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OVERVIEW
The proliferation of computers and
other sensitive devices throughout our manufacturing
and office environment has fostered the need to design
the electrical systems of buildings with an eye toward
power quality issues. There have been numerous books
and articles concerning diagnosis and remediation of
power quality problems in existing structures after
these problems have manifested themselves.
The primary focus of this presentation
will be wiring and grounding techniques and practices
that are recommended to be part of the design of new
or renovated structures. These practices will help prevent
power quality problems from occurring in the first place,
or diminish their effect to the point that they are
not significant.
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WHAT IS
POWER QUALITY
The term "power quality" means different
things to different people. One definition is the relative
frequency and severity of deviations in the incoming
power supplied to electrical equipment from the customary,
steady, 60 Hz, sinusoidal waveform of voltage or current.
These deviations may affect the safe or reliable operation
of equipment such as computers.
Thus, while not having a strict basis
of measurement, terms like "poor power quality" generally
mean there is sufficient deviation from norms in the
power supply to cause equipment mis-operation or premature
failure. "Good power quality", conversely, means there
is a low level of such deviations or mis-operations.
Because the sensitivity to such deviations
varies from one piece of equipment to another, what
may be considered poor power quality to one device may
be perfectly acceptable power quality to another.
Poor power quality affects the reliable
operation of computers and computer-based equipment,
which are now so ubiquitous. Often more important than
the physical effect on the equipment is the loss of
productivity resulting from computer equipment failure,
mis-calculations and downtime. In fact, it has been
estimated that the total cost to US businesses of this
lost productivity is a staggering $15-30 billion per
year. A recent survey by E-Source indicated that, while
most respondents did not calculate the cost of their
annual losses due to power quality (or may even erroneously
attribute power quality glitches to software or hardware
causes), roughly a third of those that did report a
loss figure said it exceeded $1 million per year1.
The vast majority of power quality
problems in a building originate within the same building.
The Institute of Electrical And Electronic Engineers
(IEEE), various government agencies and other organizations
have been studying these problems and effects for several
years. As a result, they have issued design guidelines
and recommended practices that are known to greatly
reduce, if not eliminate, the incidence and severity
of power quality related problems. In many cases, simply
installing enhanced electrical systems and better grounding
systems will prevent (or cure) the problem. Many of
the simple techniques explored in this document are
relatively inexpensive to install during construction,
or during major building renovation. Further, since
the use of a particular building, or area within a building,
may vary considerably over the years, the recommended
infrastructure improvements will serve to make the building
more useful over time, despite changes in tenants, end
uses or equipment.
Ten or more years ago, few builders
and electrical designers could imagine the level of
computerization we find today in buildings of every
sort. Who could have foreseen a PC on every desk? Business
computers were large machines located in special "computer
rooms". Lighting fixtures had low harmonics output.
Telephones were hard-wired. Motors ran only at their
design speed. An office at home was a rarity. Laser
printers were uncommon. And, considering all this, who
can predict what the future holds with respect to electronics?
Generally speaking, by following generally
well-known formulae for electrical loads to be expected
per given floor area, the designer of past decades was
reasonably assured of designing an adequate electrical
installation that could be expected to serve the needs
of the building and its occupants well into the future.
There was seldom a need to be concerned about harmonics,
or transients. But time, progress, and micro-computerization
marched on.
How often does a power quality problem
arise? According to a study of 112 sites of differing
location, size and type, performed by National Power
of Neceda, WI, the average site had 106 disruption events
per month, with the worst location having over 4,000
such events2.
Most disruptions show up in random, difficult-to-reproduce
ways, such as a PC that locks up, a PBX that loses calls
or a motor that fails prematurely.
According to the Electric Power Research
Institute, as much as 80% of power quality problems
relate to inadequate wiring or grounding,3
so as some power quality issues are examined below,
particular emphasis will be placed on wiring and grounding.
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WHAT
IS ELECTRICAL GROUNDING
The term "ground" refers to the earth,
or a large body that serves in place of the earth. The
term "grounded", then, refers to a system in which one
of the elements is purposely connected to "ground".
The British use the terms "earth" and "earthing" instead
of "ground" and "grounding", which are probably more
appropriate, but this publication will use the American
convention since the terms referencing ground appear
throughout the literature and US codes.
