From: Kevin McMurtrie
Subject: Re: Interference from power lines
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Mon, 06 Jan 2003 05:29:35 GMT
NNTP-Posting-Date: Sun, 05 Jan 2003 21:29:35 PST
"Tom Del Rosso" wrote:
>If anyone can make sense out of this, please reply.
>On Wednesday, 1/1, the power lines right outside an office fell to the
>street, still alive. They were repaired but the next day, in the office
>of 11 computers, all but 3 monitors have wavy screens to various
>degrees, from annoying to severe. The office is atop an incline, so
>it's at the height of the power lines, but they are still 50 feet away.
>The location is Long Island, NY, and the provider is LIPA. The
>interference continued at least through Friday.
>My first thought was magnetic interference, but there is no high-power
>equipment inside, except for one unplugged electric heater. The
>monitors least affected are on opposite sides of the office, and so are
>the monitors most affected. Moving a monitor made no difference to it,
>so there is no correlation with position within the room. (There is a
>relation to vertical frequency, with lots of interference on one
>previously good monitor when it was changed to low resolution (that
>monitor doesn't display frequency unfortunately), but other monitors
>were sensitive at all frequencies.)
>Because it was spread out so, I thought it might be electrically rather
>than magnetically coupled. The line voltage was a little unstable,
>varying from 111 to 115 VAC from minute to minute. I was using a Fluke
>79 (not true RMS). I tried unplugging 2 of the computers' UPSs and
>letting them run on battery but there was no change in their
>interference, so it must be magnetically coupled.
>For the short time they were on battery, both UPS outputs dropped below
>95 volts, so I guess they were left on and the batteries were killed
>during the blackout. I considered that maybe the monitors were
>permanently damaged, but unlikely I thought. In spite of the sudden
>drop in voltage when switching to battery, the monitors I tested that
>way didn't even twitch, but continued their steady low frequency waves.
>If it's still happening Monday I'll try bringing in a new monitor. I
>didn't get a chance to swap the best and worst of them, so I'd like to
>try that too. Any other suggestions? What could cause that kind of
>-If emailing please delete the first 3 words-
Ask LIPA to fix it. There's something wrong with their wiring that's
creating a long range electromagnetic field from the AC power. When
everything is working correctly, the power lines are side by side and
carrying matching amounts of power in and out. When they aren't, the
magnetic fields don't cancel.