Clarification of conductor operating temperature factors when calculating phase faults and earth faults

Things vary in practice, so always look for the 'worst case' in order to stay on the side of safety - where large currents are more of a problem than small ones, use the lowest likely resistance (cold conductors), where high resistances (e.g. Zs/disconnection times) are the issue, assume hot (full working temp) conductors.

  - Andy.

Not sure what the modern 'exam questions' expect, but it is quite common in real life for phase and neutral conductors to be warm (or even hot), but the CPC , being armour or conduit or something else nearer the fresh air than the copper carrying the current, is quite a lot cooler.
As Andy notes, the highest current flows when everything is cool and the electrons are less obstructed by thermally jiggling atoms, and yet the insulation on the cable is least likely to survive the heat of the fault current when it is already pre-heated before the fault comes on.

It all depends exactly what you want to know and why, and out in reality a design that is so close to a pass/fail limit that it matters is not a good one anyway, as small changes like a distribution network cable replacement in the street outside may tip it to the wrong side...

Mike

Not sure what the modern 'exam questions' expect, but it is quite common in real life for phase and neutral conductors to be warm (or even hot), but the CPC , being armour or conduit or something else nearer the fresh air than the copper carrying the current, is quite a lot cooler.
As Andy notes, the highest current flows when everything is cool and the electrons are less obstructed by thermally jiggling atoms, and yet the insulation on the cable is least likely to survive the heat of the fault current when it is already pre-heated before the fault comes on.

It all depends exactly what you want to know and why, and out in reality a design that is so close to a pass/fail limit that it matters is not a good one anyway, as small changes like a distribution network cable replacement in the street outside may tip it to the wrong side...

Mike