The NE fault may not trip an RCD if the fault path impedance is high enough. Here you are putting the 8 or so ohms of the fault across one of the two coils in the RCD ( and that reading is such a funny no. it may well be quite variable from day to day if it is a rusty screw not really touching copper cores very well that creaks and groans as the boiler heats up).
If the RCD coil is say 0.1 ohms and the fault path shunting it is indeed 8 ohms, then the current will split in the ratio of 80:1 - the imbalance seen by the RCD may not have reached 25mA if the flat was never heavily loaded, for example 20A was running down the live sensing coil of the RCD and 79/80 of 20A down the neutral sensing coil would give an imbalance of 20A/80 - which is about 25mA.
I have seen RCDs that seem to fire off in a load dependent way because of this sort of situation - and maybe if it only happened once or twice the occupant reset it and kept quiet having assumed they had overloaded something a bit.
As you say, it seems that both sparks before you were keen to get in and out a bit quick and do the minimum of investigation. Or maybe to be charitable, perhaps on that day the rusty screw was not in such good contact. However, after a reported shock, I'd hope you might expect a bit more rigour than the minimum.
(and that is why we check at 500V if we can, not with cheap general purpose meter with a 9V battery - which will find dead shorts and opens, but maybe not a scratchy contact where a few extra volts break the oxide barriers and reveal things like this.)
The NE fault may not trip an RCD if the fault path impedance is high enough. Here you are putting the 8 or so ohms of the fault across one of the two coils in the RCD ( and that reading is such a funny no. it may well be quite variable from day to day if it is a rusty screw not really touching copper cores very well that creaks and groans as the boiler heats up).
If the RCD coil is say 0.1 ohms and the fault path shunting it is indeed 8 ohms, then the current will split in the ratio of 80:1 - the imbalance seen by the RCD may not have reached 25mA if the flat was never heavily loaded, for example 20A was running down the live sensing coil of the RCD and 79/80 of 20A down the neutral sensing coil would give an imbalance of 20A/80 - which is about 25mA.
I have seen RCDs that seem to fire off in a load dependent way because of this sort of situation - and maybe if it only happened once or twice the occupant reset it and kept quiet having assumed they had overloaded something a bit.
As you say, it seems that both sparks before you were keen to get in and out a bit quick and do the minimum of investigation. Or maybe to be charitable, perhaps on that day the rusty screw was not in such good contact. However, after a reported shock, I'd hope you might expect a bit more rigour than the minimum.
(and that is why we check at 500V if we can, not with cheap general purpose meter with a 9V battery - which will find dead shorts and opens, but maybe not a scratchy contact where a few extra volts break the oxide barriers and reveal things like this.)
