I. Introduction to Biomedical Engineering
Electrical
Safety
• Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 1
• Industry Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 3
• BMET Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 4
• History of Electrical Safety . . . . . . . . . . . . . . . . . . . . . . . . . . .page 4
Made Easy
II. Fundamental Concepts of Electrical Safety
. . . .page 6
III. Physiological Effects of Electricity
Table Of
Contents
• Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 7
• Macroshock and Microshock . . . . . . . . . . . . . . . . . . . . . . . . .page 7
• High Frequency Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 8
• Potential Equipment Safety Hazards . . . . . . . . . . . . . . . . . . .page 8
• Equipment Safety Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 8
• Electrical Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 8
• The Electrical Power System . . . . . . . . . . . . . . . . . . . . . . . . . .page 8
• Contact with a Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 9
• Skin Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 9
• Leakage Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 10
• Electrical safety Power System Devices . . . . . . . . . . . . . . . .page 10
• Ground Fault Circuit Interrupter . . . . . . . . . . . . . . . . . . . . .page 10
• Isolation Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 10
• Equipotential Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . .page 11
• Codes and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . .page 11-12
IV. The AAMI Standard
Bibliography
Glossary
www.bapcoinfo.com
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T
he field of medical
instrumentation is by no
means new. Many
instruments were developed
as early as the nineteenth century –
for example, the electrocardiograph, first used by
Einthoven at the end of that
century. Progress was rather slow,
however, until after World War II
when a surplus of electronic
equipment such as amplifiers and
recorders became available. At that
time, many technicians and
engineers, both within industry
and on their own, started to
experiment with and modify
existing equipment for medical
use. This process occurred during
the 1950’s and the results were
often disappointing. For the
experimenters soon learned that
physiological parameters are not
measured in quite the same way as
physical parameters. They also
encountered a severe
communication problem with the
medical profession.
During the next decade many
instrument manufacturers entered
the field of medical instrumentation, but development costs were
high, and the medical profession
and hospital staffs were suspicious
of new equipment and often
uncooperative. Many developments with excellent potential
seemed to have become lost
causes. It was during this period
that some progressive companies
decided that rather than modify
existing hardware, they would
design medical instrumentation
specifically designed for medical
use. Although it is true that many
of the same components were
used, the philosophy was changed;
equipment analysis and design
were applied directly to medical
problems.
A large measure of help was
provided by the U.S. government,
in particular NASA (National
Aeronautics and Space
Administration). The Mercury,
Gemini and Apollo programs
needed accurate physiological
monitoring for the astronauts;
consequently, much research and
development money went into this
area. The aerospace medicine
programs were expanded
considerably, both within NASA
facilities and through grants to
Universities and Hospital research
units. Some of the concepts ad
features of patient monitoring
systems presently in use in
hospitals throughout the world
evolved from the base of astronaut
monitoring. The use of adjunct
fields, such as biotelemetry, also
found some basis in the NASA
programs.
Also, in the 1960’s, an awareness of
the need for engineers and
technicians to work with the
medical profession developed. All
the major engineering technical
societies recognized this need by
forming "Engineering Medicine
and Biology" subgroups and new
societies were organized, such as
the Biomedical Engineering
Society. Along with the medical
research programs at the
universities, a need developed for
courses and curricula in
biomedical engineering and today
almost every major university has
some type of biomedical
engineering program. However,
much of this effort is not
concerned with biomedical
instrumentation per se.
One of the problems of
"biomedical engineering" is
defining it. The prefix bio- of
course, denotes something
connected with life. Biophysics and
biochemistry are relatively old
disciplines in which basic sciences
have been applied to living things.
One school of thought subdivides
bioengineering into different
engineering areas – for example,
biomechanics, and bioelectronics.
These categories usually indicate
the use of that area of engineering
applied to living rather than to
physical components.
Bioinstrumentation implies
measurement of biological
variables, and this field of
measurement is often referred to as
biometrics, although the latter
Electrical
Safety
Made Easy
I.
Introduction
to Biomedical
Engineering
-1-
term is also used for mathematical
and statistical methods applied to
biology.
Electrical
Safety
Made Easy
I.
Naturally, committees have been
formed to define these terms; the
professional societies have become
involved. The latter includes the
IEEE Engineering in medicine and
biology group, the ASME
Biomechanical and Human Factors
division, the Instrument society of
America and the American Institute
of aeronautics and Astronautics.
Many new cross-disciplinary
societies have also been formed.
Several years ago an engineering
committee was formed to define
bioengineering. This was subcommittee B (Instrumentation) of
the Engineers Joint Council
Committee on Engineering on
engineering Interactions with
biology and Medicine. Their
recommendation was that
bioengineering be defined as
application of the knowledge
gained by a cross fertilization of
engineering and the biological
sciences so that both will be more
fully utilized for the benefit of man.
More recently, as new applications
have emerged, the field has
produced definitions describing
the personnel who work in it. A
tendency has risen to define the
biomedical engineer as a person
working in research or development in the interface area of
medicine and engineering, whereas
the practitioner working with the
physicians and patients is called a
clinical engineer.
One of the societies that has
emerged in this interface area is the
Association for the Advancement of
medical Instrumentation (AAMI).
This association consists of both
engineers and physicians. In late
1974, they developed a definition
that is widely accepted:
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"A clinical engineer is a
professional who brings to health
care facilities a level of education,
experience and accomplishment
which will enable him to
responsibly, effectively, and safely
manage and interface with medical
devices, instruments and systems
and the use thereof during patient
care, and who can, because of this
level of competence, responsibly and
directly serve the patient and
physician, nurse and other health
care professionals relative to their
use of and other contact with
medical instrumentation."
Most engineers go into the
profession through the engineering
degree route, but many start out as
physicists or physiologists. They
must have at least a B.S. degree
and many of them have M.S. or
Ph.D. degrees.
Another popular term, also coined
in recent years, the biomedical
equipment maintenance
technician (BMET) is defined
as follows:
"A biomedical equipment
maintenance technician (BMET) is
an individual who is knowledgeable
about the theory of operation, the
underlying physiologic principles
and the practical, safe clinical
application of biomedical
equipment. His capabilities may
include installation, calibration,
inspection, preventive maintenance
and repair of general biomedical
and related technical equipment as
well as operation or supervision of
equipment control, safety and
maintenance programs and
systems."
This was also an AAMI definition.
Typically, the BMET has two years
of training at community college.
This person is not to be confused
with a medical technologist. The
latter is usually used in an
operative sense, for example in
blood chemistry and in the taking
of electrocardiograms. The level of
sophistication of the BMET is
usually higher than that of the
technologist in terms of
equipment, but possibly lower in
terms of the life sciences.
In addition, other titles have been
used, such as hospital engineer
and medical engineer. In one
hospital the title biophysicist is
preferred for their biomedical
engineers, for reasons best known
to themselves.
These definitions are all
noteworthy, but whatever the
name, this age of the marriage of
engineering to medicine and
biology is destined to benefit all
concerned. Improved communication among engineers,
technicians and doctors, better and
more accurate instrumentation to
measure vital physiological
parameters and the development
of interdisciplinary tools to help
fight the effects of body malfunctions and diseases are all a part of
this field. The name itself is actually
not all too important; however,
what the field can accomplish is
important. With this point in mind,
we will be using the term
biomedical engineering for
describing the field in general and
biomedical instrumentation for
the methods of measurement
within the field.
INDUSTRY REGULATION
In it’s infancy, biomedical equipment was best serviced and
maintained by the original
equipment manufacturer as there
were few, if any, qualified
technicians who could do so
adequately. A lack of standards
linking the technology, an absence
of service literature, and the
general feeling from hospital
personnel that, unless trained by
these OEMs, no one would be able
to provide a suitable level of
services. Thus the early BMET was
relegated to maintain and repair
only the simplest of clinical
equipment such as centrifuges,
suction pumps and other such
equipment. Several factors soon
arose to alter this conception. First,
agencies such as the Food and
Drug Administration (FDA)
developed subdivisions under
which fell the area of medical
instrumentation. This brought
about a set of standards and
practices for manufacturing of
hospital equipment, as well as the
maintenance and repair of such.
