PQ for Diagnostic Medical Imaging Systems

advertisement
Table of Contents
What is Power Quality? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
The Healthcare Imaging System Environment: The Importance of Quality Diagnostic Imaging . . . . . . .4
Types of Imaging Systems .............................................................................................................................4
X-Ray Imaging Systems..............................................................................................................................4
Ultrasound Imaging Systems .....................................................................................................................8
Computed Tomography (CT) Imaging Systems ........................................................................................10
Magnetic Resonance Imaging (MRI) Systems .........................................................................................12
Nuclear Imaging Systems.........................................................................................................................14
Why Do Imaging Systems Require Quality Power?.....................................................................................15
Image Artifacts Caused by Poor Power Quality........................................................................................18
Imaging Systems: The Backbone for Diagnostic Services for Healthcare Facilities.....................................19
Power Quality for Support Equipment in Imaging System Suites: An Expanding Concern .......................20
Power Quality for Imaging Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Specifying Power Quality Performance for New Imaging System Installations ..........................................22
Addressing Power Quality at the Installation Planning Stage ..................................................................23
Improving Power Quality for Existing Imaging Systems..............................................................................26
What Are Imaging System Manufacturers Doing about Power Quality? .................................................26
Establishing a Partnership with Imaging System Manufacturers ................................................................31
System Compatibility Testing of Imaging Systems ......................................................................................32
The PQ Checklist: Planning, Purchasing, Installing, and Maintaining Imaging System Equipment.............33
Planning for Additional Equipment ...........................................................................................................33
Purchasing Additional Equipment .............................................................................................................34
Installing Additional Equipment ................................................................................................................35
Maintaining Equipment ............................................................................................................................35
Identifying Power Quality Problems in Imaging System Suites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Types of Audits ............................................................................................................................................36
Performing an Audit in Your Own Suite ........................................................................................................36
Assembling an Audit Plan.........................................................................................................................36
Preparing for the Audit .............................................................................................................................37
Conducting the Audit................................................................................................................................38
Interpreting the Audit Results ..................................................................................................................40
Is My Imaging System Vulnerable to PQ Problems? Questions for the Facility Engineer,
Imaging Department Director, and Imaging System Operators ..................................................................40
PQ Monitoring for Imaging Suites: Answering the Six Basic Questions .....................................................41
Common PQ Problems in Imaging Suites ...................................................................................................42
Grounding .................................................................................................................................................43
Transformers.............................................................................................................................................44
Electrical Connections ..............................................................................................................................47
Meeting the Power Quality Challenges of Imaging System Suites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47
Power Quality and the National Electrical Code...........................................................................................48
Impacts of Testing Emergency Power Systems on Imaging Systems.........................................................48
Power Conditioning for Imaging Systems....................................................................................................49
Financial Impacts of PQ Problems on Imaging Systems .............................................................................51
2
What is Power Quality?
For utilities, customers, and their loads alike, power quality is the concept of powering and
grounding electronic equipment in a manner that is suitable to the operation of that equipment. A brief reduction in voltage, as in the case of a voltage sag at the service entrance of a
hospital that reaches a branch circuit powering a diagnostic imaging system (DIS), does not
constitute a power quality problem for a healthcare provider. An imaging system may or may
not react to a sag. If the performance of the system is affected by the sag in any way to interrupt
the imaging process, or introduce artifacts into the images, then a power quality problem
occurred. The impact of a sag upon an imaging process may not be obvious at first, even if the
imaging process continued during the sag. Electrical disturbances such as sags and surges can
affect the bottom-line performance of either the imaging system manufacturer, the healthcare
provider, or the underwriter.
Power quality problems are caused by
the effects of electrical disturbances
combined with unfavorable equipment immunity to disturbances.
Disturbances that alter imaging system performance can originate inside
or outside a healthcare facility, or
inside or outside an imaging suite or
department. Facility power and
ground circuits that have no errors
and meet state and local codes can
provide very suitable paths for disturbances to reach imaging systems.
Wiring and grounding errors do not
have to be present for imaging system
power quality problems to occur.
Common wiring and grounding errors
only intensify electrical disturbances
making them more damaging to imaging system components.
In power quality, the voltage supplied
to medical equipment includes three
properties: amplitude, frequency, and
waveform. The electrical disturbances
occur whenever any or all of these
properties vary from normal. Imaging
systems may be susceptible to
changes in amplitude, frequency, and
waveform.
Amplitude. Although the amplitude of
the voltage at the point of use (i.e.,
the facility electrical system-to-imaging system interface) depends upon
3
Imaging
systems
have grown
to be very
critical to
patient care
the requirements of the equipment, voltage
should remain constant. To operate reliably,
some imaging systems require less than a
five-percent variation in voltage amplitude. A
minor decrease in voltage usually does not
damage imaging equipment but may cause
equipment to lock up and data to be scrambled or lost. An increase in voltage, especially
if combined with transients, may destroy
electronic components (e.g., power supplies
and amplifiers) and cause equipment to malfunction.
Frequency. Although utility-supplied electricity rarely deviates from 60 hertz, transferring from utility power to emergency-generator power may cause a brief shift in frequency or slow drift as the generator operates.
However, frequency variations are more likely to disturb imaging equipment than lowpowered medical equipment.
Waveform. The waveform of equipment voltage and current should be as sinusoidal as
possible. For example, non-sinusoidal voltages can cause data to be lost in medical
imaging systems and other medical equipment that depend upon data processing and
storage. Non-sinusoidal voltages can also
cause biomedical equipment such as ventilators to malfunction, imaging systems to malfunction, and lights in operating rooms to
flicker. Additionally, current drawn by typical
healthcare equipment may be distorted, producing harmonic currents, which entails
other power quality problems such as the
overheating of neutral conductors and transformers, possibly leading to widespread
equipment failures and even a risk of fire.
The Healthcare Imaging
System Environment:
The Importance of
Quality Diagnostic
Imaging
Imaging systems have grown to be very critical to patient care as they provide investigative and diagnostic devices for healthcare
professionals. Almost all healthcare facilities
have an imaging system in most available
modalities. The complex nature of imaging
systems requires that quality power be provided to their power distribution units (A
PDU in an imaging system interfaces the system to facility power and distributes power
to other subsystems in the imaging system).
Imaging systems are becoming more complex
and powerful in the amount of detailed data
they gather from the patient. These increasing complexities warrant the need for continued compatibility engineering studies of
imaging systems and improvements in power
quality to imaging suites.
Types of Imaging Systems
The various types of imaging systems used in
the healthcare industry are referred to as
modalities. Among the diagnostic imaging
systems in use today, X-ray and ultrasound
were among the first modalities used in
healthcare facilities. Computed tomography
(CT), magnetic resonance imaging (MRI),
nuclear imaging, and positron emission
tomography (PET), are four other common
modalities of imaging technologies in use
today.
X-Ray Imaging Systems
X-ray imaging systems produce X-ray images
of an object. In the healthcare industry, the
object is usually the part of the body under
investigation. Traditionally, X-ray images are
stored on a special film for viewing later by
medical staff. Many of today’s newer sophisticated X-ray machines can place the images
4
in digital memory for viewing on video monitors or sending patient data to other departments in a hospital or to another healthcare
facility.
X-ray equipment is a type of medical equipment that requires a significant amount of
power, takes up a significant amount of
space, and includes many subsystems to
operate. An X-ray machine may be portable
or a permanent installation. X-rays that are
used in medical radiography are generated
and produced by electronic equipment that
requires many different internally generated
levels of AC and DC voltages. The simplified
block diagram shown here depicts the major
components of an X-ray system.
provided at 120, 208, 220, or up to 277 volts
into a waveform more suitable for the operation of the high-voltage X-ray generator. It
also provides other forms of power for the Xray tube and other components of the X-ray
system.
The high-voltage generator converts electrical energy that the X-ray tube requires to
produce the X-ray beam. This is accomplished by imposing high voltage between
the negative cathode and the positive anode
of the tube. Whenever electrons are products
of the negatively charged cathode and accel-
The heart of
any X-ray
is the
high-voltage
generator
and the
X-ray tube.
erated toward the anode, they strike the positively charged target area of the anode, and
produce X-rays. Most of the energy used in
the production of X-rays is converted into
heat in the target. A small fraction is typically
converted into X-radiation.
Typical Block Diagram of an X-Ray Machine
An X-ray machine uses an X-ray tube that
requires a controllable high DC voltage.
Other components of the X-ray machine may
include a patient-positioning system, a system to position the X-ray source, and a filmhandling system. Portable X-ray machines
include a set of batteries with an internal
battery charger to allow the machine to be
operated in remote locations.
The purpose of an X-ray machine is to convert electrical energy into X-ray (X-radiation)
energy as specified by the X-ray technician,
and to direct the X-rays toward the area of
interest within the patient. The power supply
converts the electrical energy provided by
the healthcare facility electrical system into
high-voltage power, which the X-ray generator requires to operate the X-ray tube. The
power supply converts AC electrical power
The heart of any X-ray is the high-voltage
generator and the X-ray tube. The X-ray
machine requires many power electronic
components and systems. Before some examples of X-ray machines are discussed, some
additional information about generators and
tubes is necessary. The purpose of the X-ray
generator is to provide a controlled amount
of power to the X-ray tube that is required to
produce X-rays, and to supply power to auxiliary components of the X-ray system.
Without controlled power, X-radiation cannot be controlled. Although a standard AC
receptacle may provide sufficient power for
some X-ray applications, such as mobile
radiography, dental and podiatry radiography, and some mammography, this power
must be reshaped and precisely controlled
for use in X-ray machines.
With the exception of portable generators,
most X-ray generators are hard-wired directly
into the healthcare facility electrical system,
using 208 volts or higher, and most require a
three-phase voltage supply. This voltage is
used to produce the thousands of volts that
must be applied to operate the X-ray tube.
The ideal type of voltage to be applied to the
5
X-ray tube is a constant (zero ripple), or DC
voltage. The high-voltage generator is used to
generate this voltage.
The two principal components in a basic
generator are the transformer and the rectifier. An auto-transformer is used in the first
stage to change the voltage that is applied to
a step-up transformer in the second stage by
a factor that is variable. The second principal
component of an X-ray generator is the
power rectifier that is used to rectify the
stepped up high voltage.
made possible with advancements in power
electronics are employed in modern X-ray
machines to bring about this improvement. A
voltage sag or momentary interruption in the
AC input to the X-ray machine may impact
the operation of the high-voltage generator.
If the disturbance occurred in the positive
half of the applied high-voltage, then the
production of the X-rays would be more
severely impacted.
The X-ray tube acts as a rectifier, because it
allows current to flow only when the cathode
is negative and the anode is positive. In some
X-ray machines, during the first half-cycle of
the stepped up high voltage, the cathode is
negative with respect to the anode; this
allows current to flow and X-rays to be produced. During the second half cycle, the
cathode is positive and the anode is negative;
therefore, no current flows through the tube
and no X-rays are produced. This type of rectification circuit is called a self-rectified Xray circuit. The illustration below shows this
circuit.
Full-Wave Rectifier X-Ray Circuit
Self-Rectified X-Ray Circuit
While this scenario demonstrates a simple
(and inexpensive) method of producing Xrays, it is clearly inefficient because half of
the electrical power is not used. If the voltage
applied to the X-ray tube were a constant
voltage, with the cathode always negative
and the anode always positive, the efficiency
of X-ray production would be greatly
improved. Specialized power rectifier circuits
6
In the next level of sophistication in X-ray
generator circuits, a high-voltage full-wave
rectifier is inserted between the secondary of
the high-voltage step-up transformer and the
X-ray tube. The illustration above shows the
full-wave rectifier X-ray circuit. In this circuit
diagram, electrons can only flow in one
direction through the rectifier as in a conventional full-wave rectifier.
The use of this rectifier circuit allows X-rays
to be produced during both half-cycles of the
sinusoidal voltage that is input to the transformer. Because current flows and X-rays are
produced during both half-cycles, the efficiency of the system is substantially
improved. While the full-wave circuit provides a substantial improvement in efficiency
by using both halves of the AC power cycle,
the resulting waveform is not at all close to
the constant voltage that is optimal for the
operation of the X-ray tube. The full-wave
rectifier circuit has 100 percent ripple. A disturbance that reduces the input voltage here
would have a greater impact on the circuit
than on the self-rectified circuit.
of phase with each other. The resulting threephase, twelve-pulse waveform has only about
3 percent ripple, and is extremely close to the
desired constant voltage. The illustration
(bottom left) shows the types of waveforms,
their respective ripple factors, and the kilovolt peak (kVp) waveform applied to the Xray tube.
The next improvement in X-ray generator
waveforms uses three-phase AC power. In
principle, three full-wave rectified circuits
Nearly all diagnostic X-ray generators manufactured prior to 1980 used the 60 Hz frequency from the healthcare facility electrical
system to produce the kilovolt peak (kVp)
waveform applied to the X-ray tube. For sin-
are connected to three-phase AC power, with
the output of all circuits going to the X-ray
tube. The result is a waveform with six-pulses
per cycle (two for each half of the three phas-
gle-phase X-ray equipment, the kVp waveform varies as the 60 Hz frequency. For threephase X-ray equipment, the kVp waveform
varies at a frequency that is a combination of
es) and about 13 percent ripple.
three or more 60 Hz lines, separated from
The use of
advanced
power
electronics
systems in
X-ray systems
can increase
their
susceptibility
to electrical
disturbances
each other by slight delays. The result is a
A further enhancement of this design uses
two sets of three-phase circuits, slightly out
smoother kVp waveform.
The use of even higher frequencies to generate a very smooth kVp waveform would allow
the X-ray tube to operate at near optimal
performance. Advancements in power electronics have enabled X-ray equipment manufacturers to use higher frequencies to generate these waveforms. High frequencies are
generated by an electronic oscillator, which
are then applied to a power converter. The
use of advanced power electronics systems in
X-ray systems can increase their susceptibility to electrical disturbances; thus the need
for compatibility testing to determine where
immunity improvements are necessary.
With this type of X-ray rectification circuit,
manufacturers are able to build high-frequency generators that are much smaller and
more cost-effective than single-phase or
three-phase generators and achieve tighter
regulation of the kVp waveform and the milliamp tube current by the addition of closedloop, electronic feedback circuits. Stability of
closed-loop feedback circuits are mandatory
during an electrical disturbance upon the
input of an X-ray machine.
Types of Waveforms and Ripple Factors for X-Ray
Systems
7
The frequencies used in high-frequency Xray generators are in the range of 3 kHz to as
high as 100 kHz. The efficiency of transformers at these frequencies is much greater than
at 60 Hz. An important result is that the
high-tension transformer need not be as
large as a conventional single- or three-phase
transformer. Regulation of the kVp waveform
and tube current in high-frequency generators is accomplished by the use of a closedloop feedback circuit. The kVp feedback circuit senses the difference between the tube
kVp and the kVp set by the X-ray machine
operator. The result is a voltage that is proportional to the difference. This voltage is
input to a voltage-controlled oscillator
( VCO). The VCO provides a correction to the
original frequency used to generate the kVp
and therefore can provide close regulation of
the selected kVp. The illustration below
shows the simplified schematic of a high-frequency X-ray generator.
operating costs associated with X-ray systems that use high-frequency generators,
compatibility engineering studies should be
carried out to determine their immunity to
common electrical disturbances.
Ultrasound Imaging Systems
When comparing different types of medical
equipment, especially those with similar
functions, it is important to describe some
basics of their operation.
Ultrasound equipment is often thought of as
imaging equipment. Although this is true, it
is classified as diagnostic equipment.
Sound is mechanical energy, and thus
requires a medium such as a gas, a liquid, or
a solid for propagation. In contrast, electromagnetic energy such as that used in magnetic resonance imaging (MRI) systems does
not require a medium in which to travel.
