Devices with different power supplies will have different ground levels. This means that the potential of their ground levels is not equal. Their grounds are floating. This typically causes problems when a 0-10 Volt control signal generated by a device with it's own power supply (e.g. HVAC controller) must be connected to another device (e.g. fan speed controller). If the ground terminals of both devices are not connected, fan speed might vary at random due to the floating grounds of both devices.
To avoid this, a common zero level must be created to correctly compare all signals. To do so, connect the ground terminals of both devices. This will create acommon ground level. It will avoid floating of both grounds.
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