Why were/are there commercial vacuums made with 2-wire cords? I know all about those with the mandatory 3-wire setup, but I have seen a lot of these commercial-grade vacuums with 2-wire cords. For example, Hoover sold many uprights for the janitorial market in both flavors.
For another example, here are two Royal 501 Prince hand vacuums, for the commercial market, with 2-wire cords. I've yet to find one with a 3-wire cord (model 3-501) aside from the "Turbo Groomer" version, and I know Charlie C. (vacman1961) remembers seeing one of these in gray (or dark blue), marked "Royal Commercial" on its bag.
The Hoover commercial models with two wire cords are double insulated, so they do not require a ground to earth. The plug will also be polarized, the wider pin being the neutral. Assuming the outlet that you have is wired correctly, the feed or black wire goes to the switch, and when off, the only thing energized is the cord. Double insulated machines usually have a plastic case as well. If you've seen the switch in the handle of these Hoover's, it sits in a plastic switch case that insulates it from the handle. But if you look at the Hoover Conquest, it does have a 3 wire cord, but the ground attaches to the idler pulley bracket and basically does nothing, because that bracket is attached to the bakelite front motor housing. The 501 hand cleaner is all metal, it would have to have a 3 wire cord according to today's OSHA standards, or you may be able to plug it into an inline GFCI. Yes it will work, the gfci compares the current going out from the black wire to the current coming back on the white or neutral and when there is an unbalance it will trip. The newer class A gfci's should trip at 4-6 milliamps, some older ones took much more to trip. Note: I am not an electrician, just someone with quite a bit of knowledge on electricity. I'm sure no one has stopped using their old metal vacs just because there is no ground. They just may not be able to use them on a jobsite where OSHA comes in and checks all of the equipment, they will come in and cut the cord off. Info on GFCI's is here www.nema.org/Products/Documents/N...
Post# 329617 , Reply# 2 7/18/2015 at 22:34 (3,204 days old) by KirbyClassicIII(Milwaukie, Oregon)
I wonder why Royal never double-insulated the motors of their household metal vacuums (i.e. model 153, 602, 801 and 880) like Kirby did back in 1973 when their Classic Omega came out?
~Ben
Post# 329620 , Reply# 3 7/18/2015 at 23:22 (3,204 days old) by 3rdGenVacGuy(Columbus, Ohio )
The UL standard for double insulation systems for use in electrical equipment is UL standard 1097. Just to receive a copy of it costs $400 from what I can find. If you look at the table of contents, it is quite comprehensive, with sections on cords, strain reliefs, armature winding, commutator, internal wiring, carbon brush holders, carbon brush caps, etc. You have to design an appliance using these guidelines, then submit samples to UL to get it approved. If a copy of the standard costs over $400, I wonder what it costs to get a product approved as double insulated. Being a metal machine, it would have to be harder to achieve. I wonder how Kirby did it on the Omega, can't you unplug the cord out of the back of the switch and flip it over. Probably why they went with the cord like the Heritage 2HD, that you can only plug in one way.
(Milwaukie, Oregon) 
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