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.

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 :-)

I agree with Andy that Cc=0.9 should be applied to ref method D, for much the same reasons.

Firstly it is entirely possible to not require overload protection for a buried circuit. Feeding fixed equipment that cannot by design create an overload (as opposed to fault current) is quite common in my neck of the woods. Therefore why should the creators of the table assume that it is

Indeed the Cc factor of 0.9 is not an absolute. The Commentary to the Regs goes into its derivation and 0.9 is explicitly stated as rounding down for a general value that is deemed to comply; if you know enough about the circuit, a different - indeed sometimes lower, if you try hard enough - value is obtainable for Cc in order to achieve the basic requirement of ensuring the protective device operates before the cable is affected.

There is nothing that says Cc should not be applied to buried circuits.

Other reference methods aren't appropriate for buried circuits. Otherwise they'd say they were. You might choose to use them to obtain values by analogy but it's not in scope of Appx 4 and I would argue that requesting information from the manufacturer, or using IEC 60287 would be more appropriate.

BS7671 Appx 4§4 explicitly says that it should be applied also "To achieve the same degree of overload protection where a cable is “in a duct in the ground” or “buried direct” as compared with other installation methods a rating factor of 0.9 is applied as a multiplier to the tabulated current-carrying capacity".

IET Electrical Installation Design Guide (4.3.5) says that it should be applied: "Buried circuit rating factor Cc The buried cable ratings in Tables 4D4A and 4E4A of BS 7671 are determined at a ground ambient temperature of 20 °C (compared with 30 °C for cables installed in air). Whilst these increased ratings will result in the same conductor operating temperature at full load (70 °C for Table 4D4A), under overload conditions the conductor may exceed the limiting temperature (115 °C for Table 4D4A). The use of the rating factor prevents the cable going over this temperature during an overload, see Regulation 433.1.204"