STENGEL@pucc.Princeton.EDU (Robert F. Stengel) (02/12/91)
Inadvertently, I conducted an experiment relating to the on-going controversy regarding possible health effects of low-level electric fields and video data terminals (VDT). My new Macintosh LC computer is parked unceremoniously in the middle of my desk until I can find a better place for it. The dual-15w-tube fluorescent desk lamp that has served me well for 30+ years sits overhead, usually well out of the way. The lamp has separate "on" and "off" buttons; the "on" button must be held down to produce a higher electric field during the starting cycle. Recently, while sorting some 35mm slides, I pulled the lamp head closer to the desk top, just a few inches above the monitor screen. When I turned the computer on, the lamp tube nearest to the monitor began to fluoresce, flickering until the "off" button produced total discharge. Apparently, the electric pulse that occurred when the monitor was turned on was enough to initiate fluorescence in the lamp. The phenomenon has been repeated several times since. In the popular reporting on the subject, much has been made of average field strengths and possible repercussions on health, but little or no attention has been paid to peak transient fields, which could be expected to have increased effect. It may well be that the VDT presents a greater hazard as it is being turned on than while it is operating normally. Increased understanding of this potentially significant factor is warranted.
jmc@DEC-Lite.Stanford.EDU (John McCarthy) (02/12/91)
Possibly the interaction between the VDT and the fluorescent lamp was via the power line.
kludge@grissom.larc.nasa.gov ( Scott Dorsey) (02/12/91)
In article <12438@pucc.Princeton.EDU> STENGEL@pucc.Princeton.EDU writes: > Recently, while sorting some 35mm slides, I pulled the lamp head closer >to the desk top, just a few inches above the monitor screen. When I turned >the computer on, the lamp tube nearest to the monitor began to fluoresce, >flickering until the "off" button produced total discharge. Apparently, the >electric pulse that occurred when the monitor was turned on was enough to >initiate fluorescence in the lamp. The phenomenon has been repeated >several times since. First of all, since the pulse only occurs for a short period of time, the exposure (which is cumulative anyway) is quite slight. Secondly, the exposure from VDTs may be one of the less important elements in the environment. I have shared offices with 250 KW transmitters where fluorescent lights would burn a bright white while unplugged and held in the hand, and caused small sparks to jump off the corner of my glasses if the sharp edges were not rounded with a file. Currently I am across the street from a radar set which causes a noticeable flicker on fluorescent lamps and disconnected CRTs every time the beam sweeps in my direction. VDT exposure is in the sub-microvolt/m2 range, but there are a lot of environments where people live and work in the tens of volts/m2 range. AC power lines are a contributor to the exposure, and while the actual field strength produced by AC lines is quite small in most cases, the lower frequencies may possibly be more hazardous. Personally, I wouldn't worry about it. But it's a matter that certainly does deserve more study. --scott
mmm@cup.portal.com (Mark Robert Thorson) (02/13/91)
I'll bet the high voltage of the monitor was causing the flourescent light to glow. I've found that a neon bulb can often be made to glow by holding it near a TV set. This is a totally harmless effect, unrelated to the controversy about EM fields generated by VDT's and other devices. In particular, color monitors use very high voltages (about 25,000 volts) and at these high voltages some small amount of the electricity is discharged into the air.
jones@sj.ate.slb.com (Clark Jones) (02/13/91)
In article <12438@pucc.Princeton.EDU> STENGEL@pucc.Princeton.EDU writes: [stuff deleted] > In the popular reporting on the subject, much has been made of average >field strengths and possible repercussions on health, but little or no >attention has been paid to peak transient fields, which could be expected to >have increased effect. It may well be that the VDT presents a greater >hazard as it is being turned on than while it is operating normally. >Increased understanding of this potentially significant factor is warranted. And the designers of this equipment, in their infinite wisDUMB, put the on/off switches on the _BACK_ so you've got to get "up close and personal" to turn the thing on or off! BTW, you didn't mention whether the VDT in question is a monochrome or color. For many years, standard design of color CRT circuits has included a "de-gausser", which applies a decaying line-frequency magnetic field to the tube to eliminate residual magnetic fields (which would interfere with the image displayed). This is applied to the face end of the tube. Clark Disclaimer: The opinions expressed above are mine and not those of Schlumberger because they are NOT covered by the patent agreement!
jfa0522@hertz.njit.edu (john f andrews ece) (02/18/91)
While on the topic of EM fields and vdt's, can anyone point me to a source for
information on health effects of em fields? I have a few callers interested,
and would like to repost whatever I can find for wider dissemination.
Thanks for any comments.