Electrical systems need not be grounded
to function, and indeed not all electrical systems are
grounded. But the voltages referred to when talking
about electrical systems are usually voltages with respect
to ground. Ground, therefore, represents the reference
point, or zero potential point, to which all other voltages
refer. Indeed, as computerized equipment communicates
with other equipment, a zero reference voltage is critical
for proper operation.
The ground (earth), then, is a good
choice as the zero reference point in most cases since
it surrounds us everywhere. When one is standing on
the ground, one's body is approximately at the voltage
potential of the earth. If the building is metal framed,
the metal components of the building structure, or the
water piping (if metallic), are approximately at ground
potential.
In most cases, the electrical service
to most buildings installed over the past several decades
is "grounded". There are numerous exceptions. Whether
or not a given electric service to a building is "grounded"
- that is, purposely connected via a low impedance connection
to the "ground" - is determined by the rules of the
National Electrical CodeŠ (NEC)4 and the electric utility
serving the facility.
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WHY
ARE GROUNDED SYSTEMS PREFERRED
Primary purpose of grounding electrical
systems is to protect personnel and property if a fault
(short circuit) were to occur. In simple terms, if one
of the three hot legs (phases) of an ungrounded electric
service becomes grounded, intentionally or accidentally,
nothing happens. No circuit breaker trips, no equipment
stops running. Ungrounded electrical systems were popular
in industrial buildings of the first half of the 20th
century precisely for the reason that motor-driven loads,
which were the most common at the time, would not stop
simply because of a short.
But a consequence of this type of
system is that it is possible for the frame of a piece
of equipment to become energized at some voltage above
ground, and present a shock hazard for personnel who
may be touching the equipment and a grounded component
of the structure simultaneously.
A second purpose of a grounding system
is to provide a controlled, low impedance path for lightning-induced
currents to flow to the earth harmlessly.
The assumption in this document is
of a grounded service installed in accordance with the
National Electrical Code© (NEC). There
are some cases where this practice is not desirable,
and the NEC provides for those exceptions.
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SENSITIVE
ELECTRONIC EQUIPMENT
Earlier, the proliferation of personal
computers in the office and home environment was discussed.
That description is really a metaphor for the proliferation
of all the microprocessor-controlled equipment found
throughout the commercial and manufacturing environments.
Today, most factory environments are computer-controlled.
Concurrent with the proliferation
of these sensitive devices, the devices themselves have
been changing in ways that make them more sensitive
to power irregularities. Operating speeds have been
increasing (in the radio frequency range), making the
circuits more susceptible to (and emitting) electromagnetic
interference. Circuits have been miniaturized, with
less space between adjacent conductors on a circuit
board, increasing susceptibility to overvoltages, and
increasing adjacent-channel interference. The microprocessor
chips themselves have become smaller and more densely
packed. This decreases heat dissipation, and makes them
less robust. Operating voltages have and continue to
decrease to allow for this miniaturization. A digital
"1" may be in the vicinity of 3.5 - 5.0 volts or less,
and a "0" in the range of 0 - 1.5 volts. So smaller
overvoltages from transient conditions may result in
operating errors.
It is easy to see where it becomes
important to keep transient overvoltages and high frequency
harmonics away from the microcircuits.
As this continuing miniaturization
was taking place, a new type of power supply was developed
that offered dramatic weight and component savings,
a necessary step to development of smaller, lighter
and less costly computers. That was the "switched mode"
power supply, to be discussed in more detail shortly.
Among the types of equipment that
both can cause power quality problems, and are susceptible
to them, are:
- Uninterruptible Power Supplies
- Variable Frequency Drives
- Battery Chargers
- Large Motors During Startup
- Electronic Dimming Systems
- Lighting Ballasts (esp. Electronic)
- Arc Welders, and Other Arc Devices
- Medical Equipment, e.g. MRIs and
X-Ray Machines
This list includes equipment that
breaks a smooth sine-wave into stepped increments, for
control of the downstream device, by varying the voltage
or frequency of the output.
Arc operated devices, including general
purpose "universal" motors with brushes, arc welders,
and even arc-discharge lighting (fluorescent or HID)
can be a strong source of electromagnetic interference.
(The arc itself is rich in energy of all frequencies.)
This interference can be picked up by improperly shielded
or improperly grounded wiring, and then conducted into
sensitive devices.
