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Topic Title: Clarification of what is shown in figure 3.6.3(i) of the BS7671:2018 Onsite Guide.
Topic Summary: Single pole RCBOs in a TT installation without an upfront DP RCD.
Created On: 12 October 2018 08:40 AM
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 12 October 2018 08:40 AM
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sparkingchip

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I am trying to resolve what appears to me to be an example of ambiguity in the BS7671:2018 Onsite Guide. Ambiguity being defined as the quality of being open to more than one interpretation; inexactness.

Below the OSG figure 3.6.3(i) Consumer unit with RaCBOs, suitable for all installations (TN and TT) page 36 it states:

"Single RCBOs protect each outgoing circuit "

I presume that this should be read as individual RCBOs protect each outgoing circuit and should not be read as single (pole) RCBOs protect each outgoing circuit. Because either single or double pole RCBO s may be used.

A second point that seems to need some clarification is that the consumer unit shown in the figure does not have an upfront RCD, however presumably it is still acceptable under the regulations for single pole RCBOs to be used as circuit protective devices in all installations, both TN and TT, with the double pole main switch providing the required isolation of both of the live conductors.

Andy Betteridge
 12 October 2018 08:48 AM
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sparkingchip

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I have pinched a relevant reply by Zoomup from another discussion:

"Chapter 41 Protection Against Electric Shock. 411.5.2 says that in a TT system one or more of the following protective devices may be used, the former being preferred: (i) An R.C.D. (ii) An overcurrent protective device. The 18th edition uses the same wording.

Also 531.4.1 says that if a TT installation is protected by a single R.C.D. this shall be placed at the origin of the installation..... "

In which Zoomup presents a case for not having RCD protection at all in a TT installations, but still complying with the regulations.

Andy B.
 12 October 2018 09:31 AM
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IronFreely

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All I can say is I've presented a multi SP RCBO unit on a TT a few times on my annual inspections and the only comments I got was "that's nice and neat" and "you like RCBOs don't you"
I believe that an upfront RCD is not required on a multi RCBO board because it has no internal tails that are likely to short against the metal consumer unit. The picture in the OSG makes the option seem very clear to me.
 12 October 2018 09:43 AM
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Zoomup

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Originally posted by: sparkingchip

I have pinched a relevant reply by Zoomup from another discussion:



"Chapter 41 Protection Against Electric Shock. 411.5.2 says that in a TT system one or more of the following protective devices may be used, the former being preferred: (i) An R.C.D. (ii) An overcurrent protective device. The 18th edition uses the same wording.



Also 531.4.1 says that if a TT installation is protected by a single R.C.D. this shall be placed at the origin of the installation..... "



In which Zoomup presents a case for not having RCD protection at all in a TT installations, but still complying with the regulations.



Andy B.


Actually Andy I prefer an R.C.D. in a TT installation. An upfront double pole R.C.D. is best in my opinion. The regulation quoted above says that an R.C.D. is preferred. I agree with that recommendation in B.S. 7671.

Z.
 12 October 2018 10:28 AM
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IronFreely

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Actually Andy I prefer an R.C.D. in a TT installation. An upfront double pole R.C.D. is best in my opinion. The regulation quoted above says that an R.C.D. is preferred. I agree with that recommendation in B.S. 7671.



Z.


The problem with an upfront RCD is it offers no discrimination between circuits in the case of a N-E fault when the downstream protection is SP RCBO even if it's a delay type as occasionally that tiny potential difference between true earth and neutral is enough to throw them out, especially in a TT system.. Again I wonder if the future of RCBOs reside in the land of a single module DP SN RCBO, Wylex do one that only costs pence more than their SP one.
 12 October 2018 10:28 AM
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IronFreely

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Actually Andy I prefer an R.C.D. in a TT installation. An upfront double pole R.C.D. is best in my opinion. The regulation quoted above says that an R.C.D. is preferred. I agree with that recommendation in B.S. 7671.



Z.


The problem with an upfront RCD is it offers no discrimination between circuits in the case of a N-E fault when the downstream protection is SP RCBO even if it's a delay type as occasionally that tiny potential difference between true earth and neutral is enough to throw them out, especially in a TT system.. Again I wonder if the future of RCBOs reside in the land of a single module DP SN RCBO, Wylex do one that only costs pence more than their SP one.
 12 October 2018 10:42 AM
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sparkingchip

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Nothing percludes going over and above the requirements of BS7671.

