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

 

[telectrix]

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.

 

[telectrix]

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]

I know that. thats what Ive been going on about. Can anyone here hold their hands up and remember my initial query. I cant!

 

[ackbarthestar]

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!

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.

 

[telectrix]

don't you just love this type of discussion. beats watching constipation street.

 

[ian.settle1]

Oh sod it who's for a pint in the arms telectrix is paying:willy_nilly:

 

[SimpleSimon]

Oh sod it who's for a pint in the arms telectrix is paying:willy_nilly:

Would love to Ian. But the moderators haven't put my application through *cough*cough* lads

 

[Deleted member 26818]

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

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.

 

[Knobhead]

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!

 

[ackbarthestar]

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.

 

[SPARTYKUS]

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).

 

[Chr!s]

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

 

[Knobhead]

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.

 

[RexT]

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

 

[SPARTYKUS]

Don't worry Tony, with all the copper robbing going on you too will have your very own TN surge or difference in potential fault