- Reaction score
- 5,940
So no insulation resistance test was carried out whilst operating the switches on the two way circuit, therefore there could easily have been an IR fault that was missed.
The cable is in metal conduit, is the conduit earthed?
If the conduit was not earthed then the fault is unlikely to be a fault to the conduit.
If the conduit is earthed then this would be a likely scenario, the cable has been abraded by the conduit and has now been moved clear of the connection with conduit and so works, but still exhibits risk.
The more likely situation is as Trev says when an accessory screw or retaining bolt is tightened up the core of a conductor is trapped by it and touches the earthed screw, once the cable has been "adjusted|" the core is no longer in contact with the screw and so works fine.
The less likely scenario is if the cables in a switch box are messy and tight then cables can cross over and if a damaged L or N or a CPC that is unsleeeved or poorly sleeved come into contact then this could trip the RCD.
As an RCD trip is because of a fault to earth and not a short circuit the above a re the most likely ideas for what is happening (assuming the circuits are wired correctly, there is no borrowed neutral and so forth).
Full testing whilst there was a fault would have found this situation, which is why we test!
The cable is in metal conduit, is the conduit earthed?
If the conduit was not earthed then the fault is unlikely to be a fault to the conduit.
If the conduit is earthed then this would be a likely scenario, the cable has been abraded by the conduit and has now been moved clear of the connection with conduit and so works, but still exhibits risk.
The more likely situation is as Trev says when an accessory screw or retaining bolt is tightened up the core of a conductor is trapped by it and touches the earthed screw, once the cable has been "adjusted|" the core is no longer in contact with the screw and so works fine.
The less likely scenario is if the cables in a switch box are messy and tight then cables can cross over and if a damaged L or N or a CPC that is unsleeeved or poorly sleeved come into contact then this could trip the RCD.
As an RCD trip is because of a fault to earth and not a short circuit the above a re the most likely ideas for what is happening (assuming the circuits are wired correctly, there is no borrowed neutral and so forth).
Full testing whilst there was a fault would have found this situation, which is why we test!