Initially it would be good to get an agreement on the requirements of BS7671:2018 regarding the inclusion of RCDs in TT installations.

Then we can start to differentiate between the requirements of BS7671:2018 and the inclusion of RCDs in TT installations by designers based on engineering decisions and best practice, or even just based on imaginary requirements based on electricians folklore.

Andy B.
 12 October 2018 10:53 AM
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Zoomup

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I like the idea of a pre-metal consumer unit R.C.D. in a domestic TT installation contained inside an insulating enclosure. A 100mA time delayed type is good. Actually Wylex recommend this arrangement in their online tutorial. It also affords belt and braces protection if one R.C.D or R.C.B.O. becomes faulty or sticky.

Wylex tutorial. The section that concerns pre-consumer unit R.C.D.s is just before 4 minutes into the video.

http://www.youtube.com/watch?v=uOypkkgTEB0

Z.

Edited: 12 October 2018 at 11:13 AM by Zoomup
 12 October 2018 11:33 AM
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Zoomup

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This subject has been discussed before.

http://www.theiet.org/forums/f...id=205&threadid=12921

Z.
 12 October 2018 12:37 PM
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sparkingchip

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Originally posted by: Zoomup

This subject has been discussed before.



http://www.theiet.org/forums/f...&threadid=12921



Z.


Not really relevant, that discussion was from twelve years ago and the BS7671:2018 OSG advises that a split load consumer unit should have an upfront S-type RCD main switch due to the internal tails arrangement.

Andy B.
 12 October 2018 01:54 PM
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Zoomup

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Originally posted by: sparkingchip

Originally posted by: Zoomup



This subject has been discussed before.







"><br ">http://...t.org.../f.....2921 ...r />







Z.




Not really relevant, that discussion was from twelve years ago and the BS7671:2018 OSG advises that a split load consumer unit should have an upfront S-type RCD main switch due to the internal tails arrangement.



Andy B.


It is up to the reader to extract and apply the relevant and pertinent content of the thread. A.J.J's observations are of particular interest and relevance. An internal upfront R.C.D. within a metal consumer unit used as a main switch, does not offer the same protection as an external R.C.D. in an insulated enclosure, prior to the consumer unit, to all earthed metal work within a T.T. installation, to becoming live under fault conditions, if say a damaged incoming live tail livens up the consumer unit.

Z.

Edited: 12 October 2018 at 02:01 PM by Zoomup
 12 October 2018 01:59 PM
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Zoomup

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Perhaps we should be asking just how many N to E faults are found by us that cause an R.C.D. to trip off where single pole R.C.B.O.s are involved down stream? Is it a significant number? If it is just the odd case a year or two in a faulty appliance that can be easily unplugged to clear the fault then the problem really is not a major issue is it?

Z.
 12 October 2018 03:28 PM
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peteTLM

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That got you all talking :-)

My take was not that there was masses of live metalwork just waiting to earth out against the enclosure, but that it ensured that the neutral got disconnected in the event that single pole- don't switch the neutral RCBO's had been used.
Or that a neutral earth fault could be potentially ignored by single pole RCBO's.

Im sure it was required in say 16th/ early 17th ED and then dropped. Not been near a blue book yet.

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 12 October 2018 04:35 PM
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sparkingchip

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Can we knock that twelve year old discussion on the head, if as stated the 30 mA RCD was tripping in a split load board with a 100 mA RCD that was not tripping, the RCD main switch could have been left in place when the MCBs were replaced with RCBOs. It was probably pointless replacing it with a main switch and apart from that it was almost certainly a plastic consumer unit.

I specifically said BS7671:2018 in the original post and this discussion isn't really going to work if you try discussing examples from previous editions of the regulations. In the 17th Edition OSG there were five examples of RCDs within installations, now there are three with the main switch and 100 mA main switch split consumer units being dropped as examples, presumably as there is little you can install in a domestic property without RCD protection now that it is a requirement for domestic lighting circuits to have 30 mA RCD protection. Also the order and references of the three remaining figures have been altered, so if you are not looking at the BS7671:2018 OSG the discussion could get confusing, particularly as domestic split load consumer units are basically obsolete.

A split load consumer now would be a main switch supplying one or more RCBOs and one or more RCDs with MCBs of the type that get advertised as "Hi-integrity consumer units". Otherthat that you can have a 30 mA main switch, dual 30 mA RCD or all RCBO consumer unit and the all RCBO consumer unit could be a mixture of 100 and 30 mA RCBOs.

