Consumer unit query

[SUNRAY]

Yes it is.
I have been to several installations where DIYdave (and electricians) have mixed up the legs of two ring final circuits. this means that one leg of Circuit A goes to MCB1 and the other leg to MCB2. and one leg of Circuit B goes to MCB1 and the other leg to MCB2.
The give away for this mistake is that to isolate a circuit one has to turn off both MCB1 and MCB2 to isolate either of the circuits.

Obviously a lack of even basic testing after having done some work. Turn it on, everything works, so it must be OK.....

Going back to 60's it was fairly common to find a fusebox of 15A ceramic fusewire holders (formerly supplying 15A radials) and a ring being constructed and fed by 2 15A fuses, equally it was quite common to find a ring of junction boxes and a short spur to every socket.

 

[securespark]

Blinking 'eck, now I feel old. I've seen both of those set-ups more than once!

Sparks did love JBs in the 60s!

 

[JohnW2]

Yes it is. ... I have been to several installations where DIYdave (and electricians) have mixed up the legs of two ring final circuits. this means that one leg of Circuit A goes to MCB1 and the other leg to MCB2. and one leg of Circuit B goes to MCB1 and the other leg to MCB2.

I still think EFLI is correct, in saying that it is not possible (per BS7671 definition, as quoted by him) to have two circuits supplied by the same OPD(s), since, by that definition, they are then one circuit, not two circuits..

In the scenario you describe, what you call Circuit A and Circuit B have common origins and are protected by the same OPDs - so, per BS7671 definition, they are one circuit, not two, aren't they?

Kind Regards, John

 

[JohnW2]

I didn't notice that - 314-01-04 .... It was still 314.4 in the last book.

I'd forgotten that it was still in the current regs. You described it as 'contradictory', but I don't think it really is. It's more a matter of it being 'silly', because it is, by implication, 'outlawing' something which is, by the regs' own definition, impossible !

In other word, if one connects multiple[le 'somethings' to the same OPD, then that reg will never be violated, since the totality of all those 'somethings' are, by BS7671, one circuit!

Kind Regards, John

 

[Risteard]

I still think EFLI is correct, in saying that it is not possible (per BS7671 definition, as quoted by him) to have two circuits supplied by the same OPD(s), since, by that definition, they are then one circuit, not two circuits..

In the scenario you describe, what you call Circuit A and Circuit B have common origins and are protected by the same OPDs - so, per BS7671 definition, they are one circuit, not two, aren't they?

Kind Regards, John

They're protected by different parallel protective devices though, so it's a different scenario.

 

[securespark]

I still think EFLI is correct, in saying that it is not possible (per BS7671 definition, as quoted by him) to have two circuits supplied by the same OPD(s), since, by that definition, they are then one circuit, not two circuits..

In the scenario you describe, what you call Circuit A and Circuit B have common origins and are protected by the same OPDs - so, per BS7671 definition, they are one circuit, not two, aren't they?

Kind Regards, John

Are you saying two ring finals cross-connected to two breakers is compliant?

 

[EFLImpudence]

I can't say I have ever taken any notice of the regulation containing the option of "OPD(s)" being plural.

Odd.

 

[yorkspark]

how big is this cu?

i'd bet the 4 cores are twisted into their respective circuit pairs, as cu wasn't big enough and someone has moved 1 ring into another's mcb to free a space, probably for the obsolete shower.

 

[Jurassicspark]

how big is this cu?

8 way. 3 of them were being used; rest had exposed busbars. Bought busbar covers.

 

[Murdochcat]

In the 17th Edition, it was 314.4. Is it the same in the 18th Edition?

yup 18th amd 2

 

[JohnW2]

I can't say I have ever taken any notice of the regulation containing the option of "OPD(s)" being plural.

Interesting point, but what about FCUs (or, if one moves away from fixed installations, fused plugs) when there is another upstream OPD?

Kind Regards, John

 

[JohnW2]

Are you saying two ring finals cross-connected to two breakers is compliant?

No, I was saying that 'two ring finals' connect to one breaker would be compliant with 314.4 since, although you might regard it as 'two ring finals', per BS7671 definition it would all be "one circuit" (same origin and protected by the same OOD).

I can't think off the top of my head of what reg outlaws it, but I think we can probably agree that having one (or more) 'circuits' connected to two (or more) OPDs (such that one circuit was connected to more than one OPD) is simply 'not safe'.

Kind Regards, John

 

[Jurassicspark]

I agree

 

[EFLImpudence]

Interesting point, but what about FCUs (or, if one moves away from fixed installations, fused plugs) when there is another upstream OPD?

Well, Eric often says he thinks an FCU creates a circuit and I would by definition agree with him.

"Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by
the same protective device(s)."

Now realising the regulation contains the plural OPD(s) and "from same origin", I have no idea what it means - par for the course.


Where is the origin of a circuit if not the OPD and therefore how can there be more than one OPD if one is the origin?

 

[securespark]

So therefore, an FCU does not create a circuit.