Another difference is that while the velocity
of an electromagnetic wave remains constant
when traveling through different media, the
velocity of a sound wave varies. Also, sound
cannot be transmitted through a vacuum as
can electromagnetic energy. Sound becomes
more directional in its travel, and the beam
narrower, as the frequency increases. The
beam may be reflected, refracted or transmit-
High-Frequency X-Ray Generator
While there are some limitations to this
design, high-frequency generators are generally smaller, less expensive, and better regulated than conventional designs. The
advancements made in generator design are
the result of advancements made in power
electronics. As a result, they are becoming
increasingly popular in many diagnostic
imaging applications. In order to reduce
8
ted when crossing the boundary between two
media of different densities, and may be
attenuated or absorbed within the media.
The penetration of sound waves into a medium becomes less as the frequency increases,
which is the opposite behavior of X-rays.
Ultrasound techniques work on the principal
of sound or pressure waves traveling through
a medium, causing the molecules of the
medium to vibrate. The molecules are alternately compressed and expanded; thus the
motion is transmitted from molecule to molecule across the medium. The ultrasound
spectrum begins at 20,000 Hz, where the
audio spectrum ends. Diagnostic ultrasound,
however, is confined to the range from 50,000
Surgical team discussing images captured using an ultrasound imaging system.
Hz to 25 MHz. The velocity at which the wave
travels through any medium is dependent
upon the compressibility of the medium.
Wave velocity and compressibility are
inversely proportional by the inverse of the
density of the medium. The opposite of compressibility is hardness. Gases have the highest compressibility; liquids are next, followed
by solids. While sound waves of many different types may be generated in solids, only
longitudinal waves are transmitted through
liquids. At any given frequency, the velocity
of a sound wave will increase as the wave
passes from a gas into a liquid into a solid.
The velocity of sound in body tissue is very
close to that for water, about 1,540 meters
per second. This is not surprising, because
the structure of body tissue is primarily liquid.
Ultrasound scanners are types of electronic
medical equipment that require several separately packaged electric systems to operate.
These systems include one or more transducers, a computer mainframe, the cathode ray
tube (CRT) display device (which may be
integral or separate from the mainframe), a
keyboard, a video recording device, an additional display device, and a printer. The
transducers are external devices that are
used to transmit and receive the ultrasonic
waves. Ultrasound scanners may also include
circuitry to allow monitoring of electrocardiograph (ECG) signals.
Most ultrasound scanners will contain one or
more combination power supplies because
DC source voltages with low-noise (derived
from linear power supplies) are required to
power sensitive analog circuits and because
DC source voltages with some switching
noise (derived from SMPSs) can be used to
power digital circuits. A combination power
supply consists of a switch-mode power supply and one or more linear power supplies
with their AC voltages derived from the same
power transformer. To power the mainframe,
an original equipment manufacturer (OEM)
combination power supply will more than
likely be used. To power each transducer, a
combination power supply manufactured by
the ultrasound scanner manufacturer will
more than likely be used. The circuitry in
both the mainframe and the transducers usually require several dual DC source voltages
and thus each power supply will contain several outputs. If the CRT is internal to the
mainframe, an OEM DC-to-DC converter will
normally be used to convert a low DC voltage
from the mainframe SMPS to a high DC volt-
9
Embedded
solutions
will help
assure
that the
ultrasound
machine
remains
operative
during the
disturbance
Block Diagram of an Ultrasound System
age that can be used to drive the CRT. The
block diagram above illustrates an example
of an ultrasound system.
In this example, the power management system consists of the DC-to-DC buck converters, one boost converter, one low-voltage
drop-out (LDO) supply, a DC-to-DC controller, a supply voltage supervisor circuit,
and a voltage reference. An electrical disturbance incident upon an ultrasound system
will be felt at the AC inputs of each of these
power supply systems. The response to a distinct sag, for example, will be different for
each supply. Integrating embedded solutions
into each supply will help assure that each
subsystem in the ultrasound machine
remains operative during the disturbance as
opposed to installing a whole-system UPS on
the AC input.
Computed Tomography (CT) Imaging
Systems
Computed tomography (CT) is a type of X-
10
ray radiographic imaging system that provides digitally-enhanced sectional views of
the patient. Tomography is a radiographic
technique that selects a level in the body and
blurs out structures above and below that
plane, leaving a clear image of the selected
anatomy. In a CT system, the motion of an Xray tube must be precisely controlled.
Electromechanical devices such as servos,
positioners, etc., are used to rotate and position the complex X-ray tube in a CT scanner.
This is accomplished by moving the x-ray
tube in the opposite direction from the imaging device around a stationary fulcrum defining the plane of interest. X-ray tube movements can be linear, curvilinear, circular,
elliptical, figure eight, hypocycloidal, or
trispiral. CT is a type of tomography in which
transverse planes of tissue are swept by a
pinpoint radiographic beam and a computerized analysis of the variance in absorption
produces a precise reconstructed image of
that area. This technique has a greater sensitivity in showing the relationship of struc-
tures than conventional radiography (X-ray
machines).
The reconstruction of the internal structure
of an object from its projections is neither
new nor novel. Thousands of solid-state Xray detectors measure the X-ray beams from
the patient. The data from the detectors is
digitized, processed, and analyzed by a large
computer system. There were many investigators in the area of image reconstruction of
X-ray projections. However, it was not until
1970 that Godfrey Hounsfield, a British engineer working for EMI Limited in England,
passing X-rays through the body and detecting them with photographic film. The different tissue and body structure of the body
attenuate the X-ray beam differently and
thus vary the intensity of the X-radiation as a
function of location. These changes in intensity are what the photographic film responds
to. The CT scanner, on the other hand, consists of an X-ray tube, X-ray detectors with
photomultiplier tubes, A/D converters, computer systems, and a video display as shown
in the figure below.
Power electronic components and systems
produced the first X-ray scanning machine,
the EMI brain scanner, which basic power
electronic components made possible. For
his work in the development of the CT scan-
are used throughout the CT scanner. Highrange to very high-range AC-to-DC SMPSs
are embedded throughout the scanner to
provide power to the computer processor,
ner, Hounsfield was awarded the Nobel Prize
for Medicine in 1979. It has been said that
the data acquisition system (DAS), memory
storage devices and management systems,
there has been no comparable discovery of
and control systems. A high-voltage DC
this magnitude in radiology since Roentgen
power supply similar to the ones used in
discovered X-rays in 1895.
modern X-ray systems are used to provide a
high voltage to the X-ray tube. A separate
high-voltage power supply is used to provide
CT Scanning Techniques:
power to operate the CRTs used in the CT
The Reconstruction Process
scanner. If a separate viewing console is
An ordinary X-ray system takes pictures by
required, an additional SMPS is also used to
Simplified CT Scanner Block Diagram
11
Each power
supply used in
a CT scanner
is exposed to
electrical
disturbances
from the
healthcare
facility
power this device. Through the use of power
electronics and electromechanical devices,
the X-ray tube rotates around the patient to
transmit a series of X-ray beams through the
patient that are detected by thousands of
solid-state X-ray detectors. The detectors
measure the attenuation of the fan-shaped Xray beam by structures within the patient.
These detectors collect the information from
each projection. The information is then digitized and analyzed by a computer to reconstruct cross-sectional CT images.
ated with the word nuclear in the late 1970’s.
MRI started out as a tomographic imaging
technique; that is, it produced a thin-slice
image of the human body with NMR signals.
MRI has advanced beyond a tomographic
imaging technique to a volume imaging technique. This section presents an overview of
some of the electrical systems used in a typical MRI system.
Each power supply used in a CT scanner to
Imaging is the production of a picture,
image, or shadow that represents the object
being investigated. MRI is a type of radiography using electromagnetic energy. Certain
power one or more subsystems is exposed to
electrical disturbances from the healthcare
facility electrical system. Because each of
these power supplies are different designs
atomic nuclei with an odd number of neutrons, protons, or both are subjected to a
radio frequency pulse, causing them to
absorb and release energy. The resulting cur-
and have different loadings, they will
rent detected by a set of radio frequency coils
respond differently to common disturbances
passes through a radio frequency receiver
such as voltage sags and momentary inter-
and is then transformed electronically into
ruptions. If their compatibility with the pub-
an image. This technique is valuable in pro-
lic power system and the healthcare facility’s
viding soft-tissue images of the central nerv-
electrical environment is acceptable, then
ous and musculoskeletal systems. Other MRI
each supply should continue operating to
some specified ride-through level. The ability
techniques allow visualization of the vascular
system without the use of contrast agents.
of the high-voltage generator to survive such
However, agents are available for contrast
disturbances is critical to the life of the scan-
enhancement.
ner. Compatibility engineering studies can be
used to determine the response of each supply and its associated system to common disturbances.
Magnetic Resonance Imaging (MRI)
Systems
Magnetic resonance imaging (MRI) is an
imaging technique used primarily in the
healthcare industry to produce high quality
images of the inside of the human body.
MRI is based on the principles of nuclear
magnetic resonance (NMR), a spectroscopic
technique used by scientists to obtain microscopic chemical and physical information
about molecules. The technique was called
magnetic resonance imaging rather than
nuclear magnetic resonance imaging (NMRI)
because of the negative connotations associ-
12
Hardware Overview
The following illustration depicts a schematic
representation of the major systems on a
magnetic resonance imager and a few of the
major interconnections. This overview briefly
states the function of each component.
The components of the imager are located in
a shielded room that protects the signal generation and detection circuits of the RF coil
system. Without a shielded room, electromagnetic noise from inside and outside the
hospital environment would corrupt the
images and introduce unwanted imaging
artifacts. Improper installation of metallic
subsystems that come in contact with the
shielded room and RF leaks in the shielded
room can also produce artifacts.
The PDU is
the first to
see a
disturbance
and can
allow that
disturbance to
travel to other
power
supplies
within the
MRI system.
Major Electrical System Components of a Typical MRI System
The magnet above and below the total magnet system produces the baseline magnetic
field (Bo) for the imaging procedure. Within
the magnet system are the gradient coils for
producing a gradient in this magnetic field
(Bo) in the x, y, and z directions. The power
for gradient coils is generated by the gradient
power supply and amplifier, which contains a
high-range DC-to-DC SMPS that powers electronic circuitry used to generate the current
for the gradient coils.
The bore of the MRI system, surrounded by
the magnet of the MRI system, where the
patient rests on a movable table, contains the
gradient coils and RF coils. Within the gradient coils is the radio-frequency (RF) coil. The
RF coil produces another magnetic field (B1)
necessary to rotate the magnetic spins by 90°
or 180°. The RF coil also detects the signal
from the spins that originate within the body.
The RF power for the RF coils is generated by
the RF power supply, which contains a very
high-range DC-to-DC SMPS that powers the
RF power transistors used to switch the RF
power to the RF coils. The main power supply
for the MRI system is provided via a power
distribution unit (PDU), the heart of the MRI
system with respect to power management.
The PDU provides power to the computer,
the magnet power supply, the gradient power
supply, the RF power supply, the digitizer, the
patient table, and the RF detectors. The PDU
is the first to see a disturbance and can allow
that disturbance to travel to other power
supplies within the MRI system.
The patient is positioned within the magnet
by a computer controlled patient table that
contains a DC stepper drive. The table has a
positioning accuracy of 1 mm, for example.
The scan room is surrounded by an electromagnetically shielded room, which also confines the RF energy. The shield prevents the
high power RF pulses from radiating out
through the healthcare facility. It also prevents external radiated emissions, such as
various RF signals generated by television
and radio station transmitters and radio
communications equipment from within the
facility, from being detected by the imager.
Some scan rooms are also surrounded by a
magnetic shield, which prevents the magnetic field from extending too far into the
healthcare facility. In newer magnet systems,
the magnet shield is an integral part of the
magnet system.
On the data setup and processing side of the
MRI is the heart of the imager, a very large
13
computer system. It controls all of the components on the imager. The RF components
under control of the computer are the RF signal generator, which generates the RF signals
and the RF pulse programmer. The generator
produces a sine wave of the desired RF frequency. The pulse programmer reshapes the
RF pulses. The RF amplifier increases the
power of the pulses from milliwatts to kilowatts. The computer also controls the gradient pulse programmer, which sets the shape
and amplitude of each of the three gradient
magnetic fields. The gradient amplifier
increases the power of the gradient pulses to
a level sufficient to drive the gradient coils.
The array processor, located on some
imagers, is capable of performing a two-
not having any effect on the processes being
studied, and (2) The radionuclides being
used can penetrate tissue and be detected
outside the patient. Thus, the materials can
trace processes or “opacify” organs without
affecting their function.
Radionuclides that exit the patient must be
detected to be of use to the medical staff in
diagnosing a patient’s condition. Detection is
accomplished through the use of a variety of
radiation detectors. Imaging systems that
employ detectors are called sophisticated
imaging devices. Extensive research and
development has gone into the development
of radiation detectors with the goal of
improving the image.
dimensional Fourier transform in fractions of
Radiation detection is accomplished by
a second. The computer off-loads the Fourier
employing a transducer and associated electronics as shown in the figure below.
Radiation interacts in the transducer,
depositing energy by exciting or ionizing
transducer atoms. The transducer is physi-
transform to this faster device.
The operator of the imager gives input to the
computer through a control console. An
imaging sequence is selected and customized
from the console. The operator can see the
images on a video display located on the
console, can manage the memory storage of
the images, or can make hard copies of the
images on a film printer.
Nuclear Imaging Systems
Nuclear medicine can be defined as the practice of making patients radioactive for diagnostic and therapeutic purposes. The
radioactivity is injected intravenously,
rebreathed, or ingested. It is the
internal circulation of radioactive
material that distinguishes nuclear
medicine from diagnostic radiology
and radiation oncology in most of
its forms. The radioactivity is
detected from outside the body
without trauma to the patient.
Diagnostic nuclear medicine is successful for two main reasons: (1) It
can rely on the use of very small
amounts of materials thus usually
cally coupled to an electronics stage, where
the subtle effect within the transducer is
converted into a measurable electronic signal. This signal is processed, analyzed, and
counted by the digital computer system
which is the main part of the nuclear imaging system. The output from the electronics
stage is fed into a display or storage device
for interpretation.
The types of detectors encountered in
nuclear medicine are gas-filled detectors,
scintillation detectors, and semiconductor
Stages of Radiation Detection
14
Basic Diagram of a Photomultiplier Tube Used in Nuclear Medicine
detectors. Each one of these detectors
bias and control them. Electrical distur-
requires the use of a power supply system to
bances incident upon a system may also alter
bias the detector electronics such that the
radiation may be detected. The power supply
also biases the detection stages such that the
electronic signal may be produced.
calibration settings. Compatibility engineering studies can be used to determine the
response of each supply and its associated
system to common disturbances.
As an illustration of a detector used in scintillation, consider the figure above. This diagram shows a schematic of a photomultiplier
tube. Light photons entering the glass
entrance window impinge on the photocathode. The result is emission of one electron
for approximately every five light photons.
The electron produced is accelerated toward
a dynode chain. The accelerated electron has
sufficient kinetic energy to liberate approximately five additional electrons when it
strikes each dynode. The effective electron
gain at the collecting anode is 106 to 108.
The output of the photomultiplier is a signal
with a specific characteristic shape whose
amplitude is proportional to the number of
photons entering the photomultiplier or the
energy deposited in the crystal.
In the figure above, the high-voltage power
supply biases the tube using a series of resistors. The output at the anode is capacitively
coupled to a pre-amplifier where the signal is
prepared for processing and analysis. Similar
to the other imaging systems, each power
supply system in a nuclear imaging system is
exposed to the quality of power present within the healthcare facility. Abrupt changes in
the input voltage may shorten the life of NMI
detectors and the electronic systems used to
Diagnostic
imaging
systems (DISs)
are a compilation of
many
integrated
subsystems
Why Do Imaging Systems Require
Quality Power?
