How much separation should I leave between control and power cables?
Leave at least about 20 cm (8 inches) between power cables and control/signal cables running in parallel. Make unavoidable crossings at a 90 degree right angle. Avoid carrying them in the same cable trunking; if the same trunking must be used, choose trunking with a metal divider.
Coupling between a power cable and a signal cable occurs through two mechanisms: inductive coupling via the magnetic field and capacitive coupling via the electric field. Both weaken rapidly as distance increases; that is why separation is the cheapest EMC measure.
Rules of application:
- At least ≈20 cm on parallel runs. If the motor cable is fed from an inverter, increase this distance as far as possible.
- Cross at a right angle. At 90 degrees the coupling area is minimised.
- Separate trunking. Use separate cable trunking for power, control and signal. If a single trunking is unavoidable, choose trunking with a metal divider and earth the divider.
- Twisted pair. Carry control signals on twisted pair; the twisting reverses the sign of the inductive coupling at every twist and cancels its effect.
- Short routes. Run the cables by the shortest path; avoid long routes parallel to large metal surfaces.
- Inside the enclosure. Keep the receiver's supply and output cables away from the inverter, contactor and braking resistor area.
Making this separation at the installation stage is both cheaper and more permanent than trying to compensate later with filters and ferrites.
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