the 'laws' of Ib <= In <= Iz and I2 <= 1.45 Iz re: direct buried cable

I'm going to have to read this in details again (I'm not yet sure where * 1.1 * 1.04 comes from, I was expecting just 1.03 for 0.5m deep direct burial, and possibly 0.9 for overload protection of "20 degree ambient " buried cables).

I did think though that the 0.9 factor for overload protection of buried cables wasn't included in the tabulated numbers though - that way if you didn't need overload protection you could use the whole of It, if you did need overload protection you're limited to It * 0.9.

I can see confusion over whether that 0.9 is included in Iz - the wording of 433.1.203 suggests it isn't - which is a bit odd since all the other correction factors usually are.

Just as an aside, if Ib is only 32A, why not go for the 32A/40A/45A OPD and design out the problem?

   - Andy.

Andy, the 1.04 was from a table in another book about depth and the 1.1  I mentioned, is a comment that the figures tabled are conservative and in reality are perhaps 10% higher for the tabulated conditions  (ref clause 7.1 app4 ?).

I believe the 0.9 is related to the OCPD  check for overload protection of buried cables - in relation to I2 <= 1.45 Iz or I2 >= 1.45 In and because of the tab values being based buried ambient 20DegC (not just ambient 30 as with others) when it comes to that check,  the equation becomes I2 <=1.3Iz or I2>=1.3In  (or so I read somewhere)  and if working out Iz  from tabulated  - as in Iz = It * (correction factors) -  then that is not concerned with OCDP and only what the cable can carry given the conditions.   Once overload is present then the 0.9 seems to come into it ...  but its all clear as mud now to me  !     I started out with it 'in' then took it out based on further discussion.

however I have read elsewhere in 7671 that 0.9 has to be applied to tabulated values if overload protection is being employed.     

yes of course the OCPD could be reduced for the example Ib figure -  I was just exploring as a side exercise issue, getting as close to selectivity between a fuse and a possible MCB.   equally perhaps one might consider that overload prot is not needed and the problem goes away too :-)

Andy, the 1.04 was from a table in another book about depth and the 1.1  I mentioned, is a comment that the figures tabled are conservative and in reality are perhaps 10% higher for the tabulated conditions  (ref clause 7.1 app4 ?).

I believe the 0.9 is related to the OCPD  check for overload protection of buried cables - in relation to I2 <= 1.45 Iz or I2 >= 1.45 In and because of the tab values being based buried ambient 20DegC (not just ambient 30 as with others) when it comes to that check,  the equation becomes I2 <=1.3Iz or I2>=1.3In  (or so I read somewhere)  and if working out Iz  from tabulated  - as in Iz = It * (correction factors) -  then that is not concerned with OCDP and only what the cable can carry given the conditions.   Once overload is present then the 0.9 seems to come into it ...  but its all clear as mud now to me  !     I started out with it 'in' then took it out based on further discussion.

however I have read elsewhere in 7671 that 0.9 has to be applied to tabulated values if overload protection is being employed.     

yes of course the OCPD could be reduced for the example Ib figure -  I was just exploring as a side exercise issue, getting as close to selectivity between a fuse and a possible MCB.   equally perhaps one might consider that overload prot is not needed and the problem goes away too :-)