Diagnostic imaging systems (DISs) are a
compilation of many integrated subsystems
each of which is given life through the quality of power from the healthcare facility electrical system. Each subsystem is designed for
a specific modality and integrated into overall imaging system designs. Integrating subsystems is necessary in imaging system
design because DISs are complex and image
processing intensive, and DIS manufacturers
can save development, design, and implementation costs by not re-inventing subsystems that already exist. Their complexity and
data-intensiveness requires the use of vast
amounts of data processing power using
many central processing units (CPUs) and
memory storage devices (MSDs) among other
subsystems. Examples of other subsystems
that are critical to DISs include power supplies, amplifiers, and electromechanical
devices.
Billions of data bits are collected and
processed each second to reconstruct highresolution images representative of organs,
tissues, cellular structures and other physiological and anatomical parts of the human
15
body. DISs are used several times per hour in
a healthcare facility not only probe the body
slice by slice to display reconstructed images
for physicians, but also to store images in
MSDs that are a part of the overall imaging
system. Stored images are later used by the
physician who ordered the test and other
specialists to diagnose a patient’s condition.
More commonly now, images are transmitted
via patient data networks within a healthcare
complex and via the Internet to other geographical areas when they must be read by
other physicians.
designed, they were used on a limited basis
to protect DISs from electrical disturbances.
During the design of early DISs, active power
factor correction (PFC) techniques did not
yet exist, and designers did not use passive
techniques to address the highly non-linear
line input currents that DISs drew from the
electrical systems of healthcare facilities.
Similarly, designers used basic energy storage
components to achieve minimal filter
requirements in basic power supplies. The
lack of protection devices, current correction
techniques, and optimal energy storage in
DISs placed them at high exposure to shutdowns and damage from non-destructive and
destructive electrical disturbances. Today,
DIS manufacturers recognize that many field
failures of early DISs probably occurred
because their lack of awareness of power
quality, system compatibility engineering,
and the types of problems that disturbances
could cause to DISs.
Advancements in the design, packaging, and
application of the devices in each semiconductor category provided designers with
semiconductors of higher power, increased
switching speeds, and enhanced data storage
densities in smaller device package sizes,
allowing DISs the ability to drive more power
Semiconductor devices play many very
important roles in all DISs. Power electronic,
small signal, and integrated circuit semiconductor devices are used throughout all DIS
subsystems. Early X-ray machines and computed axial tomography (CAT) are among the
few electrical/electronic systems that
required the use of high power vacuum X-ray
tubes to generate X-ray images. All early DISs
used more individual subsystems and
devices requiring lots of interconnecting
cables, equipment cabinets and floor space
in healthcare facilities. Although some basic
protection devices such as surge protectors
were starting to become available for enduse equipment when early DISs were
16
into capturing better images and processing
more data in a shorter time frame. Moreover,
hybrid semiconductor technologies— technologies that integrated advanced control
techniques with low-loss fast power switching devices—allowed equipment designers to
generate, control and manage larger amounts
of power at higher frequencies. These
advances allowed DIS designers to achieve
DIS designs with higher imaging and storage
performance. This performance is required
to meet physicians’ demands for DISs that
could reveal more details about the human
body with faster image processing times. But,
as devices continued to advance, they also
continued to generate more heat and require
more electrical power to operate as imaging
and data processing power increase.
Although the sensitivity of early DISs to electrical disturbances was unknown to designers, installers, end-users, and maintainers,
high performance DISs, which utilized the
most sophisticated semiconductor technologies available, end users suspected that DISs
were becoming more sensitive to electrical
disturbances as their designs advanced. This
increasing sensitivity resulted from the
increased use of semiconductor devices combined with complex electronic control subsystems that required higher input power
and that were less forgiving of electrical disturbances.
This trend occurred until manufacturers of
subsystems—namely, power supplies and
subsystems that included power supplies on
their front end—began to integrate some
OEM power supplies as required by international standards. Immunity improvements in
some PDUs and power supplies resulted from
an increased level of awareness of power
quality and system compatibility engineering
applied in other industries that required
compatible systems such as manufacturing
and personal computing. Although some
improved levels of immunity to disturbances
are included in some power supplies, ridethrough performance (i.e., voltage tolerance
envelopes) is not known for DISs as a whole.
In other words, DIS manufacturers are not
aware of the weakest links in the ability of
their systems to ride through common electrical disturbances such as voltage sags and
voltage swells.
DISs manufacturers are continuously intro-
minimal level of immunity to common elec-
ducing more advanced features and
trical disturbances in their designs. However,
improved imaging performance to continue
because physicians have been focused on
obtaining higher resolution images in shorter
meeting the needs of physicians. With
time frames to decrease diagnostic time, this
trend of improving immunity did not
management, system flexibility, and high
imaging performance, today’s programmable
progress at the same rate as DIS power com-
logic devices provide the system on a chip
plexity, power requirements, and imaging
(SOC) capabilities to drive next generation
performance.
Similarly, healthcare facility engineers have
been focused on trying to provide the utilities (electricity, electromagnetically quiet
environments, water, air conditioning, heating, and medical gases) to operate DISs and
efficient utilization of space in imaging
suites. And, imaging department directors
have been focused on managing imaging
departments—staffing each DIS with trained
personnel, maintaining patient flow, and
archiving and providing access to patient
imaging data. For these reasons, limited
emphasis has been placed on characterizing
the immunity of DISs to electrical disturbances and providing quality power to imaging systems.
Awareness
of weak links
in imaging
systems will
improve their
ride-through
performance
increases in semiconductor density, thermal
imaging systems. Although increased integration within DISs equates to higher imaging
performance in smaller-sized systems, the
immunity of DISs to electrical disturbances
is still a concern.
The illustration below is an example of a data
system used in a modern DIS. Each of these
The immunity levels that exist in today’s DIS
subsystems exist because of immunity
requirements integrated into some PDUs and
Example of Complex Data Filtering, Alignment, Buffering, and Imaging
Processing in DISs
17
Signal
intensity
artifacts
interfere
with the
interpretation
of the patient’s
condition by
radiologists
and other
physicians
cards requires a DC power supply capable of
providing steady-state DC voltage and pulsed
DC current. One supply may be used to
power all cards in a system or individual supplies may be used. DC current requirements
vary among imaging system modes and
among modalities. As processing speeds continue to increase, the power requirements of
DC power supplies also increase. Immunity
issues will still remain a concern even if
power supplies are designed to provide high
frequency output currents embedded on
individual data cards.
investigations and the consideration of comprehensive compatibility testing of imaging
systems, imaging system designers, end
users, and service technicians believed that
most artifacts were caused by non-powerrelated phenomena. Injecting disturbances
into the AC power inputs of imaging systems
of various modalities may very well reveal
that the propensity for disturbances to cause
artifacts may be higher than originally
expected.
The data acquisition card, which filters
incoming data, is the most cost-sensitive system card. Usually a diagnostic imaging system will consist of multiple data acquisition
cards (in some cases up to 20 cards per sys-
discussion on imaging artifacts included
here. Signal intensity artifacts inherent in
local coil imaging include intensity gradient
and local intensity shift artifacts. The latter
tem). Once the data is filtered, it is sent to
Before examples of power quality-related
artifacts with an MR system are shown, some
can be minimized but not eliminated with
optimal coil design and tuning. Improper
the data consolidation card for buffering and
coil or patient positioning can produce sub-
data alignment. Once the data has been
tle or, in some cases, severe signal intensity
aligned, it is sent to the image/data process-
artifacts, and each is easily corrected. Signal
ing card, which is the most algorithm-inten-
intensity artifacts and image degradation can
sive card in the system because it performs
also occur in a perfectly functioning coil if
protocols are not optimized. Failure of
all the heavy-duty filtering and image reconstruction. Any damage to these cards caused
by voltage transients or electromagnetic
fields that migrate through the system may
cause card damage, image artifacts, lost
decoupling mechanisms can produce signal
intensity artifacts that will not respond to
protocol optimization and will worsen with
gradient imaging. Improper coil tuning man-
imaging data, or malfunction of
the imaging system.
Image Artifacts Caused by
Poor Power Quality
Signal intensity artifacts are
often encountered during magnetic resonance (MR) imaging
and during the use of other
modalities for image construction. Too frequently, these artifacts are severe enough to
degrade image quality.
Occasionally, they interfere with
the interpretation of the
patient’s condition by radiologists and other physicians. Prior
to carrying out power quality
Example of Signal-Intensity Artifact Caused by an Electrical Disturbance
Incident upon an MR System during a Thunderstorm
18
Example of Image with Artifact (Left) from Poor Power Quality and without Artifact (Right)
ifests as a shading artifact that can mimic
other findings. Signal degrading artifacts may
be caused by a ferromagnetic foreign body in
the imager. Signal intensity artifacts can also
result from performing ultrafast imaging with
coils that were not designed for this type of
imaging or from MR imaging system malfunction. Familiarity with the various causes
of signal intensity artifacts is necessary to
maintain optimal image quality and are typically required as part of any MR imaging
quality assurance program. Optical image
quality also involves knowledge of how electrical disturbances create artifacts and how
to avoid them.
Imaging Systems: The Backbone for
Diagnostic Services for Healthcare
Facilities
The black and white image (previous page)
illustrates a signal intensity artifact caused
by system malfunction. This is a sagittal T1-
Common DISs such as CT and MRI have
become indispensable technologies and are
used to generate critical patient data at surprisingly increasing speeds. As medical
researchers probe further into the body and
as DIS manufacturers develop more powerful
systems, continual improvements are being
made to imaging techniques. Many more
weighted brain MR image obtained in a pediatric patient that demonstrates a signal
intensity artifact caused by shim coil malfunction that resulted from an electrical disturbance incident upon the AC power input
to the MR system during an electrical storm.
The color images (above) illustrate an image
artifact (left) caused by electrical noise present on the input line of a power distribution
unit that delivers power to various parts of
an imaging system. When the noise was
removed, the artifact was not present in the
next series of images.
Practicing medicine in today’s society would
not be possible without the use of imaging
studies. As one might expect, imaging studies
are carried out through the use of diagnostic
imaging systems (DISs). Without DISs, physicians could not be alerted of a patient’s
underlying serious illness. Clinicians would
not be able to make an accurate diagnosis in
a timely manner critical to a patient’s condition. Physicians would have no way of generating follow-up images to determine if a
patient’s treatment is working.
Common
DISs such as
CT and MRI
have become
indispensable
technologies
imaging system applications in clinical and
research settings are also being developed.
In the past ten years, DISs have become an
integral part of every healthcare facility. In
major healthcare research institutions, it is
not uncommon for hundreds of physicians,
researchers and support staff to provide mission-critical care, education, research, and
prevention in radiology, nuclear medicine,
19
and various types of diagnostic imaging. As
many as 500 CT scans and 100 MRI scans
may be performed daily in a single healthcare research institution.
Poor image
quality
and the
cancellation
of studies
reduce the
efficiency
of the
healthcare
facility
and the
imaging
department
Any medical imaging director or technician
who has ever been responsible for providing
imaging services to a hospital or operating
an imaging system has experienced the consequences of imaging system downtime and
the effects of this downtime on the hospital’s
ability to serve the community. Most healthcare facilities that provide imaging services
only have one system for each modality. And,
as the demand for imaging services increases
and as new applications for imaging are
developed, hospitals and imaging centers are
having to install additional systems in the
widely used modalities. An interruption to
the imaging services at a facility will impact
the patient flow process whether the inter-
main panel that provides power to the imaging system.
A wiring or grounding problem may be easily
correctable by the facility electrician without
reaping havoc on the quality or reliability of
power to an imaging system. On the other
hand, a wiring or grounding problem may
require significant modifications to the facility electrical system which cannot be made in
the course of a day or two. Such modifications must be carefully scheduled to prevent
interrupting the operation of other critical
equipment in the healthcare facility.
Temporary branch circuits may even need to
be installed to eliminate a power quality
problem for an imaging system and provide
quality power to the system until a permanent solution can be identified and implemented in a facility while following applicable electrical codes and requirements.
ruption is brought about by having to reboot
a system just once a day or having a system
go down for a few hours or days.
If a system cannot remain operational or if a
system cannot be restarted, then the systems’
field service engineer from the imaging manufacturer must be called in to resolve the
problem. In most instances, a problem that
requires a service engineer can be resolved
within a 24-hour period. However, depending
upon the severity of the problem, there are
several instances when a system problem
cannot be resolved for several days, and
these cases are usually related to power quality.
If the service engineer addressing a problem
is trained to conduct a power quality audit,
then he or she may be able to identify a
wiring or grounding problem that may be
contributing to the system shutdown. Service
engineers are being limited as to how deep
they can probe into a facility electrical system. There are risks associated with arc flash
hazards when opening up an electrical panel
and probing around in the panel. These risks
now limit them only to investigating the
20
Power Quality for Support
Equipment in Imaging System
Suites: An Expanding Concern
The need for imaging systems, especially
MRI and CT systems, has grown dramatically
in healthcare facilities. Although advances in
MRI and CT system technologies have
occurred in the last several years, the
increasing number of applications for MRI
and CT studies have spawned a number of
related problems. Imaging studies conducted
with MRI systems can be lengthy, requiring
healthcare professionals to sedate some
patients including children and those who
cannot tolerate the procedure. Patient movement during a study can result in undesirable image quality. Moreover, patients who
are critically ill, immobile or under distress
may not be able to undergo a study or may
cause image quality to be compromised.
Poor image quality and the cancellation of
studies reduce the efficiency of the healthcare facility and the imaging department.
A patient who undergoes an MRI procedure
is under little risk. However, the need to
sedate patients for an imaging study may
increase the risk of life-threatening adverse
events resulting from sedation or the use of
anesthesia. If a patient develops a problem
or if a condition worsens, the patient may
not be able to respond or alert an imaging
system operator about the problem, a cardiac
distress, or other changes in the patient’s
physiological status. A patient requiring an
imaging study may also be dependent upon a
ventilator, which also requires the use of
patient monitoring equipment, thus the need
for support equipment in the imaging suite
and quality power to operate the equipment.
When patients are sedated or when anesthesia is used, cardio-respiratory parameters
must be monitored during the imaging study
to provide a standard of care equivalent to
that provided in the operating room.
compatible with the MRI system is brought
into the suite and placed near the patient.
Monitors that are not an integral part of the
imaging system and ventilators must be powered from a 120-volt branch circuit internal
to the imaging suite. However, healthcare
facility designers and operating engineers,
imaging department managers, and system
operators may not realize that monitors and
ventilators used in imaging suites should
also be powered from sources of quality
power. Malfunction of support equipment
caused by poor power quality in an imaging
suite may also pose interruptions to the
imaging department.
Power
quality
for imaging
systems
must be
achieved
through
well-defined
specifications
Regardless of whether the circuit is fed from
the emergency power source, monitors and
ventilators must function when needed and
require quality power to meet the needs of
Organizations specializing in the safety of
imaging have developed guidelines requiring
the healthcare professionals. Quality power
healthcare professionals to monitor critically
ill patients and patients who receive sedation
to handle a typical load consisting of a monitor and ventilator. Or, a larger dedicated UPS
or anesthesia during an MRI procedure.
may be used to provide quality power to
Most equipment used to monitor a patient’s
status or to provide respiratory assistance
cannot be used in an MRI environment
because of the characteristics of strong magnetic fields. Items, including electronic medical equipment, containing ferrous-based
materials can become dangerous projectiles
possibly injuring patients and healthcare
professionals and causing damage to the
imaging system. To overcome this magnetic
field compatibility problem, manufacturers
of patient monitors and ventilators have
designed equipment that can be used in MRI
suites.
suite.
may be provided by an individual UPS rated
every 120-volt receptacle in the imaging
Traditionally, MRI-compatible patient monitoring and other equipment used in an imaging suite is not an integral part of the imaging system, and is powered via a branch circuit separate from that powering the imaging
system. Ventilators are designed to be mobile
stand-alone equipment. When a patient
monitor or ventilator is needed, equipment
On the other hand, some imaging systems
may contain patient monitoring equipment
integral to the system. Such monitors will be
fed from the circuit(s) powering the imaging
equipment inside the suite. If this circuit is
derived from a quality power source, then the
monitoring equipment will also be fed from
quality power. Imaging department managers
should consult with the healthcare facility
designers and engineers to ensure that quality power is provided to all critical areas of an
imaging suite.
