Aiphone power supply grounding

[eddie 0]

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.