8 Dec
2006
8 Dec
'06
1:40 p.m.
On Fri, 2006-12-08 at 19:36 +0000, Tony Duell wrote:
Warren:
I worked in a calibration lab. Apparently, some equipment connects
(or USED to connect - it certainly would NOT work with a GFI system) the
neutral to ground to avoid low current ground loops that could cause
tiny measurement errors. I was never quite sure of the physics of that
whole "ground loop" business. Anyway, some of the equipment with
3-prong plugs shorted the neutral to ground. This conflicted with
ship's power, which was NOT 120 V. service, but two out-of-phase 60 volt
circuits, as produced by a motor-generator set. Shorting neutral to
ground didn't "fix" neutral at a low potential, it shorted one side. A
couple instruments even did it before the fuses. I'm not sure what the
thinking was behind that, but fortunately the power cord WILL act as a
fuse, although it is rated at quite a few amps, and kind of stinks when
it operates...
Peace,
Warren E. Wolfe
wizard at voyager.net
Interestingly, that is how U.S. Navy power
on-board ships works,
too. (Well, as of 25 years ago... *SIGH*) Two out-of-phase 60 volt
live sides. U.S. standard house wiring is, however, three wire: Live,
Neutral, and Ground at 120 volts, 60 Hz. Some equipment we used on the
ship was designed for land-based labs, and case-grounded, which meant
that when you plugged them in, you were shorting out half the mains.
Why? Since there should be no connection between the active parts of the
mains circuit and the earthed metal case, why did it matter what point of
the mains supply (one side, or the mid point) was grounded?