Power for Iris 220S

[Robert G. Brown]

Thanks to all the nice people who responded, and the folks at SG who
(eventually) arrived at the One True Word.  For we mortals who are
confused by a wiring specification that calls for 220V single phase
(ground, neutral, hot) power for a machine sold in a country where
there ain't no such beast (or at least, where it is extremely rare)
there follows a short Discourse on Wiring:

                      Feeding the Beast

The receptacle for the 220S Power Series Rack is a Nema 6L 30R, twist
lock.  Its specification is for "30A, 195-240V, 50-60Hz single phase"
or words to that effect.  The plug has three prongs:


               |_  Ground




Neutral \            / 220V

that are >>supposed<< to carry the potentials indicated in an ideal
world.  

Ground (for those ignorant of the Code), is supposed to be real, live
(or actually dead) >>ground<<.  It should be connected to the moral
equivalent of the plumbing, or the steel girders in a building that go
deep into the ground.  It should >>never<< (intentionally) carry
current.  When you touch something connected to ground, you should be
as safe (electrically) doing so as when you touch the plumbing.
Ground is >>very important<< to computers as a signal shield, too, so
your ground should be electrically "quiet" in higher frequencies.

"Neutral" in your 110 V household wiring is the white one in the
white, black, and copper triplet in standard three-wire cable.  It,
too, is connected to ground, but >>it carries current<<.  In fact, all
the current that flows "out" the black (hot) wire flows "back" to
ground on the white neutral line.  Since it carries current, it is
easy to electrocute yourself on a neutral line.  In the days before
the code, standing on a wet bathroom floor and flicking on a light
switch shorted to the neutral wire was more than adequate to get the
10 mA or so through the torso needed to defibrillate the heart.
(Today, a "ground fault" protected switch compares the current on the
black and white wires and if it is not equal -- because some of it is
being diverted through your torso, for example -- it cuts off the
line).

"Hot" in a 110V circuit or the 220V circuit shown above has a
potential difference of 110 or 220V >>relative to a grounded wire at
the transformer<<!

The thing that does "work" in an electrical circuit is the potential
difference between the two poles of the current carrying circuit.
That is the key to the solution to the problem.

The SG-220S in the rack mount has no 110V circuitry in it at all.
Nowhere does it use "neutral" as anything but a potential reference
for the "hot" wire.  The only thing that matters is the net potential
difference between the two poles.

For that reason, the following are perfectly acceptable power sources:



                    |_  Ground




110V sin(wt) \            / 110V sin(wt + Pi)


(what most humans would call 220V two phase power).  This is typical
of the power supplied to a normal house for running the range or the
drier.  In the household there may or may not be a >>fourth<< wire to
serve as a current carrying neutral which can be used to split the
line into two 110V circuits.  Note that the potential difference
between the poles is:  

110V sin(wt) - 110V sin(wt + Pi) = 220V sin(wt) at 60 cycles.


Also acceptable to the power series rack is:


                    |_  Ground




120V sin(wt) \            / 120V sin(wt + 2Pi/3)      

============================================
               (not used)
                    X 120V sin(wt + 4Pi/3)

This wiring uses two out of three legs of a "three phase 220"
circuit.  This is typical output of a "Wye" transformer and is common
in Universities and offices.  Again, there may or may not be a current
carrying neutral allowing it to be split into three 120V lines.  The
potential difference is:
120V sin(wt) - 120V sin(wt + 2Pi/3) = 207.8V sin(wt + Pi/3)
where the phase shift is completely unimportant (when >>did<< time 
start, anyway ;-).  This (208V) is well withing spec for the 220S.

The reasons for running 220V lines in this way are to minimize risk --
unless you touch two lines simultaneously you can only get a 110V
shock -- and to allow appliances to draw 30A or 45A in circumstances
where each 120V or 110V line is fused to draw no more than 15A.  15A
is code for 14 Gauge wire up to 50 feet from the distribution panel
and 12 Gauge up to 100 feet.  I recall that 20A can run on 12 for 50
feet, and 30A requires 8 or 10 Gauge, but I don't have a reference
handy and don't quote me on that.  The power cable for the 220S is 10
Gauge, three wire.


One reason for the confusion is that >>electricians<< call all three
wirings of the above plug/receptacle combo "single phase" 220V AC!
God only knows what they would call two phase or three phase.

I'm going to wrap it up here, without telling you the story of the
Ground Loop, boys and girls.  That's what you get (sort of) if you
treat a current carrying "neutral" as ground and connect it to
something that is a bit closer to "true" ground for the circuit.  This
is surprisingly easy to do, especially in a machine that is running
several lines of power with different phases and a reference neutral.
In the week or so since I've posted, I've heard lots of funny stories
about blowing up coffee pots plugged into Vax power strips, blowing up
Vaxes by connecting them when they were plugged into different phases
on different sides of the room, and lots of other stuff.  The moral of
the story is: beware ground loops.  I suspect that SG made the power
220 only in order to avoid this very problem.


The bad news, of course, is that our machine doesn't work (still) and
it isn't the power :-( But maybe by tomorrow the nice man from SG will
swap our boards and get us going.