The power cord of a lot of electronic equipment (example :  a linear 12 Vdc power supply) contains a 3-pin plug with safety ground. 
In Europe an "Euro plug". That is OK.

BUT: A plastic Euro receptacle on the back of an apparatus is NOT OK. 
-  It is isolated from the metal power supply case. 
-  Its ground pin is wrong connected :
       to the INside of the metal cabinet   OR
       directly to a ground connection on the PCB.

Creating a "PIN1 problem".
"The tiger can enter the house unseen, because the owner left the backdoor open". 

        WHY WRONG :.
The power cord safety ground wire nearly always carries RF noises, which are injected on the mains safety ground by other electric equipment, or received as RF, because main wiring acts as a parasitic antenna.

In such a cases, these ground wire related noises are not kept outside, but go directly to the electronics inside.
        Solutions :
a. Install a metal receptacle, incorporating a mains filter.
Be sure that the metal surface of such receptacle is ONLY make good contact with the OUTSIDE of the power supply housing.
Ground connection "G" at the output of the filter should NOT be connected.

         OR

b. See upper photo.
Cut the ground wire in the power cable, and make it long enough to be able to connect it to the OUTSIDE of the metal cabinet of the power supply. 

        More actions :
If all to the DC output connected equipment should be grounded onto the power supply, the "DC output minus" should be connected to the INside of the grounded power supply cabinet. This will "ground" all connected DC apparatus to this power supply.

If you want to prevent ground loops, the power supply DC output should not be connected to the (inside) of the cabinet, nor to the safety ground. 
In such cases the to the DC output connected equipment should be separately connected to a noise free ground.  For instance an active receiving antenna. 

IMPORTANT : Check that all DC rectifier diodes in power supplies are each paralleled by 100 nF capacitors. This prevents RF rattle and RF IMD.

See also : "Blocking mains supply noises".