Fourier analysis (if you remember
your calculus) tells us that a wave of any shape can
be created by a defined combination of sine waves of
varying frequency and amplitude. Very simply put, the
math tells us that square waves and quasi-square waves,
which are the output of switched mode power supplies
and variable frequency drives, contain elements of sine
waves. But rather than just the fundamental 60 Hz sine
waves, these square waves also contain many higher frequency
components, which are harmonics (multiples) of the 60
Hz fundamental, as well as spiked components that are
transient overvoltages. These harmonics can result in
heating of circuits and neutrals and possible mis-operation
of the digital logic. In addition, the leading edge
of a square wave or spike behaves like a high frequency
(radio frequency) sine wave, and can be mistaken for
such.
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THE
SWITCHED MODE POWER SUPPLY
Historically, devices requiring DC
(direct current) to operate (as all electronic circuits
do) had hefty and bulky power supplies that typically
had a stepdown transformer supplying a low voltage to
a half-wave (simple diode) or full-wave (bridge) rectifier.
The power supply was heavy, bulky, and fairly inefficient.
Recently (in the last ten years or
so), partly because of the need for lighter weight and
higher efficiency, the "switched-mode" power supply
was developed. The switched-mode power supply has a
full-wave bridge rectifier (BR1 in diagram) directly
connected to the incoming 120 V AC line.
The switching circuit draws
stored energy from capacitor C1 in short pulses (thus
quasi-square waves) before sending the now pulsed DC
on to the transformer (TR in diagram). The transformer
is now operating on high-frequency, pulsed DC, instead
of the historically used 60 Hz AC. This change in operation
enables the transformer to be made much smaller and
lighter than was possible in the 60Hz, 120 volt version.
Thus, overall power supply efficiency is greatly improved,
from about 50% in standard power supplies to about 80%
for the switched-mode type.
Figure 1.
Block Diagram of Switched Mode
Power Supply5
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THE EFFECTS
OF NON-LINEARITY ON 3-PHASE SYSTEMS
The net result of harmonic and transient
generation is possible mis-operation of sensitive electronic
equipment, and overheating of phase and particularly
neutral conductors. How does this happen?
In a balanced 3-phase circuit (equal
linear load on each phase), operating with a smooth
60 Hz sine wave voltage on each phase, the neutral carries
the vector sum of the three phase currents, which is
zero. But if one or more of the phase conductors is
also carrying significant currents at harmonic frequencies
(multiples of the 60 Hz fundamental), they may not cancel
by vector addition, but may add in the neutral. Standard
test instruments cannot even measure them.
If the harmonic currents are sinusoidal,
we find mathematically that the even multiples cancel.
But the odd multiples, because they are in phase, are
additive, and appear in the neutral, where they can
cause overheating. The current in the neutral can actually
be higher than that in any one of the phase conductors.
(Fires in fact have been reported that resulted from
harmonics.) If the fundamental or harmonics are non-sinusoidal,
such as square waves that may be caused by a pulsed
power supply, mathematical analysis becomes very difficult.
The phase wires themselves may
now be carrying a sinusoidal or non-sinusoidal 60Hz
fundamental, plus non-sinusoidal, high frequency, pulsed
currents, which may result in overheating of the phase
conductors. As predicted by Ohm's Law, these distorted
currents will cause distorted voltage wave forms in
the building wiring system, which can, in turn, cause
equipment failure in other equipment. So we have a situation
where some equipment is creating problems that can affect
other equipment in the building.
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TECHNIQUES
THAT HELP
There are a variety of techniques
that can help prevent or alleviate the effects of poor
power quality. Most simply involve better electrical
designs and installation of some additional wiring.
These techniques are inexpensive to install, especially
when a building is undergoing construction, and they
may also be cost effective during retrofits.
The most serious consequence of poor
power quality, frequently, is not the physical hardware
that may be damaged, but the lost data, reduced productivity
and costly downtime. Like most ailments, they are much
easier and cheaper to prevent than to diagnose and cure.
Most of the following techniques are
part of the current IEEE recommended practice, and are
contained in IEEE Standard 1100-1992 and/or Standard
142-19916. Unfortunately, they are not part of any required
code, although some of them should be, since safety
may be affected in some cases.
