Can I just plug it to a earth terminal of a wall socket?
Yes.
BUT be very careful that it is the earth terminal.
Also,if you use a mains plug with the wire connected to the earth terminal inside,make sure that the ESD earth wire cannot contact either of the other two terminals.
I've seen these earth wires just roughly wired into a normal OZ type power plug with no strain relief so the wire can just flop around inside the plug---AAAARRRRGGGGHHHH! ![Shocked :o](https://www.eevblog.com/forum/Smileys/default/shocked.gif)
Seconded - I was working on a project at work (here in the UK) that was designed to run from an inverter. I was doing some testing and plugged it into the mains. No problem, until I realised that I hadn't connected the earth. I quickly rectified this, plugged it in, touched the metal case and OUCH it hurt.
Turned out that the building had been mis-wired, the earth lead was connected to the live phase rather than the neutral.
Neil