OSG. Page 33. Is this right?

[simonspark]

Hello all.
The other day I was leafing through the red OSG and for no reason started to read chapter 5, isolation and switching. At point ii for every other circuit it says blah blah blah......... The device must switch all live conductors in a TT system and all line conductors in a TN system.
The latter point I have no problem with. It's the former. Does this mean ( you should read the full paragraph to be honest. I couldn't be bothered to type it!) that in a TT system all circuit breakers should be double pole? It certainly reads that way in my eyes.
I rarely have any dealings with TT systems, so I don't get involved. But one day I may need to know the answer to this. Anyone?
Simon

 

[SimpleSimon]

Your main switch at your consumer unit which is double poles will satisfy this point.

 

[claire_r]

Basically when working on tt turn the lot off!

 

[simonspark]

No. It doesnt say that. Thats what I always thought. Ok, I'm gonna type out the whole bloody thing.
ii Other than at the origin of the installation, every circuit or groups of circuits that may have to be isolated without interrupting the supply to other circuits should be provided with it's own isolating device. This device must switch all live conductors in a TT system and all line conductors in a TN system.

See what I mean?

 

[Risteard]

every circuit or groups of circuits that may have to be isolated without interrupting the supply to other circuits should be provided with it's own isolating device.

Do any circuits need to have the facility to isolate without isolating other circuits though?

 

[telectrix]

Do any circuits need to have the facility to isolate without isolating other circuits though?

that's what your MCBs are for, as well as OCP.

 

[Risteard]

that's what your MCBs are for, as well as OCP.

Not in a TT system - a single-pole device is not suitable for isolation. Hence the main switch.

 

[Knobhead]

Am I missing something here? Line and live? Has someone redefined neutral as live?

 

[Risteard]

Am I missing something here? Line and live? Has someone redefined neutral as live?

Neutral has always been live.

 

[simonspark]

Risteard. Sorry mate but Ive already typed this once before and I don't know where it went! But as I tried to say (puff!) Yes. According to the wording of the paragraph. It also cross references Regulation 537.2.1.1. Whatever that may be

 

[telectrix]

interesting... looking at page 34 of the 16th ED. OSG, ( amended 2004 ) the same paragraph reads " all live conductors in a TT system and all phase conductors in a TN system. confusing, huh. all depends on your definition of live conductor.

it would appear to suggest that for TT systems, double pole MCBs are required.

 

[Risteard]

Haven't the book on me to lookup that Reg but the point I was making is that perhaps it is not necessary for you to be able to isolate a circuit separately from the other circuits and therefore it does not apply.

 

[Risteard]

all depends on your definition of live conductor.

Not really. BS 7671 has always declared that the neutral conductor is a live conductor as it is intended to carry current. It states that, by convention, a PEN conductor is not considered a live conductor however.

 

[simonspark]

A live conductor is phase or neutral. Line is phase.

 

[Des 56]

I think the theme of Risteards reply gives reason why single pole mcbs are ok
The circuits can be isolated at the main switch and need not rely on individual mcbs as isolators

 

[simonspark]

Also, it states 'OTHER THAN AT THE ORIGIN OF THE INSTALLATION', which rules out the main switch

 

[SimpleSimon]

No. It doesnt say that. Thats what I always thought. Ok, I'm gonna type out the whole bloody thing.
ii Other than at the origin of the installation, every circuit or groups of circuits that may have to be isolated without interrupting the supply to other circuits should be provided with it's own isolating device. This device must switch all live conductors in a TT system and all line conductors in a TN system.

See what I mean?

See what contradicts in this sentence is "circuits and groups of circuits...without interrupting other circuits" so a group of circuits could be classed as your CU circuits another round in circles regulation.....

 

[Des 56]

"Other than at the origin" would mean to me to be, elsewhere double pole would be required,perhaps a remote distribution board

The mcbs are not isolators,if they were to be considered as isolators then yes they would have to be double pole,but they are not,the isolation is the main switch

 

[ackbarthestar]

My understanding is that all TN systems have, at some point, a grounded neutral; In TT systems the earth is separated by the use of a completely separate earth path, this may cause the Neutral to rise beyond the approximate 0V which would have been created by a TN system and hence becomes a true live conductor if not isolated.

 

[Knobhead]

No wonder I get confused with domestic!