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HARMONICS
 Double-Size
Neutrals, or Separate Neutrals per Phase
The sources of harmonics on building wiring have
already been discussed. Harmonics are much more than
an inconvenience or source of equipment malfunction.
They can be a serious safety concern. Fortunately,
they can be easily handled by using double-size neutrals,
as recommended by the former Computer and Business
Equipment Manufacturers Association (CBEMA), now the
Information Technology Industry Council. Alternatively,
separate neutrals can be used for each phase conductor.
At least one cable manufacturer makes a Type AC or
MC cable with oversized or extra neutral conductors
built-in. The additional cost of oversizing the neutral
is minimal. And the safety provided will be functional
even if there are changes in the equipment that affect
the frequencies involved.
Three configurations of type MC cable. Top: three phase
conductors with a separate neutral per phase. Middle:
three phase conductors (12 gage) with a double size
neutral (the 8 gage white wire). Bottom: three phase
conductors, a double size neutral, and an isolated grounding
conductor (green with yellow stripe). Note that all
three versions include a green equipment grounding conductor.
Harmonic
Filters
Filters are sometimes most cost effective in an
existing structure where rewiring is difficult or
costly. The filters are used to block or trap the
offending currents, lessening the harmonic loads
on the wiring. But the filter design is dependent
on the equipment on which it is installed, and may
be ineffective if the particular piece of equipment
is changed. Filtering characteristics need to be
carefully designed for a given installation, and
seeking professional design advice is recommended.
Filters are also fairly expensive on a per-kVA basis.
Shielded
Isolation Transformers
Shielded isolation transformers are filtering devices
that lessen feed-through of harmonic frequencies
from the source or the load. They are a plausible
retrofit technique where power problems have already
been encountered, but are also quite expensive per-kVA.
K-Rated
Transformers
K-rated transformers have beefed-up conductors and
sometimes cooling to safely handle harmonic loads.
Alternatively, standard transformers are sometimes
de-rated to allow for the extra heating due to harmonics.
Depending on the conditions encountered, a load
limit of as little as 50% of the nameplate rating
is observed. This may be adequate to handle harmonics,
but lowers effective transformer efficiency. A careful
comparison of the relative costs of K-rated vs.
de-rated standard transformers should be made.
Harmonic-Rated
Circuit Breakers and Panels
Overheating due to harmonics is
the danger here, and beefed-up components used in
these elements offer protection. Neutral buses should
be rated for double the phase current.
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GENERAL
WIRING
Separation
of Sensitive Electronic Loads From Other Equipment
A dedicated "computer"
circuit in each office is a good idea, at least
back to the branch circuit panel. A better idea,
and required in some cases, is to power sensitive
equipment from separate branch circuits emanating
from separate panel boards, fed from separate feeders
back to the main service entrance.
The neutrals and grounding conductors
need to be kept separate also. A dedicated circuit
means separate phase wires, a separate neutral,
with a separate grounding conductor, run in its
own separate metal conduit, back to the source.
See the section on conduit (below) for further discussion.
Avoid having sensitive equipment
on the same circuits, or even panelboards, as motor
loads. Such equipment as laser printers, copying
machines and fax machines should be kept separate
from computers.
An under-desk mounted outlet with a separate, clearly
labeled, orange-colored "computer" outlet, as well as
the usual brown-colored "utility" outlet. This device
is fed by two separate circuits (from separate panels),
and has transient voltage surge suppression built-in.
10
Limited
Number of Outlets per Circuit
Three to six outlets per circuit
is recommended instead of the thirteen allowed by
Code on a 20 amp circuit. This will minimize the
number and variety of sensitive equipment sharing
circuitry, tend to minimize voltage drop (discussed
later), minimize the chance for interaction, and
leave some room for later growth or equipment changes.
Metal
Conduit
Metal conduit, properly
grounded, provides shielding of the conductors from
RF energy. However, do not omit the grounding conductor
(green insulated copper wire), irrespective of the
conduit material. It is needed for safety, as well
as assurance of a continuous, low impedance path
to ground. The grounding conductor is run inside
the metal conduit, not outside.
All connections should be made
properly and maintained to avoid possible rectification
of RF at poor joints. Corrosion and joint loosening
need to be addressed on a regular maintenance schedule
to ensure low impedance electrical continuity at
all conduit joints.
