When using adiabatic equation for calculating minimum size of CPC, every example I have seen uses 0.1 second or whatever the disconnect time of the mcb element of the RCBO or MCB will be.
In a domestic sittuation most circuits are protected by RCD's with a trip time of 40mS with significant fault currents, in this sittuation why don't we use 40mS as T in the adiabatic equation?
well, even if it fails the adiabatic analysis, one may as well wait until the insulation has actually been cooked by a short circuit fault, before re-wiring - the effort is the same, and it may allow you to keep the installation running for several years longer, maybe pretty much for ever. You will be far enough away from melting the copper that there is no risk of that, just that the insulation around the CPC will be taken over the recommended temp during a 'silver spanner' short circuit.
Typical figures (Hagar)
The let-though of an MCB varies with available PSSC, as unlike a fuse it does not get faster and faster with increasing fault current but we can say something fairly intelligent.
Mike.
well, even if it fails the adiabatic analysis, one may as well wait until the insulation has actually been cooked by a short circuit fault, before re-wiring - the effort is the same, and it may allow you to keep the installation running for several years longer, maybe pretty much for ever. You will be far enough away from melting the copper that there is no risk of that, just that the insulation around the CPC will be taken over the recommended temp during a 'silver spanner' short circuit.
Typical figures (Hagar)
The let-though of an MCB varies with available PSSC, as unlike a fuse it does not get faster and faster with increasing fault current but we can say something fairly intelligent.
Mike.
