On Fri, Jan 1, 2016, at 08:18 PM, Bertho Stultiens wrote: > On 01/02/2016 01:41 AM, John Kasunich wrote: > > This is one of those cases where electrical practices on opposite > > sides of oceans differ significantly. > > Yes, there are some significant differences. That has become obvious. > > > [snip] > > Three phase or single phase doesn't matter. When a transformer delivers > > power at 120VAC one side is grounded. There are a few very limited > > exceptions in the code, such as: > [snip] > > Some more detail at: > > http://www.ecmag.com/section/codes-standards/motor-control-circuits-ground-or-not-ground > > That is a nice link because the first paragraph apparently gives the > answer afaics: > If a motor control circuit is tapped from the motor circuit and > does not leave the controller enclosure (the push buttons are > in the cover), then it need not be grounded. [90-7 ΒΆ 2, > 300-1(b), 450-1 Exc.No.2] > > The control circuit is inside and the PC with all other control logic > and electronics too. Nothing ever leaves the enclosure. So, that would > mean that my argument still holds. Or am I reading this completely wrong? >
"Tapped from" means directly, without a transformer. So if you have a three-phase 480V motor, and a contactor with a 480V coil, you could tap 480V directly from L1 and L2 to run the contactor coil, if the control circuit doesn't leave the enclosure. (Likewise with a 240V motor and a 240V contactor coil). Most of the National Electrical Code was written back when "control" meant "motor starter", and that consisted of a contactor with start and stop buttons. The exception that you cited allowed for cheap (because no transformer) motor starters, if you could live with the buttons being on the cover of the box. If the buttons were external, the normal arrangement for many years was a small 480 (or whatever) to 120V control transformer, one side grounded, start and stop buttons in the ungrounded side so an accidental short to ground would blow a fuse instead of starting the motor. Contactor coil was 120V. We have only recently started following the rest of the world in using 24V for the buttons and coils. Much safer, and the NEC doesn't have nearly as much to say about how you ground (or don't ground) a 24V circult. Except that you still need to make sure a transformer failure doesn't put primary voltage on the low voltage wiring. Grounding one side is probably the easiest way to do that. John Kasunich -- John Kasunich jmkasun...@fastmail.fm ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
