Re: Advice needed: CRT monitor 'flicker' problem caused by nearby power lines

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Do a search on Google for mu metal . It works. It is VERY expensive (around $300 for a monitor). You can probably do some good by changing the location of the monitor. You would be surprised at what a few feet can do. The strength of the field drops by the distance squared, so a small change in the distance from the source can make a huge difference in the strength of the field.

Also, assuming you are in a 60hz power location, change the scan rate of the monitor (if possible). This often helps.

Charles Perry P.E.

Did you try to increase the scan rate to about 80 Hz or something like this?

Are you located right under the power lines, or within a few hundred feet?

If the flicker is from the magnetic field of the power lines, you are correct in assuming that the flicker will vary at particular times of the day. This is correct in assuming that this has to do with the peak loading.

Your monitor CRT is shielded by a u-metal ( mew-metal ) shied according to the international safety standards. This is to meet specific requirements. Any type of magnetic field that can pass through in any direction would be considered excessively strong, or the shield in your monitor is not adequate according to established standards.

If you want to add shielding, the required type of metal is very expensive, and the design of the shield set-up would be complex. To shield out the lower frequencies, such as the 50 and 60 Hz from AC power sources is more complicated than to shield out high frequencies such as X-Ray and microwave.

My suggestion is that if you are living in a condition where the field is so strong to cause a monitor to flicker, you should strongly consider moving! These fields are known to be dangerous over time. The power companies are in denial about this, because there are not many long term studies, and many that have been done were not openly published.

I would think your problem may be more serious than just a monitor condition, if this is the case. I even have thoughts about the electrical wiring in my home, even though it is of the average, and the fields from it are very small. But, they are present, or I would not be able to have power in my home.

Effect Of Electro Magnetic Fields (ELF) on humans, World Health Organization

Power Line, Electro Magnetic Field (ELF), health concerns

I very strongly suggest you go to this site, and read the links!

Search criteria (sample)

ISO definitions

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Greetings,

Jerry Greenberg GLG Technologies GLG ========================================= WebPage

Thought so. But I love my CRT monitor! So... before I decide to throw-in-the-towel and "downgrade" to an LCD (just my personal preference/opinion), I'm going to put just a little more time into looking for some way to "magnetically shield" my monitor (i.e. the space it is in). To anyone out there technically versed in such stuff (i.e. "magnetic shielding"), I'd eagerly welcome any useful information/insight/etc. you might be able to share towards this.

Thanks,

Sodah

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Actually, assuming that the problem IS magnetic interference, this is exactly the wrong advice. Minimizing the visible effects of such interference requires setting the refresh rate as close as possible to the local power frequency or an integer multiple of that.

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Nope. International standards do not cover magnetic SUSCEPTIBILITY on the part of CRT monitors, only their magnetic emissions. Those are generally NOT addressed through internal shielding, and it is actually somewhat rare for a mainstream CRT monitor these days to contain much, if any, "mu-metal" or other magnetic shielding material. (There MAY be a bit within the CRT itself, but even that is becoming less common.)

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microwave.

Mostly because in the former case, we ARE talking about a straight magnetic field and not EM. Very different beasts.

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It doesn't take much in the way of magnetic field strength to cause visible problems on the monitor - in fact, the fields involved are generally on roughly the same order of magnitude as the Earth's own field (and you're not overly worried about THAT, I hope..:-)) The problem is that the power-line fields are AC, which causes the image on the screen to move rapidly back and forth, and thus be visually objectionable. (The Earth's own field also moves the image, but since it's a static displacement you don't notice this as a problem.) In any event, the possible health concerns over low-frequency magnetic fields have, IMHO, been grossly overstated.

Bob M.

At 60 Hz you get the slow flag wave. DUH

At 88 Hz you get a 28 Hz wave, which is faster than the human eye can follow. However, that means your eye doesn't see each change, and instead you see a wide, fuzzy vertical line rather than a wavy vertical line.

The loss of definition may bother you as much or perhaps even more than the wavy lines did!

That is essentially true, but doesn't help. The requirement is not "as close as possible", but rather "closer than is possible"! You have to *lock* the vertical rate to the power line frequency. That is because both the power line and the monitor will change enough over a day's time to cause a very slow wave unless the monitor tracks the power. (The power lines have very good long term stability, but that is an average, not an instantaneous accuracy.)

