One final thing to consider for unusually long cables are:
- If you are failing to meet the Zs for MCB disconnections times, or
- If you fail to meet 5% voltage drop at the MCB rating (even if it is OK at the fixed load's current that you intended)
They are both a warning signs that you need to consider the design in
far more detail.
Basically you might use an RCD to mitigate the high Zs situation, but you also have to consider the risk of a L-N fault also failing to disconnect quickly (as that is not considered a fault by the RCD). So you need to make sure that the over-current aspect (i.e. MCB) will prevent a fire if the cable is overloaded for a long time due to a end-of-line short not resulting in enough current to trip the MCB quickly.