Power Quality for
Imaging Systems
Achieving power quality for imaging systems
cannot be written into a given system specification or imaging suite design that stands
alone. Power quality for imaging systems
21
Comprehensive
guidelines
that address
all the causes
and effects of
power quality
problems and
recommendations to avoid
imaging system
problems do
not yet exist
must be achieved through well-defined specifications that are based on the characteristics of the real healthcare electrical environment where the system will be installed, the
expectations of the imaging department and
the imaging system manufacturer, and the
immunity (or susceptibility) of the imaging
system to common electrical disturbances
such as sags and surges. Presently, imaging
system manufacturers know little about how
their systems will react to electrical disturbances other than the impact of such disturbances on their systems costs millions of dollars in the aggregate cost of spare parts,
repair labor, and downtime. Manufacturers
are eager to learn more about how their
equipment responds to disturbances and
how to integrate embedded solutions into
the power distribution, power supply, and
critical systems to improve overall performance and reliability. Once equipment performance is known and matched closely to
the electrical environment of the healthcare
facility, most of the obstacles in purchasing,
Larger permanent systems such as MRI and
CT require well-engineered, careful, and
step-wise installations. An installation
error—on the facility electrical system side or
on the point-of-use side of the main disconnect switch at the imaging system—will foster power quality problems. While mobile
systems, also available in MRI and CT
(portable systems are also typically used for
ultrasound and X-ray) do not require as complex installation procedures, the quality of
power on circuits used to power these systems in still vitally important. Some of the
new systems are installed in place of older
ones at existing imaging suite sites, some are
new site installations, and some are used in
geographic areas where natural disasters
have occurred to provide imaging services to
communities where healthcare facilities are
inoperable. Remanufactured systems in all
modalities are also used and installed in
healthcare facilities. Regardless of the situation, specifying the level of power quality is
crucial to the successful startup of a new sys-
planning, installing, operating, and main-
tem and its continued reliable operation dur-
taining imaging systems in real electrical
ing the course of its life.
environments will be removed.
Because the successful startup and operation
Specifying Power Quality
Performance for New Imaging
System Installations
Record numbers of new imaging systems are
delivered to healthcare facilities each year.
of imaging systems is highly dependent upon
the quality of power delivered to the system,
the installation procedure, from beginning to
end, must be carried out with power quality
in mind. Knowledge of installation guidelines
that encompass all facets of power quality
and following those guidelines will help
designers, manufacturers, installers, service
people, and end users reduce installation
time and significantly reduce downtime
caused by power quality problems.
Unfortunately, comprehensive guidelines
that address all of the causes and effects of
power quality problems and recommendations to avoid imaging system problems do
not yet exist, with any imaging system manufacturer. Fairly speaking, while manufacturers have learned a lot about the do’s and do
not’s about basic power quality, installation
planning guidelines and system specifications have not reached maturity. The system
Installation of an Imaging System
22
compatibility concept, developed by EPRI
Solutions, when applied to imaging system
design will be critical in helping to develop
these guidelines. As the players—end-use
customers, manufacturers, power quality
researchers, and utilities—in the industry
learn more about power quality, the number
of power quality-related shutdowns in imaging suites will decrease.
similarities and also some distinct differences that impact the power quality related
operation of imaging systems.
Key Areas of Power Quality.
Addressing each facet of power quality at the
There are five key areas of power quality that
should be addressed at the beginning of the
site and installation planning stage. Table 1
below list these five areas, the activity that
should be carried out beneficial to the startup and continued operation of the imaging
system, and the questions that should be
answered to ensure that each area is properly
installation planning stage will minimize the
number of power quality-related shutdowns
experienced by imaging systems. If one were
to obtain a copy of the electrical system
addressed. Addressing these areas will help
manufacturers, installers, facility engineers,
and end users avoid many power quality
problems that end up causing malfunctions
specifications and installation requirements
and damage to imaging systems.
Addressing Power Quality at the
Installation Planning Stage
for the facility electrical system for any imaging system modality, one would find some
Table 1. The Five Key Areas of Power Quality and Their Benefit to Diagnostic Imaging Systems
Power Quality
Area to Examine
Activity Beneficial to Startup and Continued
Operation of Imaging System
Question to Answer
1. Voltage Quality
Over a typical year’s period, obtain knowledge of
Will the amplitude, frequency, waveshape, and rate of
what voltage disturbances to expect on a circuit pow-
occurrence of the disturbances that reach the imaging
ering an imaging system and on circuits power imag-
system affect the startup and operation of the system?
ing system accessories.
2. Wiring
Over the entire length of the power and neutral cir-
Has the entire length of each segment of the entire
cuits delivering power to the imaging system, avoid
power and neutral circuit been physically inspected for
all wiring errors, including those associated with dis-
errors? Have wiring practices in question been identi-
connects, panels, circuit protection devices, trans-
fied as errors? Have all wiring errors been resolved?
formers, and splices.
3. Grounding
Over the entire length of the grounding system that
Has the entire length of each segment of the grounding
provides a safety and equipment ground for the imag-
system been physically inspected for errors? Have
ing system, avoid all grounding errors, including
grounding practices in question been identified as
those associated with the facility, panels, subpanels,
errors? Have all grounding errors been resolved?
disconnects, and transformers.
4. Energy Storage
5. Immunity
Determine if there is a need for providing a minimum
Has energy storage been considered? If so, what are
level of energy storage to a new imaging system, and
the driving factors? Has an energy storage system been
if previous or other systems in the facility utilize
included in the site plan? If it has not been considered,
energy storage.
should it be?
Determine if the manufacturer specifies the immuni-
If immunity to disturbances is known, to what distur-
ty of the imaging system to disturbances such as
bance and to what levels is the system immune? Are
short duration variations and transients.
the immunity criteria available for review? If no immunity criterion has been established, then ask the manufacturer about immunity testing.
23
Including Power Quality Criteria in
Imaging System Specifications.
The level of detail with respect to power
quality criteria that is included within the
electrical specifications of imaging systems
varies with manufacturer. Some manufacturers take the more proactive position of
including more criteria than others. Others
may include only the basic electrical specifications for use during the site planning
stage. Research conducted by EPRI and others are helping manufacturers to improve
how to include power quality criteria in their
specifications.
Minimum Requirements.
Regardless of the approach, there are five
minimum criteria that should be included in
any imaging system specification. Table 2
outlines these minimum criteria and the
basic description of each requirement.
With respect to line voltage, most imaging
systems will require a three-phase voltage
source, require a four- or five-wire system,
will operate at 50 or 60 hertz line frequency,
and will operate within somewhat of a broad
range of steady-state AC line voltages. It is
standard practice for imaging system manufacturers to require a three-phase line voltage to be delivered to the system’s main
power distribution unit (PDU). However, not
all subsystems that are part of an overall
imaging system require three-phase power.
Within a PDU, three-phase and single-phase
power is divided up according to the individual voltage and power requirements for each
subsystem as needed. Unlike some end-use
equipment, line frequency operation at 50 or
60 hertz does not imply that the system will
operate at any frequency between these values. For example, the minimum requirements may specify a ± 1 hertz or a ± 3 hertz
range where the system can be operated
above or below the nominal line frequency
without malfunction. Many imaging systems
are designed to operate within a nominal
voltage range characteristic of healthcare
facility electrical systems found in the United
States and in other countries. Some manufacturers will impose a limit on the lower
steady-state operating voltage range, for
example 200 to208 volts, which may not be
an acceptable operating voltage for some
systems operated in the United States. At any
of the acceptable higher nominal voltage
Table 2. Minimum Power Quality Specification Requirements for an Imaging System
Minimum Power
Quality Specification
Requirement
Description of Requirement
Line Voltage
Number of phases
Line frequency
Nominal operating voltage range
Line Current Demand
Current requirement at minimum acceptable line voltage and at maximum
acceptable line voltage
Power Source
Type of transformer connection: wye-connected or delta-connected
Configuration
Power Demand
Maximum power demand and continue power demand
Power Grounding
Appropriately sized dedicated grounding conductor originating from the facility’s primary power source (main switchgear), following through each panel,
bonded to the surface of each panel with one non-insulated grounding block,
through the main disconnect panel, and terminating at the power distribution
unit of the imaging system.
24
operating points—380, 400, 415, 480 volts
AC—most systems can be operated at 10%
above or below these values.
or step-down device), connection and use of
neutral and ground conductors, and grounding. Further guidance on these issues is discussed in some of the following sections.
With respect to line current demand, systems
operated at lower line voltages will require
higher current demands, of course, resulting
in the requirement for larger sizes for power
and neutral conductors. ( The use of larger
power conductors may also require the use of
larger ground conductors.) If current demand
values for maximum momentary operation
are provided, these values will be larger than
those during continuous operation. Larger
Providing proper power and signal grounding
within an imaging system (design and installation) is one of the most critical system
requirements. In the steady-state (i.e., when
the system is powered up and ready for use),
inadequate grounding will reduce the quality
of the images captured during a scan and will
place the system at a higher exposure level
for damage resulting from electrical distur-
currents are usually required at system startup, during image capture, and/or during a
change from one system operating mode to
another.
bances. In the dynamic-state (i.e., when the
system is scanning) when a disturbance is
occurring, inadequate grounding will result
in higher levels of voltage transients whether
Providing
proper power
and signal
grounding
within an
imaging
system is
one of the
most critical
system
requirements
the transient was a true voltage surge (origiWith respect to power source configuration
and power demand, the type of transformer
used to power the imaging system and its
size is critical to powering the system correctly. In healthcare facilities, wye-connected
or delta-connected transformers may already
be in place for providing power to other
equipment or an existing imaging system.
Ideally, when a lower line voltage must be
stepped up to provide a higher line voltage to
an imaging system, a delta-wye transformer
configuration should be used. In this application, the delta side should be connected to
the low input voltage and the wye side
should be used to power the imaging system.
As a minimum, the maximum power demand
data from the system’s specifications should
be used to size the transformer instead of the
continuous demand data. If the continuous
demand data is used, the transformer will be
undersized, and the power demanded by the
imaging system will collapse (distort) part of
the line voltage during image capture.
Several important design considerations
must be addressed to prevent transformers
that are in the power conductor path from
contributing to a power quality problem with
an imaging system. These considerations
include transformer sizing, transformer orientation (using the transformer as a step-up
nating from outside the imaging suite), a
capacitor switching transient, or an embedded transient on the leading or lagging tail of
a voltage sag or momentary interruption.
With respect to power grounding, providing
the lowest possible ground impedance from
the point where the facility’s power ground is
established to the point where the PDU is
grounded will result in the best power quality
performance. The total resistance of a
grounding system used in an imaging system
consists of the sum of the resistances of each
part of the grounding system including the
resistance of each series grounding conductor, grounding block, grounding lug, and the
grounding electrode at the main service
entrance.
But practically, the grounding impedance
requirement for the facility ground to be a
few ohms or less as required by some imaging system manufacturers can be difficult to
achieve. As a part of the power quality audit
(See Identifying Power Quality Problems in
Imaging System Suites), the total resistance
and ground current of the grounding system
may be measured, identifying issues related
to ground conductor sizing, ground conductor terminations, and facility grounding sys-
25
tems that may need improvement to lower
ground currents and lower the total ground
resistance for the imaging system.
Manufacturers
have learned
electrical
disturbances
do affect the
operation
of their
systems
As a standard guideline for facility grounds,
the NEC in Article 250 requires that a single
electrode grounding system having a resistance of more than 25 ohms be augmented by
one additional NEC-approved electrode.
Grounding to the building steel (beam) at the
main disconnect panel to the imaging system
as a supplemental ground is acceptable to
the NEC, and local codes should be checked.
However, the use of a ground rod at the main
disconnect panel to the imaging system is
not recommended by imaging system manufacturers, and will create power quality problems for imaging systems.
Optimal Requirements.
There are a number of additional requirements that may be included in the electrical
specifications and site and installation planning stages that will have a definite positive
impact on the power quality for imaging systems. As imaging system manufacturers continue to learn more about how their systems
respond to electrical phenomena, improvements to electrical specifications and site
and installation planning guides are also
being made. Continued in depth power quality research and testing of how these systems
respond to electrical disturbances will foster
this process, resulting in systems that are far
more robust than ever thought. In addition
to the minimum requirements outlined in
Table 2, the optimal requirements are outlined in Table 3 on the following page. These
requirements are only listed here as a guideline for helping end-use customers and manufacturers, and for utilities to help their customers avoid power quality problems with
imaging systems.
Table 4, on page 28, outlines the categories
and typical characteristics for power system
electromagnetic phenomena (i.e., electrical
disturbances in power quality) as defined by
26
the IEEE Standard 1159-1995 (R2001). The
categories shaded in orange represent the
typical characteristics recognized by imaging
system manufacturers who require more
than the minimum, but not the optimum
power quality requirements for their systems.
Improving Power Quality for Existing
Imaging Systems
When owners and operators of multi-million
dollar imaging systems complain about system malfunctions and shutdowns, it is in the
best interest of imaging system manufacturers to listen. In response to the growing number of power quality problems with imaging
systems, manufacturers have learned that
electrical disturbances do affect the operation of their systems and are listening to customer concerns about system shutdowns.
Listening to customers complain about the
shutdown of existing systems and identifying
improvements that will reduce the number
and frequency of shutdowns is vital to the
imaging system manufacturer–healthcare
organization relationship. Some healthcare
organizations strive to keep existing systems
online for as long as they can as a backup
system or if they are unable to purchase a
new system when new systems become available.
What Are Imaging System Manufacturers
Doing about Power Quality?
With the continuing demand for imaging systems that capture a larger number of more
detailed images in a shorter imaging time,
manufacturers are focused primarily on the
performance of subsystems (software and
hardware) that allow them to produce imaging systems with higher imaging performance. Physicians and researchers are interested in systems that can reveal more detailed
information about cellular structures, complete scans in a shorter time, and allow them
to probe further into the human body (and
other forms of life) before a decision is made
to perform an invasive or other procedure.
Table 3. Optimal Power Quality Specification Requirements for an Imaging System
Optimal Power
Quality Specification
Requirement
Voltage Transients
Description of Requirement
System will continue to function when transients incident upon main power conductors:
impulsive 1 (electrical fast transients) – oscillatory (ring wave, combination wave, capacitor
switching) – level of withstand voltage should be 6 kV for ring and combination wave transients and 2.0 p.u. for capacitor switching transients from 400 Hz to 5 kHz.
Voltage Variations: Short
System will continue to function when short duration voltage variations incident upon
Duration - Interruptions
main power conductors: momentary interruptions of 0.5 cycles < d < 3 seconds and magnitude < 0.1 p.u.; temporary interruptions of 3 seconds < d < 1 minute and magnitude <
0.1 p.u.
Voltage Variations: Short
System will continue to function when short duration voltage variations incident upon
Duration - Sags
main power conductors:
1) SEMI F47 Requirement: instantaneous sags of 0.05 < d < 0.2 seconds and magnitude
> 50%; 0.02 < d < 0.5 seconds and magnitude > 70%; momentary sags of 0.5 < d < 1
second and magnitude > 80%.
