CHESTNUT

[ban-all-sheds]

However, my point is that one could use the presence of the BS1362 fuse(s) (certainly for a single socket, possibly/probably for a double one) to indicate that that the spur cable was "not likely to carry overload current".

Sorry, John - I'm really struggling to understand how one could (or would wish to) not use the presence of BS 1362 overload protection devices to show that the cable had overload protection...

If one did that, then, provided fault protection was OK, there would (per regs) be no need for any overload protection, hence 433.2.2 (number hasn't changed), with its 3m limit, would become irrelevant.

But you can't, you don't need to and you would never want to - it is those BS 1362 fuses which give you your overload protection...

As for the fault protection, I need to think about what you've said but, on the face of it, I can't see why achieving an adequate degree of fault protection should be any more difficult in this situation than it is for any other circuit (e.g. a 20A socket radial) using 2.5mm² cable.

Well in that example it isn't, and that is the whole thrust of this topic - a 20A radial using 2.5mm² cable had damn well better be protected from overload just like a 2.5mm² spur from a ring final by the fuses in the plug(s) or FCU, and from fault currents by the upstream device. And in the absence of tailored adiabatic calculations, that fault protection requires, inter alia, that the cable be no more than 3m long.

 

[JohnW2]

We should get a point of accuracy resolved - yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2, and if you have 6kA devices or manufacturers data for 10kA ones (k²S² for a 2.5mm T/E when considering a L-N fault is 82,656 which is less than the requirement for 10kA devices in BS EN 60898).
For L-N [I presume you meant L-E] faults k²S² drops to 29,756 - too low even for a 6kA device, so you're stuffed if there's no RCD (and possibly if there is - I don't know what the let-through is for those).

I agree with your figures above for k²S² for the 2.5mm² L/N conductors and the 1.5mm² CPC. However, maybe I'm missing something, but I don't see that those figures even remotely represent a problem ...

... if one takes the worst-case PSC/PSCC 'by enquiry' (230V/0.35Ω = 657A), then my understanding was that the let-through for a B32 MCB at that current would be almost an order of magnitude less than those k²S² figures, even for the CPC - as far as I am aware, I²t is of the order of 3,000 A²sec for a B32 at 657A. Indeed, I think that even at the maximum rated current (obviously 6,000A!) of a 6kA MCB, the let-through is only about 35,000 A²sec. If my belief is remotely correct, then a B32 would give more than adequate fault protection to even 1mm² T&E, let alone 2.5mm². Am I mistaken in my beliefs?

Unless I' misunderstanding something, I don't understand your "82,656 which is less than the requirement for 10kA devices in BS EN 60898 - are you sure you're not thinking of the maximum let through at the maximum rated current (i.e. 10,000A), rather than the actual let-through which would occur with a realistic domestic PSC/PSCC ?

Kind Regards, John.

 

[JohnW2]

.... as far as I am aware, I²t is of the order of 3,000 A²sec for a B32 at 657A. Indeed, I think that even at the maximum rated current (obviously 6,000A!) of a 6kA MCB, the let-through is only about 35,000 A²sec. If my belief is remotely correct, then a B32 would give more than adequate fault protection to even 1mm² T&E, let alone 2.5mm². Am I mistaken in my beliefs?

I've just had a look at let-through curves for a number of MCBs and, unless I'm misunderstanding them, what I wrote yesterday appears to be essentially correct. If I'm right, then the bottom line seems to be that that any 'domestic' Type B MCB appears to provide adequate fault protection for any size of T&E with any PFC/PSCC likely to be found in a domestic installation - even, for example, 1/1mm² cable on a B50 with a 1000A PFC. If so, that effectively removes the need to undertake calculations in any specific case. Am I misunderstanding?

Kind Regards, John.

 

[ban-all-sheds]

Thanks for the L-E correction - I did mean that.

And yes, I was using the maximum let-through figures, which are higher than reality, but the problem is what can you do without the maker's figures for the breaker in question, and without the actual PFC for the installation?

