Two physical grounds??

Also make sure the hot (phase) wire and neutral are correct. There are some older products out there that have one wire tied to the case/chassis. If it's backwards, that can be a deadly mistake.

Thank you all for your responses! As some of you guessed right the house is very old with just 2 prong sockets (ungrounded). However, I know that there is a ground bar at the fuse box which would be very difficult to fish in order to tie the two ground bars togheter. Tjtrout20 suggestion seems to be the way to go. Thanks again!

The solution is easy provided you have a metal outlet box. Simply buy a 3-pronged outlet and a package of green ground wires (pre-looped, and with green screws, called 'Equipment Bonding Jumpers'). Remove one of the old two-prongers where you want to install the PC. Install the new grounded outlet to the white/black wires and install the green ground wire to the ground screw of the outlet. At the other end of the green ground wire, screw it to the metal box with the green ground screw. Push the wires back in the box and attach the outlet to the box. You now have a grounded outlet.

Gs5556,

Metal boxes were used with old 2 wire NM sheathed cable. The metallic box is floating..... Even in the case of old BX the conductivity continuity of the armor/connectors would be poor at best.
Jim

With all due respect Gs556 should not be giving electrical advice.

"The solution is easy provided you have a metal outlet box. Simply buy a 3-pronged outlet and a package of green ground wires (pre-looped, and with green screws, called 'Equipment Bonding Jumpers'). Remove one of the old two-prongers where you want to install the PC. Install the new grounded outlet to the white/black wires and install the green ground wire to the ground screw of the outlet. At the other end of the green ground wire, screw it to the metal box with the green ground screw. Push the wires back in the box and attach the outlet to the box. You now have a grounded outlet."

No… you now have an ungrounded circuit terminating to a grounded outlet. The exact same as a two wire ungrounded outlets but now not compliant with the National Electrical Code or most International Codes.
T. J.