What year was the 3 prong outlet invented?

The current American version of the grounded plug, with two vertical blades and a round grounding pin was invented by Philip F. Labre, while he was attending the Milwaukee School of Engineering. It is said that his landlady had a cat which would knock over her fan when it came in the window. When she plugged the fan back in, she would get an electric shock. Labre deduced that if the plug were grounded, the electricity would go to earth through the plug rather than through the person holding it. He applied for a patent on 12 May 1927,[10] and was issued a US patent for grounding receptacle and plug in June 1928.[10] As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries.

Wikipedia contributors. "AC power plugs and sockets." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 16 Oct. 2012. Web. 22 Oct. 2012.