Cpc size on unfused spur

[John D v2.0]

You don't have to estimate the effect of the heating that flameport mentioned, you can get all the information you need from the chart.

in terms of a requirement for compliance, you are exactly right. I just mean that will always be the case in a consumer unit, therefore 1.5mm CPC will always be OK. Rather than having that small "gap" as I was concerned about.

And yes presumably you can back calculate the operation times at extremely high currents, but it's probably not useful to do so. Expecially since you're working at tiny portions of a 50Hz cycle at that point.

 

[JohnW2]

Not necessarily - it is also called 'branch' on a radial. I don't know why.

I forgot to comment on this, for the benefit of any readers who might be unsure....

A ring circuit is a ring, and anything connected 'radially' to that ring (i.e. not part of the ring structure), is clearly something different. We generally call that 'something different' a 'spur', the ring remaining as a ring. If that 'something different' originates at the CU (which is really just part of the ring), it can still be called a 'spur', but can equally be regarded as a radial circuit sharing its OPD with the ring.

A radial circuit is rather different, and the usual convention seems to be that what one calls 'branches' (everyday sense) depends upon whether or not they have cable with the same (or, I suppose, greater!), or lower, CSA in comparison with that in the 'main' radial circuit (that which originates from the CU, and continues in cable whose CSA does not decrease).

If, at some point, the circuit 'splits' (heads off in two directions), with both 'downstream' cables having the same (or greater!) CSA as the 'upstream' cable, neither of those split bits is any different from the other, so one could not really say that one is 'a continuation of the radial circuit' and the other is 'a spur from the radial circuit' (or even, really, 'a branch from the radial circuit') - the radial simply splits into two, 'indistinguishable', 'branches'.

On the other hands, if a 'branch' (everyday sense) connects to the 'main radial circuit' using cable of a smaller CSA, then (I suppose by analogy with ring finals), the convention seems to be to call it a 'spur' (fused or unfused, as with ring finals).

Kind Regards, John

 

[JohnW2]

You don't have to estimate the effect of the heating that flameport mentioned, you can get all the information you need from the chart.

That chart tells you nothing about how much the fault current will fall short of your calculated PEFC as a result of increased resistance of the upstream fuse. If you use the calculated PEFC, the chart will therefore give you an unnecessarily high I²t than you could use if you could estimate the current reduction due to the heating of the fuse.

... in terms of a requirement for compliance, you are exactly right. I just mean that will always be the case in a consumer unit, therefore 1.5mm CPC will always be OK. Rather than having that small "gap" as I was concerned about.

I'm a bit confused - maybe I'm being dim. As I said, the condition stated in that reg is merely the requirement which, if satisfied, enables one to have an MCB with a breaking capacity less than the PEFC, isn't it? Why does that necessarily mean that "1.5mm CPC will always be OK"?

And yes presumably you can back calculate the operation times at extremely high currents, but it's probably not useful to do so.

As my rough figures suggested, it looks to me as if it could be very useful to do so, even with modest PEFCs. If, with say a modest PEFC of 500A, and as I think EFLI was suggesting, one undertook an adiabatic calculation which 'assumed' an operation time of 0.1s (rather than the approx 0.005s back-calculated figure), one would end up estimating a required minimum conductor CSA which as about 4.5 times (actually √20 times, with those figures) larger than was actually necessary, wouldn't one?

Kind Regards, John

 

[EFLImpudence]

I forgot to comment on this, for the benefit of any readers who might be unsure....

That might be a logical explanation but the BS7671 definition of a spur is 'a branch from a ring or radial final circuit'.

 

[JohnW2]

That might be a logical explanation but the BS7671 definition of a spur is 'a branch from a ring or radial final circuit'.

I realise that, and all the examples of 'branches' from a radial in Appendix 15 are what (per what I've just written) would, indeed, conventionally be regarded as 'spurs'.

However, no matter what the definition might say, one simply cannot talk about 'a spur from a radial circuit' when all cables are the same CSA - if there are two 'outgoing' cables (of same CSA) originating from one point in a radial circuit, how exactly can one decide which one is "the radial circuit" and which one is a "branch from it" (i.e., per definition, a spur')?

Kind Regards, John

 

[ban-all-sheds]

EFLI - which sockets are on spurs/branches here?

Note - the sequence of the letters does not necessarily indicate the order in which the sockets were installed, they are just there to make it easy for you to say which one(s) are on spur(s).

 

[EFLImpudence]

EFLI - which sockets are on spurs/branches here?

1. Impossible to tell, or
2. All except 'A', or
3. Does it matter?

Can you add to a radial in a way which would not be a spur?

I have just realised there is no definition of a radial circuit, so...
The definition of a 'spur' does not mention the size of the cables.
Perhaps the definiton of 'spur' should be 'a branch of a ring or radial circuit' but it isn't.

