Further from my last topic, when using pyranaha nuts for the earthing of the armourings, I genrslly use the same size conductor as that of the line conductor, however its not something I've considered but it when seeing other works the armourings always seem to be earthed via a relatively smaller cable conductor size, hopefully someone can explain to me
Given the relativity high resistance of steels relative to copper, the effective cross section of the armour in terms of current carrying capacity is not as high as the same number of copper strands would be. Further the flying tail is not bundled with other cores, so it's cooling should be better, so a slightly reduced CPC should be OK (but no more reduced relative to the lives than would be allowed in twin and earth, or we could have trouble co-ordinating with likely MCBs) .
The biggest factor (that may not be true in a TNC-s supply) is that the CPC is not expected to carry current when there is no fault, so there is no pre-heat, so instead of assuming fault current on a conductor starting at 70C must not melt the insulation of the SWA, we are starting at perhaps 25 or 30C.
That said, I suspect it is not given much thought, and at least on smaller sizes, I'd still like the tails to be really the same diameter as the live cores, just as a robustness measure.
Given the relativity high resistance of steels relative to copper, the effective cross section of the armour in terms of current carrying capacity is not as high as the same number of copper strands would be. Further the flying tail is not bundled with other cores, so it's cooling should be better, so a slightly reduced CPC should be OK (but no more reduced relative to the lives than would be allowed in twin and earth, or we could have trouble co-ordinating with likely MCBs) .
The biggest factor (that may not be true in a TNC-s supply) is that the CPC is not expected to carry current when there is no fault, so there is no pre-heat, so instead of assuming fault current on a conductor starting at 70C must not melt the insulation of the SWA, we are starting at perhaps 25 or 30C.
That said, I suspect it is not given much thought, and at least on smaller sizes, I'd still like the tails to be really the same diameter as the live cores, just as a robustness measure.
