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

Once overload is present then the 0.9 seems to come into it ...  but its all clear as mud now to me  ! 

The way my head thinks of it is that that if In ≤ Iz then we presume that I₂ ≤ 1.45 Iz is also true - i.e. there is some assumptions/fudge-factors about both the cable current/temperature/time performance and the OPD time/current characteristics. We know the fudge isn't always true for all OPDs - e.g. not for BS 3036 fuses (or perhaps BS 1362 ones either). I think what they're saying that it's not true for cables rated for other than 30 degree ambient either - which sort of makes sense to me as at 20 degrees we can create more heat before hitting 70 degrees - i.e. can carry a larger current.  But the gotcha is that while the extra current to get the cable to its short term overload temperature might be 45% of the 30-degree rating, it's probably rather less than 45% of the higher 20-degree current rating.

Some made-up numbers to illustrate my thinking...

Say we had a cable that was rated at 10A at 30-degrees ambient, so will cope with an overload to 14.5A (for whatever short duration).

Stick that same cable in a 20 degree environment and we might say it's good for 11A continuous, the problem is that it's probably not then good for 1.45x11A = 15.95A for overload, probably something slightly larger than 14.5A since there's a slightly larger temperature difference, but not that much larger. The 0.9 factor then brings us back to something closer to 14.5A. (Numbers made up for clarity rather than accuracy).

My head might well have it wrong of course...

  - Andy.

Once overload is present then the 0.9 seems to come into it ...  but its all clear as mud now to me  ! 

The way my head thinks of it is that that if In ≤ Iz then we presume that I₂ ≤ 1.45 Iz is also true - i.e. there is some assumptions/fudge-factors about both the cable current/temperature/time performance and the OPD time/current characteristics. We know the fudge isn't always true for all OPDs - e.g. not for BS 3036 fuses (or perhaps BS 1362 ones either). I think what they're saying that it's not true for cables rated for other than 30 degree ambient either - which sort of makes sense to me as at 20 degrees we can create more heat before hitting 70 degrees - i.e. can carry a larger current.  But the gotcha is that while the extra current to get the cable to its short term overload temperature might be 45% of the 30-degree rating, it's probably rather less than 45% of the higher 20-degree current rating.

Some made-up numbers to illustrate my thinking...

Say we had a cable that was rated at 10A at 30-degrees ambient, so will cope with an overload to 14.5A (for whatever short duration).

Stick that same cable in a 20 degree environment and we might say it's good for 11A continuous, the problem is that it's probably not then good for 1.45x11A = 15.95A for overload, probably something slightly larger than 14.5A since there's a slightly larger temperature difference, but not that much larger. The 0.9 factor then brings us back to something closer to 14.5A. (Numbers made up for clarity rather than accuracy).

My head might well have it wrong of course...

  - Andy.