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

 

[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!

 

[simonspark]

MCB's are isolators. And functional switches.

 

[telectrix]

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

basically it says that in a TN system, the neutral need not be switched. it seems to me that the para in the osg means that in a TT system, isolation of both live conductors can be achieved by the D/P main switch.

 

[Risteard]

MCB's are isolators. And functional switches.

MCBs can be used as isolators (where appropriate). They don't have to be though.

 

[simonspark]

Ackbarthestar. Doesnt matter if its a tn or tt sysyem. All neutrals are live. Disconnect a neutral of an energised circuit at a consummer unit with 23 5ft flu's on it. Then touch it. It's live!

 

[simonspark]

Telectrix. It says ​other than at the origin of the installation.

 

[Mark_Burgess]

Ackbarthestar. Doesnt matter if its a tn or tt sysyem. All neutrals are live. Disconnect a neutral of an energised circuit at a consummer unit with 23 5ft flu's on it. Then touch it. It's live!

Sounds like experiance talking there!!

 

[telectrix]

the BRB considers that the neutral in a TN system is reliably connected to earth by a suitably low impedance. simon, it's only a disconnected neutral that can become live, by virtue of being disconnected from the supply, it's no longer a neutral.

 

[simonspark]

Mark. I didnt know the circuit had a time clock built in to overide the switch. I was young!!!

 

[SimpleSimon]

Telectrix. It says ​other than at the origin of the installation.

This is the OSG, does the BGB use the same wording?
The OSG is an interpretation of the regs, some things can be taken with a pinch.

 

[simonspark]

But it it is officailly a live conductor. Its not me saying this, its the Regs