Secondly, other agencies, under
which hospital equipment was
monitored and controlled, invoked
requirements such as the release of
service literature on any new
equipment purchased or manufactured to the hospital. This
literature, the law stated, must
contain the procedures for
calibration and alignment of said
equipment. Thirdly, the training of
biomedical equipment technicians
began to catch up with the
advancing technology so that the
level of competence was significantly improved. Finally, through
the feeling that they provided the
only service available, the OEMs
began to charge significantly more
for their service contracts each
year. Surprisingly, this was not a
major issue to the hospital
community until the middle of
1980 when the government, in an
effort to control the rising cost of
medical treatment, and the subsequent rise in medical insurance
claims, imposed major restrictions
upon hospitals. These restrictions,
known as Diagnostic related
Groups (DRGs) limited the amount
that medical insurance groups
would reimburse both hospitals
and physicians for each type of
illness for which a patient might
enter the hospital. Without going
into great detail about DRGs,
suffice it to say that their introduction brought with it a need for
hospitals to lower their operating
costs. One area in which these
costs could be lowered was in the
area of biomedical instrumetation.
Where once a hospital would make
a major equipment expenditure,
such as buying a new chemistry
analyzer, every five or so years, they
began keeping sophisticated
equipment such as this for longer
periods. So doing, the rising OEM
service contract prices suddenly
became an issue and the responsibility for these equipment fell
upon the BMET as a lower cost
alternative.
As more and more responsibility
fell upon the BMET within the
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Safety
Made Easy
I.
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Electrical
Safety
Made Easy
I.
hospital for larger and more
sophisticated instrumentation, new
problems arose for the hospitals. In
order for the equipment to be
adequately maintained, more
BMETs needed to be employed,
each having sufficient training in
the sophisticated equipment
involved. Hospitals were faced with
weighing the additional cost of
salaries, benefits and the costs of
attending additional manufacturers
schools and seminars with the cost
of the OEM contracts. In very large
hospital facilities, this was not such
a significant problem as it was with
smaller facilities. In many smaller
facilities, typically those under
200 patient beds, it was simply not
feasible to consider an in-house
BMET program and, thus, these
facilities were forced to pay for the
OEM contracts.
be employed on sensitive, lifesaving medical equipment.
Again, the government became
involved. Federal and state
agencies began to require all
hospital facilities to comply with
stringent safety guidelines for
patient related equipment. This
included a routine electrical safety
program under which all hospital
equipment would be tested,
evaluated and forced to comply
with. Typically, the BMET provides
one or more of the following
services to hospital facilities:
In an effort to satisfy applicable
federal and state requirements as
well as to insure that clinical
equipment are utilized both safely
and to their optimum, some BMET
shops provide periodic in-service
and education services. This often
ranges from simple electrical safety
lectures to the proper use of a
newly acquired piece of equipment
for which hospital personnel might
not yet be familiar.
BMET RESPONSIBILITIES
This brings us to the main purpose
of this document. Periodic testing
of patient and non-patient hospital
equipment to insure that they meet
the safety requirements and guidelines as set fourth by applicable
federal and state regulations. This
in no way measures the equipment
ability to perform the task for
which is was designed.
Corrective Maintenance
-4-
Non-scheduled repair of hospital
equipment other than during
Preventive Maintenance inspections. These repairs range from
replacement of minor parts or
components to total equipment
overhauls as may be required. In an
effort to further provide a cost
savings to the hospital facility,
some BMETs attempt to repair
equipment to the smallest
component level, rather than
simply replacing circuit boards or
sub-assemblies which may be very
costly. Other BMETs follow a
philosophy that component-level
repair entails far too many risks of
repeated failure or improper
component soldering techniques to
Preventative
Maintenance Inspections
Periodic testing and evaluation of
patient and non-patient related
equipment to insure that they
operate within the guidelines set
fourth by federal, state and
manufacturer guidelines. This
includes calibration and alignment
of said equipment along with the
repair and/or replacement of
component parts in order to bring
the equipment into compliance. It
is the belief of most BMETs that
good, quality preventive maintenance inspections further reduce
hospital costs by reducing equipment down-time for repairs.
Inservice & Education
Electrical Safety Inspections
HISTORY OF
ELECTRICAL SAFETY
The introduction of electricity into
commerce at the close of the 19th
century carried with it the need to
know how to deal with it safely. For
the hospital environment, much of
the electrical safety programs
started after World war II. Shortly
thereafter, federal funds via the
Hill-Burton Act increased the
number of hospital facilities.
Grounding for electrical safety was
implemented because most
electrical accidents in the home
and industry occurred because
exposed metal was energized. The
National Electrical Code stated that
"non-current carrying" metal parts
of the electrical apparatus could
prevent such accidents.
Unfortunately, by the end of the
1950’s, electrical power in the
hospitals was supplied in a
haphazard, eclectic fashion.
In the hospital environment of the
sixties, electricity came to be used
more often on, in and around
patients to a degree beyond
conception only a few years
previously. It was early in 1961 that
there appeared the first news that
"microshock" (small electric
currents applied to a conductor
near the heart) was happening in
the medical field.
In 1969, Carl Walter, M.D., who was
at the time a well-known surgeon,
stated that, "1,200 patients were
being accidentally electrocuted in
U.S. hospitals each year." Although
many engineers and health care
professionals believed that Dr.
Walter’s estimates were unrealistically high, the concept of
microshock suddenly became
publicized.
Then, on June 16th, 1970, Ralph
Nader (a household word because
of his 1965 book, Unsafe at Any
Speed) gave a speech in Detroit. In
this speech, he stated that, "1200
annual electrocutions in US
hospitals was a very least figure"
and quoted other experts
indicating that the real number
might be significantly higher. The
event was picked up by a wire
service and run under an arresting
headline the next day: Hospital
electrocutions cited Detroit (UPI) –
"Accidental electrocutions claim
5,000 lives in American Hospitals
every year but seldom get reported
due to the "close nature of
hospitals" consumer critic Ralph
Nader said yesterday.
During the 1970’s, several
proposals and regulations were
introduced to manage this
suspected problem in hospitals. In
1971, the National Fire Protection
Association published a recommended standard (76BM) to help
hospital engineers understand the
principles of electrical safety and
coordinate a program of medical
equipment electrical testing in
their facilities. In the spring of
1972, Underwriters laboratories
issued document UL544 Medical
and Dental Equipment, which was
intended to serve as a guideline for
medical equipment manufacturers.
In 1970, AAMI (the Association for
the Advancement of medical
Instrumentation) published a firstdraft standard for electrical leakage
current standards that was adopted
as an American national standard
in 1978.
Electrical
Safety
Made Easy
I.
Probably the most dramatic
proposal was the Joint
Commission on the Accreditation
of Hospitals (now the Joint
Commission on the Accreditation
of Healthcare Organizations –
JCAHO) 1976 recommendation that
hospitals maintain equipment
control programs to provide for
electrical safety training, create a
documented preventive maintenance program, and perform
semi-annual safety and performance equipment inspections and
annual inspections of electrical
receptacles.
Today, health care institutions in
the United States support clinical
engineering programs that provide
ongoing electrical safety and
performance testing as well as
preventive maintenance and repair
of medical equipment. Typically,
these programs use the most recent
editions of NFPA 70, NFPA 99,
NFPA 101, AAMI Recommended
Standards, and Joint Commission
accreditation manuals for their
reference standards.
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S
Electrical
Safety
ome of the most basic
concepts of electricity must
be understood in order to
grasp the potential hazards of
electrical devices as they are used
in a clinical setting such as a
hospital. Some of these important
ideas concerning electrical safety in
a medical environment are
explained below.
Made Easy
The Nature of Electricity
II.
Fundamental
Concepts Of
Electrical
Safety
Electrical current flowing through a
conductor is the result of electrons
moving from the outer shell of
atoms induced by an electric field
that is imposed on a conductor.
This field can be caused by any
voltage generating source, such as
a local utility company, a battery, or
a chemical reaction. In the
hospital, this voltage source is
provided by the local electrical
company and is redirected through
a series of transformers to 240, 208,
or 120 volts alternating current
(AC). The amount of current that
flows through an electrical device is
determined by the resistance that
the device is designed to provide to
the applied field. This relationship
is called "Ohm’s Law" and is
written as:
I=V
R
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-6-
Where "I" is the current in amperes
that flows through a device or
conductor, "V" is the magnitude in
volts of the applied electric field,
and "R" is the resistance that the
device or conductor has, measured
in Ohms.