2) SEMI F47 Recommendation: instantaneous sags of 0 < d < 0.02 seconds and magnitude > 0%, 0.02 < d < 0.2 seconds and magnitude > 50%, 0.02 < d < 0.5 seconds and
magnitude > 70%; momentary sags of 0.5 < d < 3 seconds and magnitude > 80%; temporary sags of 3 < d < 10 seconds and magnitude > 80%, 10 < d < 100 seconds and magnitude > 90%
Voltage Variations: Short
System will continue to function when long duration voltage variations incident upon main
Duration - Swells
power conductors: instantaneous swells of 0.5 < d < 30 cycles up to 1.8 p.u.; momentary
swells of 30 cycles < d < 3 seconds up to 1.4 p.u.; temporary swells of 3 seconds < d < 1
minute up to 1.2 p.u.
Voltage Variations: Long
System will continue to function when long duration voltage variations incident upon main
Duration Variations
power conductors: sustained interruptions of d > 1 minute and magnitude of 0.0 p.u.;
undervoltages of d > 1 minute and magnitude of 0.8 to 0.9 p.u.; overvoltages of d > 1
minute and magnitude of 1.1 to 1.2 p.u. of d > 4 hours and magnitude up to 1.5 p.u.
Voltage Distortion
System will continue to function with a distorted voltage signal composed of primarily 3rd,
5th, and 7th harmonics at 8%, 4%, and 1%, respectively; so that the overall Vthd is
approximately 10% applied to a line voltage at 110%, 100%, and 90% of Vnominal with
harmonic content.
Voltage Notching
System will continue to function with a voltage notch centered at 90° on the line-voltage
wave form with the following notch parameters: Width: 5°, Depth: 20% Vnominal .
Voltage Fluctuations
System will continue to function with a 60 Hz sine wave fluctuating supply voltage modulated with a square wave with peak-to-peak magnitude of 1%, 3%, and 5% Vrms of rated
supply voltage at a frequency of 5, 10, and 15 Hz, synchronized with the zero crossings of
the supply voltage.
Frequency Variations
System will continue to function with a 60 Hz sine wave of varying frequency from 56 Hz
to 64 Hz.
1 Impulsive transients incident upon power conductors for subsystems and control conductors within system are
not considered here.
27
Table 4. Categories and Typical Characteristics of Power System Electromagnetic Phenomena from
IEEE 1159-1995 (R2001)
Typical Spectral
Content
Typical Duration
1.1.1 Nanosecond
5 nsec rise
< 50 nsec
1.1.2 Microsecond
1 µsec rise
50 nsec – 1 msec
0.1 msec rise
> 1 msec
< 5 kHz
0.3-50 msec
0-4 p.u.
1.2.2 Medium Frequency
5-500 kHz
20 µsec
0-8 p.u.
1.2.3 High Frequency
0.5-5 MHz
5 µsec
0-4 p.u.
2.1.1 Sag
0.5-30 cycles
0.1-0.9 p.u.
2.1.2 Swell
0.5-30 cycles
1.1-1.8 p.u.
2.2.1 Interruption
0.5 cycles-3 sec
< 0.1 p.u.
2.2.2 Sag
30 cycles-3 sec
0.1-0.9 p.u.
2.2.3 Swell
30 cycles-3 sec
1.1-1.4 p.u.
2.3.1 Interruption
3 sec-1 minute
< 0.1 p.u.
2.3.2 Sag
3 sec-1 minute
0.1-0.9 p.u.
2.3.3 Swell
3 sec-1 minute
1.1-1.2 p.u.
3.1 Interruption, Sustained
> 1 minute
0.0 p.u.
3.2 Undervoltages
> 1 minute
0.8-0.9 p.u.
3.3 Overvoltages
> 1 minute
1.1-1.2 p.u.
Steady State
0.5-2%
Steady State
0-0.1%
Categories
Typical Voltage
Magnitude
1.0 Transients
1.1 Impulsive
1.1.3 Millisecond
1.2 Oscillatory
1.2.1 Low Frequency
2.0 Short Duration Variations
2.1 Instantaneous
2.2 Momentary
2.3 Temporary
3.0 Long Duration Variations
4.0 Voltage Imbalance
5.0 Waveform Distortion
5.1 DC Offset
5.2 Harmonics
0-100th
Steady State
0-20%
5.3 Interharmonics
0-6 kHz
Steady State
0-2%
5.4 Notching
5.5 Noise
6.0 Voltage Fluctuations
7.0 Power Frequency Variations
Source: IEEE Standard 1159-1995 (R2001)
28
Steady State
Broadband
Steady State
0-1%
< 25 Hz
Intermittent
0.1-7%
< 10 sec
Successful
imaging
requires
compatibility
between
continuous
quality power
and the
imaging
system
Region of Instantaneous Sags, Momentary Sags, and Temporary Sags for Voltage Sag Immunity
Testing of Imaging Systems
Imaging system manufacturers can combat
power quality problems at two basic levels—
the design level and the installation level.
With much experience in imaging system
malfunctions and damage to equipment
caused by power quality problems, combating these problems at the installation level
for existing systems in the field has almost
become a standard operating procedure for
some manufacturers. Field engineers that
routinely service and repair imaging systems
have witnessed all types of subsystem failures caused by power quality problems. A
thorough knowledge of what happens to an
imaging system as a result of an electrical
disturbance is key to a later understanding of
how and why these systems respond to similar disturbances in the power quality laboratory.
Although imaging performance is a primary
function of how well and how fast internal
features of the body are reconstructed, the
imaging process requires compatibility
between continuous acceptable quality
power and the imaging system that does not
alter or interrupt the process. Continuous
quality power does not imply that the power
is free from electrical disturbances.
Compatibility implies predictable performance of the imaging system in real electrical
environments with real power in the health-
care facility. Sine waves that contain voltage
variations such as a 5-cycle sag to 80 % voltage, for example, whether caused by electrical operations internal or external to a
healthcare facility, should have no impact
upon the imaging capture or reconstruction
process. Waveforms that contain a 1-kHz
capacitor switching transient at 1.8 per unit
(a voltage that is 1.8 times its nominal value)
characteristic of operating utility line capacitors that provide voltage stability for the distribution system and correct the power factor
also should not impact the imaging process.
Compatibility of imaging systems with the
public power system is important at both the
design and installation levels to provide reasonable protection against all types of electrical disturbances, especially sags, swells,
and transients. A sound installation of a system that has a low immunity to sags and
surges is no better than a poorly installed
hardened system that has high immunity to
these disturbances.
Like many other manufacturers of electronic
systems, imaging system manufacturers in
the last several years have become more
knowledgeable about protecting their systems from voltage surges and transients at
both the design and installation levels. This
does not come by surprise—most equipment
shutdowns and interruptions were thought to
29
Proactive
measures for
preventing
power quality
problems
with imaging
systems
should be
implemented
at the
design level
be caused solely by surges, before the days of
understanding more about how voltage sags
and momentary interruptions (i.e., distribution power quality) really impact imaging
system operation. Designers of PDUs and
subsystem power supplies for use in imaging
systems and other medical devices have integrated some level of surge protection at various circuit levels. In some cases, that level of
protection may be not enough while in others it may be too much for a specific circuit
level. Although some level of surge protection is included in most systems, an engineering analysis to determine the appropriate coordination between overcurrent protection devices, SPDs, and SPD size and location for each type of system requiring protection will result in improved imaging system
performance and more reliable protection
against surges and transients. Combining
this concept with the expected environmental exposure to surges in a healthcare facility
will reveal whether the SPDs can provide
enough protection throughout the life of the
voltage sags, knowing the immunity performance of individual power supplies is not
enough to define total system performance
and reduce the cost of system downtime and
repairs. Imaging system manufacturers, end
users in the healthcare industry, and utilities
need to know the “whole system” response to
sags and interruptions. The complex design
and dynamic operating modes of imaging
systems present various load profiles to
internal power supplies and varying load
profiles to the PDU. These load profiles will
result in a varying immunity to sags and
interruptions. Knowing the sag and interruption immunity at the minimum and maxi-
imaging system. In all, an imaging system
failure should not result from the failure of a
power chain, including the PDU, plays a role
overvoltage protection device such as an SPD
tinue to function during a common sag or
interruption.
failure, nor should a functional system be
mum “whole system” load will be important
depending upon the criticality of the system
and its intended use. Including a SEMI F47compliant power supply in an imaging system design that has unknown immunity to
voltage sags and momentary interruptions
will reap little, if any, benefit in preventing a
system from shutting down or initiating a
system reboot. Each power supply in the
in “whole system” immunity and must con-
placed at high exposure to failure from
surges and transients because of an SPD fail-
At the installation level, healthcare facility
ure.
engineers and field service engineers from
the imaging system manufacturer can initiate
and carry out proactive and reactive measures to reduce system shutdowns and damage to equipment from electrical disturbances. These measures are discussed in the
following two sections.
Providing immunity protection against voltage sags and momentary interruptions is
more challenging whether at the design or
installation level. Imaging system operators
and department directors, along with healthcare facility engineers, and even imaging system manufacturers will admit that nothing
has been done on the design level to provide
protection against voltage sags and momentary interruptions—now the most common
type of electrical disturbances traveling into
healthcare facilities across the main disconnect of an imaging system.
Although the manufacturers of some power
supplies used in imaging systems may have
as much as a SEMI F47 compliance against
30
Proactive Approach
As some medical imaging system manufacturers learn more about power quality in
healthcare facilities and why electrical disturbances affect the reliable operation of
their equipment, more proactive measures
for preventing power quality problems with
imaging systems will be implemented at the
design level. One might ask, “What is driving
their interest in taking a proactive
approach?” Well, some imaging system manufacturers have learned that minor wiring
and grounding errors such as improperly
mounted ground bus bars in a panel,
reversed neutral and ground conductors, and
redundant grounds to a system can cost
them hundreds of thousands of dollars in
service labor hours and installed replacement parts. Moreover, they have learned that
most of these problems could have been
identified and corrected in existing electrical
systems before a new system was installed if
field engineers knew what to look for in an
electrical system of a healthcare facility.
As imaging system manufacturers operate
pre-installation programs targeted towards
identifying and correcting wiring and
tors with system operating problems, reset
systems, and upload computer code among
other services when needed. If an imaging
system problem can be resolved via an activity carried out through a network connection,
then a field service engineer may be able to
avoid an unnecessary trip to a healthcare
facility.
Following a reactive approach, field service
engineers from imaging system manufacturers are routinely called into imaging departments to analyze and resolve imaging system
problems that are cannot be resoled remotely
(via a telephone or network connection).
While a system problem requiring a field
engineer may not require that the performance of the AC power system be evaluated,
cause power quality problems, they are
inspecting the condition of each component
of the AC power system may be included in
requiring that all new installation sites have
the site visit and the system’s overall evalua-
a proactive power quality audit. All errors or
tion. A “red flag” will be raised by the field
potential problems that may interfere with
the installation process or impact the relia-
engineer if a frequent system malfunction or
failure can be attributed to the malfunction
bility of an imaging system are formally doc-
or failure of a power-related subsystem such
umented and brought to the attention of the
electrical engineer or electrician at the
as an internal amplifier or power supply.
grounding errors that can contribute to or
Similarly, if a specific part is frequently
healthcare facility. Once the electrical engi-
removed and replaced, even if it is not direct-
neer or electrician reports that these prob-
ly related to any part of the system’s power
lems have been resolved, then a follow-up
supply, then the system’s evaluation will
audit is conducted at the site to verify that
the errors and potential problems have been
corrected before the installation is allowed to
proceed. Some manufacturers have even
instituted a program and reserve the right as
the servicing organization to cancel a service
contract and shutdown an imaging system to
prevent its operation until significant errors
and potential problems can be resolved.
include inspection of the healthcare facility’s
AC power system from the system’s main disconnect panel back to the facility’s service
Reactive Approach
The complexity of imaging systems warrants
a network connection from each system back
to a manufacturer’s online customer service
center. At the service center, online experts
for each modality are on standby to help
their customers resolve system problems.
Network connections are used to help opera-
Healthcare
facilities are
looking more
and more to
EPRI and their
utility to help
them resolve
power quality
problems in
various parts
of their
facilities
entrance.
Establishing a Partnership with
Imaging System Manufacturers
The cooperative effort between healthcare
facilities, equipment vendors, equipment
manufacturers, and electric utilities to prevent and resolve power quality problems in
healthcare facilities has continued to grow.
Healthcare facilities are looking more and
more to EPRI and their utility to help them
resolve power quality problems in various
parts of their facilities. This is especially true
in imaging suites where power quality problems have the potential to significantly
31
impact imaging suite operations.
System
compatibility
testing of
imaging
systems is
necessary to
determine
the response
of imaging
systems to common
electrical
disturbances
Utilities have always offered assistance to
customers in emergencies, as well as promoted new energy-efficient technologies to
improve productivity and reliability. As problems associated with new technologies were
revealed, many utilities established power
quality programs that invested in power
quality research to assist utility customers
with resolving equipment-compatibility
problems. In the area of diagnostic imaging,
new higher-performance technologies in all
modalities are frequently being introduced
into healthcare settings. The introduction of
these new technologies continues to foster
the need for power quality engineering services for the healthcare industry.
among the various imaging system manufacturers. More room for errors exist when a
facility is renovating an imaging suite or
modifying an area for a new suite where a
suite previously did not exist. Power quality
professionals can assist the healthcare facility in making sure that these requirements are
met.
As power quality professionals learn more
about the electrical environment of a healthcare facility and as imaging system manufacturers strive to learn more about the performance of their systems when subjected to
electrical disturbances, stronger relationships between power quality researchers and
medical imaging system manufacturers are
formed. These relationships boil up the
interest level of the manufacturers in efforts
Electric utilities especially recognize the
necessity of providing power quality services
to their healthcare customers. These services
to reduce the impact that disturbances have
enable healthcare staff to select the proper
power-conditioning equipment, develop
specifications for purchasing medical equipment, establish correct installation guidelines, and plan facility renovations or the
construction of new facilities.
Building strong relationships between
healthcare facilities, equipment vendors,
equipment manufacturers, and electric utilities offers many benefits. These benefits
include reducing or eliminating controllable
electrical disturbances, managing common
uncontrollable electrical disturbances,
encouraging equipment manufacturers to
design and build robust equipment immune
to most common electrical disturbances, and
avoiding citations and penalties from regulatory agencies.
Each of these efforts is extremely important
to the successful installation and operation
of a diagnostic imaging system. Not all
installations will require a power quality mitigation device, but those that have this
requirement will need the assistance of
power quality professionals to help make
that selection process a successful one. In
other cases, procurement specialists in a
healthcare facility will not know how to
interpret or what to look for in an electrical
specification for an imaging system. What
should be included? What is important? What
parameters are not good enough? What level
of compatibility between the imaging system
and the power system can be expected? Is the
electrical environment where the system is to
be installed matched to these specifications?
Regarding the level of detail in installation
guidelines, some differences are present
32
upon their equipment.
System Compatibility Testing of
Imaging Systems
System compatibility testing of imaging systems is necessary to determine the response
of imaging systems to common electrical disturbances that originate inside and outside a
healthcare facility. The System Compatibility
Research Project, created by EPRI Solutions –
PEAC Laboratory, has been used to characterize the performance of hundreds of enduse devices including semiconductor fabrication tools, adjustable speed drives, electronic
ballasts, programmable logic controllers,
consumer electronics systems, power supplies (including those for medical devices),
and many others. Each end-use device is
characterized using a pre-engineered Test
Protocol for System Compatibility. For medical imaging systems, EPRI Solutions and
imaging system manufacturers are developing a series of Test Protocols for System
Compatibility – Diagnostic Medical Imaging
Systems. Application of these protocols will
describe compatibility tests and identify the
response of a medical imaging system to various types of electrical disturbances including voltage sags, voltage transients, voltage
distortion, voltage fluctuations, and other
disturbances. EPRI Solutions is presently
working with imaging system manufacturers
to determine the immunity of various modalities to common electrical disturbances.