 

[ban-all-sheds]

3.

Yes, because if you cannot tell what is a spur and what is not then "spur" has no meaning. If it means nothing then it may not be used in definitions.

 

[JohnW2]

1. Impossible to tell, or...

As I said, I believe that is the correct answer, which is why, as has been said, 'spur' (or even 'branch') "of a radial" (or "from a radial") is meaningless.

Can you add to a radial in a way which would not be a spur?

By BS 7671 definition you could. You could say that the addition was a continuation of the radial circuit and that the pre-existing part of the circuit downstream of the 'point of split' had become 'a spur'.

Perhaps the definiton of 'spur' should be 'a branch of a ring or radial circuit' but it isn't.

I think that would be worse. I'm not sure how one can 'branch from' a ring - and the issue with radials would remain the same.

A radial is either just one straight run or it involves one or more 'branches'. In the latter case, at least if all cables are the same, there is no way of defining which of those branches was a spur/branch and which is 'the radial circuit' - it is just a 'network of branches'. I suppose one could produce a definition based on history (the order is which sockets had been installed) but that would be silly since subsequent observers would usually not know what that order was.

Whether one calls them 'spurs' or something else, there is good electrical reason to distinguish 'same CSA' from 'reduced CSA' parts of a branching radial circuit.

Kind Regards, John

 

[ban-all-sheds]

3.

Yes, because if you cannot tell what is a branch and what is not then "branch" has no meaning. If it means nothing then it may not be used in definitions.

 

[JohnW2]

indeed. As I wrote ...

As I said, I believe that is the correct answer, which is why, as has been said, 'spur' (or even 'branch') "of a radial" (or "from a radial") is meaningless.

 

[scousespark]

Not necessarily - it is also called 'branch' on a radial. I don't know why.

The Ring Final (as specifically allowed) is virtually obsolete since MCBs.
I don't know why anyone would bother with it in a commercial installation.


It would. It is up to correct design where it is protected by the fuses or does not need overload protection. See 433.

Just curious as to where can you see reference to branches off radial.
Also, why does an MCB make Ring Finals obsolete. Surely an MCB is just a different OPD.

 

[JohnW2]

Just curious as to where can you see reference to branches off radial.

Spur. A branch from a ring or radial final circuit.

Also, why does an MCB make Ring Finals obsolete. Surely an MCB is just a different OPD.

I believe that EFLI's point was that, because of the different 'fusing factors', the advantages of using a ring final circuit are much less with an MCB than they were with BS3036 fuses.

Kind Regards, John

 

[scousespark]

I'm aware of the definition forthe term 'spur' and it's descipton. We don't have spurs from a ring and branches from a radial.
There is no distinction between ring and radial and neither is there between circuits where the spur is a different size.

I'm also aware of fusing factors, but I am not seeing the death of rings.

 

[JohnW2]

I'm aware of the definition forthe term 'spur' and it's descipton.

Fair enough. In that case you presumably already you knew the answer to the question you posed ...

Just curious as to where can you see reference to branches off radial.

The answer, as you say, is that you can see that reference in the BS 7671 definition of a "spur".

We don't have spurs from a ring and branches from a radial. There is no distinction between ring and radial ...

Indeed we don't, at least as far as BS 7671 definitions are concerned. Those definitions say that both radial and ring finals have branches, and that those branches are called 'spurs'

... and neither is there between circuits where the spur is a different size.

Again, indeed not, in terms of BS7671 definitions. The situation with spurs from ring finals is fairly straightforward. With radials, as I have said, I can see the merit of having a 'special name' (and I suppose "spur" would be as good as any) for a reduced-CSA branch, but if there is no change in CSA then either 'branch' or 'spur' is meaningless - since, as has been said (and not only by me) if one has a "Y-point" in a radial, with all cables having the same CSA, there is no way one can define which downstream bit is "the radial circuit" and which bit is the 'branch'/'spur'.

I'm also aware of fusing factors, but I am not seeing the death of rings.

I didn't doubt that you were aware of fusing factors - and since you are aware of them, you presumably understand EFLI's view. No-one, EFLI or otherwise, has suggested in this thread that there is 'death' of ring finals to be seen, but the advantage they offer over radials is, I would have said, fairly marginal.

Although you are probably not interested, my personal view is that the advantage of CPC redundancy in ring finals is not to be sneezed at, but I am perfectly happy with 32A radials. As I've often said, I'm a bit less comfortable about 20A radials - since it just doesn't feel right to have umpteen 13A outlets fed by a supply that theoretically is not even enough for two 13A loads. With some circuit topologies, a ring may result in a small saving of cable/copper, but that difference is, at best, fairly modest.

Kind Regards, John