Thus, if an electromedical device
such as an aspirator is plugged into
a 120 volt receptacle and the unit
requires 3.0 amperes, then the
resistance that the aspirator
provides to the voltage is:
120 volts = 40 Ohms
3 amperes
a measure of the amount of power
that an electrical device consumes
during operation is defined as the
product of voltage and current and
is recorded in Watts.
From the previous example:
Power (watts) =
(V) (I) = (120)
(3) = 360 Watts
B
ecause the amount of
electricity that will flow
through any medium
depends on the resistance that
it encounters, current can flow
through the human body and
cause various effects. If direct
current (polarized, nonchanging)
or high frequency alternating
current passes through the body,
heating effects and ultimately
burns will occur. It is this effect that
is intentionally created when
electrosurgical generators are used
to cut tissue and coagulate fluids. If
low frequency alternating current is
applied to the body, muscular
polarization and depolarization
take place that can ultimately
create a "circus movement" in the
heart muscle, resulting in
fibrillation and death. It is this
effect that normally accounts for
death due to electric shock.
Unfortunately, the typical
resistance of the human body in
combination with the frequency of
commercially generated electricity
(60 hertz) can create a potentially
hazardous situation in the hospital
environment.
MACROSHOCK AND
MICROSHOCK
The effect of electric shock on the
human body can be anything from
barely perceptible tinges, to muscle
spasms, to death. Each can occur
from small or large currents,
depending on how the currents are
introduced into the body. Large
currents (milliamperes or larger)
that are introduced into the body
from one external point to another
(arm to leg, for example) can result
in macroshock. If small currents (as
low as 10 microamperes) are
introduced into the body from an
external source such as a catheter
or cardiac pacing wires, the
resistance to the heart muscle can
be very low, and electrocution can
occur from microshock.
The chart below describes the
effect of different levels of current
that are introduced into the human
body by creating a voltage across
each arm. For microshock, the
same effect can be produced with
current levels that are only
1/10,000 as great as those listed.
CURRENT
EFFECT
.001 Ampere (1 Milliampere)
"Tingling", threshold of perception
.020 Amps (20Ma)
Muscle Spasms, hard to release grip
.050 amps (50 Ma)
Pain, possible fainting, transient
interruption of respiration
.100 Amps (100 Ma)
Ventricular fibrillation
>5 Amps
Sustained myocardial contraction,
possible burns, temporary
respiratory paralysis
Electrical
Safety
Made Easy
III.
Physiological
Effects Of
Electricity
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Electrical
Safety
Made Easy
HIGH FREQUENCY
EFFECTS
EQUIPMENT
SAFETY TESTING
As mentioned previously, the
primary effect of high frequency
current (500 kilohertz to 2 megahertz) is to heat tissue as it is
concentrated in a certain area. The
amount of heat that is generated
depends on the amount of current
applied and the area that the
current passes through.
Tests to determine the electrical
safety of medical devices include a
measurement of the continuity as
well as the leakage current between
the chassis and the cord grounding
conductor. Equipment that has
been designed with patient leads or
contact points is also measured for
current leakage from these points.
Figures 1 and 2 show typical
circuits that may be used to
measure this leakage current.
This relationship is :
Heat = I x I
A
III.
Where "I" is the applied current
and "A" is the area that the current
flows through.
POTENTIAL
EQUIPMENT
SAFETY HAZARDS
Because current flow through the
body can be hazardous if it is of a
certain amplitude and frequency,
stray currents must be eliminated
from medical equipment. The best
method to prevent leakage currents
is to ensure that all conductive
parts of the equipment are connected to the hospital grounding
system through the power plug.
Also, to minimize patient contact
with current leakage on ECG signal
leads, electrically isolated
amplifiers should be used.
-8-
In theory, the use of line isolated
power systems helps to minimize
safety hazards by isolating the
neutral power line from earth
ground. A line isolation monitor is
also installed with these systems to
identify visually and aurally the
presence of leakage current
between the isolated power line
and the grounded conductor. Other
power system devices that help to
identify or disconnect power
sources when leakage currents are
present include ground fault
detectors (GFDs) and ground fault
interruptors (GFIs).
ELECTRICAL SHOCK
The three ingredients in the
scenario of electrocution are :
1. contact with the live
conductor of a grounded
electrical system
2. contact with a ground
3. diminished skin resistance
THE ELECTRICAL
POWER SYSTEM
Domestic voltages in the United
States are from 120V for lighting
and small appliances, and 240V for
electrical ranges and dryers. In the
hospital, outlets are 120V while
277V and 208V are often used for
fixed lighting and special
receptacles.
Typically, a voltage of 240V is
provided by the service drop to the
power meter of the hospital. This
line consists of a bare cable plus
two wires having black insulation.
The bare cable is connected to
"earth" ground via a water pipe or
grounding rod. It is this connection
that makes the electrical service a
grounded system.
NOTE : The national electric Code defines
ground as "a conducting connection,
whether intentional or accidental, between
an electrical circuit or equipment and the
earth, or to some conducting body which
serves in place of the earth."
There are three reasons to ground
electrical systems:
1. Grounding of the system
protects against introduction
into the structure, via the
wiring, of high voltages (with
respect to ground) such as
might arise from lightning or
insulation failure in a high
voltage pole transformer.
2. Tying the circuit to ground
obviates the multiple problems
that might ensue were the
domestic circuits "floating" at
some undefined but high
voltage relative to ground.
3. To facilitate operation of over
current protective devices
(fuses) which are located in the
"live" (ungrounded) side of
each circuit.
CONTACT WITH
A GROUND
For purpose of illustration, we will
use an electrocardiograph (ECG) as
our example of a medical device.
Back in the 1950s, the standard
ECG was fitted with a two-prong
plug. The patient was connected to
the chassis ground via the right-leg
electrode, and one wire of the
power cord was connected to the
chassis through a 200,000-Ohm
resistor. This connected the chassis
to ground via the natural conductor in the power system. If the
plug was inserted with polarity
reversed, then the patient was
connected to the "hot" side of the
power system. However, the 200KOhm built-in resistor limited the
current to 0.6 milliamperes if the
patient or bystander provided a
pathway to ground. Added safety to
the machine included a fivemilliamp fuse in series with the
right-leg electrode. As it turns out
this approach was not practical
because the fuse kept blowing after
the patient was defibrillated.
The codes changed this practice
and today’s ECG machines do not
ground any of the electrodes to
earth ground. In fact, the "front
end" electronics on most medical
devices have an isolated power
supply and grounding system. Plus,
the manufacturers use plastic cases
minimizing the exposed metal.
SKIN RESISTANCE
Water affects skin resistance, and
for a given voltage, resistance
determines current. When dry, skin
has a resistance of upwards
100,000 ohms. If there is an
accidental application of 120 volts
between the two hands, only
1.2 milliamperes will flow. In a wet
environment or on a hot and
humid day, that same current path
may come to have a resistance as
low as 1,000 ohms, resulting in a
current flow of 120 milliamperes.
Electrical
Safety
Made Easy
III.
If a patient is exposed to
electrical current, it takes about
1 milliampere of 60 Hz AC for a
threshold of sensation. The
sensation becomes uncomfortable
if 5 milliamperes is attained. Strong
muscle spasms appear at currents
of 10 to 20 milliamperes ("let-go"
current before sustained muscular
contraction). A current flow of
100 milliamperes or greater may
induce ventricular fibrillation and
death. These values are for currents
introduced at the body surface.
They are termed macroshocks and
require two points of external body
contact.
There is a possibility of a direct
electrical path to the heart via a
needle or catheter in an artery or
vain. This directly reduces the
resistance and current threshold.
Small amounts of current
(100 microamperes) can be
potentially lethal. Electrical shock
in these circumstances is termed
"microshock".
The frequency of the current is also
important when considering
electrical shock. If the frequency is
raised above 1 KHz, these current
levels no longer produce such
sensations or life-threatening
phenomenon. High frequencies in
the megahertz region will not cause
shock at all.
-9-
LEAKAGE CURRENT
Electrical
Safety
Made Easy
III.
All electrically operated devices
have some current that flows from
the energized electrical portions of
the device to the metal chassis.
This current is referred to as leakage current and has two components, capacitive and resistive.