The PQ Checklist:
Planning, Purchasing, Installing, and
Maintaining Imaging System
Equipment
Pre-installation checklists for imaging systems are vital to the successful operation of
n Magnet room anchors
n Multiple MR system site requirements
n Main distribution power
n Room ventilation and exhaust fan
n Telephone or broadband connections
n Cryogenic ventilation
n Radio-frequency shielding
n Lighting
Power
quality is
common to
all checklists,
regardless of
modality
n Dust-free environment
From this list, one can see that there are
many requirements including power quality
for a specific type of MRI system. Although
the list of requirements will vary with imaging system modality, there are several
requirements such as power quality that are
now common to all checklists regardless of
modality. One should not confuse this checklist with a checklist intended to guide an
auditor through the power quality audit
process when auditing an existing imaging
system installation or auditing a site for a
new installation.
all systems. Complex modalities such as MRI,
CT, vascular, and other require lengthy
Healthcare facilities routinely procure and
installation checklists. For systems such as
install equipment. Imaging systems are
MRI where a heavy magnet system is also
required, specific checklist items for the
updated on an as needed basis with an effort
to utilize the most advanced imaging equipment possible, matched to the needs of the
community and medical staff. To reduce
power quality problems between imaging
equipment and the intended electrical environment, equipment-procurement proce-
magnet system must be carried out. As an
example, listed are the items that may be
found on a manufacturer’s pre-installation
checklist for an MRI system:
n Vibration study
dures should include the following steps:
n Magnetic field study
n Structural steel requirements
n Acoustic levels
n Magnet room floor loading
n Environmental controls
n Air conditioning
n Power quality
n Water cooling
Planning for Additional Equipment
n The healthcare facility may want to
begin a sound in-house power quality
program with the purchase of a
power-line monitor. A monitor can be
used to conduct an on-site power
quality survey to identify potential
power quality problems with sensitive
medical and imaging system equipment. Consider tapping the expertise
33
of your local electric utility and other
power quality professionals to develop this program. Determine the characteristics of your facility’s electrical
system: Can it tightly regulate equipment voltage? Is voltage to equipment
continuous? Does high-wattage imaging equipment create electrical disturbances in the facility wiring? Your
local utility can also provide site-specific characteristics such as expected
voltage regulation and electrical disturbances for the area where the facility is located.
Repair
all wiring
and grounding
problems
identified
in a power
quality audit
n Request the performance require-
ments of existing imaging equipment
from the imaging system manufacturer. For new equipment, request that
performance requirements be identified through power quality/system
compatibility testing. How susceptible is each type of imaging equipment
to common electrical disturbances
such as voltage sags and transient
overvoltages?
n Set your expectations for the perform-
ance of new equipment. New, more
advanced imaging systems should
perform better than the older systems, but some type of power quality
mitigation system and/or wiring and
grounding correction may be
required. And, then ask your utility
for help in specifying facility design
features that enhance compatibility
between the imaging systems and
their intended electrical environment.
n Repair all wiring and grounding prob-
lems identified in a power quality
audit or other electrical work within
the imaging suite and the facility.
n Identify all areas where portable and
mobile (in a semi-trailer) imaging
systems may be used and the special
power requirements of such equipment.
34
n With assistance from your local utility,
identify appropriate power quality
mitigation devices for portable and
mobile imaging systems.
Purchasing Additional Equipment
n Disclose to equipment manufacturers
the power quality characteristics of
the electricity and wiring where the
new imaging equipment will be
installed. A power quality audit and a
power-line monitoring activity may
be required to fully characterize the
electrical environment in the imaging
suite.
n For all new imaging equipment, speci-
fy the voltage range (required voltage
regulation), frequency, and ridethrough performance (if known)
expected by the healthcare facility.
n Purchase equipment with an input
voltage rating matched to the voltage
at the installation site. Purchase highquality power transformers with new
equipment when the voltage ratings
of the equipment do not match the
available voltage at the installation
site.
n Make sure that power quality mitiga-
tion devices are designed for compatibility with imaging equipment. This
process will involve compatibility
testing between the mitigation device
and the imaging system.
n Make sure that all imaging and power-
conditioning equipment and installations comply with all applicable
codes, standards, and consider meeting recommended practices.
n To reduce susceptibility to common
electrical disturbances, select the
highest input voltage rating for equipment known to be sensitive to common electrical disturbances. Operate
imaging systems at their highest rated
input voltage.
Installing Additional Equipment
n Use high-performance wiring and
proper grounding techniques specified in the IEEE Standard 602-1996,
White Book (Recommended Practice
for Electric Systems in Healthcare
Facilities) and the IEEE Standard
1100-2006, Emerald Book (Powering
and Grounding Electronic
Equipment).
n For circuits connected to sensitive
electronic equipment, use singlepoint grounding, locate equipment as
electrically close to the source as possible, and make sure that neutral conductors (if needed for the specific
wiring configuration) are at least the
same size as phase conductors.
n When adding grounding conductors to
an existing facility, run the grounding
conductors parallel to the existing
power and neutral conductors to
reduce stray electromagnetic fields
and avoid other related power quality
problems.
battery maintenance program is
included in the imaging suite. Failed
batteries cannot provide ride-through
power in the event of a voltage sag or
momentary interruption.
n Regularly review equipment perform-
ance and continue the relationship
between healthcare facility staff, utility representatives, equipment vendors, equipment manufacturers, and
equipment service companies.
Imaging
systems require
quality power
to function
as expected
by the
customer
n Document all imaging system prob-
lems. Include patient schedules, the
location of equipment, the symptoms,
suspected causes, time and date of
occurrence, and any other related
events.
n Checking disturbance logs against
utility records and facility activities
which can help in revealing the
source of electrical disturbances.
These logs can also be used to specify
future equipment purchases and
determine correct installation methods.
n When installing high-wattage imaging
equipment in an existing facility,
monitor the input voltage at the proposed installation site for electrical
disturbances before initiating the
installation, if possible. The monitoring period should be a minimum of
30 days, but monitoring across each
season will reveal more information.
Maintaining Equipment
n Implement a “clean and tighten” pro-
gram for the healthcare facility and
make sure that all associated electrical distribution panels and equipment (including power conditioning
and mitigation equipment) associated
with powering the imaging system is
included in the program.
n If battery-based UPSs are used for
imaging systems, make sure that a
Identifying Power
Quality Problems in
Imaging System Suites
Imaging systems require quality power to
function as expected by the customer. Some
of the small imaging systems such as ultrasound machines require only one dedicated
120-volt branch circuit while others such as
the larger MRI systems require three-phase
power at 480 volts at tens of kVAs. Lower
power systems such as the ultrasound
machines obviously do not require complex
facility electrical systems to deliver power to
the machine. However, providing 480 volts at
100 kVA requires switchgear, properly rated
and installed transformer, emergency disconnect systems, and several branch circuits. In
addition, complex imaging systems require
as many as 12 additional branch circuits to
35
support the auxiliary systems necessary for
imaging system operation.
Those that
have extensive
experience in
conducting
audits will
likely be
more effective
in identifying
problems
Regardless of the simplicity or complexity of
the facility electrical system required to support an imaging system, some type of power
quality problem will arise at some point
within the life of the system. The occurrence
of a power quality problem suggests that an
audit is needed. Conducting a power quality
audit of an imaging system is an activity that
must be carefully planned and carried out
with safety in mind and to avoid damage and
interruption to the imaging system or surrounding critical loads in a healthcare facility. The next few sections will familiarize the
reader with some of the basics of a power
quality audit focused on an imaging system.
Types of Audits
A power quality audit of an imaging system
may be conducted for various reasons. The
customer or manufacturer may require that
an audit be conducted prior to the installation of a new system where no system was
previously installed. Or, an older system may
be slated for updating with a new system.
These types of audits are referred to as preinstallation audits.
In some cases, it is not possible to conduct
an audit on all of the electrical equipment
upstream of the main disconnect at the
imaging system before a new system is
installed. Some manufacturers and customers require that an audit be conducted
after a system is installed, especially if one
cannot be conducted prior to installation for
one reason or another. In this case, a postinstallation audit is conducted after the system has been completely installed and
turned over to the customer.
engineer may initiate the request. The director of an imaging suite or the facility engineer at the healthcare facility may also
request an audit be conducted. Other reasons for conducting an audit may surface
including frequent system malfunctions and
excessive usage of spare parts on an imaging
system where the system and site may be
escalated to a critical level where an audit is
then mandatory. In any event, regardless of
who conducts a power quality audit or the
reason for conducting it, the results usually
reveal staggering problems that need to be
corrected to improve system reliability.
Performing an Audit in Your Own
Suite
Although some extensive training is required
to effectively conduct a power quality audit
on an imaging system, imaging system manufacturers, and facility engineers in healthcare facilities may desire to conduct their
own audit outside the audits that may be
conducted by trained personnel from power
quality professionals in the utility industry
and power quality researchers. Those that
have extensive experience in conducting
audits will likely be more effective in identifying problems.
Assembling an Audit Plan
Before an audit can be conducted, an audit
plan must be assembled. This plan is usually
assembled by a power quality professional
with imaging systems experience or an auditor with experience in the imaging systems
industry. Several important components of
the audit plan must be identified before the
audit can take place. These components
include:
n Identification of imaging system
In cases with installed imaging systems, a
reactive audit may be conducted for a number of reasons. The manufacturer may
request that an audit be conducted. A customer service manager or a field service
36
under investigation
• Manufacturer
• System identification information
• Modality (CT, MRI, X-ray, PET,
Nuclear, other)
• Type of system: permanent, mobile,
or portable
• Problem and malfunction details
(from imaging system operator)
n Location of problem system
• System owner (name of healthcare
facility)
• System contact person (familiar
with imaging system operation)
• Facility engineer at healthcare
facility
n Reason for audit
• Pre-installation
• Post-installation
must be conducted during business hours.
Auditors will find that imaging suite directors
and facility engineers in the healthcare facility will be more than willing to assist if they
realize that the audit will help them improve
the reliability of their imaging systems. Their
goal will be to assist the auditors while not
interrupting any healthcare services.
Because each audit is somewhat tailored to
the electrical design and specific imaging
system in the healthcare facility, not all
audits are conducted exactly the same way.
Everyone knows that no two hospitals or
Each audit is
tailored to the
electrical
design and
specific
imaging
system in the
healthcare
facility
healthcare facilities and imaging suites are
designed and built the same way. Moreover,
most of their modifications are not well documented on the original drawings, especially
the electrical one-line diagrams. Many of the
• Reactive
n Identification of audit team members
• Power quality engineer and/or
auditor
• Technical representative from imaging system manufacturer
imaging suites have different layouts including various equipment placements. Some
audits involve inspecting more electrical distribution panels, more branch circuits and
conductors, more pull boxes, and more
wiring and grounding connections than others.
• Facility engineer
• Facility electrician or maintenance
personnel
• Imaging suite director
• Imaging system operator
Preparing for the Audit
Before a power quality audit can be conducted on an imaging system, the imaging suite
director and facility engineer in the healthcare facility must clearly understand the
audit process. They must know: (1) when the
audit should be started, (2) how long it will
take, and (3) how it might impact imaging or
other healthcare services. Because the auditor must carry out work within the imaging
suite (i.e., near the console, inside the computer room, and/or magnet room), entry into
these area is required and should be scheduled around patient schedules if the audit
After the lead auditor or other representative
has visually inspected the site, he or she will
be able to provide a summary of what areas
of the facility and what electrical equipment
should be included in the audit. This information will be helpful to the facility engineer
as electricians must be scheduled to open
electrical closets, electrical panels, and provide access to areas where panels, conduits,
conductors, and grounds are located.
Not all of the facility electrical system will
need to be audited. However, because of the
importance of the wiring and grounding
methods used in ensuring quality power, the
branch and feeder circuitry along with the
grounding system for the imaging system
under investigation will need to be traced
from the main disconnect panel of the system all the way back to the service entrance
where the main source is derived and where
the facility ground is established.
37
Conducting the Audit
Once the audit plan has been assembled and
preparations have been made to carry it out,
the audit can be conducted. The auditor will
begin the process inside the computer equipment room of the imaging suite where the
system main disconnect panel (SMDP) and
associated PDU, power supply, computer,
and auxiliary equipment are located.
The method of
bonding the
facility-side
and load-side
grounds to each
other is also
extremely
important to
the reliability
and power
quality of
imaging
systems
The SMDP must be thoroughly inspected. As
a part of the imaging system, manufacturers
provide an SMDP as an option.
Manufacturers prefer that their SMDPs are
used with their systems. Thus, the auditor
will be looking to see if the imaging system
manufacturer’s SMDP is used at the site. If
so, the layout of equipment inside the panel,
its electrical connections and safety systems
will be designed for the specific modality
system and more familiar to a manufacturer’s
auditor than an SMDP provided by another
manufacturer. The auditor will be inspecting
the breakers, fuses, and disconnect switches
to determine if they are rated correctly for
the size of the system they power. The auditor will document whether the SMDP has a
shunt trip or undervoltage detection system.
All phase voltages and currents at the SMDP
will be measured and documented. The auditor will carefully inspect and measure the
ground currents with an emphasis on facility-side ground current. Load-side ground
current will be measured with the power to
the imaging system in the ‘on’ and then in
the ‘off ’ position and within the system in
the scan mode. Care should be taken in
measuring these currents to ensure that the
appropriate ampere scale is used so that a
meaningful reading is obtained.
The sizes of the phase conductors, neutral
conductor (if present), and ground conductors are extremely important in providing
quality power to imaging systems. The
American Wire Gauge (AWG) of each phase,
neutral, facility-side ground, and load-side
ground will be documented.
38
Even if the sizes of the conductors are correct
does not mean that a wiring or grounding
problem is not present. The method of bonding the facility-side and load-side grounds to
each other is also extremely important to the
reliability and power quality of imaging systems. This method will be documented carefully. The auditor will also determine is the
neutral conductor is isolated from the
ground conductor(s) in the SMDP.
Each and every electrical connection—conductors to breakers, conductors to fuses,
conductors to disconnect switches, conductors to panel lugs, etc.—will be thoroughly
inspected for proper torque, discoloration,
and improper conductor attachment. Some
conductors are tightened down on top of
their insulation. In some cases, not all of the
conductor strands are inserted into the lugs
before tightening, some are cut off—this is
called a conductor with a “haircut.”
Auditors will also inspect cable ducts to
determine if signal cables are physically separated from the power cables by the appropriate distance. In some installations, a
power quality mitigation device will be
installed before or after the SMDP. This
device could be a power-line filter, voltage
regulator, active power-line conditioner
(APLC), uninterruptible power supply (UPS),
surge protective device (SPD) or other
device. In cases where a mitigation device is
installed, its specification, installation, and
operation must also be carefully reviewed to
determine if it is improving or degrading the
quality of power to the imaging system. In
consulting with the imaging system manufacturer, one may find that the mitigation
device (especially in the case of UPSs) is not
compatible with the imaging system. Power
quality monitoring may be useful in determining this.
With respect to the ground conductors, the
auditor will determine the route that these
conductors take back to the facility electrical
system. The type of conduit supporting the
Block Diagram and Example of Levels of Electrical Distribution System Powering an Imaging System
in a Healthcare Facility
phase, neutral, and ground conductors will
also be documented along with the current
flow through and impedance of the ground
conductor for the main facility back at the
service entrance and/or transformer loca-
Personal
protective
practice is necessary to prevent the auditor
from accidentally tripping a circuit breaker
or causing a fault on the system that will
affect the imaging system and other critical
equipment in the healthcare facility. (Before
tion. Digital photographs of various facets of
any equipment covers are removed, the audi-
the audit will compliment the auditing
process and help the auditor to discuss the
results with other power quality professionals.
tor, facility engineer, and other’s near the
equipment
(PPE) should be
worn to
protect
auditors from
arc flash
equipment should wear personal protective
equipment (PPE) to protect them from arc
flash hazards.)
