From: "Pacific Freight"
Subject: Re: Interference from power lines
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Date: Mon, 06 Jan 2003 08:40:56 GMT
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"Phil Allison" wrote in message
> "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
> > interference?
> ** 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.
> .............. Phil
I assume that the computers are networked? Might be an idea to disconnect
any network cables between computers while you are doing your testing. I
think that the network cables have a common ground pin which might
complicate matters. If someone has made a mistake in temporarily rewiring
your premises, (especially with neutral) there may be high potentials
between computers plugged into different circuits or phases. Then re-check
your test with the isolated UPS supply and let us know.