Why let through energy of cable is calculated and compared against protective device let through energy, when adiabatic equation and disconnection time check is used to verify the cable sizes? or this let thru energy verification is applied only for current limiting breakers / fuses?
Your wording is a bit unclear to me, but I think you are asking if the adiabatic equation for cables and the let-through energies of fuses etc are related.
If so, yes, actually it is the same formulae with a bit of algebraic manipulation and some letter changes to confuse the unwary.
For every conductor in the fault loop, the let through I2t represents the number of joules of heating that will occur per ohm of that part. This then relates to how hot we dare allow or want that part to get - so for plastic coated cable that is perhaps between 100 and 150C, while for a copper fuse wire we may want more like 1000C, so the figures for a given diameter end up rather different, but the idea is exactly the same - when things happen fast enough that there is no time for heat to escape, so we look at constant energy,and the thermal mass of the metal as if it was in isolation.
Your wording is a bit unclear to me, but I think you are asking if the adiabatic equation for cables and the let-through energies of fuses etc are related.
If so, yes, actually it is the same formulae with a bit of algebraic manipulation and some letter changes to confuse the unwary.
For every conductor in the fault loop, the let through I2t represents the number of joules of heating that will occur per ohm of that part. This then relates to how hot we dare allow or want that part to get - so for plastic coated cable that is perhaps between 100 and 150C, while for a copper fuse wire we may want more like 1000C, so the figures for a given diameter end up rather different, but the idea is exactly the same - when things happen fast enough that there is no time for heat to escape, so we look at constant energy,and the thermal mass of the metal as if it was in isolation.
