That thin wire behind the switch — what is it, really?
Look behind almost any switch in a smart home and you'll find two cables, not one: the usual power wire, and a slimmer, often green one running right alongside it. That second wire isn't carrying electricity to your lamps — it's carrying a quiet signal that lets your switches, lights and blinds talk to each other.
How it works, in plain terms
Two cables, not one
Behind every switch, alongside the normal power wiring, runs a second, thinner cable — usually green.
It's not powering anything
No 230V electricity runs through this thin cable. It carries a gentle signal — a kind of conversation between your switches, lights and blinds.
It goes in during renovation
While the walls are still open, running this cable is simple. Once everything is plastered and painted, adding it becomes far more disruptive.
Rooms start talking to each other
Once connected, a switch in one room can tell a light or blind in another room what to do — much like an internal phone line for your house.
Why it's a separate cable from your lighting
Two cables, two completely different jobs
One wire delivers power. The other carries conversation between devices. That's exactly why they never share the same path.
Power or conversation — never both
The 230V wire feeds your lamps and appliances. The thin green cable only carries a low-voltage signal that lets devices talk to one another.
Its own route through the wall
For safety and reliability, the bus cable is kept physically separate from the 230V wiring for its entire run — never bundled in together.
Usually green, so it's easy to spot
A certified KNX bus cable is typically green, so an electrician can recognise it at a glance among all the other wiring.
Fitted while it's still easy
This cable is run during renovation, while walls are open — adding it later would mean opening them back up.
Branches like a tree, never loops
Devices branch off the main cable one after another, like branches from a trunk — never a closed loop — which keeps the whole system simple and dependable.
What's actually inside that little cable
A quick, jargon-free look under the jacket.
- {'t': 'The red and black pair', 'd': "This pair carries both the bus's own power and its signal — the pair devices actually 'talk' through."}
- {'t': 'The white and yellow pair', 'd': 'A spare pair, usually left unused at first — simply room to grow into later.'}
- {'t': 'The green outer jacket', 'd': 'A recognisable colour, so the cable is never mistaken for ordinary electrical wiring behind the wall.'}
- {'t': 'A certified standard', 'd': 'The cable follows the international KNX standard (ISO/IEC 14543-3, EN 50090), so any certified installer knows exactly how to work with it.'}
- {'t': 'Low-voltage by design', 'd': 'The signal running through it sits at a gentle, low-voltage level — built for safe conversation between devices, not for powering them.'}
Frequently asked questions
Can't you just use the wire that already goes to the lights?
No — the signal needs its own, separate cable so the whole system stays safe and reliable for years to come.
Why can't it be added later, once renovation is finished?
Technically it can, but in practice that means opening the walls back up — running it now, while they're still open, is far simpler.
Is this wire dangerous to touch, like a power cable?
No — it only carries a gentle signal, not 230V electricity, so it's not dangerous to touch.
Why is it always green?
That's the recognised, certified colour for this type of cable — it helps an electrician instantly tell it apart from the rest of the wiring.
What happens if it's wired the wrong way round (red and black swapped)?
It's a common but easy-to-spot mistake — it gets checked and corrected before the system is switched on, so there's nothing to worry about.
Want the plain-words version for your own home?
If you're planning a renovation and wondering whether KNX makes sense for your family, let's talk — no jargon, just a clear answer.