Theorectical discussion on fusing down smaller cables

Overload protection (if necessary) for the 'small' cable can obviously be addressed by downstream fuses, so it's achieving adequate fault protection which is the potential issue (and that's likely to be a theoretical problem with 2.5mm² cable protected by a 1200A OPD). However, pragmatically speaking, if the situation is such that the probability of a fault (short) in that smaller cable is very low (e.g. if it is in steel conduit), then I think there may well be a good argument for applying 'common sense discretion'.

Kind Regards, John
Correct and that is precisely why I'll not be offereing further advice or opinion in this thread as someone is bound to do something silly if I say it is OK and it's bound to start a silly arguement.
 
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Correct and that is precisely why I'll not be offereing further advice or opinion in this thread as someone is bound to do something silly if I say it is OK and it's bound to start a silly arguement.
Fair enough - and, in any event, as you have said, I don't think that anyone (not only you) would be able to say that "it is OK" in a specific case without having seen, understood and considered the precise situation.

I've never been totally sure - is it your view that 434.2.1 is saying that fault protection downstream of a reduction in CSA (e.g. the fuse in a plug or FCU) is acceptable, provided that the conditions specified in that reg are satisfied?

Kind Regards, John
 
I've never been totally sure - is it your view that 434.2.1 is saying that fault protection downstream of a reduction in CSA (e.g. the fuse in a plug or FCU) is acceptable, provided that the conditions specified in that reg are satisfied?
I would say yes, but of course,

the plug fuse does not afford fault protection for a fault in that part of the circuit, hence the other methods/conditions.
 
I would say yes ...
That's always been how I read it, but ....
... but of course, the plug fuse does not afford fault protection for a fault in that part of the circuit, hence the other methods/conditions.
Indeed - which I suppose is why I 'wondered' a bit about intended meaning. In particular, what's so special about a 'reduction in CSA' - what about the situation in which the CSA of a cable is too small to be adequately protected against fault by the upstream OPD all the way from that OPD to a downstream one? Is there a reg that says that can be acceptable, if 'conditions' (such as those in 434.2.1) are satisfied?

Kind Regards, John
 
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Indeed - which I suppose is why I 'wondered' a bit about intended meaning. In particular, what's so special about a 'reduction in CSA' - what about the situation in which the CSA of a cable is too small to be adequately protected against fault by the upstream OPD all the way from that OPD to a downstream one?

Kind Regards, John
the plug fuse does not afford fault protection for a fault in that part of the circuit, hence the other methods/conditions.
Something like a 2.5mm²on a 32A MCB which is expressly included in the regs as a spur on a ring final where the downstream plug fuses are within the rating of the cable?
 
Something like a 2.5mm²on a 32A MCB which is expressly included in the regs as a spur on a ring final where the downstream plug fuses are within the rating of the cable?
Yes, something like, but in that case the PFC should be determined to see if the CPC can cope should the spur cable suffer damage.
If it cannot, I presume that the spur cable should be protected in the ways stated in 434.2.1.

Can anyone think of an situation where it would be necessary to install a cable which was inadequate for its own fault current - as in John's question.

This is why the 3m. is often misquoted as a limit for all such spur cables.
I personally think that such a misreading is the reason for the DNOs stating that tails over 3m. must have a fuse - even though that fuse can be the same rating as the cut-out fuse which is obviously pointless.
 
You're not on that Facebook group are you?
 
Can anyone think of an situation where it would be necessary to install a cable which was inadequate for its own fault current - as in John's question.
Good question. SUNRAY has mentioned one case in which we would install a cable whose (upstream) overload protection was theoretically 'inadequate' (a 2.5mm² spur from a 32A ring final which originated at the CU) - so the question remains as to how adequate (I would suspect very adequate!) the fault protection was ....

... I think it's probably true to say that most of us (certainly myself) do not usually give any appreciable thought to the adequacy of fault protection of a cable, our assumption being (almost certainly correctly) that if the overload protection is adequate (easy to determine), then fault protection will also be adequate.

If one does want to confirm adequacy of fault protection, then one has to undertake an adiabatic calculation and I think one of the main reasons why most of us don't do that (even when the 'confirmation' would be nice to have) relates to uncertainty as to what value of disconnection time ('t') to use for the OPD convened, since such figures don't really exist in the sources we usually look to (BS7671, OSG etc.).

