Why is split RCD not appropriate for TT?

[westward10]

Prior to the non combustible consumer unit the use of a metal cased consumer unit without prior rcd protection on a TT earthing system was a big no no in fact the NICEIC used to make a big deal of it, it would be Catergory 1 on a periodic. Things change.

 

[buzzlightyear]

Thats what the thread was about why it's ok with rcbos and not ok using split rcds

Where split consumer units are used with a separate double-pole main switch and 30 mA RCCBs to protect the final circuits, the interconnecting cables which connect the load side of the main switch to the supply side of the RCCBs must be provided with double or reinforced insulation to comply with Regulation 531.3.5.3.2.201.
If this isn’t the case, this particular arrangement wouldn’t be suitable for TT installations because the interconnecting cables wouldn’t be provided with adequate fault protection. If the metal enclosure became live due to a fault, it would cause considerable inconvenience as RCCBs don’t have integral overcurrent protection.

A recommended consumer unit arrangement with RCBOs which is suitable for use on TT installations .

 

[mainline]

Where split consumer units are used with a separate double-pole main switch and 30 mA RCCBs to protect the final circuits, the interconnecting cables which connect the load side of the main switch to the supply side of the RCCBs must be provided with double or reinforced insulation to comply with Regulation 531.3.5.3.2.201.
If this isn’t the case, this particular arrangement wouldn’t be suitable for TT installations because the interconnecting cables wouldn’t be provided with adequate fault protection. If the metal enclosure became live due to a fault, it would cause considerable inconvenience as RCCBs don’t have integral overcurrent protection.

A recommended consumer unit arrangement with RCBOs which is suitable for use on TT installations .

Maybe a good time for you to read the complete thread, so as to avoid saying the same things.

 

[buzzlightyear]

Maybe a good time for you to read the complete thread, so as to avoid saying the same things

I have read the post, but some peaple
Would like it clarifyed what should be done.

 

[Dartlec]

The same reg also mentions split RCD though so i don't think it means RCBO's are required in residential premises.

For Class I enclosures in TT systems where RCD protection is used on outgoing circuits, double
or reinforced insulation of all live conductors (incoming cables, extension terminals, etc.) on the supply side of the
incoming device, e.g. main switch, shall be used. Insulated and non-metallic sheathed cables are deemed to meet the
requirements of double or reinforced insulation.

The added note shows how often they have to fudge things to clear up inconsistencies, like meter tails not technically being double insulated OR having reinforced insulation.

I wonder whether, if they started again from scratch, they could come up with a book about 1/4 of the length of the current ones, just by being able to not have to clear up all the loopholes that develop over time.

I imagine that is likely impossible now though, with all the cross links to other standards and definitions.

 

[nicebutdim]

It makes it compliant with regard to isolation

every circuit...that may have to be isolated without interrupting the supply to other circuits

should be provided with its own isolating device.

The device must switch all live conductors in a TT system

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

 

[Dartlec]

every circuit...that may have to be isolated without interrupting the supply to other circuits

should be provided with its own isolating device.

The device must switch all live conductors in a TT system

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

Every circuit that may have to be isolated without interrupting the supply to other circuits

I guess there is an argument as to whether any circuit in a domestic dwelling has to be isolated without interrupting the supply to other circuits.

In the absence of key life support systems etc, is there any circuit that really can't be isolated if necessary by turning everything off? Inconvenient obviously, but is that enough to require anything more in terms of that reg?

Fixed appliances can be isolated with the appropriate switch if needed, (if they have been installed of course), so it's the circuit that's being discussed in that regulation.

Of course RCBOs are the answer in almost all cases - especially now that many of them are changing to 1P & switched neutral. Cost is also becoming less and less of a reason to choose them. But the Regulations specify a minimum standard that is considered acceptable, so it's still useful to discuss where that standard is - something that isn't always as clear as it should be...

 

[mainline]

every circuit...that may have to be isolated without interrupting the supply to other circuits

should be provided with its own isolating device.

The device must switch all live conductors in a TT system

DP main switch meets requirement for swithcing of all live conductors. Perhaps I'm tired and unable to see what's in front of me, but how does this meet the requirement that every circuit be provided with "its own" isolating device?

That statement I think is the OSG interpretation

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 prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributor's incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices


“Provision may be made for isolation of a group of circuits by a common means”

 

[swaRRR]

That statement I think is the OSG interpretation

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 prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributor's incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices


“Provision may be made for isolation of a group of circuits by a common means”

Where are you getting this statement from? I've searched the regs and that wording isn't in there.

 

[mainline]

Where are you getting this statement from? I've searched the regs and that wording isn't in there.

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

 

[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