eddie

Hello, I'm a home user and just finished installing 2 doors - 2 inside units of Aiphone JF-2MED it comes with an 18VDC power supply which have + - and Ground screws on it. I don't see in the instructions where do I connect the ground to, do I need to connect it to a water pipe or what? thanks for any help. The system is working without any problems and my wife and me both happy, I just want to make sure I do it right. thanks again.