Discuss TT Supply MCBs in the Electrical Forum area at ElectriciansForums.net
I guess as we have traditionally considered having correct polarity an essential aspect of an installation. Our plugs are only line-fused, and many switches are line-only, so polarity matters whether TT on TN and so no advantage in safety terms for the board MCBs to be double-pole.
But on a TT system the incoming RCD would be all-pole so there is some protection for high N-E voltage faults even if unlikely.
Yes it would be a vast improvement if everyone followed suit.Realistically the SP+SN option for RCBO is more useful in terms of allowing N-E faults to be identified and circuit testing performed without much cable removal. but not very common outside of the Wylex/Crabtree ones in the UK it seems.
Personally I would prefer to see DP or SPSN MCBs. They are much more widely used in mainland Europe where TT is more common. We seem to have a general disinterest in them, perhaps because we are generally pretty consistent about preventing significant N-E potentials at source and would gain little in practice.
We always have DP isolation in all SP+N CU's though as PC1966 points out.
Do you have a picture of these new consumer units? Sounds like what we are doing in the US.Yes it would be a vast improvement if everyone followed suit.
I am annoyed that schneider didn't take the opportunity to include this in the new range of Acti9 PoN boards, when they changed to a neutral busbar and plug in contact instead of flying leads on the RCBO I'm sure they could have included switching of the neutral too.
Your point about the time delay is valid as a neutral to earth fault will disconnect the main time delay rcd but there’s actually no requirement for the time delay rcd in most domestic installations unless it maybe supplies a distribution circuit , in which case I’d use a dual rcd CU, as DP rcbos can be hard to come by.It's common practice to use a time delayed main switch on TT systems for back up earth fault protection and to protect metal cased DB's, in which case DP or SN RCBO's are essential if RCBO's are being used. Standard RCBO's would not be compliant but are often used.
My post concerned where a time delayed RCD is used as a main switch which you appear to have overlooked.There no requirement to switch or disconnect the neutral in TN or TT arrangements during fault conditions except for those laid out in part 7. There is a need to be able to isolate the neutral on TT arrangements but this is done by incorporating a 4 pole main switch disconnector in the DB or as what’s required anyway for single phase installations by use of ordinary persons , a DP switch is used regardless of the earthing arrangement.
Rcbos do not have to be DP on TT arrangements unless required to be by the extra requirements laid out in part7.
Yeah I deleted the first part of the post as I realised I had misunderstood what you were saying.My post concerned where a time delayed RCD is used as a main switch which you appear to have overlooked.
In that circumstance an earth fault on a final circuit will operate a SP RCBO but the N will not be disconnected probably resulting in the RCD main switch also operating, disconnecting the entire installation. That would be non compliant because division of circuits and circuit protection must be designed for minimum disruption in the event of a single fault.
The only way to prevent that is to fit RCBO's which disconnect the N.
So the installation of SP RCBO's where a T/D RCD main switch is used to protect the metal cased DB is non-compliant.
RCD is a term which covers a group of similar devices.How is that possible? I thought RCDs did not respond to over current.
That clears it up a bit more. When I think of RCD I only think of the literal differential device.RCD is a term which covers a group of similar devices.
RCCB (residual current circuit breaker) works only on residual current and is what is often referred to as an RCD
RCBO (residual current breaker with overcurrent protection) combines residual current and overcurrent operation, often described as an MCB and RCD combined.
SRCD (socket outlet incorporating residual current device) does what it says on the tin (similar to a USA GFCI outlet but does not provide any protection for outgoing cables which I believe they do in the USA)
There are others, but I can remeber all the acronyms or what the stand for, CBR, RCM spring to mind.
All of them are RCDs,
Yes, but at 500 mA, Explanation by Legrand here translated:So really more of an RCBO?
Hager also make them and are available in Ireland, but can't find them in the UK: Electrical Wholesaler - Hager RCBO 2P(1P+N) 6kA B-32A 30mA A Class AD932J - https://www.electricalwholesaler.ie/products/switchgearfuses/switchgeardistribution/rcboresidualcurrentbreakers/hagerrcbo2p1pn6kab32a30maaclassad932j, but anyone thinking of using them in France the connections are reversed and the use of strip bus bars will not work due to the top connections not being staggered, however for use in the UK there is a staggered bus bar that fits across the bottom, if I remember correctly it's 13mm pitch.Realistically the SP+SN option for RCBO is more useful in terms of allowing N-E faults to be identified and circuit testing performed without much cable removal. but not very common outside of the Wylex/Crabtree ones in the UK it seems.
This is because in overhead networks the neutral can become disconnected?France is rather spread out with small villages miles apart, so it's more cost effective to run the power supplies overhead as phase and neutral and legislate that it's the occupiers responsibility to provide the earth, different in large towns, but even they are spread out.
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