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

 

[Lucien Nunes]

There's a short between N and E between room 1 (that worked) and room 2.

This doesn't go any way to explaining why the MCB trips.

Just a thought could be N + L reversed between the first and second pendant

If nearer the supply than the N-CPC fault, the switch and lamp would have no effect and the RCD would trip immediately the circuit was energised. If further away, it would not alter the polarity at the N-CPC fault and hence not cause the MCB to trip. Also, because this installation uses sheathed singles with lines looped at the switch, it's most unlikely.

So why did the MCB trip?

 

[Richard Measures]

Hi all.
Thanks again for your thoughts on this.
I ran tests again and found a definite low reading between Rn and R2 on the link between the 1st and 2nd room. Despite having to lift quite a lot of the landing, I laid in a new T&E (using just 2 conductors) to replace, and.....

All is well!

No sign of MCB tripping during this round of tests so I'm putting this down to a poor MCB, although it is working now.

I guess long term operation will be the proof of the pudding but so far so good.

 

[Pete999]

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

Why is it a bonkers system?

 

[UNG]

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

There are plenty of properties with the live looped at the switch, quite a few of the councils local to me insisted on live looped at the switch on rewires so there was no permanent live at the fitting so meddling tenants didn't kill themselves

 

[Richard Measures]

I say bonkers because earth and neutral are already looping around the fittings, so doing the same with live seems obvious. I'd question the council's logic - what if they take a switch off?

 

[Pete999]

I say bonkers because earth and neutral are already looping around the fittings, so doing the same with live seems obvious. I'd question the council's logic - what if they take a switch off?

??

 

[UNG]

I say bonkers because earth and neutral are already looping around the fittings, so doing the same with live seems obvious. I'd question the council's logic - what if they take a switch off?

I don't normally take a switch off to change a light fitting may be you do, the council's logic is no looped live at the fitting reduces the risk of a tenant harming themselves, calling them bonkers or any other names won't matter to them they have their reasons for this specification

I find it bonkers that you limit access to your profile on here so it gives us no clue to who or what you are but that is life

 

[Richard Measures]

Sorry Ung, not trying to be contentious.
Forgive use of language - just find the single wire system over complex - guess that's why it's been phased out, no?
Didn't realise my profile is limited - I'll look into that, but needless to say I'm pretty new to this...

 

[Pete999]

I don't normally take a switch off to change a light fitting may be you do, the council's logic is no looped live at the fitting reduces the risk of a tenant harming themselves, calling them bonkers or any other names won't matter to them they have their reasons for this specification

I find it bonkers that you limit access to your profile on here so it gives us no clue to who or what you are but that is life

One of my Pet hates as well UNG.

 

[Bobster]

One of my Pet hates as well UNG.

There was a period of time with new members signing up that profiles were hidden as default. I'm sure this has been changed since.

 

[telectrix]

just find the single wire system over complex -

not complex at all. advantages are:
1. only a single conductor from switch to light.
2. switch off, light has no L. ( like in 3 plate).
3. less use of expensive copper cable.
4.confuses anyone under 50.

 

[UNG]

Sorry Ung, not trying to be contentious.
Forgive use of language - just find the single wire system over complex - guess that's why it's been phased out, no?

Excuse my ignorance when was it phased out as I still occasionally use it when wiring wall lights and other situations where it is or will be difficult to hide a lot of cables

If you find the single wire system complex what are your views on the many different ways of wiring a 2 way lighting circuit

 

[telectrix]

Excuse my ignorance when was it phased out as I still occasionally use it when wiring wall lights and other situations where it is or will be difficult to hide a lot of cables

If you find the single wire system complex what are your views on the many different ways of wiring a 2 way lighting circuit

touche.

 

[Pete999]

Excuse my ignorance when was it phased out as I still occasionally use it when wiring wall lights and other situations where it is or will be difficult to hide a lot of cables

If you find the single wire system complex what are your views on the many different ways of wiring a 2 way lighting circuit

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

 

[telectrix]

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

you still got any teeth left? stripping VIR singles wreaks havoc on the incisors.