Single core and earth lighting fault

[Richard Measures]

Could the better and wiser assist with an odd one...?

I've just finished a big CU swap on a 1950's house that uses single core & earth lighting circuits. All but one of these are fine, but the faulty one is pretty funky;

There are 3 accessories on this circuit in 3 separate rooms. The first room/switch/pendant works fine. The later 2 have the same issue - with no lamp in the fittings all is well, but put a lamp into either with the switch on and 3 things happen:
- The bulb glows very briefly (30mS?)
- The RCD covering this half of the CU trips
- The 6amp type B MCB covering the circuit trips

If this was just an RCD issue I'd be confident of a neutral/borrowed neutral issue, but how can a bulb (which doesn't blow/fail during this) cause an MCB to trip?

It's got me going round in circles so any clarity and advice is most welcome.

Thanks

 

[UNG]

Cut my teeth wiring new builds in singles, easy peasy method.

The training was a lot better back then

 

[Pete999]

The training was a lot better back then

True very True, back when UNG? I'm talking early 60s, 1960s for those who need to ask.

 

[UNG]

True very True, back when UNG? I'm talking early 60s, 1960s for those who need to ask.

I got through just before they started really screwing the training up at the beginning of the 80's it has all been on the slide since

 

[Lucien Nunes]

There is a minor disadvantage in wiring in singles not in conduit which is that where the 'go' and 'return' are not in the same cable, i.e. the currents are not balanced, noisy LED drivers (and before them, noisy CFLs) emit more radio interference. Also happened with traditional fluorescents but those were less common in a domestic install.

No sign of MCB tripping during this round of tests so I'm putting this down to a poor MCB,

Sorry, doesn't make sense. The MCB only knows how much current is going into the load, not where it goes afterwards. I'm not saying you haven't fixed the problem, but as yet there's no explanation of why the MCB was tripping, be it rich, poor or completely skint. I don't like unexplained phenomena.

 

[davesparks]

Yes Charlie, live looped through switches (2-way in the second room) and neutrals/earths looped via pendants - bonkers system IMHO

It's the most logical system and replicates the way lighting is wired in conduit.
I say it's the most logical because you take each conductor to where it is needed rather than taking it to places it is not needed and then jointing it.

 

[Lucien Nunes]

It is, however, harder to alter, in a way that does not mirror conduit work. E.g. adding a point to a normal looped conduit system, you are almost guaranteed a route through the existing conduit to get the two sides of the circuit together ready to head off to the new point. But with singles run separately in a domestic situation, you may well have to access two locations; getting the neutral from the rose might be easy, but then you have to open up elsewhere to get at the line, and if you can't add a cable down to the switch, you are forced to add a junction box, possibly in an undesirable position. Neither of these are necessary if the line is looped at the rose too.