TT Isolation

[chiplard]

Pg 33 of OSG 5.1 para (ii) refers to isolation for every circuit. For a TT system it states that for every circuit or group of circuits that may need isolating without interrupting the supply to other circuits, the isloating device must switch all LINE conductors ( other than at origin ).

Can someone confirm that a split load ( dual RCD ) CU meets this criteria. Obviously you can isolate 1 half of the board without interrupting the other half but you cannot isolate all LINE conductors for each individual circuit without interrupting another circuit on the same RCD.

Any thoughts greatly appreciated.

 

[Spudnik]

If you look at the Reg that it refers to, the OSG is not exactly correct IMO.

As far as im concerned, yes, in a TT system, there should be a means of isolating both live conductors, which is the main switch.

On a TT CU change i did for my assessment last year, i wasnt even questioned on it by my assessor.

Sintra is your man for this really.

 

[chiplard]

Just looked at the reg and I agree but Reg 537.2.1.1 refers to a TN-C-S or TN-S not requiring isolation of a Neutral conductor if it has a suitably low impedance to earth.

Is it me is this an open ended statement. Where would this stand with a TT system.

It seems open to various interpretations!!

 

[Spudnik]

It is indeed and this is what makes this job so hard!

The way I see it is that in a domestic situation, a normal cu with a DP main switch meets the requirements.

 

[chiplard]

Reading 537.2.1.1 i have to agree. Lesson learnt, The OSG is a GUIDE, the BRB is the BIBLE (the ambiguous Bible that is) !!

 

[pushrod]

Pg 33 of OSG 5.1 para (ii) refers to isolation for every circuit. For a TT system it states that for every circuit or group of circuits that may need isolating without interrupting the supply to other circuits, the isloating device must switch all LINE conductors ( other than at origin ).

Can someone confirm that a split load ( dual RCD ) CU meets this criteria. Obviously you can isolate 1 half of the board without interrupting the other half but you cannot isolate all LINE conductors for each individual circuit without interrupting another circuit on the same RCD.

Any thoughts greatly appreciated.

Your original quote is wrong - for a TT system all live conductors need to be interruptible, eg a double pole isolator. What the OSG is saying is that if you need to isolate some circuits, say for maintenance, with a TN system then a single pole device is acceptable but with a TT system any isolator that is used for such isolation has to be double pole. However at the origin of the installation in a normal domestic situation it has to be double pole regardless of whether it is TN or TT.

It is not saying that all individual circuits on a TT system need DP isolators, unless you want to be able to regularly switch that individual circuit off. I think the clue is in the word "may" from your quote. If you need to work on a circuit in a TT installation then it may well be that you are switching it off at the DP main switch.
It seems you need a degree in English to interrpret some of these regs

 

[chiplard]

I did mean LIVE conductors otherwise there was no point starting the thread!!!

Thanks for clearing that up for me pushrod, been a great help.

 

[Vessy]

I read it as "circuit or group of circuits that may have to be isolated WITHOUT interrupting supply to other circuits" not many in domestic properties that cannot be isolated during work. so the isolator or RCD meet regs.

Wish they would write the regs so they make simple sense!!

If you look at page 24, figure 3.2 would be wrong! I suspect one day RCBO's will be the min requirement.

 

[La Poste]

I suspect one day RCBO's will be the min requirement.

I agree and double pole ones at that.