RCBO's on TT

[steptoe]

sorry Batty, for not being so clear, and M107 please take note as well.

1, NICEIC do NOT make the REGS. (ie, BS7671)

2. BS7671 is NOT statutory

3. YOU are the person that has to stand in court and say that your wiring did not kill the little kid, hiding behind 7671 is a good defence but not a good get out for your head. and I would really like to find/speak to/ask the NIC guy what his reasoning is behind NOT requiring to disconnect the neutral.

although 7671 is NOT legal a good/reasonable defence in law can be made by complying with them, as I dont really know them inside Im sorry Batty as to be able to quote numbers, but Im sure it states that the live must be disconnected in a fault condition, and that includes neutral, as now classed a live conductor.

ask the DNO why they specify disconnection of N on TT and wait for the answer,(they may even ask if you should be asking that question if you dont know the answer)in a polite(!?) way of course.

if no one else can confirm I will try find the number in the comic on sunday(my day off),

otherwise, I hope this helps.

 

[Apache]

I'm really enjoying this thread

 

[steptoe]

I'm really enjoying this thread

paddy, if I send you a copy of the 7671 will you look up numbers for me?

 

[Apache]

paddy, if I send you a copy of the 7671 will you look up numbers for me?

no!

 

[Andy™]

no!

If i sent you a CD of it, with full search feature, would you look up numbers?!

 

[Apache]

If i sent you a CD of it, with full search feature, would you look up numbers?!

ok

 

[steptoe]

but Im more entertaining(argumentative)

 

[Apache]

but Im more entertaining(argumentative)

Damn right

 

[M107]

Nope cant find any regs stating dp rcd is required on TT system.

Did find this

431.2.1 TN or TT system

Where the csa of the neutral conductor is at least equal to or equivalent to that of the line conductors, it is not necessary to provide overcurrent detection and an associated disconnecting device for the neutral conductor.

Steps I know niceic dont write the regs but seen as they are my chosen scheme operator I thought I'd run the question past them.

 

[batty]

There are certain installs that do require double pole RCD protection ie caravan parks, jettys and a few others in the regs.

Batty

 

[binky]

but Im more entertaining(argumentative)

For the sake of a good argument then....

Neutral is obviously the return path for the line conductor, and therefore when under use is live, however when the line conductor is disconnected neutral is not in service and although you may get some back eddy votages from the rest of the circuits, it is not actually carrying a full 240V, and can therefore be considered as not being a live conductor, ergo single pole RCBO is perfectly adequate for purpose.

 

[Apache]

For the sake of a good argument then....Neutral is obviously the return path for the line conductor, and therefore when under use is live, however when the line conductor is disconnected neutral is not in service and although you may get some back eddy votages from the rest of the circuits, it is not actually carrying a full 240V, and can therefore be considered as not being a live conductor, ergo single pole RCBO is perfectly adequate for purpose.

Now Binky I would believe you if we were talking about DC, but with AC the live is carrying as much as the neutral as the electrons are moving back and forth. Therefore something could become live from the neutral?

 

[binky]

More likely to be live from a borrowed neutral, but overall current flows from high potential to low potential - think of it as being like water flowing through a pipe, otherwise Neutral would as live as Line, and I would have been belted many times whilst changing light fittings switched off at the wall plate only.

 

[steptoe]

but if we are talking about SP RCBOs (multiple) then we have multiple neutrals to consider, not just the one belonging to the circuit with the fault on, so we can therefore have a backfeed present.

I will deffo have a look tomorrow and find the reference. (unless its in my (big)head!!!)

binky, as I said, under normal circumstances neutral can be considered as at the same potential as earth, its under fault conditions on TT that it needs to be disconnected, remember the fault current will take the easiest path to ground, similar to the reasons we do NOT export TNCS.

 

[binky]

I take it you mean with no earth connected steps, but that's no different to any other system, especially TNC-s

 

[steptoe]

I take it you mean with no earth connected steps, but that's no different to any other system, especially TNC-s

even with an earth connected binky you will generally find on TT the neutral will still provide a path of less resistance to earth than the spike/rod/electrode. remember the neutral is still earth at the transformer so will almost always (in my experience) be less resistance than a rod.

 

[steptoe]

I take it you mean with no earth connected steps, but that's no different to any other system, especially TNC-s

the reason I have replied to this separately is that it is a world apart, and one of the reasons I hate TNCS,

the fact you are relating no neutral on a TT to having no neutral on a TNCS shows a massive lack of knowledge on supply systems.

NO neutral on a TNCS means you have no earth, hence the possibility of everything metallic in the dwelling going to a potential of 230v, or 400v if 3ph.

no neutral on a TT means the DP protective device will have tripped off saving any fault possibility. (imbalance between line&neutral)

does anyone else agree with me or am I still talking sh1te?

 

[green-hornet]

I agree with the imbalance between live and nuetral.

TNC-S systems cause more problems especially when trying to export an earth which I have seen some electricians do.

If I had my way I would make all installations tt. I would make builders to ulra the foundation metal cages and plate or spike the whole area.Problems solved no great differentials and a better earthing arangement for additions than pme.

 

[steptoe]

I agree with the imbalance between live and nuetral.TNC-S systems cause more problems especially when trying to export an earth which I have seen some electricians do.

If I had my way I would make all installations tt. I would make builders to ulra the foundation metal cages and plate or spike the whole area.Problems solved no great differentials and a better earthing arangement for additions than pme.

couldnt agree more,

 

[Apache]

I can see steps and horny are going to get on.............

:x