Hans.... I may be wrong in my explanation to the following question you posed but I will give it a shot.....You said: I don't know any reason why the system has to be keyed, I always assumed the US development of keyed plugs was triggered by old DC systems (since in DC Eq. it is necersarry to know the orientation), but not a new addition. Strange, we dropped all orientationwhen switching to AC (with Protective Ground). Could anyone give give a reason why the system has to be keyed _IF_ a Protective Ground is included (other than define the Protective Ground pin) If you hook an scope up circuit such as a TV and hook the ground probe up to signal ground but you have the plug reversed the circuit will short out and you will blow the device you are testing. That is because you just provided a current path to frame ground for the signal which you thought was signal ground. TV repairman who have used scopes have done this more than once. Tony Niemann 2606 Merriwood Avenue Louisville Kentucky 40299 tniemann(a)msn.com 502-267-9233