Securespark, the R1+R2 at the furthest point on the lighting circuit is 2.01 ohms, the measured Ze is 0.51 ohms (TN-S). So Zs = 2.52 ohms still bellow 6.4 ohms for type B 6A breaker. As there is no fault on the circuit why would it trip the RCD on this circuit and not on the ring etc.