Why is split RCD not appropriate for TT?

[swaRRR]

Isolation and Switching Chapter 53 BS 7671

Key Points of… Isolation: 537.2 Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to preven…

the-regs.co.uk

It doesn't say that on the link you provided, either. So where are you getting the quote from?

The only rebuttal to what the regs clearly says has so far been something that's not in the regs at all. IMO it confuses things greatly. Could we stick to what's in the actual brown book instead of someone else's interpretation of it? Think it would help fuel the discussion instead of confusing matters.

 

[mainline]

It doesn't say that on the link you provided, either. So where are you getting the quote from?

Do you mean the

Provision may be made for isolation of a group of circuits by a common means, ?

 

[swaRRR]

Do you mean the

Provision may be made for isolation of a group of circuits by a common means, ?

Yes you're using that quote as the basis for saying the quoted reg isn't 100% correct but the quote doesn't appear anywhere in the regs.

 

[Pretty Mouth]

With RCBO's the supply is via the busbar. It's in a fixed position so can't come into contact with the casing.

Just thought I'd offer another perspective on this:

The busbar is fixed, but uninsulated, in an enclosure containing a number of sleeved CPCs. Sleeving, I'm told, is for identification only and isn't officially rated as an insulator (though real world I'm pretty sure it does offer some level of insulation). Anyway, the uninsulated CPCs could make contact with the uninsulated busbar, resulting in a L-E fault that won't disconnect.

This makes me think that this risk for TT installations is possibly worse for an RCBO board, vs a dual RCD board, where the cables feeding the RCDs at least have proper insulation.

 

[mainline]

Yes you're using that quote as the basis for saying the quoted reg isn't 100% correct but the quote doesn't appear anywhere in the regs.

I can't find where it is and i haven't got the book, but it says:

“Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this”

 

[mainline]

Yes you're using that quote as the basis for saying the quoted reg isn't 100% correct but the quote doesn't appear anywhere in the regs.

Things are starting to get a bit confusing, what quoted reg have I said isn't correct ?

 

[cireland]

Just thought I'd offer another perspective on this:

The busbar is fixed, but uninsulated, in an enclosure containing a number of sleeved CPCs. Sleeving, I'm told, is for identification only and isn't officially rated as an insulator (though real world I'm pretty sure it does offer some level of insulation). Anyway, the uninsulated CPCs could make contact with the uninsulated busbar, resulting in a L-E fault that won't disconnect.

This makes me think that this risk for TT installations is possibly worse for an RCBO board, vs a dual RCD board, where the cables feeding the RCDs at least have proper insulation.

The same could be said for a dual RCD board.

 

[swaRRR]

I can't find where it is and i haven't got the book, but it says:

“Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this”

It doesn't, i've searched an electronic version of the regs and that's not in there.

The reg i quoted was written verbatim from the book.

 

[Pretty Mouth]

The same could be said for a dual RCD board.

The busbars would be RCD protected on the dual RCD board wouldn't they? But not for the RCBO board (this all assuming TT, and no upfront RCD)

 

[cireland]

The busbars would be RCD protected on the dual RCD board wouldn't they? But not for the RCBO board (this all assuming TT, and no upfront RCD)

Yep. My mistake.
Too early.
Need coffee.

 

[mainline]

Just thought I'd offer another perspective on this:

The busbar is fixed, but uninsulated, in an enclosure containing a number of sleeved CPCs. Sleeving, I'm told, is for identification only and isn't officially rated as an insulator (though real world I'm pretty sure it does offer some level of insulation). Anyway, the uninsulated CPCs could make contact with the uninsulated busbar, resulting in a L-E fault that won't disconnect.

This makes me think that this risk for TT installations is possibly worse for an RCBO board, vs a dual RCD board, where the cables feeding the RCDs at least have proper insulation.

I think that the cables feeding the rcds are the problem, not the busbars.


Regulation 531.3.5.3.2.201 requires that, where Class I enclosures are used (that is, earthed metalwork) on TT systems with RCD protection on outgoing circuits, all live conductors on the supply side of the RCD – that is, the internal cable links – must have double or reinforced insulation.


As each RCBO is supplied by a busbar, the risk of the busbar becoming loose and making contact with the metal enclosure is minimal.

 

[mainline]

It doesn't, i've searched an electronic version of the regs and that's not in there.

The reg i quoted was written verbatim from the book.


Yes you're using that quote as the basis for saying the quoted reg isn't 100% correct but the quote doesn't appear anywhere in the regs.

Like I said earlier, I'm a bit confused as to what part of the regs I'm saying is wrong ?
I only have the 17th edition green book to hand, but it's certainly in there.

537.2.1.2 : Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this

 

[Dartlec]

Like I said earlier, I'm a bit confused as to what part of the regs I'm saying is wrong ?
I only have the 17th edition green book to hand, but it's certainly in there.