Once the audit process is completed at the
SMDP inside the computer equipment room
for the imaging system, the auditor and team
will progress to trace the feeder and branch
circuits (phases, neutral, and ground) back
through the facility to the next upstream
piece of electrical equipment. That equipment may be a electrical distribution subpanel, step-up transformer, step-down transformer, isolation transformer, utility transformer, UPS, emergency generator, automatic transfer switch (ATS), or main switchgear
(see the illustration above).
Regardless of the type of equipment, the
auditor should make it a practice to allow the
facility engineer or electrician to remove any
covers or doors from the equipment. This
hazards
At each upstream electrical device, the auditor will record the following information:
n Identification of electrical equipment
type
n Location of electrical equipment in
healthcare facility and in the facility
electrical system
n Manufacturer of electrical equipment
n Healthcare facility identification code
n Current rating
n List of breakers with rating greater
than 50 amps and load names listed
on the panel
39
n Physical distance from SMDP
n Transformer configuration (if applica-
ble)
n kVA rating
n Percent regulation
n kW rating
n Type of source feeding panel or other
equipment
Care should
be taken
not to leave
electrical
panels with
covers
removed and
unattended for
safety reasons
All voltages and currents for all phase, neutral, and ground conductors should also be
recorded for each upstream electrical equipment. In addition, the sizes of the conductors
and the integrity of their connections will
also be documented as in the inspection of
the SMDP.
The auditor will continue this process until
the service entrance of the facility is reached
at the secondary of the utility transformer.
This process will entail tracing and photographing the conduits (containing the phase,
neutral, and ground conductors, if applicable) through facility interstitials, basements,
suspended ceilings, electrical closets, and
electrical equipment rooms. The process will
not be simple as conduits can take routes
through concrete floors, walls, and ceilings.
The auditor should be able to place his or her
eyes on each inch of the conductors outside
the conduits to ensure their continuity
throughout the circuit. If pull boxes, junction
boxes, wiring or cable trays are encountered,
then their covers should be removed to
inspect the integrity of the connections and
conductors. Care should be taken not to
leave electrical panels with covers removed
unattended for safety reasons.
Once the audit is complete, all electrical
panel covers and doors should be secured in
their original position such that no safety
regulations or requirements are violated.
the visual inspections and electrical steadystate measurements are complete, the audit
results may be complied into a form that can
be reviewed by a professional power quality
engineer. The audit results should identify
any potential installation problems with any
of the facility electrical equipment and the
imaging system as well as any unusual voltage, current, or impedance measurements
during imaging system operating conditions.
The reviewer will need to be able to quickly
identify the problems such that corrective
measures can be listed, and those that are
likely to correct or improve the problems can
be investigated and later implemented.
Is My Imaging System Vulnerable to
PQ Problems? Questions for the
Facility Engineer, Imaging
Department Director, and Imaging
System Operators
In every case where an imaging system is
found to be suffering from power quality
problems, healthcare facility CEOs, facility
engineers, imaging department directors,
and imaging system operators ask these
questions:
1. Is the healthcare facility vulnerable to
power quality problems?
2. What causes the facility to be vulnerable to power quality problems?
3. After several years of operation, why
are we just now learning that the
facility has this vulnerability?
4. Why does a million dollar imaging
system have to be shutdown for a day,
or longer, for us to know that our
imaging systems are vulnerable to
power quality problems?
5. What is the cause of these power
quality problems to our imaging systems?
6. How are the problems going to be
solved?
Interpreting the Audit Results
Once the data from the audit is gathered and
40
7. Whose responsibility is it to solve the
power quality problems with our
imaging systems?
8. How will we know that our imaging
systems will not again suffer from
power quality problems?
9. And, why does the $40,000 UPS that
we purchased with the imaging system not protect our imaging from
power quality problems?
is needed?
2. If monitoring is needed, then how
many monitors should be used?
3. Would they need to be located temporarily or permanently?
4. Where should a monitor be located in
an imaging suite?
5. How can one ensure access to the
monitor for setup and data down-
These are only the basic, but typical direct
questions, which healthcare facility personnel ask after they have experienced only one
shutdown of an imaging system. Often, these
questions are aimed at all parties that were
involved with the imaging system from the
time it was first discussed during the planning stage to the time when it was commissioned after its installation. These parties
include:
n Imaging suite director
n Facility engineer
n Facility electrician
n Imaging system operator
n Field service engineer (imaging sys-
tem manufacturer)
n Field installation team (imaging sys-
tem manufacturer)
n Electrical contractor
n Installation specialist (consultant)
Identifying answers to these questions may
end up involving each party.
PQ Monitoring for Imaging Suites:
Answering the Six Basic Questions
In determining the need for power quality
monitoring at an imaging suite, one might
ask “What are the six basic questions?” that
need to be addressed before utilizing power
quality monitoring. These questions are:
1. How would one know that monitoring
loading?
In determining
the need for
power quality
monitoring at
an imaging
suite, one might
ask “What are
the six basic
questions?”
6. What is the future for monitoring
imaging systems and suites?
The complex nature, procurement costs,
operational costs, and cost of downtime
associated with diagnostic medical imaging
systems may very well warrant the need for
power quality monitoring. Monitoring may
be put in place as a proactive or reactive
power quality tool. Traditionally, imaging
system manufacturers and power quality
professionals have used monitoring as a
reactive tool to help identify the cause of system damage and shutdowns. Monitor usage
has been limited in imaging suites due to
monitor cost and availability.
However, with the advancement and
improved cost effectiveness of monitors, they
are increasingly being used as a proactive
tool. Some manufacturers will place one or
two monitors at a single healthcare facility
with a focus on the imaging suite to capture
power quality data during normal imaging
system operation. System damage and shutdowns may be more closely correlated to
electrical disturbances. This type of information is useful to manufacturers in identifying
the cause of a power quality problem and a
resolution to the problem. If two monitors
are used, one might be located near the service entrance or further upstream close to the
point where the 480-volt bus for the imaging
suite is derived on the primary side of a
transformer, for example. The other monitor
might be located at the main disconnect for
the imaging system. In cases where only one
41
Imaging system
manufacturers
may incorporate
low-cost monitor
systems into the
front ends of
their systems
monitor is used, it might be located at the
480-volt derivation point or at the imaging
system disconnect.
remote monitor access and control. Most
monitor users are still relying on an analog
telephone connection for remote operation.
Several inherent problems exist in locating
monitors in healthcare facility for imaging
system investigations. For monitors that are
contained in larger enclosures, auditors may
have a difficult time finding a place for their
temporary location out of the way of imaging
system operators and field service engineers.
Space in equipment rooms for imaging systems is usually very limited. The power and
data connection cables for a monitor should
In the future, imaging system manufacturers
may incorporate low-cost monitor system
into the front ends of their systems or utilize
digital signal procerssing (DSP) hardware to
implement embedded monitoring at the PDU
level. Some of these systems may only record
voltage to control monitor cost and size. This
would prevent the use of bulky monitors outside the system boundaries. Setup and data
management would also be streamlined and
be located such that they do not interfere
with the operation or safety of the main disconnect switch.
included in the systems operating platform.
Even though an imaging system may contain
a built-in monitor, it may not be activated on
a reactive basis until a system problem
The monitor setup and data retrieval
occurs.
processes also present challenges for auditors. If the monitor cannot be setup for
remote operation, then the auditor will need
to revisit the site to make necessary setup
adjustments and download data. Remote
monitoring is becoming more of a standardized feature incorporating user friendly controls. However, most healthcare facilities will
not allow penetration into their firewall for
Remote monitoring may also find a widespread usage in imaging suites to monitor
several modalities in conjunction with monitoring at the main switchgear level. The illustration on page 43 demonstrates a monitoring system designed to capture power quality
data at five imaging system locations plus at
the main switchgear level where the 480-volt
is derived. This type of system would provide
enough data to determine in an electrical
disturbance occurred upstream or downstream of the main switchgear or specific
imaging system.
Common PQ Problems in Imaging
Suites
Any power quality professional or auditor
with auditing experience knows the basic
types of problems that are encountered in a
power quality investigation. Quality power
containing even minor everyday electrical
disturbances can reap significant problems
on an imaging system if the electrical system
is plagued with wiring and grounding errors.
Wiring and grounding systems with errors
will make any electrical disturbance at the
source worse at the load, thus increasing the
likelihood of more damage to a system.
Inside the Main Disconnect Panel for an Imaging System
42
Example of Remote Monitoring in an Imaging Suite
Audits on imaging systems and their electrical system infrastructures typically reveal
imaging system require low-impedance
ground paths back to the voltage source to
many of the same types of problems. These
problems are usually associated with the
techniques used to design and install the
wiring and grounding systems that are used
to power and protect an imaging system.
Some examples of typical errors are included
in the sections below.
prevent ground currents from creating high
potentials across electrical devices where
they can cause damage to the system’s electronics. Power quality auditors on imaging
systems are consistently interested in the
integrity of the grounding systems.
Grounding
Grounding methods and practices are
extremely important to the reliable operation
and life of an imaging system. Grounds for
imaging systems are required for safety and
help to protect the sensitive electronics
inside the imaging system. Electrical disturbances that are created inside or outside the
Grounding
methods and
practices are
extremely
important to
the reliable
operation
Some imaging system manufacturers are now
requiring pre-installation power quality
audits for all new MRI, CT, cardiac X-ray, and
vascular X-ray imaging systems. With such
programs in place, service record trends
began indicating that many of the damaged
and shutdown systems involved grounding
errors inside the imaging suite associated
with the grounding system all the way back
to the service entrance.
and life of
an imaging
system
43
Older
healthcare
facilities are
known for
multitudes of
renovations,
additions, and
changes to the
electrical
system
Auditors have found that ground conductors
were missing from the branch circuits feeding the imaging systems. In some cases, the
conduit ground (i.e., ground continuity
established by the connection of the conduit
system) was the only ground return path. In
the event of a electrical disturbance such as a
voltage surge or voltage sag, any ground current would find it more difficult to return to
the source when it depended upon the conduit as a ground conductor. In cases where a
grounding conductor was available, most of
the time it could be traced back to a grounding point in the facility other than the
ground point associated with the phase conductors.
Older healthcare facilities are known for multitudes of renovations, additions, and
changes to the electrical system. These activities take their toll on the electrical systems.
Some facility engineers in older facilities are
unable to locate their earth-grounding electrode (i.e., ground rod) because it had been
installed for a very long time and no one ever
needed to examine it. When electricians are
unable to identify a reliable ground, they are
inclined to install an additional grounding
electrode for each new piece of imaging
equipment. In cases like this, a facility with
several grounding electrodes in different
locations cannot establish a equipotential
ground reference during an electrical disturbance. This often results in failed equipment
caused by potential differences developed
across the various grounding electrodes.
n Randomly appearing, intermittent
software errors
n System lock-ups that require imaging
system operators to reset or reboot
the equipment
n Random imaging artifacts that come
and go in intensity and frequency
n Hardware failures
n Interaction with or interference from
unrelated imaging systems in the
facility
The following photograph illustrates an
example of multiple grounding errors found
in a main disconnect panel for a CT imaging
system. Notice from the figure that there are
multiple grounding conductors with some
not connected to any ground bus. In addition, the facility-side and load-side grounding conductors are connected together
through the use of two lugs. However, these
lugs are mounted on top of a painted surface
inside the disconnect panel. Mounting
grounding bars on top of painted surfaces is
not good practice. This creates a capacitive
effect between the ground bars and the metal
of the panel. Such effects prevent high-frequency voltages and currents in the grounds
from finding a low-impedance path back to
the source. This type of grounding error
In other cases, auditors found that the
ground for an imaging system had not been
connected to the grounding system of the
facility. Some architects, electrical system
design engineers, and facility engineers even
recommend that isolated grounds be used
when installing new imaging systems. The
use of isolated grounds is not recommended
for these types of installations. Grounding
errors can cause many types of imaging system problems including:
Example of Grounding Errors Found in a Main
Disconnect Panel of a CT Imaging System
44
would make a voltage transient developed at
the source more damaging for the front end
electronics at an imaging system.
effective and most widely used, but can present significant power quality problems if the
neutral and ground conductors are not configured correctly on the primary and secondary sides.
In providing
power to
imaging
systems,
transformers
are often
necessary to
match the
source voltage
to the input
voltage
requirements
for the system
Transformers
In providing power to imaging system, transformers are often necessary to match the
source voltage to the input voltage requirements for the system. Some of the imaging
systems are designed for multiple input voltages ranging from 200 to 480 volts, 50 and 60
hertz. One-hundred and twenty (120) volt
requirements are also included but are usual-
The figure below illustrates an example of
step-down transformer used as a step-up
transformer to provide a 480-volt source to
an imaging system. The commonly provided
208/120-volt secondary voltage throughout
the facility is used as the primary voltage for
this transformer. However, a neutral conductor is pulled from the secondary of the 480-
ly derived from separate sources. Systems
that provide a range of input operating voltages should be powered from the higher voltage (i.e., 480 volts). Utilizing the higher volt-
volt delta to 208/120-volt wye transformer
and connected to the ‘primary’ of the
208/120-volt wye to 480-volt delta transformer. The use of this neutral conductor
age will result in the use of phase and neutral
(dashed line) should be avoided to prevent
(if needed) conductors that have lower current ratings. Important to power quality, utilization of the higher voltage will result in
fewer shutdowns caused by voltage sags and
momentary interruptions.
power quality problems with imaging systems.
In some imaging suites, the higher voltage of
480 volts is not readily available, meaning
that no 480-volt circuit is within a reasonable
distance of the imaging suite where distribution subpanels are located. If a new imaging
system is specified for such suites, then a
problem in supplying the correct voltage to
the area is encountered. Most suites have
three-phase 208/120-volt circuits readily
available with the appropriate ampacity ratings.
The options for providing 480-volts to an
imaging suite are: 1) install a new 480-volt
circuit from the main switchgear, possibly
requiring a long run; 2) install a step-up 208volt delta to 480/2770-volt wye transformer,
requiring a special order transformer; or 3)
install a step-down 480-volt delta to 208/120volt wye transformer with the 208/120-volt
side connected as the primary and the 480volt side as the secondary. Options 1) and 2)
are the most costly and present the longest
installation times. Option 3) is the most cost
Transformation of Utility Voltage to Secondary Voltage to Imaging System Voltage
with Improperly Connected Neutral Conductor between the Two Transformers
The figure on the following page illustrates
another example of step-down transformer
used as a step-up transformer to provide a
480-volt source to an imaging system. The
commonly provided 208/120-volt secondary
voltage throughout the facility is again used
as the primary voltage for this transformer.
However, a conductor is attached from the
neutral of the primary side of this transformer to the ground. The use of this neutralto-ground conductor (or bond) should be
avoided to prevent power quality problems
with imaging systems.
45
Transformation of Utility Voltage to Secondary Voltage to Imaging System Voltage with Improperly
Connected Neutral-to-Ground Conductor at ‘Primary’ of the Reversed Step-Down Transformer
Incorrect
transformer
installations
can burn up
phase, neutral,
and ground
conductors
during a fault
condition
These types of installations can burn up
phase, neutral, and ground conductors dur-
ductor should also be avoided to prevent
power quality problems with imaging sys-
ing a fault condition. How does this happen?
The neutral on the primary side of the transformer provides a path for the fault current.
This current in the primary windings of the
tems.
Here are a few more precautions to consider
regarding transformers:
transformer sets up a current in the secondary windings of the transformer, which in
turn tries to drive the load. Then, these secondary circulating currents try to “pull up”
the primary phase that is down, and the neutral current rises too high. You can avoid this
problem by eliminating the neutral connection on the primary winding of the transformer.
