When i was a teenager, i had a desktop computer – after couple of years, i realized that every time i touch the metal casing – my arm, palm and fingers will feel numb, not much of a good feeling – but everything works – doesn’t seem like something has broken.. made me wondered for years.
Looking back – i got to say the cause must be due to the fact that the “Earth” wire (Green/Yellow) was broken – this cause voltage to build up on the metal chassis – and when i touched it, i was the “Earth” connection, and current actually flowed through my body to return to its source (for how this work see the other blog entry: http://electroniccircuitdesignsharing.blogspot.com/2013/03/3d-perspective-of-test-pen-and-how-it.html) .
My experience dated back in 1990’s, nowadays there are more appliances (computer, printer, tv, entertainment set…) being power up by 2-prong AC power (no earth connection), this is what being called appliance class II stuffs (A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground).)
Below is a Class II symbol (from http://en.wikipedia.org/wiki/Appliance_classes)
So i guess it is getting very common for folks to experience such sensation. And it is supposed to be harmless