From: "Phil Allison"
Subject: Re: Interference from power lines
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 6 Jan 2003 19:04:46 +1100
NNTP-Posting-Date: Mon, 06 Jan 2003 18:54:32 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Tom Del Rosso" wrote in message
> 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
** Do you have DMM or clamp meter available - there may be high
currents flowing in the ground wire of the AC outlets due to a high
resistance or missing neutral. The problem may be due to AC voltage
differences ( 100 mV or more) on the ground from one outlet to the next.