537.2.1.2 : Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this

It appears to have moved in the 18th to 422.3.13:

422.3.13​

Except as permitted by Regulation 461.2, every circuit shall be provided with a means of isolation from all live supply conductors by a linked switch or a linked circuit-breaker. NOTE: Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this. ​

 

[swaRRR]

It appears to have moved in the 18th to 422.3.13:

422.3.13​

Except as permitted by Regulation 461.2, every circuit shall be provided with a means of isolation from all live supply conductors by a linked switch or a linked circuit-breaker. NOTE: Provision may be made for isolation of a group of circuits by a common means, if the service conditions allow this. ​

I stand corrected.

So it's a case of EVERY circuit SHALL be provided with a means of isolation, except for when they aren't. And make sure you don't read the other contradictory rule about this in a completely different section of the book where it says each individual circuit must be individually switched. Clear as custard once again.

The more i read these books the more i think the IET aren't fit for purpose - contradictions, ambiguity, spelling errors abound. Even some of their very important equations needed for testing are wrong in the books.

No wonder the rest of the world laughs at our electrical practices.

 

[mainline]

I stand corrected.

So it's a case of EVERY circuit SHALL be provided with a means of isolation, except for when they aren't. And make sure you don't read the other contradictory rule about this in a completely different section of the book where it says each individual circuit must be individually switched. Clear as custard once again.

The more i read these books the more i think the IET aren't fit for purpose - contradictions, ambiguity, spelling errors abound. Even some of their very important equations needed for testing are wrong in the books.

No wonder the rest of the world laughs at our electrical practices.

But if everything was as clear as glass, there wouldn't be forums like this where gentlemen discuss these things in a friendly, happy manner.

 

[swaRRR]

But if everything was as clear as glass, there wouldn't be forums like this where gentlemen discuss these things in a friendly, happy manner.

Hey it's been a good discussion i think!

My problem was i thought you were referring to the sentence being in the same reg, i didn't know it'd be in a completely different part of the book!

I doth my cap to you; you were right.

 

[davesparks]

I stand corrected.

So it's a case of EVERY circuit SHALL be provided with a means of isolation, except for when they aren't. And make sure you don't read the other contradictory rule about this in a completely different section of the book where it says each individual circuit must be individually switched. Clear as custard once again.

The more i read these books the more i think the IET aren't fit for purpose - contradictions, ambiguity, spelling errors abound. Even some of their very important equations needed for testing are wrong in the books.

No wonder the rest of the world laughs at our electrical practices.

There aren't as many errors or contradictions in BS7671 as you seem to think.
There is often more of an issue with people not reading it properly, not understanding the language used and sometimes people will just read what they think is written and not actually what is there.

Where in the rest of the world are our electrical practices laughed at? We are far ahead of a lot of countries in electrucal safety and standards

 

[timhoward]

The more i read these books the more i think the IET aren't fit for purpose - contradictions, ambiguity, spelling errors abound. Even some of their very important equations needed for testing are wrong in the books.

A more general point...
It's very difficult to take the 'painting by numbers' OSG (which is a simplified set of rules that can be followed exactly to comply with BS7671) and then try and work backwards to BS7671. Trying to guess the full version from a particular method of complying is almost like trying to reverse engineer the regs.

I took a break from the industry during most of the 17th edition era, and hence fairly recently did an 18th edition course. I was actually strongly advised to NOT read the OSG until I'd passed the exam, as the layout and manner in which it sets out things is not conducive to learning the regs. I think it was good advice!

The brown book for all it's complexity is laid out in a certain structured way, and learning to think in terms of this structure is the best way forwards. So I guess what I'm saying is that should be where the focus goes.

(I'm far from an expert on the regs, but I believe I understand the structure and then it's a case of slowly filling in the blanks!)

 

[Dartlec]

A more general point...
It's very difficult to take the 'painting by numbers' OSG (which is a simplified set of rules that can be followed exactly to comply with BS7671) and then try and work backwards to BS7671. Trying to guess the full version from a particular method of complying is almost like trying to reverse engineer the regs.

I took a break from the industry during most of the 17th edition era, and hence fairly recently did an 18th edition course. I was actually strongly advised to NOT read the OSG until I'd passed the exam, as the layout and manner in which it sets out things is not conducive to learning the regs. I think it was good advice!

The brown book for all it's complexity is laid out in a certain structured way, and learning to think in terms of this structure is the best way forwards. So I guess what I'm saying is that should be where the focus goes.

(I'm far from an expert on the regs, but I believe I understand the structure and then it's a case of slowly filling in the blanks!)

A lot of sense in that approach.

The OSG is not so much a guide to the regs as a simplified set of standard installation methods that will comply (within the limitations given).

It's also true though that the Regs have become a bit of a behemoth, partly because of the way additions and notes have been added to 'clarify' things, to the point where it is not always clear what the requirements actually mean.

It's not helped by the interconnection between it and many other standards, which most people have no access to due to cost.

Then you have the various electrical bodies guidance put out which doesn't always agree or in some cases is conflicting, leading to unnecessary confusion.

 

[Tart83]

The busbars would be RCD protected on the dual RCD board wouldn't they? But not for the RCBO board (this all assuming TT, and no upfront RCD)

this chap has made a very good point i hadnt thought of before