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Topic Title: Dual RCD plus RCBO consumer unit
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Created On: 27 July 2015 11:11 AM
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 27 July 2015 11:11 AM
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Lieuwe

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Joined: 21 July 2015

Having had some interesting responses to my first post on this forum, I got curious about something else.

We have a BTL property and the other day our tenant rang to say the lights had stopped working. Turns out a blown bulb had tripped the MCB, but the CU had me puzzled.

It is a big Wylex unit. It has got 2 80A/30mA RCD's protecting 5 MCB's each (or 4 & 6 .. not sure now). But it has also got 5 RCBO's.

- Why are those extra RCBO's there? Could it not simply have had those 2 RCD's protecting all 15 ways? I guess if the RCBO trips it only trips that specific circuit, but is it worth the extra expense of buying those rather than MCB's?

- There is a green sticker under all the circuits that are RCD protected, but there is a red sticker under the 5 RCBO's, saying they are not RCD protected .. that is not technically correct is it? .. and RCBO is basically an MCB + RCD isn't it?

The system is TT .
 27 July 2015 11:51 AM
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Delbot321

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You didn't say how old the unit is as this can be quite relevant - both as to the standards it was installed to and also as to what components were avaiable at the time.

RCBO is MCB & RCD combined, however it is only more recently you ger RCBOs, initially the MCB and the RCD had to be purchased separately and configured to suit your installation needs.

It sounds like the unit was made and intended for a TN earthing system where you would have had half the board with RCD protection and half without. Then it has been installed on a TT system so they needed overall RCD protection - probably 100mA (or 300mA) on that switch - they simply didn't cross out or remove the no RCD protection lable.

They may have run out of ways on the 30mA protection side and added some more RCDs to other circuits as the MCBs were already in place - hence the extra ways used.

For a protection and incovinience perspective it's much better to have separate RCDs and MCBs as you can see what has tripped - and therefore know what the liekly faults are. However from a cost perpective - which is how customers buy everything - the absolute minimum will win hence why we now end up with a split board with each half on a separate RCD with individual MCBs as this is the cheapest option.
 27 July 2015 11:57 AM
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Baldyhugh

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Lieuwe

I would be interested to see where the rcbo's are being supplied from. Is there a main switch in the consumer unit and they could be supplied from the output of that? There is a possibility that the rcbo's are being supplied from the busbar of the mcb's adjacent to them in which case they are also being protected by another 30ma rcd which is pointless. Also if the rcbo's are single pole which they probably are then they are not permitted in TT systems as they only switch the phase conductor. There is a bit of debate on this as the osg shows various consumer unit setups and shows one with a main switch and all single pole rcbo's suitable for TN and TT systems, however the regulations state that switching on TT must isolate all line conductors which for TT means live and neutral.

Peter
 27 July 2015 12:28 PM
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AJJewsbury

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It is a big Wylex unit. It has got 2 80A/30mA RCD's protecting 5 MCB's each (or 4 & 6 .. not sure now). But it has also got 5 RCBO's.

Sounds like an ordinary "high integrity" split load setup - the ways not protected by either of the two main RCCBs (RCDs) are used for loads that you'd want to stay on even if there's a fault elsewhere in the installation - e.g. freezers or smoke alarms. On TN installations they'd usually be populated with MCBs (hence the red label strip), on TT or later 17th Ed installations where RCD protection is needed for cables concealed in walls, RCBOs are used instead of MCBs (in which case the red strip should properly have been replaced with a green one).

however the regulations state that switching on TT must isolate all line conductors which for TT means live and neutral.

Automatic disconnection for shock protection and isolation are different things. ADS doesn't require the N to be disconnected even on TT systems. MCBs/RCBOs don't need to provide isolation if operation circumstances permit other devices (e.g. the main DP switch) to be used to isolate a group of circuits together.

- Andy.
 27 July 2015 12:29 PM
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Lieuwe

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We took the house on from a family member, it hasn't been wired in the last 2 years is all I know. There is the main switch on the right, then there are these 5 RCBO's (I don't even remember exactly what they are protecting, I think one of them is the 40A shower one), then an RCD followed by 4 or 5 MCB's, then another RCD and then the rest of the MCB's. I haven't opened the unit but I assume all RCD's and RCBO's are supplied from the switch? Do I need to get a spark in to check this if you say a TT system (its got a stake) shouldn't have these single pole RCBO's?
 27 July 2015 12:38 PM
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Baldyhugh

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however the regulations state that switching on TT must isolate all line conductors which for TT means live and neutral.

