Bit of an odd one.

[JohnW2]

There probably isn't any advantage, these were in the days when houses were wired using singles so avoiding the need for 3 core wasn't a consideration.

As I implied, I can't really think of any significant 'advantages', but I can think of several 'disadvantages' (more work, plus danger) - so I wonder why it was ever done. I must say that, until now, I had not realised that such a wiring method had ever been utilised!

Kind Regards, John

 

[endecotp]

I'm making this up, but imagine a 5-year-old me having two-way lighting wiring explained by my father:

"So that's the theory, but it needs a 3-core cable, and we don't have any of that in the shed. So we've got two choices. Either we can use the earth wire as the third core, which is a bit dangerous, or we can use this alternative circuit, which is also dangerous in a different way. So remember, if you ever need to change a bulb that's controlled by two switches, take care where you put your fingers."

 

[EFLImpudence]

I'm not sure that is a valid explanation.

Your 'unusual' example requires unlikely numbers of conductors between the switches and light which would not occur in normal installation methods or premises.

The two-strapper conventional method leads to the quite frequent borrowed neutral which would be preferable - especially before RCDs.

 

[JohnW2]

"So that's the theory, but it needs a 3-core cable, and we don't have any of that in the shed. So we've got two choices. Either we can use the earth wire as the third core, which is a bit dangerous, or we can use this alternative circuit, which is also dangerous in a different way. So remember, if you ever need to change a bulb that's controlled by two switches, take care where you put your fingers."

Hmmm. I'm not particularly impressed with a choice between two 'dangerous' options - and, to be frank, if I were somehow forced to choose one of them, I think that using a bare CPC as a live conductor is probably less dangerous (certainly to the user) than the ('diabolical', IMO!) alternative.

In any event, there is a third option, namely to use two 2-core cables (and ignore one of the cores), if that's all you have in your shed. That's essentially what often happened with the 'conventional' method of two way switching - with 2-core between the switches and only a single cores needed between the light and one switch, between line and the other switch and between the light and neutral. As EFLI has said, that's how/why 'borrowed neutrals' often arose.

Kind Regards, John