Lollipop final circuit

[ericmark]

My sons main objection was it was not a standard method so anyone coming across it could make a mistake and easy break the ring without realising it was a ring.

I considered as long as a circuit diagram was clipped into the space provided on the consumer unit then what anyone does in the future was their concern not ours.

As I said it got heated and it was his signature so what he said went.

One could say the installation is intended to be under the supervision of a skilled or instructed person and as such slightly different criteria exists. But as some time you will leave the house we don't live forever so unless the house is demolished at some time some one will have to work out what you have done.

Forget the BS7671 do a risk assessment. Is there really any risk? And if there is would it be reasonable to take that risk? I personally see no problem but it's your signatory not mine.

 

[ricicle]

My sons main objection was it was not a standard method so anyone coming across it could make a mistake and easy break the ring without realising it was a ring.
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Anyone who couldn't grasp such a simple variation to a standard circuit arrangement shouldn't be allowed anywhere near electrical work !

 

[holmslaw]

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[ban-all-sheds]

But then you decide you want one circuit on an RCBO ...

I'd have to make changes, just as I would if I wanted to put an RCBO in the CU today.

More to the point if you common the Ns then they are not separate circuits are they?

They are already commoned in the neutral bars in the CU - - all I'm doing is, in effect, moving the existing neutral & earth bars out of the CU enclosure into another one.

and you probably have oodles of spare 2.5mm etc to hand, but not the larger sizes?

I'm fine for cable availability up to 10mm².

 

[Paul_C]

I don't see the arrangement as being any less satisfactory than a conventional ring, so long as the "handle of the lollipop" is wired with suitably sized conductors.

I would say one could look at it this way: Imagine you run a feeder from the main board to a sub-panel, and to obviate any potential arguments about earth size, assume that it's 3 x 6 sq. mm singles in conduit or something equivalent. That feeder is protected by a 32A MCB, and at the board it feeds you install another 32A MCB to which the two 2.5 sq. mm live conductors of the ring are connected. So in principle it's no different than any other sub-panel which everybody would find perfectly acceptable, except that it feeds only a single final circuit and there would be the issue of lack of discrimination between the two 32A MCB's.

For all practical purposes, the only difference between that and the proposed scheme is that you are doing away with the 32A MCB at the junction between the 6 sq. mm feeder and the 2 x 2.5 sq. mm ring conductors. As there is already a 32A MCB upstream anyway, there is no loss of protection since that second 32A MCB was already redundant.

As for arguments about it being unconventional and that it might confuse somebody coming along in the future, I'm firmly with the camp which believes that's not your problem. If somebody else who comes along in a few years' time can't figure out what's been done, then it's up to him to learn a little more to work it out or get somebody else in who can.

 

[bernardgreen]

Adding a bit more to the debate from a lollypop I saw a few years ago.

Where the single was divided into the two legs of the ring there were two fuses, one for each leg of the ring. I cannot recall the sizes and ratings but I am certain the fuse in the fuse box was rated to protect the cable of the lollypop "stick" and the maximum safe current for the ring and the two others to protect each leg of the ring if there was a problem or fault in the ring.

If the design was a good one then the fuse in the fuse box was the overall protect for gross overloads and the other two would protect the legs from an overloaded leg caused by all the load being at one end of the ring or a failed connection in the ring. If one of them popped of gross overload then likely the other one would then pop as it took the full load. Or in the event of a broken ring one leg would stay working if it was not overloaded.

 

[JohnW2]

Where the single was divided into the two legs of the ring there were two fuses, one for each leg of the ring. I cannot recall the sizes and ratings but I am certain the fuse in the fuse box was rated to protect the cable of the lollypop "stick" and the maximum safe current for the ring and the two others to protect each leg of the ring if there was a problem or fault in the ring.

The 'lollipop' circuit is essentially a red herring here. Although (as you go on to argue), is is probably safe in most predicatable scenarios, I suspect that most people would be at least a bit uneasy about separate protective devices for each end of the ring - whether that happened at the CU or the dividing point of a 'lollipop'.

Kind Regards, John.

 

[GCarnegi]

I don't know how relevant this is here, as I'm not an electrician, but I seem to remember that current doesn't flow uniformly across the sectional area of a conductor.

Something about current being a flow of electrons which are all negatively charged and try to repel away from eachother, meaning more of the electrons gather around the outside of the copper, and the core of the conductor has very few. (that's one of the reasons why multi strand wires can carry more current than single strand/solids.)

As such the current carrying capability of a conductor is more related to its circumference than its cross sectional area.

The circumference of a 2.5mm2 conductor is 2.8mm, while the circumference of a 6mm2 conductor is 4.3mm. The combined circumferences of TWO 2.5mm2 conductors is therefore 5.6mm, which means that this will carry MORE current than a single 6mm2 conductor.

Anybody still awake? Sorry!!!

 

[Paul_C]

I don't know how relevant this is here, as I'm not an electrician, but I seem to remember that current doesn't flow uniformly across the sectional area of a conductor.

The effect of greater current flowing on the outer surface of the conductor is commonly known as skin effect, and is well known (and often made use of) in radio-frequency applications. It becomes more noticeable as frequency increases.

For the size of conductors under consideration here at the low frequency of 50Hz, it's not a significant factor.

 

[ban-all-sheds]

At 50Hz the skin depth of copper is about 9mm, so the effect you describe is of no practical consideration with conductors which are under 2mm dia....

 

[holmslaw]

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[Paul_C]

That is basically true but can be ignored at low voltages the effect becomes more pronounced at very high voltages.

It's the frequency which is the controlling factor, not voltage.

 

[bernardgreen]

GCarnegi is correct.

The current density is higher on the surface of a conductor than at the centre. The difference ratio increases as the frequency increases.

At 50 Hz the difference is small enough to be almost insignifcant. But not completely insignificant.

 

[GCarnegi]

Thanks for setting me straight. I think I fell asleep in that class after about 20 minutes. The second half hour probably said what you guys just did.

 

[bernardgreen]

Looks like a few people have been woken up !