non-RCD circuit tripping RCD

[winston1]

Give it a rest, Winston, your arguments are pathetic.

No valid. Some vacuum cleaners are nearly 3KW.

 

[very_pointy_ears]

I don't know who installed the CU in this house but it's a bit random at best, and a complete maze of wires which makes it very difficult to see what goes where.There are even a few choc blocks in it to join several neutrals and lives together - God knows why.

I'm gradually working my way through it and ripping out old stuff to replace with new. It won't surprise me if some non-RCD circuits have their neutrals on the RCD side. I've already re-cabled some lights that took their live from one circuit and their neutral from a completely different one. Also, a lot of the lighting cable here is twin core stranded (and single core stranded) so I've been replacing that with T+E and re-routing it more appropriately.

The kitchen shared its 'ring' (using that term loosely given that the kitchen wiring appears to have the topology of a "6" with 4 sockets on the tail) with 1.75 other rooms (in that one of the rooms it shared the ring with had one socket on a different circuit). I've separated the kitchen now and have ripped out the rest of what was that ring main, including a 2.5 T+E cable running under the edge of the upstairs carpet in close company with the carpet gripper.

Given the issue that the UPS appears to be causing, and being wary of this sort of thing in general, I'm now considering putting my office on its own ring with an RCBO on the non-RCD side.

 

[very_pointy_ears]

No valid. Some vacuum cleaners are nearly 3KW.

Then presumably they'd trip the 6A breaker?

 

[ericmark]

If you have been renewing cables then you will have a means to test them I assume? So you can test with your insulation tester and simply identify the fault. I was to start with under the impression this fault had started to happen without you doing anything.

To try to guess what has happened when you are swapping cables is pointless. You need to publish the reading you have taken and explain what you did before the fault started. Still think it will be a neutral fault, but it would seem if you have renewed cables you have made an error some where, pictures may help.

 

[JohnW2]

Some vacuum cleaners are nearly 3KW.

They are, but I said "most", and I still believe that is essentially true. Indeed, I don't think that 3kW, or anything like it, can be legally sold any more, can they?

... and, in any event, as I and others keep saying, even if someone did plug a 3kW load into a 6A circuit, all that would happen (if the 'overload' lasted long enough) would be taht the OPD would operate - just as it would if someone plugged, say, a couple of substantial loads into a 16A or 20A radial sockets circuit.

Kind Regards, John

 

[winston1]

... and, in any event, as I and others keep saying, even if someone did plug a 3kW load into a 6A circuit, all that would happen (if the 'overload' lasted long enough) would be taht the OPD would operate -

and in the loft case plunge you into darkness in a place where there may be no floor.

 

[winston1]

Then presumably they'd trip the 6A breaker?

and plunge you into darkness

 

[ban-all-sheds]

They are, but I said "most", and I still believe that is essentially true. Indeed, I don't think that 3kW, or anything like it, can be legally sold any more, can they?

... and, in any event, as I and others keep saying, even if someone did plug a 3kW load into a 6A circuit, all that would happen (if the 'overload' lasted long enough) would be taht the OPD would operate - just as it would if someone plugged, say, a couple of substantial loads into a 16A or 20A radial sockets circuit.

John - why are you still trying to reason with Winston? He does not do that, and he does not have a single opinion arrived at in that way.

 

[JohnW2]

and in the loft case plunge you into darkness in a place where there may be no floor.

Regardless of any details of the electrical installation, I would never go into a loft without a torch. Over the years, probably most of the (few) times I have been 'plunged into darkness', anywhere in my house, have been due to power cuts, not anything to do with the electrical installation.

Kind Regards, John

 

[very_pointy_ears]

If you have been renewing cables then you will have a means to test them I assume? So you can test with your insulation tester and simply identify the fault. I was to start with under the impression this fault had started to happen without you doing anything.

To try to guess what has happened when you are swapping cables is pointless. You need to publish the reading you have taken and explain what you did before the fault started. Still think it will be a neutral fault, but it would seem if you have renewed cables you have made an error some where, pictures may help.

Sorry if I misled you.

I have a tester on order but it has not arrived yet as I only ordered it yesterday.

I think you're probably right about a neutral fault.

What I've done so far (I've had a few glasses of wine now so forgive me if this is a bit triceratops but with less spinach):

• traced route of original ring that included the kitchen, identified cables (made easier by the fact that most were on view as the downstairs ceilings are in fact the underside of the upstairs floorboards)
• identified cable from CU that went straight to kitchen ring
• identified cables leaving CU on this ring and marked which one went via snug and other room and which one went directly to kitchen
• identified junction box (let's call this David) in ceiling void above kitchen connecting the ring cable coming in from snug to kitchen
• removed cable going from snug to David
• installed a new run of 2.5 T+E from CU to David and connected it where the snug->David cable had been connected
• installed new B32 breaker in unused slot of RCD protected side of CU, connected live cores to this, moved neutrals and CPCs to corresponding positions on connector blocks; kitchen is now on its own circuit
• for the moment, replaced the B32 breaker that fed what is now a single 2.5 T+E feeding snug sockets with a B16 one
• (several days pass)
• removed all ring main cabling from snug and the room above, as these rooms are being gutted and re-done (B16 now redundant)

Meanwhile (back at the ranch)

• computer room is on a different circuit, but one that happens to feed one of the sockets in the room that was otherwise was on the kitchen/snug ring
• after some testing, realise that the two 2.5 T+E cables whose live cores connect into a B32 breaker aren't in fact on a ring but are instead two separate strings of spur sockets, ie broken ring
• rip out one of these strings of sockets and cabling, which happens to include the socket that feeds my UPS
• as a temporary measure, plug in the UPS via an extension cable to one of the sockets on the same breaker, but on the other string of sockets
• replace the B32 breaker feeding the remaining string of sockets with the B16 one that was previously in temporary use in the snug

And on lighting:

• identify junction box in ceiling void above kitchen where lighting power enters kitchen
• remove cable feeding that, as it's in an ugly place
• feed new 1mm T+E cable directly from CU on a new B6 breaker to the junction box so that the kitchen now has its own dedicated lighting circuit (the main reason for this is because the previous feed, which came from the snug lighting circuit, was routed in a place that made it look awful)

And then ... notice issues with RCD tripping. Without LSD.

 

[very_pointy_ears]

and plunge you into darkness

And of course there's your sense of touch and a glimmer of light coming from the loft hatch.

 

[ericmark]

I expect at some point we have all done jobs without doing all the testing laid out in the famous book 3, there are it seems 7 books in a series from the IET but book 3 covers what to do when inspecting and testing.

I have a shared meter between my son and I, so often there is a delay between finishing a job, and officially commissioning it. In the main not a problem, but every so often one does make an error.

In your case I would switch off the mains and the RCD and all MCB's and test with a 500 volt insulation tester between neutral and earth, if down then one by one test each neutral, until faulty circuit located, then slowly divide the circuit until fault is found.

I think most testing for faults falls into two stages, stage one is we test likely causes, as said before unplug everything, look for water in outside lights etc. If this fails then stage 2 is to test from origin and step by step isolate where the fault is, I think you have reached stage 2.