According to the IEEE Standard
142 (Green Book), rigid steel conduit offers better
performance as a grounding conductor than aluminum,
if a separate copper grounding conductor is not
used. But the best advice is to always use a separate,
full-size copper grounding conductor, irrespective
of the conduit material, due to the concern for
corrosion and loosening.
Voltage
Drop
Although the NEC allows
up to a 3% voltage drop in a branch circuit, recommended
practice is to design for no more than a 1% voltage
drop at full load on branch circuits feeding sensitive
equipment. Feeder voltage drop should not exceed
2%.
That means conductor gages should
often be larger than required as code minimums.
But a side benefit of larger conductor gage is that
larger conductors frequently save enough energy,
due to their lower resistance, to compensate for
higher initial cost, with a short payback. Copper
Development Association Inc. has free information
on upsizing conductors to save energy, available
on request.
Another factor to be considered
in computing voltage drop is the crest factor (ratio
of peak to average value of the wave shape.) In
a sine wave, the crest factor is 1.414 ( ), and
most tables, formulae and codes are based on this
common traditional waveform. But a non-sinusoidal
waveform, containing harmonics and irregular shapes,
may have a crest factor of 3, 4, or higher.
Thus, the voltage drop at the
current peaks may be several times higher than usually
expected from the sinusoidal case. The question
arises as to the value of current to employ when
computing the voltage drop, as well as the value
of circuit impedance at the higher harmonic frequencies.
One engineer has suggested using three or four times
the nameplate loads of the connected equipment to
account for this increased crest factor and to compensate
for the skin-effect and higher inductive reactance
of the higher frequency components of current that
may be present. This degree of conservatism may
not be required in most cases, but prudence would
suggest that phase conductors not be loaded to their
published ampacity limits.
The combination of upsizing
conductors beyond the gage needed for the load,
combined with a 1% design voltage drop limit, should
preclude excess voltage drop in the branch circuit
in most cases. Again, it is a case of the extra
materials being an inexpensive part of the overall
installation cost during construction.
Conductor
Material
The chances of problematic
connections which could cause voltage fluctuations
in mild cases, and catastrophic failure in extreme
cases, are decreased with the use of copper conductors.
Copper is the standard conductor metal against which
all other conductor materials are measured, And for
good reason. It has lower electrical resistance for
given gage size. That means smaller gages and conduit
sizes for a given load requirement. Copper oxide is
a relatively good conductor, whereas aluminum oxide
is an insulator. Special installation precautions
are not needed, and maintenance requirements are reduced
when using copper. Special corrosion inhibitors are
not needed. Because of its superior connectability,
there is less risk of a power quality-related failure.
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GROUNDING
CONSIDERATIONS
Metallic
Enclosures
All metal objects that enclose
electrical conductors, or are likely to become energized
in the event of a fault or electrostatic discharge,
should be effectively grounded to provide personnel
safety, as well as equipment performance. It is
best to use solidly grounded AC supply systems.
All metal enclosures, raceways,
equipment grounding conductors and earth grounding
electrodes should be solidly joined together into
one continuous electrically connected system. All
structural building steel should be bonded into
a single electrically conductive mass, and connected
to the required electric service ground at the service
entrance, as well as the equipment grounding conductor
system and the metallic cold water system. Ground
in accordance with Article 250 of the NEC.
Isolated
Grounds (IG)
Isolated grounding is a
loosely defined technique that attempts to reduce
the chances of "noise" entering the sensitive equipment
through the equipment grounding conductor. The exact
methods used in IG wiring vary somewhat from case
to case, and there is no defined standard method.
In a typical branch circuit,
the grounding conductor of the equipment is connected
to the metallic outlet box through the connection
of the grounding conductor screw to the mounting
yoke (mounting strap), as well as to the green grounding
conductor for that circuit. It is then further connected
to the metallic panelboard enclosure where the branch
circuit originated. There, it can pick up noise
from adjacent circuits sharing the panelboard.
In the case of an IG receptacle,
usually orange colored and identified with an orange
triangle symbol on its face, the grounding pin is
not electrically connected to the device yoke, and
so is not connected to the metallic outlet box.
