I quote from Sec. 1991 of the 1951 NEC Handbook, comments:
Figure 28 shows a duplex receptacle and an attachment plug intended for use where it is desired to provide for grounding the frames of small portable appliances. These devices are rated 15 amp, 125 volts. The receptacle will receive standard two-pole plugs, so grounding is optional with the user. The grounding contacts in the receptacle are electrically connected to the supporting yoke so that the connection to ground is automatically provided if a metal-enclosed type of wiring is used. (italics mine)
I assume the earlier crowfoot receptacles (like today's NEMA 10 devices) did not have the "ground" contact bonded to the yoke.
The comment continues:
For use on circuits of voltages higher than 125 volts, a duplex receptacle and attachment plug has been developed which is exactly the same as the design shown in Fig. 28, except that the current-carrying blades of the plug and contacts of the receptacle have the tandem arrangement instead of being parallel. These devices are rated 15 amp, 250 volts. It is expected that a change will be made in the Underwriter's Laboratories' Standard to require a rating of 250 volts only for 10-amp attachment plugs and receptacles of the tandem type. This will make it possible to ensure compliance with subparagraph 1 of section 2111 where attachment-plug receptacles of the grounding type or other type are installed on circuits having voltages higher than 150 volts between conductors. (italics mine0
The editor, Arthur Abbott, and his successor, Frank Stetka, were still waiting as of 1959 for this UL change to take place. I don't know when it finally happened, as my '62 and '65 books are not Handbooks. It was a non-issue by '68 (and of course forgotten today).
[This message has been edited by yaktx (edited 01-05-2006).]
[This message has been edited by yaktx (edited 01-05-2006).]
