Was at a mates and he has an outdoor ornamental lamp post, when it is switched on the bulb lights for a few seconds and then goes out by tripping the RDC at the main board. We checked the lightbulb socket that looks ok, we checked the socket where the feed comes from (no loose wires) and thats ok. We checked most of the armoured cable and that looks intact a few feet of the cable is under concrete so left that for the moment, the cable is buried mainly in a gravel drive. When the lightbulb is removed it still trips the RDC. The lamp is fed off the ground floor socket ring.
What would be the way forward for identifying the problem, he will have a multi meter next week.