Not sure that "I've looked at a number of let-through curves and they were all OK so I never need to do calculations" is a good way to show compliance with 434.2.2....

 

[JohnW2]

Thanks for the L-E correction - I did mean that. And yes, I was using the maximum let-through figures, which are higher than reality, but the problem is what can you do without the maker's figures for the breaker in question, and without the actual PFC for the installation?

I don't see it as a problem. For a start, you can measure PFC (indeed, you would effectively be obliged to as part of (Zs) the I&T of the circuit being installed). In any event, the regs generally allow PFC to be determined by enquiry and, as I said, the highest PFC you'll get by that method is 657A. Let-through curves for any make of MCB are available. Protection against L-E faults by an RCD will certainly provide adequate fault protection (with TT, but also usually with TN).

Not sure that "I've looked at a number of let-through curves and they were all OK so I never need to do calculations" is a good way to show compliance with 434.2.2....

I did say 'effectively removes the need' to undertake the calculations. What that means is that, whilst one obviously should do the calculations, it would appear that one knows in advance that, with any T&E, any domestic Type B MCB and any likely domestic PFC, those calculations will always confirm compliance with 434.2.2.

So, if you accept that, do you now agree that a combination of 433.3.1(ii) and 434.2.2 means, in practice, that no overload protection is theoretically required for a spur cable supplying one socket (certainly single, probably double), and hence that the 3m limit does not apply?

Kind Regards, John

 

[JohnW2]

Sorry, John - I'm really struggling to understand how one could (or would wish to) not use the presence of BS 1362 overload protection devices to show that the cable had overload protection...

So, if you accept that, do you now agree that a combination of 433.3.1(ii) and 434.2.2 means, in practice, that no overload protection is theoretically required for a spur cable supplying one socket (certainly single, probably double), and hence that the 3m limit does not apply?

BAS, are you still struggling ?

Kind Regards, John

 

[ban-all-sheds]

Sorry - The topic hadn't been showing as having a new post.

1) Re the 3m limit - I did say, earlier, "yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2...". I may have overestimated the difficulty in doing that, but I do still maintain that you would actually have to do it, using verifiable data, not just say "I've looked into it before and it has never appeared to be a problem".

2) Re the overload protection - it makes no difference to the requirements for fault protection, so I do still struggle to understand why, given that you do have overload protection as per 433.2.2 you would wish to perversely ignore that and claim that you didn't need it as per 433.3.1.

 

[JohnW2]

1) Re the 3m limit - I did say, earlier, "yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2...". I may have overestimated the difficulty in doing that, but I do still maintain that you would actually have to do it, using verifiable data, not just say "I've looked into it before and it has never appeared to be a problem".

You did indeed say that, and I agree that one does have to undertake the calculation. However, as I recently suggested, provided I've not gone wrong somewhere (and no-one has challenged or corrected me yet), it would appear that the calculation will confirm satisfactory fault protection for any size of T&E with any size of domestic Type B MCB and any likely domestic PFC/PSCC.

2) Re the overload protection - it makes no difference to the requirements for fault protection, so I do still struggle to understand why, given that you do have overload protection as per 433.2.2 you would wish to perversely ignore that and claim that you didn't need it as per 433.3.1.

Well, yes, you can argue in one of two possible ways - but, either way, there is no 3m length limit. Firstly, as you have been doing, one can argue that overload protection is being provided by the downstream fuses per 433.2.2 - but, if you have adequate fault protection (see above), you can utilise 433.2.2(i) [rather than 433.2.2(ii)] and therefore not have a 3m limit. Alternatively, given adequate fault protection, you can invoke 433.3.1(ii) to demonstrate that overload protection does not have to be provided. 433.2.2 (which is about positioning of an OPD, if one has one) and, with it, the 3m limit in 433.2.2(ii), then ceases to be applicable ...

...so, provided only that one can demonstrate that there is adequate fault protection and that it is is unlikley that the cable will have to carry more than 20A [rendered unlikely by the plug fuse(s)] then there is no 3m limit, regardless of which of those two arguments one chooses to use. Do you agree?

