A builder hands over a flat. Six months later the owner’s CCTV picture rolls every time the pump starts, and the broadband drops out in the evening. An electrician gets called, checks the router, blames the ISP, and everybody is annoyed for a year.
The actual fault was decided during construction, when somebody pulled a camera cable and a 2.5 mm² power circuit down the same 25 mm conduit because it was there.
Why it goes wrong
A current-carrying power conductor produces a magnetic field around it, and an alternating one produces an alternating field. Any signal conductor sitting inside that field has voltage induced into it. On a 230 V lighting circuit the induced level is tiny in absolute terms, but a CCTV signal or a data pair is working at millivolts, so tiny is enough. Motors, dimmers, VFDs, inverters and pumps make it far worse, because they switch hard and produce sharp edges rather than a clean sine wave.
Data cable defends itself with twisted pairs and, in shielded types, a screen. That defence is real but it is finite. Push a camera cable hard against a submersible pump feeder for thirty metres and it will lose.
The rules that actually work
- Separate conduits, always, if you have the choice. This is the whole answer. Everything below is damage control for buildings where it wasn’t done.
- Distance beats everything. Induced voltage falls off quickly with separation, so even pushing the two routes apart by a few hundred millimetres inside the same ceiling helps a great deal.
- Cross at right angles. Where a power route and a signal route must meet, cross them at ninety degrees rather than letting them run alongside each other. A crossing couples almost nothing; a parallel run couples along its whole length.
- Keep parallel runs short. Coupling accumulates with distance. Two metres side by side is usually survivable, forty is not.
- Never share a conduit between a motor or pump feeder and any signal cable. Switching loads are the worst offenders by a distance.
- Earth a screen at one end only. Earthing both ends of a screen creates a loop that carries current between two earth points and injects noise instead of removing it. Same principle as the drain wire in an instrumentation cable.
- Don’t overfill the conduit. Cramming cables together removes the one thing keeping them apart. The conduit fill chart gives the sensible limits.
Exact minimum separation distances vary with the wiring regulation, the voltage and the installation method, so take the number from your project specification or consultant rather than from a rule of thumb. The principles above are stable regardless of which code you’re working to.
Choosing cable that tolerates a bad route
Sometimes the route is fixed and unpleasant, and cable choice has to make up for it. Properly built LAN cable and coaxial CCTV cable with correct construction and consistent impedance will ride out interference that cheap cable won’t. On the power side, the house wiring itself should be a known quantity, which is the argument for a specified grade like HR-FR-LSH house wire rather than whatever was on the van.
Our structured cabling basics guide covers how to plan LAN, CCTV and CATV routes in a home or small building before the conduits go in, which is the only time this is cheap to fix.
If you’re building or renovating right now
Run one dedicated ELV conduit to every room, even if you don’t know yet what will go down it. It costs a few hundred rupees during construction. Retrofitting the same conduit after the plaster is on costs somebody a wall.