Old 2.5mm2/1.0mm2 cable for ring protected by 32A breaker

[JohnW2]

It is a long time since I looked at those graphs in anger. ... But I have to admit in 30 years, I have never run an adiabatic for a ring final circuit.

None of that surprises me, and I would be very surprised if the same was not true of the vast majority of electricians.

However, we quite often see it being suggested that an 'adiabatic calculation is necessary', yet, without looking at such graphs (and understanding them more confidently than I do!), it's not possible to undertake a proper adiabatic calculation.

IIRC, it was realised circa 1977 (my memory may have it wrong) that you wouldn't get the correct disconnection times using 2.5/ 1.0 on a 3036. ... According to Table 2A in my 17th OSG, 2.5/ 1.0 will comply on a 32A MCB or 30A 1361 fuse, but not a 30A 3036. The quick fixes that were adopted (on Wylex Standard boards anyway) were either swapping out the 30A 3036 for a 20A 3036 or a 30A plug-in breaker.

I can't find my 17th OSG, but I can't work out what is an equivalent Table in the 18th ed. one (there is no 'Table 2A') - what is the title of the Table?

Kind Regards, John

 

[securespark]

In the 16th and 17th OSGs, it's in Appendix 2, Table 2A

 

[securespark]

16th Ed:

17th Ed:

 

[JohnW2]

In the 16th and 17th OSGs, it's in Appendix 2, Table 2A

That, in itself, doesn't help me,since I never had a 16th ed OSG and can't find my 17th ed. one. However ...

16th Ed: <photo> 17th Ed: <photo>

... that certainly does help. Thanks. That has become Table B1 in Appendix B (letters, not numbers, for Appendices now) of the 18th OSG, namely:

... which is conceptually the same as your 17th ed. one, other than the max Zs figures have reduced slightly (presumably due to "Cmin").

So, you're right. The OSG, then and now, say that a 1.0mm² CPC with a 30A BS3036 is 'Not Permitted'. It doesn't say why, but I assume that, as you have implied, I presume it must be the result of an adiabatic calculation. If I can find an I²t figure for a BS3036 fuse, I'll see if I agree with them!

What I do not understand is the fact that those tables make assertions about the adequacy of the CPC without regard to the PEFC. Both the 17th and 18th ed versions say that a 1.5mm² CPC is OK for a 30A BS3036 (well, at least, they don't say that it is 'NP = Not Permitted'). However, if the Zs were so low (hence PEFC so high) that I²t was greater than 29,756 A²sec (if that is possible), then, in terms of an adiabatic calculation, a 1.5mm² CPC would not be adequate.

Kind Regards, John