Capacitive leakage current results
from distributive capacitance
between two wires or a wire and a
metal chassis case. Components
that cause capacitive leakage
currents are RF filters, power
transformers, power wires, and any
device that has stray capacitance.
Resistive leakage current arises
from the resistance of the
insulation surrounding the power
wires and transformer primary
windings.
ELECTRICAL SAFETY
POWER SYSTEM
DEVICES
Several techniques are available to
protect clinicians and patients
from electrical shock. The most
common ones are the ground fault
circuit interrupter (GFCI), the
isolation transformer and
equipotential grounding.
GROUND FAULT
CIRCUIT INTERRUPTER
The GFCI acts like a circuit breaker
when it senses an inequality of as
little as 6-ma between the "hot"
and neutral wires of the circuit. It is
mandated by the national electric
code where electrical outlets are so
situated that simultaneous contact
with a grounded surface is
especially likely. "Wet locations"
such as a whirlpool bath or
bathroom are examples of GFCI
usage.
-10-
In the GFCI, current in the "hot"
wire passes through one
transformer coil on the same
transformer core. From the design,
the net flux is zero, when the
currents are equal. If the current is
unbalanced (i.e. current flowing
through ground) a net flux is
induced across the third coil. This
current will trigger a switch
opening the "hot" line.
The GFCI affords economical
protection against electrical shock.
However, the GFCI depends upon
an active system, and the integrity
of it’s mechanical operation is
crucial.
ISOLATION
TRANSFORMER
The Isolation transformer offers
electrical safety by converting
grounded power into ungrounded
power. This is accomplished by
grounding the primary winding of
the transformer and not grounding
the secondary winding. Isolation is
not perfect for two reasons. First,
the isolation transformer has some
stray capacitance to ground.
Second, every medical device that
is attached to the transformer
possesses stray capacitance which
causes some degree of coupling
between its power-carrying wires
and the grounded frame. To
monitor the system isolation, a line
isolation monitor (LIM) is employed. Its function is to analyze
the entire isolated circuit and
quantify its degree of isolation from
ground. The LIM provides visual
and audible alarm signals when the
predicted ground-seeking current
exceeds a specified magnitude. The
LIM does not indicate an existing
current flow, rather it predicts the
current that would flow if a short
circuit were to develop between
isolated wire and ground.
In the operating room, isolated
power systems were first installed
as a measure directed against
sources of ignition rather than
electrical shock. Flammable
anesthetics such as diethyl ether
and cyclo-propane were used.
Today, most anesthesiologists use
non-flammable anesthetics and
isolated power is not required.
EQUIPOTENTIAL
GROUNDING
Another technique that reduces
electrical shock is equipotential
grounding. This is accomplished by
adding another grounding wire
from each chassis to a central point
that is in parallel with the third wire
in the power cord. If the chassis of
all equipment is at the same
potential there will be no current
leakage to the heart. This technique
has its advantages and disadvantages and is not typically used in
today’s health care environment.
CODES AND
STANDARDS
In the 1960s, all aspects of hospital
activities involving fire and
explosion hazards (including
electrical shock and emergency
electrical power) were seen to be in
need of some guidelines and/or
standards. As it turns out, in the
standards arena of the 1970s,
patient safety was twice corrupted.
First, standards were generated in
ignorance (solutions were imposed
before the problems were defined)
and second, safety was blatantly
exploited for ego-serving,
bureaucratic, and commercial
gains.
Today, there are two standards that
specify electrical safety: NFPA 99
and the AAMI Standard for Safe
Current Limits.
The 1996 edition of NFPA 99
includes numerous technical
changes that relate to leakage
current and it now more closely
correlates to the international
standards. Originally, NFPA 76B
covered the electrical wiring system
and electrical appliances. The
summary of the standard as it
relates to electrical safety is in the
Table on the following page.
Electrical
Safety
Made Easy
III.
One further comment about
standards. On may 30, 1972,
Underwriter’s laboratories (UL)
released the first edition of UL-544,
Standard for Safety for Medical and
Dental Equipment. UL-544 deals
with details regarding enclosure
safety, mechanical stability, and
integrity of insulation. It’s leakage
current, isolation requirements,
and test methods are similar to but
not identical with those of AAMI
and NFPA.
-11-
NFPA - 99 1996
Electrical
Safety
Made Easy
III.
Chassis Source Current,
Cord Connected (Portable)
Ground
Open
Ground
Intact
With Isolated Patient Connection
300µa
300µa
300µa
100µa
With Nonisolated patient Connection
300µa
300µa
300µa
100µa
Likely to Contact Patient
300µa
300µa
300µa
100µa
No patient Contact
300µa
300µa
300µa
100µa
Chassis Source Current,
Permanently Connected
Ground
Open
Ground
Intact
Ground Ground
Open
Intact
With Isolated Patient Connection
5000µa
5000µa
5000µa
100µa
With Nonisolated patient Connection
5000µa
5000µa
5000µa
100µa
Likely to Contact Patient
5000µa
5000µa
5000µa
100µa
No patient Contact
5000µa
5000µa
5000µa
100µa
Lead to Ground Current
Ground
Open
Ground
Intact
Ground Ground
Open
Intact
With Isolated Patient Connection
50µa
10µa
50µa
10µa
With Nonisolated patient Connection
100µa
100µa
100µa
50µa
Ground
Open
Ground
Intact
With Isolated Patient Connection
50µa
10µa
n/a
n/a
With Nonisolated patient Connection
50µa
50µa
n/a
n/a
Sink Current (Isolated Test)
Ground
Open
Ground
Intact
With Isolated Patient Connection
n/a
20µa
n/a
n/a
With Nonisolated patient Connection
50µa
50µa
n/a
n/a
Ground
Open
Ground
Intact
Ground Ground
Open
Intact
Ground
Intact
Ground Ground
Open
Intact
Ground
Open
4 Oz
Ground
Intact
Ground Ground
Open
Intact
Ground
Open
Ground
Intact
Ground Ground
Open
Intact
Lead to Lead Current
Ground Impedance
0.5 Ohms
Existing System
0.2 Ohms
New Construction
0.1 Ohms
Receptacle Indicators
Ground
Open
Hospital Grade
Emergency System
Isolated Ground
Retention Force,
Ground Blade
-12-
AAMI/AAMI ESI
1993
GFCI Trip Current
Ground Ground
Open
Intact
Ground Ground
Open
Intact
Ground Ground
Open
Intact
Listed
Red
Orange
6mA
T
he AAMI Electrical Safety
Committee began
deliberating the issue of
safe risk current limits for
electromedical apparatus in 1967
and first published recommended
limits and test methods in 1971.
The first edition of this American
National Standard was approved by
the American National Standards
Institute (ANSI) in 1978, and a
second edition was approved and
published in 1985. Work on this
third edition involved the
dedicated efforts of concerned
health care professionals, industrial
scientists, and government
representatives. AAMI expresses its
gratitude for the service of all
persons involved in the development of this standard.
fault conditions and provides risk
current levels for each of these
states. Also included in this
standard are test methods for
equipment with nonconductive
enclosures and for doubleinsulated equipment as well as
limits for earth leakage or earth
risk currents.
In this third edition of the standard,
the risk current limits have been
raised to be compatible with,
although not identical to, the limits
set forth by the International
Electrotechnical Commission
(IEC) in its standard, Medical
electrical equipment—Part 1:
General requirements for safety
(IEC 601-1-1988). As in the IEC
standard, this edition of Safe
current limits for electromedical
apparatus (ANSI/AAMI ES1—1993)
introduces the concepts of normal
operating conditions and single
Suggestions for improving this
standard are invited. Comments
and suggested revisions should be
sent to Technical Programs, AAMI,
3330 Washington Boulevard, Suite
400, Arlington, VA 22201-4598.
This standard is intended primarily
for the testing of electromedical
apparatus intended for use in or
near the patient care area.
This standard should be considered
flexible and dynamic. As technology advances and new data are
brought forward, the standard will
be reviewed and, if necessary,
revised.
NOTE — This foreword does not
contain provisions of the American
National Standard, Safe current
limits for electromedical apparatus
(ANSI/AAMI ES1—1993), but does
provide important information
about its development and
intended use.
Electrical
Safety
Made Easy
IV.