-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) snipped-for-privacy@barrow.com

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Very true!! In fact I'm keeping my refresh rate at 60 hz (during "peak hours", that is) as it does make quite a difference ... The image stops shaking around (which, btw, further confirms the, "it's the powerlines", explanation...). (Other available refresh rates don't do the trick.) Unfortunately, 60 mhz itself inherently appears as 'strobe-like' (rather than appearing to my eye as 'continuous and uninterrupted'), thus similarly productive of eyestrain/fatigue/etc ... thus not a long term solution.

Thanx, Sodah

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By the way, don't assume that those big power lines you see outside are the cause. They often aren't. This type of interference is caused by magnetic fields, which are caused by current. The strength of the field decreases by the square of the distance. Those big lines often have no more current than the low voltage ones attached to your home and they are much farther away. It is very likely that the conductors causing the problem are low voltage (600v or less). It could be the service conductor to the house (or apartement building). It could also be a transformer. It could be wiring in the wall.

Charles Perry P.E.

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Well, I'll just add that the builder of the house had to ground all of the window frames (which are aluminum, of course) when this house was constructed. The other day when the cable guy came to install my internet connection, he was mystified and daunted when he saw all the cabling seemingly "coming out of everywhere". I had to explain to him that it was not television (or other coaxial) cable...

Sodah

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Why did he have to ground them? Who told him he had to ground them? What was grounding them going to fix?

Charles Perry P.E.

High electric field produces a voltage differential across a distance toward the conductor (0 V at ground, $HIGH V at wire). In some cases, unattached fluorescent tubes will illuminate.

Probably the codes, or the power company.

Buildup of charge, or maybe a humming noise.

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Exactly, none of which have anything to do with the flickering monitor! Electric fields cause the voltage problem, magnetic fields cause the monitor problem. Like I said in a previous post, just because you have a high voltage line does NOT mean you will have a high B field outside of the right of way. The origonal poster gave the "E field protection" answer as justification that the line was causing his problem.

Charles Perry P.E.

Has anyone suggested to you the use of a "Faraday cage"? I'm really not an electronics type, but IIRC just a grounded metal grid separated from your monitor by a distance twice that of the aperture size of the grid should isolate your appliance from RFI. Of course that would also mean surrounding your monitor with something like "hardware cloth" (square apertures) or "chicken wire" (not a good idea), INCLUDING THE FRONT. Probably impractical, but it would be interesting to see whether doing such would make any difference.

'Sporky'

Sodah wrote:

Yes, probably high-current wiring and/or a transformer. I remember one job for a test group where we had a bank of systems/monitors along a wall, on the other side of which was the electrical room for the entire floor of a rather large building, with a large (guessing) 480V-120/208 volt transformer. (just outside it and a floor below was a BIG transformer on a pad which fed it) Anyway, all those monitors' images danced all day long. As they were all test/burn in systems nobody ever had to use them for long so it didn't really matter.

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For buildings in such close proximity to high-tension power lines (e.g. in the city of Surrey, British Columbia, Canada) as this house is, the above is required by the local building code. The grounding is ostensibly to prevent arcing and/or other electrical discharge (a potential fire hazard???) along the metal window frames, owing to their close proximity to said power lines. I'm sorry I can't give you a more rigorous or comprehensive answer, as I haven't seen the building code specification for myself. This information was provided by word of the real estate agent prior to purchase. However, I suggest if you are genuinely motivated to learn something new or interesting here (whether it be about _true science_, or merely municipal _politics_, I don't know) that you research this for yourself. I didn't post here in order to debate anything with anybody.

Sodah

"Try to second guess all you like, but what I've described here are my discrete observations; scrupulously kept independent of and uncorrupted by the interpretation I've expressed thereof."

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What I was getting at was that the grounding helps an electrostatic problem, that is related to high voltage lines but has nothing to do with the problem of the monitor flickering. The flickering is caused by magnetic, not electric, fields. A high electric field does NOT indicate a high magnetic field. People (particularly the news media) equate a high voltage line with high magnetic fields and it is just plain incorrect. The magnetic field from your service conductor is almost always higher than that from a high voltage line outside of your home.

Charles Perry P.E.

The problem is, this ISN'T RFI. It's magnetic interference, and a Faraday cage won't do diddly for that.

Bob M.