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-----------------------------------------------------------------------------henry@zoo.toronto.edu (Henry Spencer) (02/20/91)
In article <1991Feb13.002236.8087@sj.ate.slb.com> jones@sjs.sj.ate.slb.com (Clark Jones) writes: >And the designers of this equipment, in their infinite wisDUMB, put the >on/off switches on the _BACK_ so you've got to get "up close and personal" >to turn the thing on or off! This in turn is a safety issue. Keeping the 110VAC wiring in a tight little clump at the back makes it substantially easier to pass safety standards for shock hazard, especially when you can use a plug/fuse/filter/switch/etc. module built by somebody else instead of having to do all that high-hazard wiring yourself. -- "Read the OSI protocol specifications? | Henry Spencer @ U of Toronto Zoology I can't even *lift* them!" | henry@zoo.toronto.edu utzoo!henry
jones@sj.ate.slb.com (Clark Jones) (02/22/91)
In article <1991Feb19.232959.28401@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >In article <1991Feb13.002236.8087@sj.ate.slb.com> jones@sjs.sj.ate.slb.com (Clark Jones) writes: >>And the designers of this equipment, in their infinite wisDUMB, put the >>on/off switches on the _BACK_ so you've got to get "up close and personal" >>to turn the thing on or off! > >This in turn is a safety issue. Keeping the 110VAC wiring in a tight little >clump at the back makes it substantially easier to pass safety standards for >shock hazard, especially when you can use a plug/fuse/filter/switch/etc. >module built by somebody else instead of having to do all that high-hazard >wiring yourself. >-- >"Read the OSI protocol specifications? | Henry Spencer @ U of Toronto Zoology >I can't even *lift* them!" | henry@zoo.toronto.edu utzoo!henry Henry, have you forggotten entirely about the rather dangerous voltages that these things require to work? Furthermore, having to reach around to the back _DECREASES_ safety (I rather suspect that you could get a U.L. engineer to agree that reaching around a burning terminal to shut it off is not as safe as being able to turn it off from the front ;-), not to mention the increased danger of back strain from that reach and the chance of knocking the whole box off onto the floor (have you ever seen a CRT implode? Flying glass _everywhere_, not a nice thing to be bending over). But to really blow your argument out of the water, Henry, a quick check of the two monitors on my desk (a 19" color from Sun and a DEC VT-100) reveals that _neither_ of them make use of a third-party switch/fuse/plug/filter module. (The VT-100 does have a plug/filter/voltage-select module, but the switch is separate.) No, it is not a safety issue, just plain anti-ergonomics. Clark Disclaimer: The opinions expressed above are mine and not those of Schlumberger because they are NOT covered by the patent agreement!
henry@zoo.toronto.edu (Henry Spencer) (02/23/91)
In article <1991Feb21.183853.28327@sj.ate.slb.com> jones@sjs.sj.ate.slb.com (Clark Jones) writes: >>This in turn is a safety issue. Keeping the 110VAC wiring in a tight little >>clump at the back makes it substantially easier to pass safety standards... > >Henry, have you forggotten entirely about the rather dangerous voltages that >these things require to work? Of course not, but what difference does that make? You're *still* safer if the 110VAC is restricted to a small section. It is, if anything, a bigger hazard than the high voltages, since it can supply a whole lot more current. >Furthermore, having to reach around to the back _DECREASES_ safety... Perhaps. Terminals do not ignite very often. :-) You have to balance the awkwardness of getting at the power switch when it's put at the back against reduced hazards from physical abuse and manufacturing error when high-power wiring is minimized. You are also ignoring cost considerations; note the word "easier" in my comments. All the safety features in the world don't help if your boss won't buy it because something else is cheaper. People are seldom willing to pay extra for arguably-better safety in very unlikely situations. >But to really blow your argument out of the water, Henry, a quick check of >the two monitors on my desk (a 19" color from Sun and a DEC VT-100) reveals >that _neither_ of them make use of a third-party switch/fuse/plug/filter >module... I hate to point this out, but my argument still floats solidly. :-) For one thing, although you don't specify the make of Sun so I can't say about it, the VT-100 design is 15 years old. It actually took quite a while for reasonably comprehensive modules to appear. For another thing, you are talking about products built in huge volume by large manufacturers, who are very conscious of the long-term merits of spending a little extra to build custom solutions rather than using off-the-shelf parts that cost five cents more each. >No, it is not a safety issue, just plain anti-ergonomics. Contrary to popular belief, manufacturers do not take sheer malevolent joy in making equipment hard to use. There are usually good reasons, or at least reasons that are perceived as good, for these decisions. Keeping the AC power handling concentrated in one small area is a safety improvement, at least by UL/CSA/etc rules. Keeping it out of the equipment entirely is a still bigger win, which is why lots of things now use external "brick" power supplies (which let the power-supply manufacturer do all the worrying about AC). -- "Read the OSI protocol specifications? | Henry Spencer @ U of Toronto Zoology I can't even *lift* them!" | henry@zoo.toronto.edu utzoo!henry