The AAMI
Standard For
Safe Current
Limits For
Electromedical
Apparatus
-13-
Electrical
Safety
Made Easy
IV.
SAFE CURRENT LIMITS
FOR ELECTROMEDICAL
APPARATUS
1 SCOPE
1.1 Inclusions
This standard sets risk current
limits and referee test methods for
electromedical apparatus intended
for use in the patient care vicinity
and also sets limits for nonpatient-contact electromedical
apparatus.
The standard applies to line- and
battery-powered apparatus and to
apparatus used singly or with
properly connected accessory
equipment. When more than one
electromedical apparatus is
powered by a single power cord,
the equipment assembly acts like a
single apparatus in terms of risk
current limit requirements, and
shall be considered as such for the
purposes of this standard. The
safety and performance criteria
defined in this standard are
intended for use in design qualification by the device
manufacturer.
NOTE — The referee test methods
of Section 5 are intended to provide
means by which conformance with
the standard can be established.
These tests are not intended for use
in verifying the performance of
individual devices in routine
quality assurance inspections. Also,
referee tests allow for the use of
alternative methods for design
qualification, provided that devices
so qualified will also meet the
requirements of this standard when
tested in accordance with the
referee methods.
-14-
1.2 Exclusions
This standard does not set limits
for the composite risk current
when several devices are performing different functions for the
same patient and are independently connected to the utility power
system. This standard does not
apply to therapeutic currents.
In addition, this standard does not
apply to apparatus designed
primarily for nonmedical
applications and used in
conjunction with electromedical
apparatus, but located outside of
the patient care vicinity.
NOTE — As indicated above, all
devices cannot be readily covered
by this standard. Equipment such
as video cassette recorders or
computing devices are now being
used in health care facilities. Such
equipment is designed to other
standards. If such equipment is
not located in the patient care
vicinity, such devices are not
considered to pose a risk as they
are not likely to contact the patient.
2 DEFINITIONS
For the purposes of this standard,
the following definitions apply.
2.1 accessory
Device produced or recommended
by the manufacturer of an electromedical apparatus, and intended to
be electrically connected to that
apparatus in order to make the
apparatus useful or to improve its
efficacy or versatility, and not a
modular part of that apparatus.
2.2 auxiliary apparatus
Electromedical apparatus used in
conjunction with other
electromedical apparatus to
achieve a common purpose.
NOTE — Auxiliary apparatus
includes both interconnected
apparatus and noninterconnected
apparatus.
2.3 composite risk current
Total risk current derived from the
risk currents of all the apparatus
associated with the patient that can
flow through the patient, medical
staff, or bystander.
NOTE — This definition is included
for reference only. A method of
derivation and limits for composite
risk current are not covered in this
standard.
2.4 electromedical apparatus
Instrument, equipment, system, or
device that directly or indirectly
uses electricity for any medical
purpose.
NOTE — Also included are all parts
that are connected to such
equipment and are required for the
normal use of the equipment,
including associated patient wiring
or cables.
2.5 enclosure
Exterior surface of the electromedical apparatus, including all
accessible parts, knobs, grips,
and shafts.
2.6 exposed electrically
conductive surface
External metal or otherwise
electrically conductive surface that
is connected to the internal
circuits, mechanisms, or enclosure
of an electromedical apparatus.
2.7 input part
Part of the electromedical
apparatus, other than a patientapplied part, that is intended to
receive input signal voltages or
currents from other equipment.
2.8 isolated patient connection
Connection between the patient
and the electromedical apparatus
that is isolated from power ground
(earth)1), the utility power system,
and other supporting circuitry to
such a degree that the risk current
flowing through the connection
does not exceed the limits given in
Table 1—Summary of risk current
requirements in rms microamperes
(mA), provided in Section 4.2.
2.9 modular apparatus
Electromedical apparatus that
includes modules in its
construction.
2.10 module
Self-contained assembly that
performs a function or class of
functions in support of the major
function of an electromedical
apparatus.
NOTE — Modules can generally be
removed or replaced without
affecting the operation of other
assemblies in the apparatus.
2.11 nonoperational
environmental conditions
Temperature, humidity, altitude, or
acceleration limits specified by the
manufacturer for storage or
shipment.
2.12 normal condition (NC)
Condition in which all means
provided against safety hazards are
intact and the device is operating
as desired.
2.13 output part:
Part of the electromedical
apparatus, other than a patientapplied part, that is intended to
deliver output signal voltages or
currents to other equipment.
Electrical
Safety
Made Easy
IV.
2.14 patient-applied part
Entirety of any part of the
equipment that comes
intentionally into contact with the
patient via a patient connection.
2.15 patient-applied risk current
Current flowing from the
electromedical apparatus through
the patient to power ground (earth)
or between patient-applied parts.
2.16 patient care vicinity
Space, within a location intended
for the examination or treatment of
patients, extending 6 feet (ft)
(1.8 meters [m]) beyond normal
location of the bed, chair, table,
treadmill or other device that
supports the patient during
examination and treatment. The
patient care vicinity extends
vertically to 7 ft, 6 inches (in)
(2.3 m) above the floor.
2.17 patient connection
Deliberate connection that can
carry current between an electromedical apparatus and a patient.
This can be a surface contact (e.g.,
an ECG electrode), an invasive
connection (e.g., an implanted wire
or catheter), or an incidental longterm connection (e.g., connective
tubing).
-15-
Electrical
Safety
Made Easy
IV.
NOTE — As used in this standard,
"patient connection" is not
intended to include adventitious or
casual contacts, such as push
buttons, bed surfaces, lamps, and
hand-held appliances.
2.18 patient isolation risk current
Current flowing from the patient to
power ground (earth) through a
part applied to the patient due to
the unintended introduction of a
voltage from an external source on
the patient.
2.19 risk current:
Nontherapeutic current that can
flow through the patient, medical
staff, or bystander as a result of the
use of electromedical apparatus.
2.20 single fault condition (SFC)
Condition in which a single means
of protection against a safety
hazard in equipment is defective, a
component failure could increase
the risk current, or a single external
abnormal condition exists.
2.21 sink current:
Current that flows into a device or
any part thereof, when an external
voltage is applied to it.
2.22 source current
Electrical current that flows from
any part of an electromedical
apparatus to any other part or to
power ground (earth), when no
external voltages are applied.
2.23 therapeutic current:
Current that is intentionally
applied to the patient for treatment
of disease or disorder.
3 CLASSIFICATION OF
ELECTROMEDICAL
APPARATUS AND
MEASUREMENT
CONDITIONS
-16-
The following Sections define
specific classes of electromedical
apparatus and measurement
conditions and detail how these
classifications are applied in this
standard.
3.1 Classification of
electromedical apparatus
For purposes of this standard, four
categories of electromedical
apparatus have been defined. For
each category, risk currents are
established. These four categories
are listed below :
a) Electromedical apparatus with
isolated patient connection:
Electromedical apparatus
intended to be connected to
the patient with the patient
circuit isolated from power
ground (earth), utility power
systems, and other circuitry.
b) Electromedical apparatus
with nonisolated patient
connection: Electromedical
apparatus intended to be
connected to the patient.
c) Electromedical apparatus
likely to contact the patient:
Electromedical apparatus that
does not have a patientapplied part, but that is
intended for use in the patient
care vicinity.
NOTE — See Section 2 for
definition of patient care vicinity.
d) Electromedical apparatus with
no patient contact:
Electromedical apparatus that
is intended for use outside the
patient care vicinity and that
has no patient connections.
3.2 Classification of
measurement conditions
The following Sections define
normal and fault conditions.
3.2.1 Normal operating conditions
Under normal operating
conditions, a device is operating as
designed with all means provided
for protection against safety
hazards intact, connected properly
and securely to an approved power
source and, if the device includes
patient-applied parts, with such
parts applied according to the
manufacturer's instructions. The
following are considered normal
operating conditions:
a) power switch on/power switch
off;
b) power polarity normal/power
polarity reversed (cordconnected apparatus only);
c) patient grounded; patient not
grounded.
3.2.2 Single fault condition
A single failure of a device's
protection mechanism against a
safety hazard or the failure of a
single device component can
introduce a hazard condition or
lead to the existence of an external
hazardous condition. The
following are considered to be
single fault conditions :
a) power ground (earth)
conductor open;
b) short circuit of either barrier of
double insulation;
c) failure of a single component
that can produce a hazardous
current;
d) (for equipment that is not
intended to be grounded) the
application of line voltage to
an input or output part or to
accessible conductive
hardware of the enclosure;
e) (for electromedical apparatus
with isolated patient
connections) the application of
line voltage on a patientapplied part.
