We distinguish 2 different situations:
 
1. A passive potentiometer or 10KΩ potentiometer can be connected to a fan speed controller or EC fan via 3 wires. In this case the voltage is supplied by the fan speed controller or the EC fan. The potentiometer does not have a separate power supply. The potentiometer generates a 0-10 Volt signal by varying the resistance.
 
2. When the 0-10 Volt control signal is generated by an external device (e.g. PLC) with a separate power supply, it is very important to connect the ground of the external device to the ground of the fan speed controller's analogue input.
 
Devices with different power supplies both have different ground levels - so called “virtual grounds”. To guarantee the stability of a control signal, both virtual grounds must be connected to make them "common" by connecting their GND terminals. This will keep their ground or zero levels equal. If both ground terminals are not connected, the control result can become very unpredictable. Fan speed might vary at random due to the virtual grounds that start floating.