Cables Underground.

Are you thinking of 433.1.203, which downrates the OCPD (and thus, of course, the circuit) by a factor of 0.9?

Hi Chris. I was looking at Appendix 4, 5.1.1, which is the same in principle as 433.1.203. 

But in the tables, there is also a rating for cables underground. 

There is a correction factor Cc of 0.9 for cables buried, or in ducts underground.

However, in the current carrying capacity tables, there is a column for Ref Method D; direct in ground or in ductings.

Would you still apply the Cc to these figures, and if so, why aren't they adjusted in the tables?

If the circuit requires overload protection, then yes you do, if however only fault protection is required (say if you're supplying a fixed load) then the 0.9 factor need not be applied.

Overload protective devices (both fuses and MCBs) don't just open immediately as soon as there's an overload, but they take time to open - the quicker for large overloads and slower for small ones - as suggested by the graphs in appendix 3. That's both deliberate and useful - as small overloads of short duration (e.g. during motor starting or switch-on surges) can be safely accommodated by the cables, as they take time to heat up.

As I understand it there's an amount of co-ordination between the way cables heat up and how long overload protective devices will stay open, for all the various overload currents, to ensure the cable (really the plastic insulation of the cable) remains protected. And that co-ordination has been done using the assumption that the conductors are in a 30 degree ambient temperature.

It's pretty unusual for it to be 30 degrees underground in the UK however. Normally the maximum temperature is much lower - hardly ever over 20 degrees. That means you can push more current through the cable without it overheating - which at times can be quite useful - and that's the current the tables show. The problem is that the co-ordination between cable and protective device isn't quite the same - a say 10% overload will produce more additional heat in the cable if you're starting with the higher 20-degree ambient rating than if you started with the lower 30-degree ambient rating. So for that situation you'd ideally have a protective device with a higher nominal rating, but a somewhat quicker response to overloads. But we don't have such devices, they're all made to the "normal" design - so to compensate (in a very rough ball-park way) we derate by a 0.9 factor which should result in the normal design of overload protective devices offering adequate protection to the cable.

It might have been simpler and clearer just to have two method D columns in the tables - one where overload protection is required (including the 0.9 factor) and the other where it isn't (the currently listed values), but probably would have been too easy.

   - Andy.

Andy, thanks.

I missed the bit about overload protection on the previous page. That explains it all nicely.