4 REQUIREMENTS
4.1 Labeling and documentation
requirements
4.1.1 Isolated patient
connections (labeling)
Patient connections that meet the
requirements of this standard for
isolated patient connections shall
be identified as being isolated at
the connector of the apparatus.
NOTE — Labeling of isolated
patient connections shall comply
with symbol number 3 of table DII,
page 329, in the International
Electrotechnical Commission (IEC)
standard 601-1, second edition
(IEC, 1988).
4.1.2 Information manuals
The manufacturer shall supply the
user with operating and
maintenance instructions
specifying how the electromedical
apparatus should be operated and
maintained to prevent the device's
risk current from increasing
beyond the limits set by this
standard for its particular category
(refer to Section 3.1). In addition,
the manufacturer shall disclose the
risk current category for which the
apparatus is designed and shall
identify the specific limits defined
by this standard for that category.
Electrical
Safety
Made Easy
IV.
4.2 Risk current requirements
(general)
Electromedical apparatus shall
meet the applicable risk current
limits of this standard under
normal conditions and under the
single fault conditions specified in
the test methods of Section 5.
Table 1 (see next page) summarizes
these requirements.
4.2.1 Apparatus interconnection
Electromedical apparatus shall
meet the risk current limits of this
standard when manufacturerdesignated auxiliary apparatus,
modular apparatus, or accessories
are attached in the quantity and
combinations stipulated by the
manufacturer. The manufacturer
shall supply the user (and shall
label the apparatus) with
limitations and with directions for
the interconnection of modular
apparatus, accessories, and
auxiliary apparatus, and with
directions for the use of
convenience receptacles.
4.2.2 Cleaning and sterilization
Electromedical apparatus shall
meet the risk current limits of this
standard after exposure to any
disinfection or sterilization process
specified by the manufacturer.
-17-
Electrical
Safety
Made Easy
4.2.3 Environmental conditions
Electromedical apparatus shall
meet the risk current limits of this
standard after exposure to the
nonoperational environmental
conditions (e.g., storage, transportation, etc.) and under the
worst-case environmental
operating conditions specified
by the manufacturer.
4.3 Enclosure risk current
4.3.1 General
Enclosure risk current, when
measured with the AAMI standard
test load, is that current that flows
between power ground (earth) and
IV.
a) exposed chassis conductive
surfaces or hardware; or
b) a 200 cm2 (centimeters
squared) foil in contact with a
nonconducting enclosure.
NOTES :
1) The frequency-weighted
network compensates for the
allowable increase in risk
current limits with increasing
frequency. For measurement
purposes with a voltmeter as
shown, the limit remains
constant at 1 mA/mV,
independent of frequency.
With the meter connected, the
entire circuit is called the "risk
current tester."
2) Refer to Section 5.7.2 for
component requirements and
tolerances.
4.3.2 Risk current limits
Limits for enclosure risk current for
all categories of electromedical
apparatus, whether batterypowered, cord-connected, or
permanently connected, and under
both normal and single fault
conditions, are shown in Table 1 on
next page.
10,000 Ω
0.015 µF
INPUT
MILLIVOLTMETER
1,000 Ω
100 Ω
Figure 1
AAMI standard test load .
-18-
Category
Normal
Condition
PatientPatient
CordApplied Risk Isolation
General
Other
Connected/ PermaCurrent Risk Current CordCordPermaBatterynent
(source
(sink
connected connected)1 nent2)
current)
current)
100 mA
100 mA
10 mA
N/A
500 mA
2,500 mA 5,000 mA
ISOLATED
Single Fault
Condition
300 mA
5,000 mA
50 mA
50 mA
1,000 mA
5,000 mA 10,000 mA
Normal
Condition
100 mA
100 mA
10 mA
N/A
500 mA
2,500 mA 5,000 mA
NON-ISOLATED
Single Fault
Condition
300 mA
5,000 mA
100 mA
N/A
1,000 mA
5,000 mA 10,000 mA
100 mA
N/A
N/A
500 mA
2,500 mA 5,000 mA
LIKELY TO CONTACT PATIENT
Single Fault
Condition
300 mA 5,000 mA
N/A
N/A
1,000 mA
5,000 mA 10,000 mA
Normal
Condition
Normal
Condition
100 mA
100 mA
100 mA
N/A
N/A
500 mA
2,500 mA 5,000 mA
NO PATIENT CONTACT
Single Fault
Condition
500 mA
5,000 mA
N/A
N/A
1,000 mA
5,000 mA 10,000 mA
Electrical
Safety
Made Easy
IV.
Table 1
Summary of risk current requirements in rms microamperes (dc to 1 kHz)
1) Equipment that has no
protectively grounded
(earthed) accessible parts and
no means for protective
grounding (earthing) of other
medical equipment and which
complies with the applicable
requirements for enclosure
leakage current and patient
leakage current; also mobile
x-ray equipment and mobile
equipment with mineral
insulation.
2) Equipment specified to be
permanently installed with a
protective power ground
(earth) that is electrically
connected and secured at a
specific location so that the
connection can only be
loosened or moved with the
aid of a tool.
-19-
4.4 Patient-applied risk current
(source current)2)
Electrical
Safety
Made Easy
IV.
4.4.1 General
Patient-applied risk current, when
measured with the AAMI standard
test load, is that current that flows
between any patient-applied part
and :
a) power ground (earth);
b) exposed chassis conductive
surfaces or hardware;
c) a 200 cm2 foil in contact with
a nonconducting enclosure; or
d) any other patient-applied
parts.
Patient-applied risk current is also
that current that flows between all
patient connections tied together
and (a), (b), (c), and (d) listed above
when measured with the AAMI
standard test load.
NOTE — The current shall be
measured at the patient end of the
cable when connected to the
device.
4.6 Earth risk current
4.6.1 General
Earth risk current, when measured
with the AAMI standard test load, is
that current that flows in the
protective earth conductor (ground
conductor).
NOTE — Not applicable to double
insulated devices using a two-wire
power cord.
4.4.2 Risk current limits
Limits for patient-applied risk
current for all categories of
electromedical apparatus, whether
battery-powered, cord-connected,
or permanently connected, under
both normal and single fault
conditions, are shown in Table 1.
4.6.2 Risk current limits
Limits for earth risk current for all
categories of electromedical
apparatus, whether cordconnected or permanently
connected, and under both normal
and single fault conditions, are
shown in Table 1.
NOTES :
4.7 Risk current limits
versus frequency
The risk current limits specified in
Table 1 are for frequencies from dc
to 1 kilohertz (kHz). Above 1 kHz,
the limit is increased
proportionally to a maximum value
100 times the limit at 1 kHz. Above
100 kHz, the limit is that which is
determined for 100 kHz (see Figure
2). The use of the AAMI test load
automatically compensates for
frequency.
1) These limits are not applicable
to electromedical apparatus
without direct patient-applied
connections.
2) The current shall be measured
at the patient end of the cable
when it is attached to the
device. The cable is specified
by the manufacturer.
4.5 Patient isolation risk current
(sink current)
4.5.1 General
Patient isolation risk current, when
measured with the AAMI standard
test load, is that current that would
flow into a patient-applied part if
the patient came into direct
contact with a potential of 120 volts
(V), 60 hertz (Hz) with respect to
power ground (earth).
-20-
4.5.2 Risk current limits
Limits for patient isolation risk
current for electromedical
apparatus with isolated patientapplied part(s), whether cordconnected or permanently
connected, are shown in Table 1.
5 Tests
This Section contains referee tests
and procedures by which
compliance with the requirements
of Section 4 and Table 1 can be
determined.
WARNING — These tests can
expose personnel to hazardous
electric shock and must be carried
out with caution.
Electrical
Safety
100 mA
NORMALIZED CURRENT
Made Easy
1 mA
500 Hz
1000 Hz
10 KHZ
100 KHZ
1 MHZ
10 MHZ
FREQUENCY (HZ)
Figure 2 — Normalized current limits versus frequency
5.1 Compliance with the
labeling requirements
Compliance with the labeling and
documentation requirements of
Section 4.1 shall be verified by
inspection.
