Surge and lightning protection for KNX bus systems

KNX devices are sensitive to surges. A direct lightning strike will destroy them, but so will a nearby strike (induced surges propagating along bus or mains). Layered protection prevents the loss.

Mains-side protection. Class B surge protectors at the building entrance, Class C at the distribution board, and Class D at the cabinet level. The mains side is the most common source of surges on KNX, because the PSU connects KNX to the mains. A surge that destroys the PSU can also reach the bus before the PSU fails.

Bus-side protection. Surge protection devices specifically rated for KNX bus install on every line where the bus enters the building from outside (for example, a garden lighting circuit) or runs near outdoor cable trays. These devices clamp surges to safe levels and are inexpensive (typically 30 to 80 euros per unit).

Earthing strategy. Equipotential bonding is critical. The earth references for the KNX PSU, the building earth bar, and the lightning protection system should all bond at a single point. Multiple separate earths invite ground loops and induced surges.

Device protection. Many KNX devices include internal TVS diodes for transient suppression, but these only handle small surges. Do not rely on them as the only line of defense.

Common failure mode after a surge. The PSU survives because it has its own protection, but downstream actuators are dead. The customer reports half the lights stopped working, and replacing the PSU has no effect. Diagnosis: walk the cabinet and check status LEDs on every actuator. The dead ones got hit, the live ones did not. Replace the dead ones, and inspect the surge protection components (some are sacrificial and need replacing after a hit).

Document the surge protection plan. The handover pack should include a single page describing what surge protection is installed where, and how to verify it is still functional.