Kind Regards, John.

 

[Electrifying]

Agreed.....there is no way that an un-fused 2.5mm spur off a 2.5mm ring final is restricted in length to 3 metres........which is essentially what is being suggested.

 

[JohnW2]

Agreed.....there is no way that an un-fused 2.5mm spur off a 2.5mm ring final is restricted in length to 3 metres........which is essentially what is being suggested.

Thanks. If I have succeeded in convincing BAS (he hasn't actually said so yet!), it's taken a long time. However, I think he is technically right in saying that, for either of the arguments/approaches, is is theoretically necessary to undertake the adiabatic calculation - although, as I've said, I think that calculation will always confirm that all is well.

Kindest Regards, John.

 

[Electrifying]

Sorry, I should also give my answers to the OPs question, instead of just butting in.

1/ A ring final 2.5/1.5 T & E with six twin sockets on a 32A B MCB?

Compliant - Yes........subject to consideration of equal loading of 'Ring'

Safe - Yes........subject to consideration of equal loading of 'Ring'

2/ As above with a spur from each point to a twin socket in 2.5/1.5 T & E

Compliant - Yes........subject to consideration of equal loading of 'Ring'

Safe - Yes........subject to consideration of equal loading of 'Ring'

3/ As above with five of the six spurs at junction boxes on the ring.

Compliant - Yes........subject to consideration of equal loading of 'Ring'

Safe - Yes........subject to consideration of equal loading of 'Ring'

4/ As above with 5 spurs on the ring or at a point.

Compliant - No

Safe - No

5/ As in 1/ but with one spur at the fuseway

Compliant - Yes

Safe - Yes

6/ A radial 2.5/1.5 to one twin socket.

Compliant - Yes

Safe - Yes

7/ A number of radials (say 6 for example) 2.5/1.5 each to one twin socket providing of course that the fuseway terminal and its corresponding N terminal will accept all the conductors mechanically & electrically.

Compliant - Yes

Safe - Yes

Of course all floor areas served, insulation, ambient temperature factors have already been addressed[/quote]

 

[JohnW2]

4/ As above with 5 spurs on the ring or at a point.
Compliant - No
Safe - No

What if one had a meaty enough JB that could take all the conductors satisfactorily (mechanically and electrically), at or near to the mid-point of the ring? I certainly don't see why that would be unsafe and, despite common assertions to the contrary, I don't think I've seen anything in the regs which explictly prohibit more than one spur being connected at one point in a ring.

Kind Regards, John.

 

[Electrifying]

4/ As above with 5 spurs on the ring or at a point.
Compliant - No
Safe - No

What if one had a meaty enough JB that could take all the conductors satisfactorily (mechanically and electrically), at or near to the mid-point of the ring? I certainly don't see why that would be unsafe and, despite common assertions to the contrary, I don't think I've seen anything in the regs which explictly prohibit more than one spur being connected at one point in a ring.

Kind Regards, John.

Yes, perhaps I should have used the same answers as for the earlier questions - i.e. with consideration of loading.
What you suggest would be fine - just as it is in Point 7 (All run from MCB)


"I don't think I've seen anything in the regs which explictly prohibit more than one spur being connected at one point in a ring."

No, there isn't.
Nothing wrong with taking two spurs from the back of a socket.......bugger getting 4 conductors in each terminal, though.

Once again, it's more a design consideration with regards overloading one leg of the ring.

 

[ebee]

sorry just to clarify 4/ the spurs are all at different places or from different points not the same place or point.


I suppose having a few connected at midpoint would be no worse than having a few at the fuseway.
However it might be a problem assuring you are right ion the midpoint and not some distance away from it

 

[JohnW2]

I suppose having a few connected at midpoint would be no worse than having a few at the fuseway. However it might be a problem assuring you are right ion the midpoint and not some distance away from it

Not difficult if you have a means of measuring resistance - and, if you don't you shouldn't be installing circuits

Kind Regards, John