5.2 Compliance with the
risk current requirements
(general test procedures)
The risk currents of electromedical
apparatus shall be measured by the
methods described in this Section.
5.2.1 Test equipment
and power system
5.2.1.1 Measuring instruments
The risk current tester consists of
the AAMI standard test load and a
millivoltmeter as shown in Figure 1.
The millivoltmeter shall measure
true rms volts; however, it may be
calibrated to true rms microamperes (mA) by employing a
conversion factor of one microampere per millivolt (mV). The
millivoltmeter shall have an input
impedance of at least 1 megohm
and have a bandwidth of dc to at
least 1 megahertz (MHz) (–3
decibels). In the band from dc to
100 kilohertz (kHz), the indicated
measurement shall not display an
error of greater than 5 percent of
reading, and shall resolve a signal
as small as 1 mV.
IV.
Instruments that indicate true rms
microamperes and have internal
frequency compensation identical
to that shown in Figure 1 meet the
requirements of this Section if the
measurement indicated does not
display an error of greater than 5
percent of reading and resolves a
signal as small as 1 mA in the band
from dc to 100 kHz.
5.2.1.2 Power source
5.2.1.2.1
For line voltage powered
equipment, the tests shall be
performed on a grounded power
system at the rated line voltage
plus 10 percent. In the grounded
system, the potential between the
neutral and grounding conductors
at the receptacle selected for the
test shall not exceed 3 V.
5.2.1.2.2
The power ground (earth) terminal
used in these tests shall be the
grounding terminal of the specific
receptacle powering the
instrument under test.
5.2.1.2.3
Battery-powered apparatus shall be
tested while powered by the type of
battery recommended by the
manufacturer and, if applicable,
while connected to line power.
-21-
5.2.2 Test conditions
Electrical
Safety
Made Easy
IV.
5.2.2.1 General
First, the apparatus shall be
disconnected from all other
apparatus except auxiliary
apparatus, modular apparatus, or
accessories, as defined in the
normative definitions (see Section
2). A single- or multi-function
apparatus in a cabinet or in
multiple cabinets with a single
power cord connection is tested as
a single apparatus. Each individual
apparatus shall also be tested
independently if described by the
manufacturer as a stand-alone
apparatus. Tests shall be conducted at the rated line voltage
plus 10 percent.
5.2.2.2 Nonconducting enclosure
The risk current shall be measured
from an electrically conductive foil
the size of the enclosure, but not to
exceed 200 cm2, in immediate
contact with the enclosure. The foil
shall be placed at a location—
determined by experimentation—
such that the current measured to
power ground (earth) is a
maximum. If exposed chassis
hardware is likely to be touched by
personnel, then the hardware shall
be treated as an exposed
electrically conductive surface.
Testing of nonconductive exposed
surfaces of patient wiring and
cables is not required.
delivers therapeutic energy to the
patient (e.g., a pacemaker), the
therapeutic energy shall be zero
during the test. Otherwise, the
instrument shall be in the active or
operable mode; i.e., with output
switches closed, with electrodes
properly connected to dummy
loads, and with final circuit stages
properly functioning but without a
physiological drive signal.
5.2.2.4 Operation
During the test, the apparatus shall
run through a normal cycle and
activate all accessories and/or
auxiliary apparatus.
5.3 Enclosure risk current
5.3.1 Application
The enclosure risk current tests
shall apply to cord-connected, linepowered apparatus, to batterypowered apparatus with the
charger connected, and to
permanently connected apparatus.
5.3.2 Cord-connected, normally
grounded apparatus
5.3.2.1
Using the test circuit of Figure 3,
the enclosure risk current shall be
measured:
5.2.2.3 Controls
During risk current tests, all
operator-accessible controls shall
be adjusted to yield the largest risk
current found by experiment. If the
electromedical apparatus normally
Reversing
Switch
S1
a) between enclosure and power
ground (earth);
b) between electrically
conductive surfaces and
power ground (earth);
c) between a 200 cm2 foil in
contact with the nonconducting enclosure and
power ground (earth).
200 cm2 fail
Rated
Line
Voltage
+10%
Apparatus
Under
Test
Select
Per
5.3.2.1
Ground
M
Ground Switch
S2
-22-
Ground Wire
Exposed
Conductive Surface
Risk Current
Tester
Figure 3 — Enclosure risk current test circuit (normally grounded)
NOTE — The 200 cm2 foil line
leading from the "select" box to the
apparatus under test refers to the
connective mode with insulated
apparatus. The line with an arrow
leading from the "select" box to the
apparatus under test refers to
connections made with conductive
enclosure.
5.3.2.2
Each measurement is performed
when:
a) the utility electricity supply
polarity is normal and when
the utility electricity supply
polarity is reversed (by
reversing S1). These are
normal conditions;
b) the apparatus power switch is
on; the apparatus power
switch is off. These are
normal conditions;
c) the ground switch (S2) is open;
the ground switch is closed.
The first is a single fault
condition; the second is a
normal condition;
d) each barrier of double insulation is short circuited. These
are single fault conditions.
NOTE — The test methods for all
measurement conditions are not
supplied in this standard because
they are device- and circuitspecific.
5.3.2.3 The power on/power off
test also applies to apparatus with
nonrechargeable batteries.
Rated Line
Voltage
+10%
Reversing
Switch
S1
5.3.3
Cord-connected, normally
ungrounded apparatus
5.3.3.1
Using the test circuit of Figure 4
(see next page), the enclosure risk
current shall be measured:
a) between enclosure and power
ground (earth);
b) between electrically conductive surfaces and power
ground (earth);
c) between a 200 cm2 foil in
contact with the nonconducting enclosure and
power ground (earth).
5.3.3.2
Each measurement is performed
when:
Electrical
Safety
Made Easy
IV.
a) the utility electricity supply
polarity is normal; the utility
electricity supply polarity is
reversed (by reversing S1).
These are normal conditions;
b) the apparatus power switch is
on; the apparatus power
switch is off. These are
normal conditions;
c) each barrier of double
insulation is short-circuited.
These are single fault
conditions.
NOTE — The test methods for all
measurement conditions are not
supplied in this standard because
they are device- and circuitspecific.
200 cm2
fail
Apparatus
Under
Test
Select
Per
5.3.2.1
Risk Current
Tester
Exposed
Conductive Surface
Ground
Figure 4 — Enclosure risk current test circuit (normally ungrounded)
-23-
5.3.3.3 The power on/power off
test also applies to apparatus with
nonrechargeable batteries.
Electrical
Safety
Made Easy
e) all patient connections
shorted together and power
ground (earth);
f) all patient connections
shorted together and any
exposed, electrically
conductive surface;
g) all patient connections
shorted together and a
200 cm2 foil in contact with
the nonconducting enclosure;
h) any patient connection and all
other patient connections
connected together.
5.3.4 Permanently
connected apparatus
Before line-powered apparatus is
permanently installed, the
enclosure risk current shall be
measured according to the
procedures described in 5.3.2.
5.4 Patient-applied risk current
(source current)
IV.
5.4.1 Application
The patient-applied risk current
tests of this Section shall apply to
line-powered and battery-powered
electromedical apparatus that has a
patient connection(s).
NOTE — The test methods for all
measurement conditions are not
supplied in this standard because
they are device- and circuitspecific.
5.4.2.2
Each measurement is performed
when:
5.4.2 Apparatus with isolated
patient connection
5.4.2.1
Using the test circuit of Figure 5
(see next page), the patient-applied
risk current (source current) shall
be measured between:
a) any patient connection and
power ground (earth);
b) any patient connection and
any exposed, electrically
conductive surface;
c) any patient connection and a
200 cm2 foil in contact with
the nonconducting enclosure;
d) any patient connection and
any other patient connection;
a) the utility switch (S1) is
normal/the utility switch is
reversed. These are normal
conditions;
b) the apparatus power switch is
on/the apparatus power
switch is off. These are normal
conditions;
c) the ground switch (S2) is
open/the ground switch is
closed. The first is a single
fault condition; the second is a
normal condition;
120K
Select Per
6.4.2.2.d
Reversing
Switch
S1
Rated Line
Voltage
+10%
Ground Switch
S2
120V / 60Hz
Patient
Connection
Apparatus
Under
Test
200 cm2
fail
Select Per
5.3.2.1
Select Per
5.3.2.1
Exposed
Conductive Surface
-24-
Figure 5 — Patient-applied risk current test circuit
Risk Current
Tester
d) line voltage is applied to an
input or output part or to
accessible conductive hardware of the enclosure, if not
grounded under normal
conditions. These are single
fault conditions;
e) each barrier of double
insulation is short circuited.