So are 100 mA S-type RCD main switches going to be phased out, unless there is a distribution circuit or some other specific reason for installing one?

There is another reason for installing an upfront RCD, but I'll leave that until later in the day.

Are we all agreed that a TT installation can have all single pole RCD devices with a double pole main switch and comply with the requirements of BS7671:2008?

Andy B.

Edited: 12 October 2018 at 04:45 PM by sparkingchip
 12 October 2018 04:39 PM
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mapj1

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TT is unusual however in that during faults the voltage on the earth electrode moves up to meet the live voltage, rather then in TNS or TNC-s where in general the 'live' voltage gets pulled down towards the earth. This higher touch voltage during fault is the driver for the reduced ADS times in such cases.
There is an associated risk , at least with the smaller "pole-pig" transformers, perhaps only supplying a few isolated cottages, that the electrode resistance effects are equally serious at the supply end, as well as the consumer.
If the transformer has the 20 ohm electrode resistance that is permissable for a worst case, and the house holder earth resistance is the same, then 230v is shared, and the system neutral at the TX , (and for all other transformer users too) is now 115 above terra-firma, even though everyone still sees an L-N difference of 230 and may not even realise there is a fault in progress.

It is not utterly incredible that one house with lots of outdoor earthed metal work may actually be better earthed than the transformer. Now it gets really fun if there is a pre-RCD L-E fault at that property; then it may almost reverse the roles of live and neutral for everyone else on that phase of the transformer in terms of which supply pole is nearest true earth voltage, until the fault is cleared. (and if you like to be scared, now imagine what happens to users on another phase of that transformer and on the same neutral - they too see no fault in their L-N difference, but what is their L-E voltage doing...)

As Alan Capon lives on that famous "TT" island, I guess he will have an input on how often this sort of weirdness actually occurs when he gets in from the day job.

In summary, there is a very good reason for the advice to treat neutral as a live conductor, generally, and on TT it becomes more important. If you have a system that is playing up, the ability to break L and N may save a life.

-------------------------
regards Mike


Edited: 12 October 2018 at 04:46 PM by mapj1
 12 October 2018 04:53 PM
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sparkingchip

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You present a strong case for installing an upfront double pole RCD or using double pole RCBOs in a TT installation, but does BS7671:2018 actually require it?

And if BS7671:2019 doesn't require an upfront DP RCD in a TT installation, is there any other enforceable requirement?

Andy B.
 12 October 2018 04:55 PM
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sparkingchip

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 12 October 2018 05:01 PM
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mapj1

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I fear BS7671 does not require it, but I think it is releated to the reason the authors appear not to understand why a fault from a pre RCD live to a TT earthed metal consumer unit is really bad both for you and the neighbours either - things that are specific to the DNO are considered out of scope, and cannot introduce a problem.

Then you get really funny things like that earth electrodes for TNC-s nonsense that was in the 18th edition draft and then got pushed out again, none of it re-assures me that the technical side of the regs gets the top level of attention except when someone squeals.

edit...
Yes, Schneider is aiming at the world market, and places like France and Italy that have larger regions of TT users do indeed now insist on DP isolation of final circuits, and in the French case have for a while. (when fuses became MCBs roughly)
A collective memory of olden times and split phase supplies may be partly responsible for that attitude, although it does not seem to assist in consistant preservation of LN polarity at socket outlets

-------------------------
regards Mike


Edited: 12 October 2018 at 05:15 PM by mapj1
 12 October 2018 05:05 PM
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AJJewsbury

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Are we all agreed that a TT installation can have all single pole RCD devices with a double pole main switch and comply with the requirements of BS7671:2008?

Provided you can arrange that the entire installation up to the RCBOs still provides adequate protection against electric shock - usually by double/reinforced insulation (e.g. if the CU is at the origin and is either insulating or an approved class II kit is used). Trickier if the CU isn't at the origin.

Personally I prefer a 100mA S-type incomer and DP RCBOs for TT - just because where something critical like ADS in concerned (rather than "just" additional protection) I prefer a 0.49% failure rate to a 7% one. But that's just my preference, not a BS 7671 requirement.

- Andy.
 12 October 2018 05:10 PM
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AJJewsbury

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Schneider thinks SP RCBOs are unacceptable in TT installations.

Methinks there's been a too literal translation from French requirements (100 Ohms max Ze, DP switching at MCBs etc all have a certain Gallic flavour, not to mention the Cedex address)
- Andy.
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