Monitor malfunctions and electrical problems

[Bill Horne]

TWIMC,

I asked for a professional opinion on the shaking monitor thread 
and followups.  Herewith:  the word from my favorite Master 
Electrician. Ahem.

Bill Horne

-----Original Message-----
From:	Thomas D. Horne
Sent:	Friday, May 05, 2000 11:11 AM
To:	Bill Horne
Subject:	Re: FW: last on X for now

> Very few offices have three-phase power. This is only used to 
operate heavy machinery with
> large motors.  It would be uncommon in an office environment, 
although sometimes HVAC or
> refrigeration equipment needs it.  No light fixtures ever run 
on three-phase power, at
> least not unless you are talking about things like stage 
lighting.
>
[snip]  In multistory buildings ALL power
is three phase: this allows for very large cost savings in 
circuit
layout and installation.  The power to the lighting circuits in 
large
office buildings in the US is not only multiphase but very often 
a
higher voltage than the receptacle circuits.  The reason that is 
true is
that the watts available to provide lighting is the product of 
the
ampacity of the circuit times the voltage of the source.  A 120 
volt
circuit breakered at twenty amps can provide a theoretical 
maximum of
2400 watts.  The same circuit supplied from a 277 volt circuit 
can
supply 5540 watts.  One four wire, three phase, twenty amp, 
circuit can
supply 16620 watts of power.  This is why three phase power is 
the rule
rather than the exception in large office buildings.  The 
construction
cost savings are too great for any competent electrical engineer 
to pass
up.    [snip]

> The most common cause of monitor shake is RF hash.  I would 
doubt very
> seriously that there is an serious electrical problem causing 
it.
> Sometimes a ground loop can do it, especially on an unbalanced 
network
> topology (coax Ethernet).
>

One relatively rare cause of monitor malfunction is high voltage 
on the
neutral of a multi phase circuit.  This can be brought on by 
excessive
voltage drop in the neutral caused by harmonic currents 
generated by
things like computer power supplies.  Since the neutral, or 
grounded
conductor, has no over current protection, these harmonic 
currents can
raise the current flow to a level higher than the circuit can 
carry
without excessive voltage drop.  The excessive drop can raise 
the
voltage on the neutral, at the outlet, to twenty or more volts. 
 Since
the voltage available to the monitor is the difference between 
the phase
conductor and the neutral conductor voltage any excessive 
voltage drop
on the neutral will lower the apparent voltage at the outlets 
effected.
This can cause erratic monitor performance without noticeably 
effecting
the computer's performance.

Tom

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