prior to the 1947 NEC equipment could be grounded to the grounded conductor only by special permission. In Sec. 2560 in the 1947 NEC it was specifically permitted to ground an electric range to the grounded conductor. This could result in a small voltage between the range frame and adjacent grounded surfaces, but was rarely a problem. It is my understanding that in 1947 the Utilities were interested in building load, and to make range installation less costly the 3-wire rather than 4-wire connection was permitted. One argument in favor is that if the grounding conductor to a range is open there will be no indication of it until it is needed, but with the old 3-wire connection if the neutral is open the clock does not work, the oven light does not work, and it is obvious that something is wrong, and it will be repaired. Now only existing 3-wire range outlets are permitted, new installations requiring 4-wire.
The 3-wire concept was extended to dryers. If you need to know when, I will look it up.
Hope this helps. Creighton
