RGS200-12P / RGS100-5P: Switch is not powering up the PoE Ports - what can it be?

RGS200-12P / RGS100-5P: Switch is not powering up the PoE Ports - what can it be? In case your RGS-Switch does not Power up the PoE-Ports, a very common mistake is that the power-supply towards the switch is dimensioned too low:

The power ratings give away that you can input power as low as 12V. Looking at the specifications in regards of current of stating approx. 6 A, we have a maximum power consumption of ~ 72 Watt. This already indicates, that looking at the max. PoE Budget of 240W, that this means when powering up the switch via 12V, it does not power PoE. 

In fact, on the label of the device, it states:


Zyxel can not offer a standard power supply to connect with the RGS Series. The kind of power supply is provided by a professional electrician person/company, who can enable the wiring properly to both devices.