SWA to T&E

[EFLImpudence]

Apart from that, could the length be increased by using omission of overload as the cable cannot be overloaded because of the plug fuse(s)?

If you're talking about 433.3.1(ii), that also invokes the requirement for "fault protection per 434'.

Yes, but it complies with 434.2.2 and not 434.2.1 (the three metre restriction).

 

[JohnW2]

Apart from that, could the length be increased by using omission of overload as the cable cannot be overloaded because of the plug fuse(s)?

If you're talking about 433.3.1(ii), that also invokes the requirement for "fault protection per 434'.

Yes, but it complies with 434.2.2 and not 434.2.1 (the three metre restriction).

Fair enough. Since, in turn, that approach invokes 434.5.2, it requires an adiabatic calculation - although I assume that will rarely present a problem.

This approach obviously at least requires that there is no more than one double socket (or two single ones) at the end of the cable (i.e. max of 2 x 13A fuses) and that the CCC of the cable is at least 26A (i.e. 2.5mm² 'clipped direct'). I wonder, however, whether having 26A worth of downstream BS1362 fuses is considered to render it "unlikely that the cable could carry overload current", given that the fuse(s) would obviously allow more than 27A to be carried indefinitely, and much higher currents for appreciable periods of time? With a single socket, there would presumably be no concern.

Kind Regards, John

 

[EFLImpudence]

I wonder, however, whether having 26A worth of downstream BS1362 fuses is considered to render it "unlikely that the cable could carry overload current", given that the fuse(s) would obviously allow more than 27A to be carried indefinitely, and much higher currents for appreciable periods of time?

No more than an unfused <3m spur.

With a single socket, there would presumably be no concern.

So, you agree that the 3m rule can be overcome by this principal.

It is dependent on correct design.

 

[JohnW2]

I wonder, however, whether having 26A worth of downstream BS1362 fuses is considered to render it "unlikely that the cable could carry overload current", given that the fuse(s) would obviously allow more than 27A to be carried indefinitely, and much higher currents for appreciable periods of time?

No more than an unfused <3m spur.

Yes, but the matter of unfused spurs (from ring finals) is totally different, regs-wise, from what is being discussed in this thread. The concept of unfused spurs from ring finals, for which cable with a CCC as low as 20A is allowed to be protected by a 32A OPD, is simply 'allowed' by the regs (aided by App 15), without any need to invoke 433.2.2 (to justify the situation on the basis of downstream overload protection) or 433.3.1(ii) (to justify it on the basis that overload protection is not required, no matter what the length of the spur (I think - I'm not aware of any restriction in the regs).

On the other hand, if you want to invoke (in a situation which is not that of a spur from a ring final) 433.3.1(ii) in order to allow a >3m cable (of inadequate CCC in relation to the upstream OPD), on the basis that overload protection is not required (at all), then the designer has to be satisfied that "the characteristics of the load or supply are such that the cable is not likely to carry overload current". Hence, as I asked, is 26A worth of BS1362s necessarily enough to satisfy the designer that, say, the CCC of 2.5mm² cable (at best 27A) is 'unlikely' to be exceeded? I really don't know what they intend here.

With a single socket, there would presumably be no concern.

So, you agree that the 3m rule can be overcome by this principal.

Probably yes - but in this case I would say that it's pretty reasonable to invoke 433.3.1(ii), since is is surely 'unlikley' that one will get more than 27A through a 13A fuse for a significant period of time? As above, it's rather different with 2 x 13A fuses.

Kind Regards, John

 

[EFLImpudence]

But surely overload is not the problem as an unfused spur <3m. is subject to the same loading as one >3m; they are both protected by the same rated OPD.

Perhaps Appendix 15 relies on 433.3.1(ii) and 434.2.2 because it does not mention a 3m limit.

 

[JohnW2]

But surely overload is not the problem as an unfused spur <3m. is subject to the same loading as one >3m; they are both protected by the same rated OPD.

As I said, I am unaware of any length limits in relation to unfused spurs from ring finals.

Perhaps Appendix 15 relies on 433.3.1(ii) and 434.2.2 because it does not mention a 3m limit.

Similar considerations might well have been in their minds, but I don't think they really need to invoke any specific regs. 433.1.103 simply allows ring finals "with or without unfused spurs", and I would think that would supersede any other detail regs which might appear to undermine what 433.1.103 'allows'.

It is perhaps worth pointing out that 433.1.103, which is, AFAIAA, the only actual regulation which mentions unfused spurs from ring finals (and doesn't even refer to Appendix 15) not only does not mention any length limit but also does not say anything about what may, or may not, be fed by an unfused spur - the only mention of the latter is in the ('informative', 'provided as guidance', App 15)!

Kind Regards, John

 

[scousespark]

Hi Novice

Did you get an electrician to look at this job?
If so, what has he said about your planned circuit?