Automatic disconnection for shock protection and isolation are different things. ADS doesn't require the N to be disconnected even on TT systems. MCBs/RCBOs don't need to provide isolation if operation circumstances permit other devices (e.g. the main DP switch) to be used to isolate a group of circuits together.


I stand corrected Andy

Thanks

Peter
 27 July 2015 01:59 PM
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leckie

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It is true that SP RCBO's can comply even in a TT system, however if we have a TT installation that has for example a 100mA S type Rcd as the main switch, I don't like to see SP RCBO's downstream. The problem being that a neutral to earth faulty will not clear if an SP RCBO trips and so the main RCD will see the fault and turn the lot off. So the lack of discrimination is a problem.

Doesn't sound as if this is the case in the OP's example but I just thought I would mention it?
 27 July 2015 03:41 PM
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potential

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Originally posted by: AJJewsbury

It is a big Wylex unit. It has got 2 80A/30mA RCD's protecting 5 MCB's each (or 4 & 6 .. not sure now). But it has also got 5 RCBO's.


Sounds like an ordinary "high integrity" split load setup - the ways not protected by either of the two main RCCBs (RCDs) are used for loads that you'd want to stay on even if there's a fault elsewhere in the installation - e.g. freezers or smoke alarms. On TN installations they'd usually be populated with MCBs (hence the red label strip), on TT or later 17th Ed installations where RCD protection is needed for cables concealed in walls, RCBOs are used instead of MCBs (in which case the red strip should properly have been replaced with a green one).
- Andy.


I think you are right.
Wylex did make a unit with 3 separate sections.
2 of those sections were protected by their own separate RCD and 1 section was unprotected.

The unprotected section was expected to contain ordinary MCBs to supply circuits that at that time didn't require RCD protection.
The RCBOs that replaced them in the Wylex unit are directly connected to the main switch so will not trigger either RCD.
 27 July 2015 05:47 PM
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phantom9

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Originally posted by: leckie

It is true that SP RCBO's can comply even in a TT system, however if we have a TT installation that has for example a 100mA S type Rcd as the main switch, I don't like to see SP RCBO's downstream. The problem being that a neutral to earth faulty will not clear if an SP RCBO trips and so the main RCD will see the fault and turn the lot off. So the lack of discrimination is a problem.

Doesn't sound as if this is the case in the OP's example but I just thought I would mention it?


I see exactly your reasoning and I agree with it. But I wonder why a main switch on a TT system with modern dual RCD CUs needs to be anything other than a standard double pole switch? I understood the reason for a time delayed RCD main switch on the older 16th edition split load CU because it gave 100mA RCD protection to the circuits that were not otherwise RCD protected. The time delay would discriminate with the 30mA on the split.

It would be satisfactory to have a standard main switch with all RCBO circuits on a TT (notwithstanding the need for precaution where the CU is metal, and lets face it, it will be now). There is, however, no point having a 30mA RCBO and a 30mA RCD main switch, just pointless. You can use 16th edition CUs to good advantage having half all 30mA RCBO on the main switch and the other half all mcb on a 30mA RCD.

Just thought I'd mention it.
 27 July 2015 06:18 PM
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weirdbeard

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Originally posted by: phantom9

There is, however, no point having a 30mA RCBO and a 30mA RCD main switch, just pointless.


Twice the additional protection against user neglergance

-------------------------
:beer)
 27 July 2015 07:30 PM
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ebee

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Originally posted by: weirdbeard

Originally posted by: phantom9



There is, however, no point having a 30mA RCBO and a 30mA RCD main switch, just pointless.




Twice the additional protection against user neglergance


And (possibly) some mitigation against RCD failure

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 28 July 2015 09:19 AM
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mapj1

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Given that some popular designs of RCD are engineered down to cost, rather than up to a particular performance, I'd be in favour of retaining the set up of cascading them, and ignore the discrimination thing, especially if its all in the same cupboard - if one was in a locked box 300 yards up a muddy farm track then there may be an argument about the wisdom of having to set off in the dark to get the lights back on (though then I might just make the one far away from the load a time delay type unless I was dead sure that the impedances were low enough to blow a fuse or MCB reliably, and put an EM fitting above the consumer unit ).

The RCD nearest the supply ought to be double pole breaking, but further down single pole may be pest sometimes, but the currents you get with an N_E fault in a TT system tend to be lower, so the one-out all-out thing does not always happen, like it does on TNS or TNC-s.
Consider that with a typical single electrode resistance of say 30 ohms, to push 30mA you need 0.9V of N-E offset voltage.
On PME this is nothing, but on a lightly loaded TT system it would be possible but less common, and if the front end RCD was a 100mA or 300mA type, then the N-E offset has to be more like 3V or even 9V (unlikley even on PME) to take out the main RCD with an N-E fault.
Equally if the TT earth is really solid and just a slack handful of ohms, then you will see that tripping back up the chain, but it's still no worse than with TN. supply.
It may not be a text book installation, but it sounds perfectly sensible for your purposes, unless there are other things that make you suspsicoius.

