Why must power and signal cables be run separately?
A power cable switched with PWM produces rapidly changing magnetic and electric fields around it; this field induces voltage into a signal cable running parallel alongside it. The practical measure is to leave at least 15 cm of separation between power and signal, to cross them at right angles where a crossing is necessary, and to use screened cable on the signal side.
When two cables run in parallel, an unwanted capacitive and inductive coupling forms between them. The strength of the coupling depends on four things: the distance run in parallel, the spacing between them, the rate of change of the current and the impedance of the signal circuit. You can influence three of these during installation.
- Spacing: separate the power and signal routes, in practice leaving at least 15 cm. Coupling falls off quickly with distance.
- Parallelism: long parallel runs are the worst case. If unavoidable, use an earthed metal divider in between.
- Crossing: if routes have to cross, cross them at 90 degrees; at a right-angled crossing the mutual coupling falls to a minimum.
- Twisting: twist the outgoing and return conductors. A twisted pair produces the induced voltage with opposite sign in successive twists so that they cancel each other.
- Shield: use screened cable for analogue signals and connect the shield to chassis at one end only, with 360-degree contact.
DC motors producing brush arcs and switching driver outputs in particular radiate broadband noise; running these lines inside the same conduit as a sensor or communication cable is one of the most frequent installation mistakes.
The full topic: Installation, redundancy and field continuity