Lighting circuit connected to two MCBs

[eBatch]

I went to change a light fitting in my mother-in-law's house.

Tried turning off each of the 6 amp MCBs in turn to isolate the fitting in question. No joy - the light always worked.

A few combinations later, I found that turning off two adjacent MCBs had the desired effect.

Any good reason why a circuit might be wired in to two MCBs?

 

[securespark]

No. That needs investigation!

 

[JohnW2]

Any good reason why a circuit might be wired in to two MCBs?

Definitely not. That's clearly potentially dangerous, and needs attention.

Kind Regards, John

 

[eBatch]

Have to say that my immediate reaction at the time can't be repeated in decent company! But I thought I'd ask just in case there was some explanation.

Begs the question as to how (or rather why) it has happened. Haven't opened up the consumer unit (and didn't note the brand), but it sets me thinking about the wiring inside. Did someone deliberately bridge the two MCBs? Or is it possibly just the exposed tail of the live core that is sticking out where it shouldn't and touching the adjacent connector?

I didn't check any further so not even sure as to whether any other similar issues with the other circuits.

 

[flyingsparks]

It could be within the CU but I expect more likely the two circuits have been linked together somewhere else. Either way certainly needs attention!

 

[Iggifer]

I too, doubt it's in the CU, but that doesn't mean it isn't. Would certainly be an easier fix if the fault was in the CU.

How are the circuits 'split' by labels up/down or front/back?

I would check the hallway switch that has up/down switches in it, check the circuits haven't been inadvertently linked in that switch

 

[Robin0577]

Yup, hallway/landing switches are the odds on favourites for this kind of fault.

 

[ban-all-sheds]

Any good reason why a circuit might be wired in to two MCBs?

"good reason" as in "sensible design"? No.

But as in "credible explanation"? Yes - an idiot has been fiddling with your electrics. I wonder what other surprises he has left for you, and if any of them will cause fires or electric shocks?

 

[ericmark]

Oddly I have seen on many times where a ring has two MCB's clearly the person wiring it had no idea of how a domestic ring final worked and had wired it incorrectly.

I have also seen lighting wired as a ring no good reason why there was no real volt drop problem some one had clearly thought that was how it was done.

It has been pointed out before a ring main and a ring final are very different yet it is so easy to call a ring final a ring main or extra low voltage low voltage or a fuse box a consumer unit the words have swapped around and hence DIY people make errors.

 

[ban-all-sheds]

It is hard to see how confusion over terminology, or thinking that a lighting circuit should be a ring, etc, would lead someone who actually knew enough about what they were doing to think that a circuit should be supplied by two protective devices in parallel.

 

[JohnW2]

It is hard to see how confusion over terminology, or thinking that a lighting circuit should be a ring, etc, would lead someone who actually knew enough about what they were doing to think that a circuit should be supplied by two protective devices in parallel.

Good question - although in cases such as we are discussing, one imagines that it's far more like that there is an erroneous cross-connection somewhere in the circuit, rather than deliberate connection of one circuit to two protective devices.

If I may be allowed to 'fantasise' in terms of ring final circuits, and although it would not have been an option when they were introduced (when protective devices were fuses), I could see electrical merit in having the two ends of the ring (wired in cable with CCC ≥20A) each connected to separate, but mechanically linked, 20A MCBs - since that would remove one of the 'objections'/concerns about the rings which we actually have!

Kind Regards, John

 

[ban-all-sheds]

Good question - although in cases such as we are discussing, one imagines that it's far more like that there is an erroneous cross-connection somewhere in the circuit, rather than deliberate connection of one circuit to two protective devices.

That's true.