Question
What is the electrical classification of the KNX TP bus power supply?
Answer
C — 30 V DC SELV
The KNX TP bus is powered at 30 V DC SELV (Safety Extra Low Voltage). This voltage simultaneously powers the low-consumption devices and carries the differential telegrams superimposed on the DC level. Choice A (230 V AC) is the mains voltage, not the bus. Choice B (12 V DC) is too low and not a KNX value. Choice D (48 V DC PoE) is an Ethernet-related standard with no KNX TP equivalent. The SELV classification permits routing in the same conduit as other low-voltage cables.
Preparation tip
Anchor 30 V DC SELV in memory — both the value and the SELV classification are recurring exam triggers for cabling rules.
All proposed choices (exam context)
- A.230 V AC
- B.12 V DC
- C.30 V DC SELV✓
- D.48 V DC PoE
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