desimone@frodo.columbia.edu (Sal Desimone) (01/29/88)
Hi...we're in the process of specifying a preliminary wiring plan for a new building. The architects have given us a cable-routing scheme utilizing cable trays along hall-way walls. Their scheme seems acceptable to us except for one point: the cable trays will be mounted in an aluminum tray above the hallway lights (they are flourescent, I believe). The question I have is will the fluorescent lights cause interference along the data lines? Is the aluminum tray an adequate shield? The architects have never installed a cable routing scheme like this so they can't give us any references. We will probably run RS-232 stuff and perhaps twisted pair ethernet abd appletalk, and probably others in the cable tray. Any ideas? Salvatore DeSimone Columbia University
wyatt@cfa.harvard.EDU (Bill Wyatt) (01/29/88)
> [...] The architects have given us a cable-routing scheme > utilizing cable trays along hall-way walls. Their scheme seems > acceptable to us except for one point: the cable trays will be mounted > in an aluminum tray above the hallway lights (they are flourescent, > I believe). The question I have is will the fluorescent lights > cause interference along the data lines? Is the aluminum tray > an adequate shield? The architects have never installed a cable > routing scheme like this so they can't give us any references. > We will probably run RS-232 stuff and perhaps twisted pair ethernet > abd appletalk, and probably others in the cable tray. I can't actually comment on the above question, but I have a warning that may not have occurred to you: Make SURE you have separate circuits for your computers and other quiet power units (including separate GROUND at the outside water main or whatever) that isolate from the lights and other noisy equipment. In our building, built in 1972-3, the fluourescent lights leak a voltage into the neutral wire, so that there's 1.5 volts between the neutral and ground. This is usually OK, but can apparently affect the life of our computers. Since DEC specifies a limit of 0.7 volts, we've sometimes had hassles with field service over the issue. Unfortunately, rewiring is completely out of the question. -- Bill UUCP: {husc6,ihnp4,cmcl2,mit-eddie}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: cfairt::wyatt
kaufman@Shasta.STANFORD.EDU (Marc Kaufman) (01/29/88)
In article <5253@columbia.edu> desimone@frodo.columbia.edu (Sal Desimone) writes: >Hi...we're in the process of specifying a preliminary wiring plan for a new > building. The architects have given us a cable-routing scheme > utilizing cable trays along hall-way walls. Their scheme seems > acceptable to us except for one point: the cable trays will be mounted > in an aluminum tray above the hallway lights (they are flourescent, > I believe). The question I have is will the fluorescent lights > cause interference along the data lines? Boy, can they ever! Fluorescent lamps in commercial buildings are often run on 208 volts, 288 volts or higher. If you must run RS-232 above such fixtures, 1) use fully shielded cable, 2) use line drivers for noise reduction if running any distance. > Is the aluminum tray > an adequate shield? No. No more than the fixture case itself. However, if it can be used to give you more space between the data cable and the fixture, that will help. I used to own a Protocol Converter company, and had to help customers find problems. In a large number of noise cases, the data cable was routed overhead and just thrown over the light fixtures. You should always try to keep low level signals away from them. Marc Kaufman (kaufman@Shasta.stanford.edu)
eshop@saturn.ucsc.edu (Jim Warner) (01/29/88)
In article <865@cfa.cfa.harvard.EDU> wyatt@cfa.harvard.EDU (Bill Wyatt) writes: > >I can't actually comment on the above question, but I have a warning >that may not have occurred to you: Make SURE you have separate >circuits for your computers and other quiet power units (including >separate GROUND at the outside water main or whatever) that isolate >from the lights and other noisy equipment. I disagree that computers are electrically quiet. But more important, DO NOT ASK FOR SEPARATE GROUNDS. The purpose of grounding electrical equipment is primarily for safety. Separate "isolated" ground systems are a violation of the NEC. If you are responsible for spec'ing the power for a new installation, be sure to get a copy of "Guideline On Electrical Power for ADP Installations", FIPS Publication 94, U.S. Dept of Commerce, National Bureau of Standards. There are no clean grounds and dirty grounds. Ground is ground. But rather than go on and on...just get the book. > >In our building, built in 1972-3, the fluourescent lights leak a >voltage into the neutral wire, so that there's 1.5 volts between the >neutral and ground. This is usually OK, but can apparently affect the >life of our computers. Since DEC specifies a limit of 0.7 volts, we've >sometimes had hassles with field service over the issue. >Unfortunately, rewiring is completely out of the question. > I'm not sure I believe this. The resistance of 12 gauge copper wire is 1.6 ohms/1000 ft. At its rated load (20 Amps), DEC's 0.7 volt rule would require your circuits to be no more than 21 ft from the panel. One thing you should ask for is dedicated outlets for the custodians to use for their floor waxers and vacuum cleaners. This is especially important as workstations start to show up in offices. We've seen the following: a custodian plugs his waxer into a circuit being used by a computer. The waxer, along with the computer, exceeds the rating of the breaker and it opens. At the instant the breaker opens there is lots of energy stored in the magnetic field in the motor. The surge from the motor can't go back to the public utility because the breaker is open. Guess where it goes.... jim warner Sr Development Engineer University of Ca, Santa Cruz
wyatt@cfa.harvard.EDU (Bill Wyatt) (01/29/88)
in article <1769@saturn.ucsc.edu>, eshop@saturn.ucsc.edu (Jim Warner) says: > Xref: cfa comp.dcom.lans:1025 comp.dcom.modems:1506 > > In article <865@cfa.cfa.harvard.EDU> wyatt@cfa.harvard.EDU (Bill Wyatt) writes: >> >>I can't actually comment on the above question, but I have a warning >>that may not have occurred to you: Make SURE you have separate >>circuits for your computers and other quiet power units (including >>separate GROUND at the outside water main or whatever) that isolate >>from the lights and other noisy equipment. > > I disagree that computers are electrically quiet. Right. But I really meant *other* sources of noise that pollute the computer's power. > But more important, > DO NOT ASK FOR SEPARATE GROUNDS. The purpose of grounding electrical > equipment is primarily for safety. Separate "isolated" ground systems > are a violation of the NEC. All grounds are supposed to terminate at the outside water main. I was asking for sepaarate CIRCUITS, not GROUNDS. I should have said `separate ground wires' to make that clear. > If you are responsible for spec'ing the > power for a new installation, be sure to get a copy of "Guideline > On Electrical Power for ADP Installations", FIPS Publication 94, > U.S. Dept of Commerce, National Bureau of Standards. > >>In our building, built in 1972-3, the fluourescent lights leak a >>voltage into the neutral wire, so that there's 1.5 volts between the >>neutral and ground. This is usually OK, but can apparently affect the >>life of our computers. Since DEC specifies a limit of 0.7 volts, we've >>sometimes had hassles with field service over the issue. > I'm not sure I believe this. The resistance of 12 gauge copper wire > is 1.6 ohms/1000 ft. At its rated load (20 Amps), DEC's 0.7 volt > rule would require your circuits to be no more than 21 ft from the > panel. What don't you believe? That fluorescent lights leak voltage? I'm quoting the building electrician on that. That DEC's specification is 0.7v? I'm quoting the head of DEC's F.S. for Boston on that. > One thing you should ask for is dedicated outlets for the custodians > to use for their floor waxers and vacuum cleaners. [...] Absolutely. Also, we buy noise filter/surge supporessor power strips to plug in to *all* our computers. At ~$100 per, it's worth it. -- Bill UUCP: {husc6,ihnp4,cmcl2,mit-eddie}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: cfairt::wyatt