If one considers a radial circuit with a Zdb of 0.35Ω (hence 'worst case' PFC, at the CU, of 657A at 230V or 723A at 253V) wired in 2.5mm² T+E and protected by, say, a B32, the adiabatic calculation (using 'k'=115) appears to indicate that the maximum acceptable disconnection time would be 0.191s at 230V/657A or 0.158s at 253V/723A - which I imagine is within the capabilities of a B32.

Looked at the other way around, if we consider PFC of 723A at 253V, then, if we assumed a disconnection time of 0.1s (the lowest plotted in the BS7671 graphs), then that would be adequate to protect a conductor down to about 1.99mm², whilst if we looked at 0.01s (some graphs go down that far), it would protect a conductor down to about 0.63mm².

However, interpretation of all that relies on knowledge of what 't' (disconnection time) we should be considering, which probably requires access to "I²t graphs".

Kind Regards, John
 
Good question. SUNRAY has mentioned one case in which we would install a cable whose (upstream) overload protection was theoretically 'inadequate' (a 2.5mm² spur from a 32A ring final which originated at the CU) - so the question remains as to how adequate (I would suspect very adequate!) the fault protection was ....

Kind Regards, John
I confess I struggle with the 2.5mm² spur on a ring final being compliant as a DSSO 2.5mm² radial on a 32A OCD is not

It's good to see a well presented thread for a change on a subject that I've encountered far too often. Most of my electrical work has been commercial, specifically control panels. Typically the incoming feed (which will be from 6A up to thousands) will end up on a buss of some sort and from there will be downstream protected by MCB or BS88 fuse.
 
I confess I struggle with the 2.5mm² spur on a ring final being compliant as a DSSO 2.5mm² radial on a 32A OCD is not.
I would agree, but only IF you were talking about a 2.5mm 32A radial which served only one double socket.

In fact, given that the dispensation for a spur from a ring final seems to be assuming that a double socket represents a maximum load of 20A (since the reg in question only requires that the cable CCC be ≥20A), I would say that (applying the same assumption) it's quite easy to argue that a 32A 2.5mm² radial supplying just one double socket is compliant with regs. The concern, of course, would be that someone might subsequently extend that radial by adding further sockets.

Kind Regards, John
 
In fact, given that the dispensation for a spur from a ring final seems to be assuming that a double socket represents a maximum load of 20A (since the reg in question only requires that the cable CCC be ≥20A),
I would argue that the spur is not part of the ring and therefore the ≥20A (20A minimum) does not apply and it should be correctly designed.

I.e. 1mm² could be used were it not specifically prohibited for "power" circuits.
 
I would argue that the spur is not part of the ring and therefore the ≥20A (20A minimum) does not apply and it should be correctly designed.
Whilst I can understand that interpretation, I think that's probably debatable (my emboldening) ...
BS7671 said:
Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, protected by a 30 A or 32 A protective device ... The circuit shall be wired with copper conductors having line and neutral conductors with a minimum cross-sectional area of 2.5 mm2 ... Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (Iz) of the cable is not less than 20 A
Furthermore, the fact that Appendix 15 is taken to be suggesting that one double socket is acceptable, whereas two single ones are not, seems to be indicating that they are assuming that the maximum load represented by a double socket is less than 26A (hence probably 20A).
I.e. 1mm² could be used were it not specifically prohibited for "power" circuits.
Agreed - but, as you say, and for whatever reason, that is specifically 'prohibited'.

For a spur supplying just one single socket, normal design consdierations would say that 1.5mm² would be OK.

Kind Regards, John
 
Whilst I can understand that interpretation, I think that's probably debatable (my emboldening) ...
Possibly, but I would say that ", with or without unfused spurs," separated by commas means that the spurs are separate and not being considered.

Furthermore, the fact that Appendix 15 is taken to be suggesting that one double socket is acceptable, whereas two single ones are not, seems to be indicating that they are assuming that the maximum load represented by a double socket is less than 26A (hence probably 20A).
Agreed - but, as you say, and for whatever reason, that is specifically 'prohibited'.
Yet, it only states that while showing 2.5mm² conductors - which could handle two single sockets.

For a spur supplying just one single socket, normal design consdierations would say that 1.5mm² would be OK.
Yes, but 1.5mm² has a CCC of 20A, anyway.
 

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