One of those Ignorance is Bliss / Wish I'd Never Started This stories.
The boss wanted an Induction Hob and no spare slots on 8-way Consumer Unit. Also EXTREMELY old (although probably as safe as it ever was - I thought), so change it for a CU with more slots. And, given that MCBs are now the most basic option and not wanting the risk of a single RCD with no clue where the trip(s) might be coming from, I decided to use RCBOs. Ok - expensive. But you can always immediately tie problems to just one circuit.
So yesterday I stuck the thing in. And everything is OK including the new hob (with its dedicated radial using 6mm cable to meet its max load of 32A!!) but EXCLUDING a lighting circuit that immediately tripped the RCBO. Investigating revealed the cause straight away: a Neutral to Earth fault that had probably existed for a VERY long time but without any RCDs in the system, remained undetected.
Worse, several of the connections that might be wrong are lost in ceiling voids: the roses on several of the pendants have only one TC&E cable in them, indicating that some cowboy in the distant past moved pendants but (presumably) substituted junction boxes in the void instead of extending all the cables to the new positions. Understandable but irritating, since I've no idea of the original locations and the JBs are almost certainly under fitted carpets, bathroom floor, etc.
In the computer networking world, there's a gadget called a TDR for locating (Ethernet) cable damage but as far as I know, the only tool available for mains wiring is an ohm meter. At two locations where I've found a full complement of cores in the ceiling rose, the resistance Neutral to Earth is about 2 ohms. This is about the same value as on the end of the cable at the consumer unit.
Needless to say, being lighting and quite old, it's a radial circuit, not a ring. But I can't at present work out what the route is.
I guess the only good thing to come out of this is that 8 out of 9 circuits are working OK with RCBOs! If I'd chosen a split CU with 2 RCDs, I'd have spent a lot longer fiddling around to isolate the faulty circuit.
Anyone recommend an effective process for localising the fault to a smaller area than the whole circuit?
The boss wanted an Induction Hob and no spare slots on 8-way Consumer Unit. Also EXTREMELY old (although probably as safe as it ever was - I thought), so change it for a CU with more slots. And, given that MCBs are now the most basic option and not wanting the risk of a single RCD with no clue where the trip(s) might be coming from, I decided to use RCBOs. Ok - expensive. But you can always immediately tie problems to just one circuit.
So yesterday I stuck the thing in. And everything is OK including the new hob (with its dedicated radial using 6mm cable to meet its max load of 32A!!) but EXCLUDING a lighting circuit that immediately tripped the RCBO. Investigating revealed the cause straight away: a Neutral to Earth fault that had probably existed for a VERY long time but without any RCDs in the system, remained undetected.
Worse, several of the connections that might be wrong are lost in ceiling voids: the roses on several of the pendants have only one TC&E cable in them, indicating that some cowboy in the distant past moved pendants but (presumably) substituted junction boxes in the void instead of extending all the cables to the new positions. Understandable but irritating, since I've no idea of the original locations and the JBs are almost certainly under fitted carpets, bathroom floor, etc.
In the computer networking world, there's a gadget called a TDR for locating (Ethernet) cable damage but as far as I know, the only tool available for mains wiring is an ohm meter. At two locations where I've found a full complement of cores in the ceiling rose, the resistance Neutral to Earth is about 2 ohms. This is about the same value as on the end of the cable at the consumer unit.
Needless to say, being lighting and quite old, it's a radial circuit, not a ring. But I can't at present work out what the route is.
I guess the only good thing to come out of this is that 8 out of 9 circuits are working OK with RCBOs! If I'd chosen a split CU with 2 RCDs, I'd have spent a lot longer fiddling around to isolate the faulty circuit.
Anyone recommend an effective process for localising the fault to a smaller area than the whole circuit?