-------------------------
regards Mike
 28 July 2015 06:27 PM
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phantom9

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But all of that is just your own ideas and is not required by the wiring regulations. Just saying like.
 28 July 2015 08:50 PM
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mapj1

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Wiring regs only require one RCD - if you can convince yourself false tripping will be rare, you could just have one 30mA unit at source, and in a small place like a flat that is similar in size to a caravan, it may be all that is needed actually. But in the case of the OP he already has a mix of RCD and RCBO, and I claim there is no need to undo it.

-------------------------
regards Mike
 29 July 2015 03:58 PM
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potential

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Originally posted by: mapj1
.............
But in the case of the OP he already has a mix of RCD and RCBO, and I claim there is no need to undo it.


I agree.
The only problem seems to be one of labelling.
 29 July 2015 06:51 PM
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phantom9

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Well, that leads me to question the abilities and understanding of the Regulations, of the person, or persons, unknown who installed that mismatched pile of carp in the first place. So your take seems to be "well it isn't Regs compliant but I can find no reason not to leave it in". Do you remember that argument I had with everyone on here about minimising inconvenience in the event of a fault? People started using the old definitions tack to convince themselves dual RCD CUs were compliant with this Reg. And you are basically saying that even though there are too many 30mA RCDs and in series that this is acceptable because if one fails it will be backed up by another? Haha. Totally flawed argument, has no basis for being acceptable and contravenes best practice. And to say that the only problem is that of labelling. Haha. Really? You surprise me. I thought you were more technically proficient than that, potential.
 29 July 2015 06:53 PM
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phantom9

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What about testing the RCBOs and RCD on the same side of the CU? Right messy set up to do it. And when you have 30mA RCDs one or the other or both could trip in the event of a fault, thus contravening minimizing inconvenience. Take out the RCD covering the RCBOs. That's the right thing to do. RCBOs are superior to one RCD covering multiple circuits anyway.

Edited: 29 July 2015 at 07:00 PM by phantom9
 29 July 2015 07:23 PM
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weirdbeard

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Phantom, as AJJ said, sounds like the op has a normal 17th ed hi integrity board.

-------------------------
:beer)
 29 July 2015 07:26 PM
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potential

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Originally posted by: phantom9

Well, that leads me to question the abilities and understanding of the Regulations, of the person, or persons, unknown who installed that mismatched pile of carp in the first place. [IMG][/IMG] So your take seems to be "well it isn't Regs compliant but I can find no reason not to leave it in". Do you remember that argument I had with everyone on here about minimising inconvenience in the event of a fault? People started using the old definitions tack to convince themselves dual RCD CUs were compliant with this Reg. And you are basically saying that even though there are too many 30mA RCDs and in series that this is acceptable because if one fails it will be backed up by another? Haha. Totally flawed argument, has no basis for being acceptable and contravenes best practice. [IMG][/IMG] And to say that the only problem is that of labelling. Haha. Really? You surprise me. I thought you were more technically proficient than that, potential. [IMG][/IMG]


I think you misunderstand the OP's description of the CU phantom9.

As I understood the description, there are 2 sets of MCBs (4 in one & 6 in the other) each section protected by their own RCD and another separate unprotected section, next to the main switch, containing 5 RCBOs, making a total of 15 ways in all.

The RCBOs are fitted in the section not protected by any RCD.

None of the RCDs or RCBOs are in series with one another so the problem of discrimination does not exist.
 29 July 2015 09:16 PM
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OMS

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LoL -

I suspect,P9, that you've never lived out in the sticks with nothing but a single front end 30mA RCD between you and certain death (or at least inconvenience and hazard)

At one point, I actually had a wylex 30mA in an enclosure in the tails feeding my basic non split MCB consumer unit with a single 30mA front end RCD acting as the board main switch - so two 30mA devices in series - so at least some protection from a common mode failure of the RCD's

I know your a great believer in following the regulations without deviation or interpretation - but even you would have to ask the question how we got from section 314 to the so called called 17th edition consumer unit without gross manipulation of the regulations by thiose chaps who just happen to make a penny by selling RCD's

So, from that point, having a few RCBO's downstream of an RCD isn't exactly a major problem in a simple domestic dwelling

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
IET » Wiring and the regulations » Dual RCD plus RCBO consumer unit

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