In other cases an isolation transformer like
that shown in the illustration below, is used
in a facility to isolate the primary from the
secondary. This type of installation seeks to
isolate incoming voltage transients from
reaching the output. In many of these installations, a neutral conductor is carried from
the primary side of the transformer to the
secondary side. The use of this neutral con-
n Do not use three transformers to cre-
ate a wye-wye transformer bank.
Because the transformers do not
share the same iron core, the third
harmonic circulating magnetic fields
do not cancel out, creating a pseudo
voltage of more than 300 volts and
damaging any system or piece of
equipment connected to it.
n Do not use an ungrounded delta
transformer to power diagnostic
imaging equipment. This configuration can actually act like a voltage
doubler. In this type of installation,
auditors have documented phase-toground voltage measurements as high
as 960 volts on a 480-volt system. To
Use of an Isolation Transformer in a Imaging System Installation with Improper Use of
a Neutral Conductor from the Primary to the Secondary Side of the Transformer
46
eliminate this problem, use a wye or
corner-grounded delta transformer to
power diagnostic imaging equipment.
Electrical Connections
Although some power quality professionals
are debating where the line is drawn in the
old rule of thumb that 85 to 90 % of all power
quality problems are caused by wiring and
grounding errors, these errors, when present,
still pose significant problems for the
dynamic loads and sensitive electronics in
diagnostic imaging systems.
Wiring and grounding problems include
improperly grounded electrical equipment
(see Grounding above); undersized phase,
neutral, and ground conductors; improperly
terminated conductors, and loose conductor
connections. Two out of these four problems
will be found in almost every imaging suite
site with most of them containing some loose
connections.
Loose connections present an additional
impedance in the circuit that will impact the
shape of the voltage and current during
steady-state and dynamic conditions when
an electrical disturbance is occurring. A visual inspection of a connection will not be
enough to tell if it is actually loose. Some
conductors may be so loose that when
touched may fall out of their lugs. For this
reason, it is important to check for loose
connections very carefully so as not to trip
Eighty-five to
ninety percent
of all power
quality
problems
are caused by
wiring and
grounding
errors
quality audit of an imaging system. This
loose connection was found in the load-side
phase conductors. This site experienced a
number of power quality problems including
failed power supplies and computer boards.
These connections were tightened to the
appropriate torque during the audit.
Meeting the Power
Quality Challenges of
Imaging System Suites
There are a number of challenges that must
be met in order to understand and improve
the power quality of imaging systems. These
challenges are related to the design, installation, maintenance, and repair of these systems. Meeting these challenges will allow end
users and manufacturers to reduce the cost
of operating these systems through fewer
service calls, fewer spare parts used, reduced
customer downtime, and improved patient
care services. Because each of these challenges are related to the technical backbone
of power quality—delivery of electrical power
through a healthcare facility electrical system
on a 24 x 7 x 365 basis, those involved in
addressing these challenges must understand
equipment off line or run the risk of getting
shocked. Loose connections can be found by
using a 1,000-volt insulated UL- and OSHAapproved screwdriver (in good condition) to
check the torque of the connections. A facility electrician or auditor may be able to turn a
lug screw 2 to 6 times on some loose connections. Tightening all connections, terminals,
and fittings will help to resolve many power
quality problems.
The photograph (right) illustrates a loose
connection that was found during a power
Example of Loose Conductor on Load Side of Fused Disconnect in
Main Disconnect of CT Imaging System
47
the facets of each challenge. These facets
involve application of the National Electrical
Code, methods of providing backup power,
good power quality practices, and the financial impacts of power quality problems on
imaging systems.
requirements and codes have come up
against power quality issues, parties have
always been able to resolve the issues without violating the NEC or standard power
quality practices. Continuing to utilize this
approach will foster improvements in imaging system performance related to power
quality.
Power Quality and the National
Electrical Code
The NEC
does recognize
the technical
areas of
power quality
The purpose of the National Electrical Code
(NEC) addresses four areas: practical safeguarding, adequacy, intention, and relation
to international standards. In practical safeguarding, the purpose of the NEC is to protect “persons and property from hazards arising from the use of electricity”. In adequacy,
the NEC “contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in
an installation that is essentially free from
hazard but not necessarily efficient, convenient, or adequate for good service or future
expansion of electrical use”. In intention, the
NEC “is not intended as a design specification or an instruction manual for untrained
persons”. With regard to standards, the NEC
addresses the fundamental principles of protection for safety as also recognized in safetyrelated international standards.
Impacts of Testing Emergency Power
Systems on Imaging Systems
Much of the focus on testing emergency
power systems in healthcare facilities has
been on how often the generator should be
tested and whether it should be tested under
actual load or under a real simulated load
(i.e., load bank). Regardless of testing conditions, national standards and state and local
governments require that the generator be
started, up-to-speed, and carrying the
healthcare facility load with 10 seconds.
Imaging systems are used very frequently in
healthcare facilities. The chances of a power
outage occurring during imaging system use
are real. Facility engineers and imaging suite
directors must be assured that their systems
will be functional on emergency power in the
event of an interruption, outage, or natural
disaster.
The NEC does recognize the technical areas
of power quality but simply states that poor
power quality resulting from voltage drops
during load startup and distorted output
voltages may affect the operation of a facility.
However, the NEC does not recognize various
types of electrical disturbances nor does it
specify specific wiring and grounding methods as improving the power quality to a facility or a load. Although this may be true, the
healthcare facility engineer, imaging system
manufacturer, and the power quality auditor
should strive to ensure that all relevant NEC
requirements and codes, including those that
are recognized by the state and local governments, are met. In no way should an NEC
violation be instituted to resolve a power
quality problem. In all cases where NEC
48
Other than the issues of generator starting
and fueling, there are three basic concerns
with using emergency generators to power
any sensitive electronic equipment in a
healthcare facility. These are 1) disturbance
generation resulting from crossing over
(transfer) to emergency power from utility
power, 2) voltage distortion when operating
on emergency power, and 3) disturbance
generation resulting from crossing back to
utility power. Imaging systems simply do not
have enough inherent ride-through capability to remain operational during the crossover
and during the cross back. The transition
from utility power to emergency power will
create some discontinuities in voltage
including transients and distortions. In some
cases, these discontinuities will damage the
sensitive power supplies on the front ends of
imaging systems. More importantly, imaging
systems will need to be restarted after
crossover or cross back. And, in some cases
by the time a system is restarted and operational again, the utility power has already
been restored.
n Load sizing
n Dynamic load testing
n Physical sizing
n Installation requirements
n Battery maintenance and reliability
X-ray, MRI, and CT systems present various
During generator operation, voltage distortions and frequency shifts may occur. In
most cases, voltage distortions (as long as
they are not too severe) will have less of an
impact on imaging system operation than
frequency shifts. Generally, imaging systems
can withstand a shift in frequency from ± 1
hertz to ± 3 hertz. Cases have been reported
where imaging system operation from emergency power resulted in damage and shutdown to imaging systems caused by disturbances and frequency shifts.
Power Conditioning for Imaging
Systems
Various types of power conditioners may be
used to provide quality power to imaging systems. Several conditioners installed on the
inputs of imaging systems during laboratory
problems that must be considered when sizing a UPS for these systems. Each of these
As another example, a vascular X-ray generator capable of delivering 100 kW of energy to
an X-ray tube requires 171 kVA of input ener-
tests and field installations were unable to
gy. This is also a very dynamic load because
support the dynamic load characteristics of
the vascular system will only have a continu-
imaging systems resulting in a collapse of the
voltage and/or shutdown or permanent damage to the conditioner and/or imaging system. In some cases, the installation of a
power conditioner actually further degraded
the power quality. For these reasons, it is
imperative for any conditioner to be validated for proper operation with an imaging system before it is specified, procured, and
installed.
ous power demand of 5 to 10 kVA. The power
demand for a vascular system could go from
5 kVA to 171 kVA for 10 to 40 milliseconds
and back to 5 kVA. This load cycling may be
repeated up to 12 times per second. These
described loads are very dynamic and in a lot
of cases are not compatible with a number of
UPS system outputs.
Despite the obstacles in validating a uninterruptible power supply (UPS) type conditioner
for use with an imaging system, UPSs have
been the most popular conditioner. Several
issues must be carefully considered during
the selection process. These issues include
Any power
conditioner
should be
validated
for proper
operation with
an imaging
system before
it is specified,
procured, and
installed
systems has very dynamic load characteristics and voltage regulation requirements that
must be considered. As an example, a typical
CT system will have a continuous power
demand of 20 kVA but a maximum power
demand of 90 kVA from a few milliseconds
up to possibly 10 to 20 seconds. During this
time frame, the input voltage must be within
6% of the nominal line voltage. From this,
one can see that the CT system has a very
dynamic power demand. Interestingly
enough, some imaging systems are even
more dynamic with respect to their power
demands.
To identify the proper UPS, the operating
characteristics and dynamic load behavior of
the imaging system (e.g., CT, MRI, X-ray, etc.)
need to be clearly defined by the manufacturer. This information should be provided to
the UPS manufacturer or to the power quality researchers studying the compatibility in
order to fully understand what type of loads
will be placed on the UPS.
49
Following is a list of the critical operating
characteristics that need to be provided to
the UPS manufacturer for the specific imaging system:
magnetic interference problems
resulting in ultrasound imaging problems.
n Nuclear medicine cameras can have
an imaging head gantry that is driven
n Continuous power demand
n Instantaneous or maximum power
demand
n The cycle time for these power
demands
Power
conditioners
can be
validated
for use with
imaging systems
by conducting
specialized
compatibility
testing
• Definition of how long the maximum load demand will be required
• Definition of how often the maximum demand will be repeated
n Voltage regulation requirement
n Front-end rectifier design used in the
diagnostic imaging system that will
be requiring power from the UPS, for
example, does the imaging system use
a 6 or 12 pulse design
All diagnostic imaging systems do not have
dynamic power demands. Nuclear medicine,
positron emission tomography (PET), ultrasound, and information technology systems
(used to store and manage imaging data)
have a fairly constant power demand. Thus,
as a general rule, it is easier to size a UPS for
these loads based on nameplate power specification data and an appropriate UPS sizing
philosophy for non-linear loads.
by motors that could require large
momentary current demands that
might be larger than the kVA available
from a UPS. Because of this dynamic
load requirement, the UPS will possibly need to be sized significantly larger than the continuous power
demand that the system would
appear to require.
Power conditioners can be validated for use
with imaging systems by conducting specialized compatibility testing on the candidate
conditioner electrically paired with the imaging system requiring conditioned power.
Compatibility engineers at EPRI Solutions
have significant experience in this area and
working relationships with most conditioner
manufacturers and some imaging system
manufacturers. Compatibility testing will
identify problem areas specific to the conditioner and the imaging system that may otherwise be resolved with system settings or
design changes.
Some common misconceptions regarding the
use of UPSs with imaging systems included:
n “If the diagnostic imaging system has
Although this is true, there are a couple of
caveats that need to be considered:
n Ultrasound systems are very sensitive
to radio-frequency (RF) noise. RF
noise, which can compromise the
operation of an ultrasound system,
can be either radiated or conducted.
Radiated noise travels through the air
and conducted noise can travel
through any metallic medium. The
inverters in a UPS can potentially be
a major source of RF noise for the
ultrasound system and cause electro-
50
a continuous load of 20 kVA, then a 20
kVA UPS should be able to support
the system.” Wrong, the continuous,
average or normal power rating listed
for most diagnostic imaging systems
is the power demand when the system is in an idle state. When a patient
imaging procedure is carried out on
the system, the power demand could
significantly increase to many times
larger than the continuous value listed.
n “The load is only ‘momentary’ so the
UPS can be undersized.” Wrong, the
diagnostic imaging system has voltage tolerance variations for which the
system was designed to operate. If a
UPS is undersized, the peak load of
the system will cause the input voltage to go out of that tolerance range
and cause image artifacts, system
errors, lockups, and possibly hardware failures.
n “If the diagnostic imaging system
load is 60 kVA, then just doubling the
size of the UPS up to 120 kVA will take
care of the voltage regulation concerns.” Wrong, the ability of the UPS
inverters to handle the dynamic step
load is the deciding factor. Doubling
or tripling the size of the UPS might
not enable the UPS to meet the voltage regulation requirements of the
diagnostic imaging system.
Financial Impacts of PQ Problems on
Imaging Systems
There is no doubt that power quality problems financially impact the operation of
imaging systems in healthcare facilities.
Financial impacts may be created by several
factors:
es overhead costs. Systems that cannot be rebooted must remain idle
until online or field service engineers
from the imaging system manufacturer can resolve the problem.
n Damaged hardware – Systems that
experience damaged hardware as a
result of power quality problems may
be out of service for a few hours up to
a few days or a week. In some cases, it
may be difficult to determine all of
the hardware components that were
damaged by electrical disturbances
until the system can be brought back
up to some operational level.
Additional parts may be required during the repair to render the system
fully operational.
In all, it is very difficult to determine the
exact cost associated with imaging system
downtime. Manufacturers typically keep very
good system records which are used to determine which systems require a significant
number of spare parts and service time. With
imaging tests costing anywhere from a few
thousand dollars to several thousand dollars
and with the rising costs of healthcare, the
cost of system downtime is definitely not
decreasing.
n Missed patient schedules – Patients
having to reschedule an imaging test
will have an extended stay in the
healthcare facility.
n Repeated imaging tests – Patients that
must have their imaging tests repeated due to corrupted imaging files or
artifacts, for example, must have their
tests repeated. This increases the cost
for that patient and reduces the available time slots for other patients.
n System reboots – Systems that must
be rebooted as a result of shutdown
due to a power quality problem
require the attention of imaging system operators and suite directors. The
time required for these reboots is
billed back to the facility and increas-
51
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES
PROPRIETARY NOTICE:
THIS DOCUMENT, IN WHOLE OR ANY PART THEREOF, CONTAINS PROPRIETARY INFORMATION, AND IS
INTENDED FOR THE SOLE USE, OF SOUTH CAROLINA ELECTRIC AND GAS COMPANY (A SCANA COMPANY).
COPYING THIS DOCUMENT, OR ANY PART THEREOF, IS EXPRESSLY FORBIDDEN WITHOUT PRIOR WRITTEN
APPROVAL OF SOUTH CAROLINA ELECTRIC AND GAS COMPANY.
NON-ENDORSEMENT NOTICE:
THE USE OF DATA OR SPECIFICATIONS IN THIS DOCUMENT IS NOT AN ENDORSEMENT OF ANY PRODUCT OR
PROCESS. THERE MAY BE OTHER PRODUCTS FROM DIFFERENT MANUFACTURERS THAT MAY BE EQUALLY
ACCEPTABLE IN ANY GIVEN APPLICATION. THE DATA AND SPECIFICATIONS ARE PRESENTED FOR COMPLETENESS TO AID THE READER IN ASSESSING THE PRESENTED RESULTS.
LEGAL NOTICE:
THIS REPORT WAS PREPARED BY THE EPRI SOLUTIONS, INCORPORATED (ESI). NEITHER THE EPRI SOLUTIONS, EPRI, MEMBERS OF EPRI, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY
WARRANTY, EXPRESS OR IMPLIED, WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS,
METHOD, OR PROCESS DISCLOSED IN THIS DOCUMENT; OR (B) ASSUMES ANY LIABILITIES WITH RESPECT
TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE OF ANY INFORMATION, APPARATUS, METHOD
OR PROCESS DISCLOSED IN THIS DOCUMENT; OR (C) GRANTS ANY RIGHT, TITLE, LICENSE, OR PERMISSION
TO USE ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DESCRIBED IN THIS DOCUMENT. USE OF
INFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS DOCUMENT MAY INFRINGE PRIVATELY OWNED RIGHTS.
Copyright © 2006 EPRI Solutions, Inc. All rights reserved.
52
Download