It is true that the grounding prong was at
the top of the triangle for some years; Philip
Labre (US patent 1672067A) was the inventor of
this particular type of grounded plug. Before
Labre's patent, there was another patent by
George Knapp (US1179728A) of a grounded
outlet that had the grounding prong on the side.
This design is still used in some parts of the
world. As to the differentiation between
consumer (for the home) and industrial or other
usages of the grounded sockets, it is not clear
that there was ever a socket that was NOT for
consumers. If we don't see grounded outlets in
American homes that have the grounding pin on top,
that may be due to a lag between the
invention/patenting of something and its
commercial application, during which time the
design may be improved and the old design rendered
Here's a link about the history of grounded
It is said in this link that grounded outlets were
required in 1947 for laundry appliances.
Here's a picture of 3-prong vs 4-prong dryer outlets:
3 prong vs 4 prong outlet
Those pictures and the text below are from a site
about power plugs and sockets.
The grounded consumer plug has several claimants
to its invention. The earliest patent for a
grounded plug appears to be one applied for on
January 11, 1915 by George P. Knapp, on behalf
of the Harvey Hubbell company and granted on
April 18, 1916. This patent covers the use of a
grounding pin which extends further than the other
two contacts to ensure that it is engaged first.
However, the suggested configuration of the pins
was that found in the Australian plug used today
primarily in Australasia and China, which was not
interoperable with existing two-contact ungrounded
plugs. Other grounded plugs that are widely used
today were developed later by others so as to be
interoperable with ungrounded plugs.
The Schuko-system plug was invented by
Albert Büttner, who patented it in 1926.
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
(MSOE). 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 figured out
that if the plug were grounded, the electricity
would go to Earth through the plug rather than
through his landlady. He applied a patent on May
12, 1927 and was issued a US patent for
grounding receptacle and plug in June, 1928. As
the need for safer installations became apparent,
earthed three-contact systems were made
mandatory in most industrial countries.
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