KNX Actuator

The division of roles in KNX is simple: the sensor sends, the actuator receives and acts. A push button sends a telegram onto the line, every actuator listening to the same group address evaluates it and switches its relay. Between the two there is no computer, no central unit and no software mediating.

This is why an actuator does not know the push button that operates it. It only knows the group address. Four push buttons in four rooms can switch the same luminaire without anything being changed at the actuator, and one push button can address eight actuators at once.

The separation is not always clean, though. A switching actuator with a status object sends by itself as soon as its output switches, and at that moment it is a sender. The terms therefore describe the task of a communication object, not the category of the device.

An actuator switches load, and load comes from the distribution board. This is why the DIN rail form is the normal case: short paths to the circuit breaker, room for terminals, and the mains voltage is present anyway. The bus is added as a two-wire line.

From this follows a planning rule that many only notice on site: **every switched circuit needs its own conductor running to the distribution board.** There is no classic two-way switching with travellers in KNX: instead of two conductors between the switches, every circuit runs individually to the actuator, and the link is only created in the ETS.

There are exceptions. Flush-mounted actuators sit behind the push button or in the luminaire box and are the route for retrofits where no new cable reaches the board. They cost more per channel and are harder to reach when something fails.

Actuators are sold by channel, meaning the number of circuits one device can switch. Common counts are 2, 4, 8, 12 and 16. The price per channel drops as the channel count rises, because housing, bus coupler and electronics are paid for only once.

The planning mistake is almost always the same: fitting exactly as many channels as are needed on day one. Anyone wanting to add a circuit later then buys a second device, needs space in the board and has two devices to maintain instead of one. Spare capacity on the DIN rail is cheaper than a second actuator.

The switching actuator is the base case, one relay per channel, on or off. The dimming actuator sets a brightness value and needs a lamp that matches its dimming method. The blind actuator moves and positions, usually with two relays per channel and an interlock that never allows up and down at the same time.

For heating and cooling there are actuators with electronic outputs that drive valve actuators via a pulse width instead of switching them hard. Added to these are special forms such as RGB dimmers, ventilation controllers and actuators with integrated energy metering that report their own consumption per channel onto the bus.