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?
As others above have said the adiabatic can be re-arranged as I have done in my post above.
Yes we have to verify that earth faults have to disconnect with the times set out in Chapter 41 but we also have consider the Chapter 43 requirements for over current. As Andy has pointed out above there is a requirement to protect the cable against near end and far end cable faults. Usually, but not always, the far end short circuit fault live to neutral will be a more onerous "slow blow" situation. The I squared t calculated from the neutral line short circuit current at the far end of the cable. Then the "t" time for the over current device from the time current curves in Appendix 3 and the calculated short circuit current. The "S" is the CSA of the cable and the K value comes from Table 43.1 in Chapter 43 for disconnection times up to 5s. The magjc adiabatic equation only works for calculated vales between 0.1s and 5s. For disconnection times of less than 0.1s you need to use manufacturers let through data and above 5s the heat radiation from the cable has to be considered.
A common error is to use an RCD to provide fault protection on a very long cable without considering if the over current protection will provide protection for short circuits. at the far end of the cable.
As others above have said the adiabatic can be re-arranged as I have done in my post above.
Yes we have to verify that earth faults have to disconnect with the times set out in Chapter 41 but we also have consider the Chapter 43 requirements for over current. As Andy has pointed out above there is a requirement to protect the cable against near end and far end cable faults. Usually, but not always, the far end short circuit fault live to neutral will be a more onerous "slow blow" situation. The I squared t calculated from the neutral line short circuit current at the far end of the cable. Then the "t" time for the over current device from the time current curves in Appendix 3 and the calculated short circuit current. The "S" is the CSA of the cable and the K value comes from Table 43.1 in Chapter 43 for disconnection times up to 5s. The magjc adiabatic equation only works for calculated vales between 0.1s and 5s. For disconnection times of less than 0.1s you need to use manufacturers let through data and above 5s the heat radiation from the cable has to be considered.
A common error is to use an RCD to provide fault protection on a very long cable without considering if the over current protection will provide protection for short circuits. at the far end of the cable.
