Tom Székely, P.E., LEED AP

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September 20, 1993

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Isolated Grounds, Dedicated Circuits, and the Third Harmonic Syndrome - or, Hysteria of the Week

Twenty-five or thirty years ago, when personal computers were the stuff of science-fiction, IBM, in its infinite wisdom put out a System 360 Design Manual for the unwashed hordes to use in preparing a holy place for the installation of the latest mainframe from Armonk. The document specified stuff like where under a given piece of equipment, access through the raised floor was required for power and (in some cases) cooling water connections, the type and rating of the equipment's connection plug, its heat rejection, and other information deemed important for the building's Architect or Engineer to know.


One of the things writ large therein was the need for a clean ground, preferably derived directly from building steel, with an illustration showing embedment of 10 square feet or thereabouts of 1/4" thick plate in the floor slab as an alternate. Remember, these were the days when you had to maintain room temperatures and humidifies within limits that would do a NASA clean room proud. Though I haven't been around any mainframes lately, I think the thing most supercomputer manufacturers are concerned with is that you provide enough cooling to keep the heavy iron from frying itself.


With the advent of PC's life has become simpler. If a person is comfortable in a room, the computer will do just fine. The only thing you really have to watch for nowadays is static electricity. It's simply not a good idea to install (especially nylon) carpeting or use an office chair with plastic casters on a wood floor in a room which houses personal computers. If you absolutely must do either of these things, an antistatic mat beneath your chair (or static discharge mat at your keyboard) is called for.

I know because I've blown one keyboard and one I/O card from zapping the computer with a lightning bolt when sitting down to it. (The crackle of a static discharge from your finger slams you and the device you've grounded yourself to with a discharge of several thousands of volts.)


The funny thing is, that though the computer, monitor, and laser printer are fed from a surge suppression power strip plugged into the same outlet which feeds my office copier, and is on the same circuit as the outlets the maid uses when she plugs in the vacuum cleaner, I've never had any of these motor loads cause any problem with the computer. This is even though the vacuum cleaner's startup voltage dip has not only dimmed room lights, it once actually caused my monitor's image to contract. (The reason it's never caused it to contract again is because I've come to realize the futility of trying to work when Brunhilda -I swear to God, that's her name- is wielding the vacuum.)

Did you know a fully loaded PC's power supply will crank out the 5 and 12 volts DC required by the machine as long as the AC input voltage stays between 90 and 135 volts? If you've got a 230 watt power supply in your PC, and it has the standard complement of 2 floppies and a hard drive as well as a video and I/O cards, chances are it'll keep cranking with much lower input voltages, which is probably why my computer didn't hiccup when my monitor did.


If you owned (excuse me, leased) horrendously expensive mainframe computers in the days before PC's, or still do so because you run a WAN (Wide Area Network - the thing that your LAN's [Local Area Networks] all across the country report to), you did so because, even at the outrageous cost incurred, you were making money because the amount of transactions you were processing electronically dwarfed what you could do with the best of people manually. For this to have any utility, you could not afford any mistakes. ("To err is human; to really foul up requires a computer").


This led to the first and second commandments of Electronic Data Processing systems. "Thy systems shall remain in operation all day, every day", and "Thou shalt not let thy data become corrupted".


IBM could not post hefty charges if their equipment spewed forth the wrong answers, and financial institutions could not take advantage of "float" on electronic transactions if the transactions dried up because of continual errors. It is the fear of such downtime and errors that led to the development of the UPS (Uninterruptible Power System) and the orthodoxy of dedicated circuits for all computers.


Dedicated circuits, or more properly, what I'd call semi-dedicated circuits, actually have some utility in being used to provide power to outlets that feed PC's. When the coffee machine repairman comes into your office and starts flipping breakers off, you don't want him to take down PC's by accident, so you wire, say 2 or 3 workstation outlets to a single circuit breaker, label it "Computers - do not turn off" and provide a handle lock on the circuit breaker making same impossible, so it becomes as near damnfool proof as human endeavor can make it.


OK, so it's reasonable to dedicate a circuit to 2 or 3 computers (not one), but what's this about isolated grounds? Beats me. Maybe it's left over from the days when MIS personnel walked in white coats on raised floors (do you think there's supposed to be an analogy to walking on water?) An isolated ground has utility in attenuating analog noise so amplifiers have a clean background in recording studios, but what does this have to do with digital computers?


Making sure that the in-wall outlet which feeds your computer plug-in strip is itself a surge suppressor immediately identifies it as being on a dedicated computer circuit just as an isolated ground outlet would, and it provides additional protection upstream of the surge suppressors in the plug-in strip, making itself infinitely more useful in the real world than an outlet whose claim to fame is isolation between its ground prong and mounting strap.

So why the continued preoccupation with Dedicated (one outlet on a breaker) Circuits? Wait for the next issue, and hear all about the gremlins of Third Harmonics.


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