It is, therefore, "isolated" from the green wire
ground. A separate conductor, green with a yellow
stripe, is run from the insulated grounding pin
of the outlet to the panelboard with the rest of
the circuit conductors, but usually is not connected
to the metallic enclosure (Figure 4). In some cases,
the isolation may terminate here. Instead it is
insulated all the way through to the ground bus
of the service equipment or to the ground connection
of a separately derived system, i.e., an isolation
transformer.
In the opinion of many designers,
the IG wiring method sometimes helps reduce power
quality problems, and sometimes it makes them worse!
Thus, one may consider installing the IG conductor,
to be available if needed, but experiment with reverting
to a solidly grounded method if proven superior.
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LIGHTNING
Lightning
Protection Systems
In simple terms, if part of the "path of least
resistance" to ground the lightning sees is through
your wiring or equipment, that is where it will
flow. Lightning produces very high currents, for
a short time interval, but enough to cause fires
or to destroy microcircuits even miles away. The
idea of air terminals, or lightning rods as commonly
known, goes back to Benjamin Franklin. The purpose
is to provide a convenient, controlled point for
lightning to strike, and then be safely conducted
to ground. To provide the least resistive path,
heavy-gage copper wire should be employed in the
leaders and down conductors.
Grounding
of Lightning Systems
The down conductors tie
directly to the ring ground described above, or
other grounding electrode system, along with all
building steel and electric service grounds. Use
heavy-gage copper conductors to minimize impedance.
Detailed design considerations
covering lightning systems are found in the National
Fire Protection Association's Code #780, Code For
Protection Against Lightning.
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CONCLUSIONS
By following the recommendations above,
the chances of power-quality problems are minimized.
During construction or major renovation, when structures
are exposed and workmen are on-site, the cost of extra
materials or larger conductors is minimal. The potential
savings in lost production and downtime make these precautions
a good investment.
In cases where power quality problems
are encountered in an existing facility, a careful study
will be necessary to determine the best course of action.
Solutions may be as simple as moving some loads between
branch circuits, some minor rewiring, or additional
branch circuits. In some cases installation of shielded
isolation transformers or harmonic filters may be the
best course of action. In difficult cases, professional
engineering assistance is recommended.
Ring grounds, combined with vertical
rods, are recommended for new construction. They are
usually not practical for retrofits, especially in urban
areas or where there is limited space. In those retrofit
cases the best solution may be a lengthy vertical ground
rod or a chemically enhanced ground rod (or rods). Make
sure any chemicals or backfill materials placed in the
earth are environmentally acceptable and approved by
such organizations as the National Sanitation Foundation
and the relevant state environmental agency.
In diagnostic testing, be sure to
use test instruments capable of accurately measuring
harmonic frequencies (usually called "True RMS Meters").
Additional information on power quality,
including a bibliography of information sources and
a video on harmonics, is available from the Copper Development
Association Inc. (1-800-CDA-DATA, or www.copper.org).
Power quality problems frequently
can be avoided entirely by careful design of building
systems. In existing buildings, they are sometimes alleviated
or eliminated through simple, often inexpensive, changes.
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1.
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 Bill
Howe, "Corporate Energy Managers Express Their Views
in Second Annual E-Source Survey", E-Source Strategic
Memo, SM-97-5, July 1997
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2.
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Dr.
Edwin Brush, private communication, BBF and Associates,
September 1995
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3.
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"Wiring
and Grounding for Power Quality", Electric Power Research
Institute, Palo Alto, CA
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4.
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©
National Fire Protection Association, Batterymarch Park,
Quincy, MA 02269 (1-800-344-3555)
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5.
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From
"EC&M Practical Guide to Quality Power for Sensitive
Electronic Equipment," Intertec Electrical Group, Overland
Park OK (1-800-543-77771)
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6.
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Power
and Grounding of Sensitive Electronic Equipment, Standard
1100-1992 (Emerald Book)©, Institute of Electrical
and Electronic Engineers, Piscataway, NJ 1992 (1-800-678-IEEE)
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7.
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Grounding
of Industrial and Commerical Power Systems, Standard
142-1991 (Green Book)©, Ibid.
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8.
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Courtesy,
AFC Cable Systems, New Bedford, Mass.
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9.
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Courtesy
Sacramento Municipal Utility District
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10.
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Courtesy
Sacramento Municipal Utility District
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11.
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Courtesy
Sacramento Municipal Utility District
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12.
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Intertec
Electric Group, op. cit.
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