Thread by @soilandreyes

Tight coupling is not just in software..
So as our upstairs carpet is getting changed I thought it should be easy to add wiring for second lamp in hallway.

The hall switch (left) is next to 2-way switch for stairs. I could not measure to find a neutral line here (except earth!), so left cable is just intermediate to lamps live effectively regular 1 way switch)

Existing hall lamp. The black with silver tape is from switch, brown lighting bulbs, blue return to neutral black. You see the red part of hall switch cable join the other always-live-reds in the middle (unrelated to the bulb). Presumable one red/black pair is feed from supply.

So then let's follow back the always live red cable on the left, bridge to feed two way switch on right, which decides which color goes live in right pair, black (L2) or red (L1).

Upstairs two way switch. Black L2 or Red L1 coming up, toggles which is fed to red going up. To complicate matters two leds are connected in serial between L1 L2; it lights when lamp is off!
Notice upper pair black is not connected.

In stair lamp. We find the pair with disconnected black, which red live goes to lamp brown, blue back from bulb to a different black neutral. Again unrelated live reds in middle, one of which is paired with the neutral black. (Second middle pair probably goes in to bathroom lamp)

Debugging the cables (here taped black for my sanity) we find one goes from hall switch to hall lamp, one goes from hall lamp to stairs lamp (!), and one goes from downstairs switch to upstairs switch. We remember there is also the lone red from upstairs switch to stairs lamp.

So it is (puh) a pretty normal 2 way switch circuit, just in British style with the lamp holders playing dual role of also propagating the supply feed (middle reds) and joint neutrals (right blacks).

So to add a new lamp spur (close to downstairs switch) it should be sufficient to split the two taped cables and add junction box. I will need live from the switch-lamp cable and neutral back to the lamp-lamp cable. I do not need to touch the 2way cable between switches.

Because of tight coupling (or being out of touch with British "best practice" I had to debug the supposedly independent 2 way system.
Good thing is I don't need to pull new cables through wall (switch doesn't have neutral!), nor add long 4th pair to daisy chain from hallway lamp

Split feeder cable from hall lamp to stairs lamp. As you see not much room for manoeuver (or cutting mistakes) on this one.
Fuse on, triple check this was wired correctly as later we need this neutral.

Btw, before cutting a cable (I only took off fuse for lighting circuit) it is crucial to test with a power tracer the line is dead. Here shown on the separete cabling for socket, still live, do not cut!

Disconnecting the red live I verified it gave all the power to upstairs light, except stairs lamp, which as you remember get its live from the second cable from downstairs switch. So time to split it!

My tip for when stripping cables, roll back outer insulation to check the inner pair insulation is not damaged. Here I left enough spare cable for second cut in case I did a mistake.

Minor plan P detail, always add yellow/green earth sock. Cut to measure!

If we had no clue what was going on, or wanted loose coupling, we could reconnect switch cable in separate terminals like this.

Remember switch gets live feed on red, and optionally return live on black. So this black is special, and keeping with regulations we mark with red tape to warn future engineers. (Equivalent: code comment!)

This could however get confusing to future visitors as it could also mean this is an old mis-coloured cable with live black and neutral red. Best practice is to also tape mark those if found and not yet fixed.

Design of these terminals include a nice edge so I can measure exactly where to cut each cable in advance.

Yes yes, unit testing I know...!
Look. Red is only connected to black from switch if it is "on" (fuse is always off when measuring resistance)

Refactoring complete, integration test successful. And as you see the lamp above me is indeed lighting up, so we did not break feed to the 2-way circuit (which switch cable we did not touch).

Obligatory counter example; strangely the new cable was less resilient!
Btw, red/black cabling dates from <2004, after that EU-regulated brown/blue pairs is required on new cabling. electriciancourses4u.co.uk/useful-resourc…
Perhaps post brexit it will change back to red/black?

After refactoring, adding the new feature is straight forward. So brown=red~=live and blue=black~=neutral. (In other countries both pairs may be live against earth during AC, but it is still useful to think of it in this fashion)

Live in production.
Next scrum will replace prototype with something nicer ;)

Released (e.g. junction box fixed to beam).
Code smell left underneath. How often should one hover under the floor..?

Whaah! Just as floorboard is put back (carefully not to squeeze the cables), shouts from downstairs: "New light stopped working!"
Panic over, replaced buggy prototype with classic fool-proof prototype.

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