lights on ring mains??

[JohnW2]

What I am saying is with a B32 MCB the current needs to be 160A to trip in 0.01 of a second and unless the earth loop impedance is below 1.44 ohms it will not trip on the magnetic part of the trip ... So likely with a B6 MCB 100A will flow for 0.01 of a second but with a B32 MCB it will flow for 12 seconds before disconnecting. That's 1200 times the energy which will need dissipating as heat to that using a B6 MCB.

Fair enough - with those numbers/assumptions I would agree. In fact, I think the disconnection time of a B32 at 100A is nearer to 25 seconds than to 12 seconds, so we could be talking nearer to 2,500 times the energy.

However, that's because you have assumed a fault current of 100A - i.e. 'a fault loop impedance' (Zs+fault impedance) of about 2.3 Ω (at 230V). If the fault current was nearer to (or equal to or greater than) 160A, then the disconnection time, hence energy involved, would be much the same with a B6 and B32.

However, you raise an interesting point. As I said at the start of my post, it seems that many electricians would say that that the MCB was there only to protect the cable, not accessories or loads (so would be 'happy' provided that a B32 was affording adequate fault protection the the cable, which it probably would) - but you are thinking of 'protection' of the lamp/fittings fed by the cable (i.e. to prevent the welding you experienced). As you will be aware from my mutterings, I have always felt that it is very reasonable to use the lowest possible rated OPD to 'protect' a low current load (many of which don't have much/any internal protection), even if a much higher-rated one would be adequate to protect the cable (e.g. I often use 1A fuses in plugs for small electronic things) - but it seems that many electricians stick to their view that the OPD needs only to protect the cable.

Kind Regards, John

 

[Thornwhite]

Sorry for delayed response - no internet over the weekend.

I can get pictures tonight, but the situation is largely as davelx suggested.

the ring mains has been looped through the ceiling roses and that the "spur" is the switch drop. i.e. standard loop-in/out way of wiring the rose, but on the RFC instead of on a lighting circuit.

There are other socket outlets on the downstream side of the second rose.

The 'Ceiling Rose' is just the base of a bog standard plastic light pendant that is being used as a junction box. I would guess that it is probably too lightweight - electrically - to be used like this.

To summarise, then, I think.
This wiring design, whilst not ideal, can be made to work OK (safely).
Protection needs to be fitted in the form of fuses to protect the light circuitry.
The light switch needs to be uprated to cope with 32A
Ceiling roses should be replaced by proper junction boxes that are rated for 32A

Is this it or have I gotten lost somewhere.

Appreicate all of the comment so far.

 

[AndyPRK]

I'm just amazed someone would wire it up like that!

Why !!??

 

[JohnW2]

I'm just amazed someone would wire it up like that!

Indeed - if it really is as described (and it sounds as if the OP is fairly certain), it's crazy.

Why !!??

I suppose 'convenience', on the part of a person who didn't understand the difference between a sockets circuit and a lighting one, and just regarded them both as as 'just electricity'! Definitely not the sort of person who should be even dreaming of undertaking such work!

Kind Regards, John

 

[AndyPRK]

Sounds like its in a 1980s extension.

Maybe it's limited to the extension and only a ring is supplied to the extn and builder done the electrics.

 

[Thornwhite]

I switched off the MCB and then went round the house finding out which sockets were no longer active
It seems that this ring main seems to supply most of the sockets - but not all???

from the replies, I guess it would it be much ore sensible to run a new circuit for these two lights or try and tap into another lighting circuit. Will have to see what can be accessed

 

[AndyPRK]

If its only 2 lights in an extension it's not too bad.

As already said a fused light switch would protect the bulb holder for those that worry about welded bulb holders!

I guess the question from me is:

A) is it really a ring in the extension ? Or just a spur off the ring circuit?

If a spur and little load on sockets in the extn fit a fuse for the whole extension which it should have anyway!

 

[JohnW2]

If its only 2 lights in an extension it's not too bad. As already said a fused light switch would protect the bulb holder for those that worry about welded bulb holders!

Indeed, from the point of view of the lights, I personally wouldn't be too concerned. I'd probably be a bit more worried about a sockets circuit (and we are told that there are socket(s) downstream of the second rose), whether ring or radial, going through connections in ceiling roses. Again, probably OK in practice (without considering 'compliance'!), but it certainly "doesn't feel very nice".

... is it really a ring in the extension ? Or just a spur off the ring circuit? ... If a spur and little load on sockets in the extn fit a fuse for the whole extension which it should have anyway!

Indeed so.

Kind Regards, John

 

[PrenticeBoyofDerry]

The main concern would be that you have a 32A circuit, connected to accessories that are rated at possibly 10A. Even if you down fused via S/FCU (swapping out existing switch), you still have terminals/conductors connected to the 32A circuit within the ceiling rose loop. That is if I am understanding the set-up correctly.

 

[JohnW2]

The main concern would be that you have a 32A circuit, connected to accessories that are rated at possibly 10A. Even if you down fused via S/FCU (swapping out existing switch), you still have terminals/conductors connected to the 32A circuit within the ceiling rose loop.

Indeed, I think that's what I said (wrote) about an hour ago....

I'd probably be a bit more worried about a sockets circuit (and we are told that there are socket(s) downstream of the second rose), whether ring or radial, going through connections in ceiling roses. Again, probably OK in practice (without considering 'compliance'!), but it certainly "doesn't feel very nice".

Kind Regards, John