Shaking monitors and power feeds, continued

[bhorne at banet.net (Bill Horne)]

TWIMC,

I'm going to post this reply from my brother, and then take the 
issue off-list:  it's getting kind of far afield from Linux.

My original advice still stands:  if you have a problem with a 
monitor, and changing outlets cures it, call an electrician.

FWIW, YMMV, etc.

Bill

-----Original Message-----
From:	Thomas D. Horne
Sent:	Saturday, May 06, 2000 7:23 PM
To:	Bill Horne
Subject:	Re: RF vs. AC as cause of monitor shake

> ?I think the confusion is what we mean by an "office 
 environment."  Most suburban offices get
> single-phase drops just like a residence.   Even non-factory 
office buildings which have
> three-phase drops will  rarely carry it around into office 
space, and may only have it easily
> available in some place like a boiler room.
>

[snip] The economies of construction using three
phase power in place of single phase are tremendous.  If three 
phase
power is available to a building it will be extended to every 
floor and
suite.  The cost savings to be had from running one third fewer 
branch
circuit cables to every space to be served are too great to pass 
up.  A
four wire armored cable will deliver three times the power, at 
any given
voltage and current per leg, as a two wire armored cable.  ( i 
use
armored cable in my example because it is the wiring method most 
often
used in multi story office buildings and because there is no 
ground wire
to complicate the example since the ground path is provided by 
the
cables metal jacket.)  A two wire, 277 volt, 20 ampere, lighting 
circuit
can supply 24, 40 watt, four tube, fluorescent, light fixtures. 
 A four
wire, three phase, circuit operating at the same voltage can 
carry three
times the current and light 72 such fixtures.  The use of the 
multi wire
branch circuit will save about half the cost of installing the 
three two
wire circuits.  The choice of three phase power distributed in 
multi
wire branch circuits is a no brainer for the design team.  If 
three
phase power is available it will be used.

> In a large office building, the drop will be three-phase, but 
 the lighting circuits themselves are
> not.  There may well be three separate  legs, taken from the 
three-phase drop, but -- and this is
> where the  confusion lies -- the power is single-phase from 
the perspective of any particular  light
> fixture.  There is effectively no such thing as three-phase 
 office lights.
>

It is true that each ballast is connected to only one phase. 
 The branch
circuits supplying them will be three phase right up to the 
point at
which each set of fixtures is tapped off of it.

> The important point is that, in a multi-phase system, the 
 neutral is common to all legs.  However,
> the total current into loads is  always going to equal the 
total current out of loads -- this is
> actually Kirchoff's Law.  One of the main purposes of 
multi-phase service is to  "timeshare"
> the neutral so that instantaneous current never exceeds its 
 instantaneous ampacity rating.  If the loads have a
> phase changing effect,  which is to say that they are highly 
inductive or capacitive, then the
> current on the neutral can become excessive and this, in turn, 
can lead to low  voltage at the loads
> because of the resulting drop.  Nevertheless, computer  power 
supplies are strictly
> power-consuming devices and it is  something of a misnomer to 
talk about them as "generating"
> current, harmonic or otherwise, although almost no computer 
power supplies are  linear.
>

Computer power supplies are second only to high efficiency 
fluorescent
lighting ballast as a cause of harmonic current overloading of 
the
neutrals in three phase, wye connected, power distribution 
systems.  If
you're hung up on the use of the word "generate" just substitute 
"cause".


> Voltage supplied to the monitor which is too low can cause 
 shake. However, it will also usually
> cause all sorts of other problems  that show up before shake, 
such as pincushion distortion and
> dimness.     Also, the voltage must drop quite far, usually 
below 90V, before  noticeable problems
> occur.  This is highly specific to each monitor: large 
monitors  will be more susceptible to low
> voltage than small monitors, good  brand-name monitors seem to 
be less susceptible than
> off-brand monitors,  and so on.
>
> I do disagree that low voltage to this extent will leave 
 computers unaffected, and some brands --
> especially Compaq -- may even  refuse to turn on.  In any 
environment where a UPS is commonly
> used,  low-voltage will result in an immediate warning long 
before it reaches a stage  where actual
> performance of either computers or monitors will be affected.
>

Well, he already said that it varies by brand.  The fact is that 
most
computer power supplies are somewhat more forgiving of low 
voltage than
the monitor is likely to be.

> While what this electrician says is all technically accurate, 
I  still think it is very unlikely that an
> actual case of monitor shake is  caused by building power 
problems, and the most likely solution
> is to put some ferrite material on the cables to filter hash 
from the computer.
>

I did say rare did I not?
--
Tom

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