is unlikely anyone will be told you got it wrong.

The same applies with type AC, A, F, B RCD's only with TT is it really a problem, as with TN the RCD is additional protection.

I does seem to me a battle of English, the writers of BS7671 do their best to write it so it can't be misunderstood, the the manufacturers and scheme operators do their best to misinterpret what it says.

Take "The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any, other purpose." seems quite clear to me, but the next sentence says "Single-core cables that are coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be over-marked at their terminations" and because of this second part, people seem to think the first part only refers to single-core cables and not multi-core.
To my mind there is a full stop after purpose, but the point is made why then even refer to single core, however it continues ", except as permitted by Regulation 514.4.3.
In this combination one of the colours shall cover at least 30 % and at most 70 % of the surface being coloured, while the other colour shall cover the remainder of the surface.
A bare conductor or busbar used as a protective conductor shall be identified. where necessary, by equal green-and yellow stripes, each not less than 15 mm and not more than 100 mm wide, close together, either throughout the length of the conductor or in each compartment and unit and at each accessible position. If adhesive tape is used, it shall be bi-coloured." so the reference to single core is to qualify its use as a PEN conductor, as does not refer to first statement. But this has caused arguments again and again, I have seen specially with central heating many times where the bi-colour combination green-and-yellow has been over sleeved. Even if the regulations do allow it, it's a silly thing to do, but it seems we need to be English students now it does not matter what makes electrical sense.

However it is not simply cost, back in 2001 finding a consumer unit made out of metal with three neutral bars was near impossible. Today finding a single module width RCBO type F is also like looking for rocking horse muck, but I would think in 10 years time they will likely be the norm.

At the moment no one seems to be able to say if using a powerline adaptor should be permitted with anything less than a type F, but since you can't buy type F RCBO's then it would mean outlawing the powerline adaptor, maybe not a bad thing, but my point is there is no perfect, it is a compromise, and one has to accept some risk.

 

[JohnW2]

My understanding is with TT the neutral needs switching, with TN it does not ...

I think that, at least in terms of explicit regulatory requirements, you may be confusing switching and isolation. In terms of isolation, BS7671 starts by saying that isolation requires both L and N to be interrupted, but then goes on to say that the neutral does not have to be interrupted (for isolation) in TN installations (hence essentially, certainly for domestic, just leaving TT). It even gives a 'rationale' for that, although I don't find it convincing, particularly with TN-S.

However, that's all in relation to isolation, not switching, and I am not aware of anything in BS7671 which, in general, requires DP switching.

If some of the final circuits in a TT installation don't have RCD/RCBO protection then there is a 'need' for an overall up-front (usually time-delayed) RCD and, in that situation, there is a 'need' (probably rarely satisfied in the case of RCBOs) for DP RCDs/RCBOs for final circuits, since otherwise an N-E fault which causes one of those devices to trip will also trip the up-front RCD, thereby killing the entire installation and hence probably violating 314.1.

However, my understanding is that if all final circuits in a TT installation are protected by residual current devices (as in 'dual RCD' CUs and all-RCBO ones, or a combination thereof) then, unless the meter tails are long (and hence need RCD protection), there is no need for an up-front TD RCD - in which case I can see no reason why there would be a need for DP devices in such a TT installation any more than in a TN one.

... only makes sense if you say the RCD is forming a circuit, however I was told many times on this forum I was wrong, and a RCD did not form a circuit as it does not measure over current, going over 30 mA it seems was not going over current?

I don' t know why you persist with this and, as I've said, I think it undesirable that it should be repeatedly mentioned in a DIY forum, where many people are confused, or have misconceptions' about the difference between an 'over-current device' and a 'residual current device'.

You know the difference as well as the rest of us who are 'in the know' - and you therefore know that no-one (other than you) who knew what they were talking about would call a residual current device an 'over-current' one!

Kind Regards, John

 

[flameport]

So does the 18th edition require amost all RCBO's now in a CU.

For that, the only thing that changed with the 18th edition is that people / manufacturers / other decided to actually read what 314 states and act upon it.

Other than some very minor wording changes and references to other chapters/sections, the same content of 314 has been in BS7671 since the 17th edition was published well over a decade ago.
Why it was ignored by many for so long is unknown. Just as with many other things, including why some countries banned the use of Type AC RCDs at the end of the last century, yet in Britain they are still being installed as the standard item.

That same 17th edition from 2008 also included requirements for surge protection on some installations, so that isn't new either - although that was substantially altered and expanded in the 18th edition.

