I have books from the '20s showing electric ranges, although I've read that they did not become common until GE invented the Calrod element in 1932. The 1936 GE Supply catalog shows the device now known as a NEMA 10-50, including a grounding version with clips that mount on the outside of the cap and make contact with slots in the wallplate. (The 10-50 may be even older than that, for all I know.) The 10-30 configuration was apparently created after WWII for dryers, which did not even exist until 1947 or so.
The exception to the normal rules of equipment grounding, in the case of ranges and dryers (which is expressed for grandfathered pre-1996 installations as 250.140 in the 2002 NEC) was first fully legitimized in 1947. In the 1940 NEC, the practice of grounding a range through the neutral required the special permission of the AHJ. It has been said that wartime shortages of copper persuaded AHJs to grant this permission more freely than in the past, and in 1947 the requirement for special permission was dropped.
Dawg, what I meant is that "U-ground" devices only existed in the 5-15 and 6-15 configurations in the '50s. The 5-20 devices were here by '64 at least, and the 30A and 50A U-ground devices came later.
I don't know what year the NEMA 14 devices were created, but Article 550 (mobile homes) did not enter the Code until 1965. Presumably the adoption of this article created a need for a three-pole four-wire device with a separate ground. I'm sure most stateside ECN members know this already, but the requirement to use NEMA 14 devices in buildings other than mobile homes is only ten years old.
(Again, the fact that the NEC did not mention mobile homes until 1965 in no way means that they didn't exist before that time. They just weren't regulated. The entire mobile home industry, one might argue, is another legacy of WWII.)
[This message has been edited by yaktx (edited 12-22-2006).]
