Arduino with external power, linking ground

Hi all,

I'm planning to use an Arduino Uno board (rev3) with an external power source.
I would like to link the grounds of the board and the extrenal source (https://www.velleman.eu/products/view/?id=349761)
The arduino is powered from my desktop usb, that is in the same (not grounded socket) as the external power.
I would therefore assume that both are grounds are not refferenced and i can link the two grounds, am i correct?

Kind regards,

It would appear that you can connect them together. To verify this, you can check if the grounds are separate by measuring the resistance between the two with an ohmmeter. If the resistance is high then you can connect them together.

Its at least larger than 2000k, Ill give it a go.
Thanks

..and nothing exploded