Path: utzoo!attcan!uunet!seismo!ukma!tut.cis.ohio-state.edu!ucbvax!PHY.DUKE.EDU!rgb From: rgb@PHY.DUKE.EDU ("Robert G. Brown") Newsgroups: comp.sys.sgi Subject: Power for Iris 220S Message-ID: <9002140322.AA05845@physics.phy.duke.edu> Date: 14 Feb 90 03:22:52 GMT Sender: usenet@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 137 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<>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.