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

 

[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

 

[scotsparky]

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.

 

[vernam616]

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

 

[simonspark]

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.

 

[Knobhead]

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.

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

 

[simonspark]

Whats a £SD

 

[telectrix]

used to be a little white pill

 

[telectrix]

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.

if only carlsberg made CUs.

 

[JUD]

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

It's in the definitions....

Live conductor (see Live part).

Live part A conductor or conductive part intended to be energised in normal use, including a neutral conductor but, by convention, not a PEN conductor.