Is a Split Load/Dual RCD CU mostly the same thing

Split load generally refers to a consumer unit where some of the circuits are covered with a single RCD, and other circuits have no RCD at all. Popular with some when the 16th edition was current, mainly because at the time, only some circuits required an RCD and doing that was the cheapest way of doing it.

Dual RCD is half of the circuits on one RCD, the other half on another one. Misleadingly called a '17th edition' by certain manufacturers and vendors. Another cheapo way of attempting to comply for those where saving a few quid on the cost of the consumer unit was top priority.

Another variant was the ridiculously named 'high integrity' effort which had 2 RCDs and a third section for circuits with no RCD. One or two RCBOs could be fitted in that section if desired.

Surge protection is required for the vast majority of installations. It is not optional.
I do not see these being sold as standard in CU's ... if they are not optional then why not ?

What some vendors offer for sale is primarily driven by profit, as in what can be sold in the largest quantities for the lowest prices. They really couldn't care less whether it complies with BS7671 or not, as that is entirely down to the person installing it.

 

[flameport]

in which case I can see no reason why there would be a need for DP devices in such a TT installation any more than in a TN one.

Neutral in a TT installation is not reliably connected to Earth, and there can be a significant voltage between N & E. Disconnecting line only could leave parts of the installation energised relative to Earth.

461.2 general requirements for isolation and switching, Table 537.4 where circuit breakers are listed as devices for isolation and switching.

 

[JohnW2]

Neutral in a TT installation is not reliably connected to Earth, and there can be a significant voltage between N & E. Disconnecting line only could leave parts of the installation energised relative to Earth.

That is the 'rationale' I referred to implied by BS7671 - but, as I said, I'm not convinced that lets TN-S off the 'DP isolation' hook very comfortably, does it?

The partial sentence of mine which you quote was not about isolation - but about the ('functional') 'need' (or lack of it) for DP protective devices in a TT installation which has all circuits RCD/RCBO-protected and which does not have a 'whole installation' up-front Type S RCD.

461.2 general requirements for isolation and switching,

I don't really get why that reg refers to "isolation and switching" (rather than just isolation - which I seem to recall was how is was done in 17th). Taken at face value, since it is stating the conditions for neutral switching/isolation not being required with TN-S or TN-C-S, the implication seems to be that DP switching is always required (for any/all 'switching') in a TT (or IT) installation, yet 463.1.2 (Functional switching) starts by saying (as one would expect) "Functional switching devices need not necessarily switch off all live conductors of a circuit.", without any reference to the type of earthing system

Table 537.4 where circuit breakers are listed as devices for isolation and switching.

Indeed, and note that a footnote to the 'Isolation' column of that Table says "(4) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors. See Regulation 462.2". We know that, and it seems perfectly reasonable - but, as I said, I would probably be even happier if it applied to TN-S as well as TT & IT - so the fact that a device appears in that Table with a 'Yes' in the Isolation column does not necessarily mean that it is suitable for isolation in a TT (or IT) installation.

However, in any TT installation there will always be a means of DP isolation available - if nothing else, an RCD (if there is one) or main switch. 462.2 does say "Provision may be made for isolating a group of circuits by a common means, if the service conditions allow this.". So, returning to the point under discussion, to my mind isolation is therefore not an issue, the only actual ('functional') 'need' for DP RCBOs in the CU of a TT installation arsing if there is a 'whole installation' RCD upstream of the CU (because of the potential problem with N-E faults, if one has SP RCBOs in that situation).

Kind Regards, John

 

[frank999]

What some vendors offer for sale is primarily driven by profit
Umm are you better off going with vendors that charge more - and would be more ethical in there sales approach then ? I have been looking at Wylex gear, I guess mid market ??

Just called Wylex, tech were very helpfull, it appears they sell a 'split load flexible' board (NMRS17S12SLMA) that will allow 5 RCBO's, several MCB's protected by an RCD, plus SPD - took a phone call to the manufacturer, the wholesaler wasn't aware of such a board, and led me to beleive such things were unavailable.
Not sure if the aboce CU's come with an RC D, but the Wylex catalogue states any RCD's RCBO's supplied are Type 'A'.

including why some countries banned the use of Type AC RCDs at the end of the last century
My wholesaler slid an AC RCD across the counter, and suggests its the default.

He also suggested that the majority of CU's going out the door have SPDs, as you suggested they are not optional.