KNX/bus installations — SELV separation of the bus cable from 230 V wiring

Home and building automation systems based on a two-wire bus — KNX being the most widespread standardised example in the Netherlands — use a bus cable that carries both communication and a low-voltage supply for connected sensors at 29 V DC, specified in EN 50090-2-2. That places the bus circuit in the SELV (Safety Extra-Low Voltage) category described in the [SELV/PELV/FELV guide](/guides/nen-1010/414-selv-pelv-felv) — but in practice, the bus cable is routinely run through the same trunking, conduit, or wall box as ordinary 230 V lighting and socket circuits, which is exactly the situation NEN 1010's separation requirements for extra-low-voltage circuits are designed to address.

Why SELV status is not automatic just because the cable is rated for it

A KNX bus cable itself, and the equipment connected to it, are designed and rated for SELV operation. That design rating alone does not, however, guarantee that the installed circuit actually remains SELV: SELV status depends on the low-voltage circuit being kept electrically separated from higher-voltage circuits throughout its entire route, not merely on the connected equipment's own rating. A bus cable installed without the required separation from an adjacent 230 V circuit can, in the event of an insulation fault in that 230 V circuit, have mains voltage impressed onto the low-voltage bus wiring and everything connected to it — defeating the safety purpose of using SELV in the first place.

What separation actually requires

Consistent with the general SELV/PELV separation principle (see the SELV/PELV/FELV guide) and the proximity provisions for non-electrical and other services (see the guide on §528 proximity to other services), a KNX bus cable sharing a route or enclosure with 230 V wiring must be kept separated by one of the recognised methods:

Physical separation: routing the bus cable in its own compartment of a multi-compartment trunking system, or maintaining a defined clearance from the 230 V wiring, so that a fault in one cannot bridge to the other.
A cable construction providing basic insulation rated for the higher voltage on either the bus cable or the mains cable, applied consistently so that the SELV conductor is not relying solely on the basic insulation of the adjacent mains cable to stay isolated from it.
An additional insulating sheath or barrier between the two, where physical separation or upgraded cable insulation is not practical (for example inside a shared conduit run).

Simply bundling a standard bus cable together with 230 V circuit cable inside the same conduit, with no additional measure, does not meet this requirement — even though both cables individually appear undamaged.

Inside the distribution board: the actuator itself

A KNX actuator (for example a switching or dimming actuator controlling 230 V loads) is typically mounted in the same distribution board as the mains circuits it switches, and internally connects both the SELV bus terminals and the 230 V load terminals. This does not by itself defeat SELV separation: the actuator is designed and certified with internal separation between its bus-side and mains-side terminals equivalent to the basic/reinforced insulation requirements of SELV equipment. What does need attention is the external wiring entering and leaving the device — the bus cable and the mains cable approaching the actuator terminals must still be kept from touching or chafing against each other outside the device itself, and terminated so that a loose or displaced conductor from one side cannot bridge to the other.

Note: this article covers the separation principle as it applies specifically to a KNX/bus installation coexisting with mains wiring. The full requirements for cable construction, clearances, and approved separation methods are set out in NEN 1010 §528 and the general SELV/PELV provisions of §414; for a specific installation, the manufacturer's KNX installation guidelines (which implement these requirements in practical detail for their specific products) should also be consulted.

Practical relevance

When designing or inspecting a KNX (or comparable bus-based) automation installation, it should be verified that the bus cable is genuinely kept separated from 230 V wiring wherever the two share a route — a multi-compartment trunking system, a defined physical clearance, or an appropriately rated cable/sheath — rather than assuming that the bus cable's own SELV rating is sufficient protection on its own, and that actuators combining bus and mains terminals are wired so that the external cables approaching them do not compromise that separation.

Common mistakes

Assuming a KNX bus cable's own SELV rating is sufficient regardless of how it is routed relative to 230 V wiring — SELV status depends on separation throughout the installed route, not only on the cable's design rating.
Bundling a bus cable together with 230 V circuit cable in the same conduit without an approved separation measure (physical separation, upgraded insulation, or an additional sheath).
Treating a KNX actuator's internal SELV/mains separation as covering the external wiring too — the device's internal design handles the separation between its own terminals, but the approaching bus and mains cables outside the device still need to be kept apart.
Not consulting the specific manufacturer's installation guidelines for the exact approved separation methods for that product range, relying instead on a general assumption about SELV wiring practice.