OSG. Page 33. Is this right?

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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
 
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?
 
simonspark said:
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.
Click to expand...

Do any circuits need to have the facility to isolate without isolating other circuits though?
 
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
 
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.
 
Last edited:
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.
 
telectrix said:
all depends on your definition of live conductor.
Click to expand...

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.
 
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 said:
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?
Click to expand...

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.....
 
"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
 
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.
 
simonspark said:
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
Click to expand...

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.
 
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!
 
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.
 
reading between the lines, it is saying that on a TT system there must be the facility for isolating both live conductors, i.e. a double pole isolator, but in a TN system the main switch/isolator can be single pole.
 
the neutral is a live conductor as it carries current , but it is not a line conductor ( phase conductor in english, before the eurotrash interfered)
 
simonspark said:
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!
Click to expand...

I don't have a problem with a neutral conductor being live since by definition it carries current under normal conditions.
But I think we are not talking about touching a disconnected neutral while the circuit is still energized. That's painful, but after the single or triple pole isolator has been turned off.
 
don't you just love this type of discussion. beats watching constipation street.
 
simonspark said:
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
Click to expand...
Yes, they should be double pole, if they are intended to be used for isolation purposes.
Unfortunately such devices are very rare in the UK, very common in Europe.
 
OK I may be off track with this, but from your comments due to the potential voltage difference between neutral and earth on a TT system a MCB should break both the line and neutral. I can see the logic of it. So are all recently install 17[SUP]th[/SUP] edition boards on a TT systems technically wrong then?

Out of interest are RCBO’s PD or SP?

Now how do you convince the customer that you fitted a nice shiny new board for last week that he now needs a new board!

Europe strikes again! We’ll end up with those monstrous DP boards!
 
I think as long as the front end isolator isolates all the live conductors as opposed to just the line conductors then there would be no need to provide extra DP RCBOs. All single phase DBs are likely to have double pole switching.
 
Interesting post.

If you look at pages 24-26 as well, the suggested diagrams also seem to contradict that which simonspark has pointed out, as in, they show the layout of every new TT board in the country as Tony points out (btw Tony RCBO's are single pole .... ? 99% anyway).

I have also seen the double pole MCB/RCBOs in a french made van :)

They look a bit like a contactor with tiny little terminals. And a switch on of course.

So not like a contactor at all.



I wonder what the answer to this is. Maybe its just that they are guidelines and this particular one has fell by the wayside (apart form their being a DP isolator up front).
 
The Regs allow us to isolate a group of circuits for isolation, its for the designer to make a risk assessment and consult with the client and decide on how safe isolation shall be achieved. If this was a say a working farm how convenient and safe will it be to isolate a group of circuits. For safety and for minimal disruption individual circuit isolation may be appropriate or it may not.

Regards Chris
 
Until the thread it’s something I’d not considered. There is the possibility of a considerable PD between neutral / earth. It does throw even more reliance on to the RCD, which I’m not keen on in the event of a fault. With a line / earth fault you have the MCB to back up the RCD. A neutral / earth the entire fault is in the RCD’s domain, and considering the PD a large fault current can ensue. DP MCB’s are beginning to make sense now.
With the number of faulty RCD’s you guy’s report I wouldn’t be happy.

Just glad I don’t live out in the sticks now.
 
Hi Tony,

I believe the regs book does say somewhere that the neutral conductor is treated as if it was a live conductor.

Best wishes

Rex
 
Well not got the BRB to hand so had a look at my Shinney BGB and in that it Reg 537.2.1.1
says Every circuit shall be capable of being isolated from each of the live supply conductors . Ina a TN-S or TN-C-S system the it is not necissary to isolate or switch the neutral conductor where it is deemed to have a reliably connection to earth by a suitably low impedance

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



What i take from that is on TN systems you can islolate phase conductors only but TT its Phase neutral and 1 switch can cover all.
 
Would explain why the porter cabins that i wire a temporary supply up to, all have double pole mcb's installed in the internal D/Bs
 
Double pole mcb's are brilliant. Hager makes a board that uses a twin bus bar. One live, one neutral. These are offset so that you just drop the dp mcb (or rcbo or cartridge type fuse holder) onto the twin bars.The dp breakers take up the same space as a single pole one. No neutral bar required. No more huffing and puffing trying to work out which neutral goes with which phase when testing.
All boards should be made this way. And, except in certain circumstances, mcb's should be done away with and every final circuit should be protected with rcbo's.
 
simonspark said:
Double pole mcb's are brilliant. Hager makes a board that uses a twin bus bar. One live, one neutral. These are offset so that you just drop the dp mcb (or rcbo or cartridge type fuse holder) onto the twin bars.The dp breakers take up the same space as a single pole one. No neutral bar required. No more huffing and puffing trying to work out which neutral goes with which phase when testing.
All boards should be made this way. And, except in certain circumstances, mcb's should be done away with and every final circuit should be protected with rcbo's.
Click to expand...

I wondered how long it would take the £SD brigade to jump on the bandwagon!
 
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