Intermediate switching

Retroelectric · Apr 9, 2021

Hi
I was asked to change an intermediate and 2 way switch for chrome and some other bits , so fairly straightforward I thought.
About 10 years ago someone converted the landing 2 way to an intermediate by taking a 3 core across the other side of the landing from the switch at the top of the stairs.
This switch is wired by the twin upstairs method and bridging a live across from the downstairs 2 gang switch and a single switch wire is at the intermediate.
So I have 2 way that's - 3 core - intermediate that has 3 core twin from downstairs and single switch to pendant - 2 way 2 gang downstairs - twin and a perm live taken from other switch.
Apparently its not been working for a while but I'm not sure how it worked at all.
I was on autopilot at the time and just wired it the same but it still does not work.
I think it needs another core but anyone know different?
Excuse the drawing.

Mike Johnson · Apr 9, 2021

Don't see how the intermediate ever worked with only three core?

Retroelectric · Apr 9, 2021

I cant see how it worked.
The intermediate has a 3 core from the 2 way, a single that's the switch wire and a twin going down to the other 2 way so there is 6 cores at the intermediate.

SparkyChick · Apr 9, 2021

If I understand correctly... you have the following situation...

2W switch downstairs... permanent live to COM, L1+L2 up the twin to the intermediate.
Intermediate... single to the light, twin from downstairs, 3C to the other 2W
Other 2W 3C from the intermediate.

If this is the case....

Perm Live -> 2W Com => L1 and L2 (on the 2C to the intermediate) -> L1 and L2 on one side of the intermediate, L1 and L2 on other side of the intermediate to 2 cores of the 3C => L1 and L2 on the other 2W switch, COM to the 3rd core of the 3C -> intermediate -> single to the light

SparkyChick · Apr 9, 2021

It's not ideal because you'll be generating a magnetic field that won't be cancelled out by the return but it will work perfectly fine.

Retroelectric · Apr 9, 2021

Thanks for that ?

Mike Johnson · Apr 9, 2021

SparkyChick said:
It's not ideal because you'll be generating a magnetic field that won't be cancelled out by the return but it will work perfectly fine.

Is that not the problem that cause's flashing of LED's where a condenser is needed to cancel it out?

Lucien Nunes · Apr 9, 2021

No that's due to capacitance between the cores that allows a small current to flow from perm L to switched L when the switch is off. The unbalanced current thing is where the 'go' and 'return' (e.g. PL and SL or L and N) carrying the same current take different routes instead of along two cores of the same cable. If the current flows around a loop with conductors spaced widely apart, they behave like a single turn coil that radiates electrical interference. Not so important when it was just 50Hz to a filament lamp but LED drivers and dimmers can produce wide band interference that radiates more readily.

Many people ignore this completely, e.g. with 'conventional' 2-way and T+E for strappers as in this case, also with single red / single black installations. My experience and involvement with professional audio and studio electronics, where electrical noise sources such as current loops can cause headaches, makes me rather conscious of it so I would never install that way if avoidable.