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venfree/Wiki/KNX Bus Voltage

KNX Bus Voltage

KNX TP devices are powered directly from the bus, the bus cable simultaneously transmits data and energy. The bus voltage is SELV (Safety Extra Low Voltage) and harmless to persons.

Voltage Values and Tolerances

Nominal voltage: 29 V DC. No-load (no device): approx. 31 V. Rated load: 27–29 V. KNX devices must operate at 21–31 V, below this they shut down. The power supply keeps the voltage within these limits even under varying loads.

SELV means: galvanically separated from 230 V, safe to touch. KNX cable may be installed in the same conduit as 230 V conductors (per EN 50174-2), because the SELV concept excludes any hazard.

Key Facts

  • Nominal voltage: 29 V DC SELV
  • Permissible range at device terminals: 21–31 V
  • SELV: galvanically separated, safe to touch
  • Short-circuit protection: power supply shuts off, bus restarts automatically
  • KNX cable may be installed with 230 V cables (EN 50174-2)

Related Articles

KNX TP – Twisted Pair
KNX TP is the most widely used transmission medium. A twisted pair simultaneously transmits power (29 V DC SELV) and data (9,600 baud), all devices on a line share the same cable.
KNX Power Supply
The KNX power supply provides the TP bus with 29 V DC SELV and delivers operating current to all bus participants. At least one power supply is required per line; with many devices, two can operate in parallel.
KNX Topology
KNX TP networks are hierarchically structured: devices form a line, lines are grouped into areas, areas are connected via the backbone. This structure supports up to 14,400 devices in a pure TP network.

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