These are single fault conditions.
NOTE — The 120 K resistance is
intended to protect the test
operator.
5.4.3 Apparatus with nonisolated
patient connection
The patient-applied risk current
shall be measured by the
procedures described in 5.4.2.
5.5 Patient isolation risk current
(sink current)
The patient isolation risk current
shall be measured in each
individual patient connection that
is labeled "isolated" when a
potential of 120 V rms, 60 Hz, is
applied through a series 120 kilohm
resistance to the labeled patient
connection, as shown in Figure 6.
The patient isolation risk current is
measured with respect to power
ground (earth). For solely batterypowered apparatus, the patient
isolation risk current is measured
with respect to an electrically
conductive surface on which the
apparatus is positioned, and with
an exposed conductive surface or
other external electrical connection
on the apparatus grounded. This
test shall be performed with the
apparatus both on and off and
properly connected to its electrical
supply. The patient cable shall be
placed 20 cm away from a
grounded surface.
NOTE — The 120 K resistance is
intended to protect the test
operator.
Electrical
Safety
Made Easy
5.6 Earth risk current
5.6.1 Application
The earth risk current test shall
apply to cord-connected apparatus,
to battery-powered apparatus with
the charger connected, and to
permanently connected apparatus.
IV.
5.6.2
Cord-connected, normally
grounded apparatus
5.6.2.1
Using the test circuit of Figure 7
(see next page), the earth risk
current shall be measured in the
protective power ground (earth).
5.6.2.2
Measurement shall be performed
when:
a) the utility electrical supply is
normal/when the utility
electrical supply is reversed (by
reversing S1). These are normal
conditions;
1:1
Rated
Line
+10%
Apparatus
Under
Test
120V
60Hz
Select Per
5.5
120K
Ground
Risk Current
Tester
Figure 6 — Patient isolation risk current test circuit
-25-
b) the apparatus power switch is
on/the apparatus power switch
is off. These are normal
conditions;
c) each supply conductor is
interrupted, one at a time
(opening S2 and S3 in turn).
This is a single fault condition;
d) each barrier of double
insulation is short-circuited.
This is a single fault condition.
Electrical
Safety
Made Easy
IV.
5.6.3 Permanently connected
apparatus
Before the line-powered apparatus
is permanently installed, the earth
risk current shall be measured
according to the procedures
described in 5.6.2.
5.7 Risk current limits
versus frequency
5.7.1 General
When multiple risk currents of
various frequency and phase
relationships are present during a
single test, the resultant risk
current is related to the voltage
across the AAMI standard test load.
The risk current of an apparatus
shall be the largest current
measured during any of the
required tests and conditions. The
apparatus must meet all applicable
limits of Table 1.
5.7.2 AAMI standard test load
As shown in Figure 1, the test load
shall be constructed using metalfilm resistors with a tolerance of
1 percent or better, and a mica- or
plastic-dielectric (extended foil)
capacitor with a tolerance of 5
percent or better. The AAMI
standard test load has an
impedance frequency characteristic (Figure 8) which is the
approximate inverse of the curve of
Figure 2, which shows risk current
versus frequency.
5.7.3 Risk current calculation
Using the AAMI standard test load
of Figure 1 and a voltmeter
calibrated to indicate rms
millivolts, the weighted risk current
is read directly from the meter,
because:
V(mV rms)
Z(k ohms)
I(mA rms) =
Line Interruption Switches
S2•
Reversing
Switch S1
Rated Line
Voltage
+10%
S3•
Apparatus
Under
Test
Protective
Earth
(Ground
Conductor)
Risk
Current
Tester
-26-
Relative Magnitude (dB)
Figure 7 — Earth risk current test circuit
Figure 8 —
Relative
frequency
characteristics of
millivoltmeter
reading in AAMI
standard test
load of Figure 1
0
-20
-40
10
101
102
103
104
105
A.1 General
The rationale discusses the need
for the standard and describes the
basic underlying principles,
empirical data, assumptions, and
sources that support the
requirements and test methods
adopted in the standard.
A.2 Need for the standard
This standard seeks to reduce the
risk of inadvertent electric shock
from medical devices. In
particular, it concerns itself with
the risk of injury from the small
currents that inevitably flow from
or to electromedical apparatus.
The intent of the AAMI Electrical
Safety Committee was to develop a
general baseline standard. The
extent to which the standard
should be applied is to be
determined by individual
institutions, standards groups, and
other authorities.
A.3 Classification of
electromedical apparatus and
measurement conditions
Changes from the second edition of
the standard (AAMI, 1985) have
been made in keeping with
changes to the requirements given
in Section 4.
A.4 Rationale for the specific
provisions of the standard
A.4.1 Labeling and
documentation requirements
A.4.1.1 Isolated patient
connection (labeling)
Fault conditions can contribute to
patient risk due to source and sink
currents. The greatest risk is with
direct cardiac applications; such
applications should utilize isolated
patient connections. In order to
better manage patients requiring
direct cardiac connection, the user
should be able to readily identify
electromedical apparatus with
isolated patient connections.
Therefore, labels should appear on
the electromedical apparatus itself.
The standard does not require
nonisolated patient connections to
carry any special labeling.
A.4.1.2 Information manuals
By identifying the risk current
classification and risk current
limits, the manufacturer is
informing the user of the device's
intended purpose as that purpose
relates to the risk of electric shock.
Any special user actions required to
ensure that the risk current limits
are maintained throughout the life
of the equipment should be
described in the operating
instructions or maintenance
manuals.
Electrical
Safety
Made Easy
IV.
A.4.2 Risk current requirements
(general)
The committee felt that grounding
should not be the primary
approach to limiting the risk of
electric shock because it is possible
to have a single fault failure in the
grounding system. Redundant
means of grounding are possible
but are controlled by the user and
not the manufacturer of the
electromedical apparatus. If the
grounding is lost or if other safety
means fail, the risk current
available from the enclosure should
not represent a substantial hazard
to the patient.
The risk current limits were
changed for certain categories as
compared to the previous version
of this standard (ANSI/AAMI ES1—
1985). These changes have been
made in order to bring this
document into closer harmonization with risk current limits
specified in the International
Electrotechnical Commission
standard, Medical electrical
equipment—Part 1: General
requirements for safety (IEC 601-1,
1988).
-27-
Electrical
Safety
Made Easy
IV.
-28-
In its review of the allowable risk
current levels, the committee
considered the following:
a) Likelihood of stimulation of
excitable tissue. The likelihood
of stimulation of excitable
tissue depends upon:
1) the location of the sites at
which current enters and
leaves the body;
2) the area of contact;
3) the amount of current
flowing;
4) the susceptibility to
mechanical stimulation;
5) the presence of a fault
condition;
6) the probability of the
current having a given
value. Medical devices have
been classified into the risk
categories described in
Section 3 because of the
different magnitudes of risk
associated with these
categories.
b) Survey of published data.
During the several years since
the publication of the previous
version of this standard
(ANSI/AAMI ES1—1985),
experience has been gained
with respect to the incidence
of problems related to risk
current and the probability of
occurrence of the potential
hazard. The available published data on currents
causing ventricular fibrillation
in humans have also been
reexamined. The following
have been noted:
1) The combination of an
open power ground (earth)
wire and a person touching
both a conductive part of
the enclosure and the
patient is a low probability
event.
2) The combination of an
open power ground (earth)
wire and a person touching
both a conductive part of
the chassis and the distal
end of an invasive cardiac
connection is a low
probability event.
3) In one study, Raftery (1975)
found that the smallest
current that produced a
disturbance in rhythm in
humans was 80 mA. In a
second study, Watson
(1973) found that the
smallest current that produced ventricular fibrillation in humans was
15 mA.
4) Mechanically induced
ventricular fibrillation has
been observed during
cardiac catheterization at
zero current.
5) The human data obtained
by Starmer (1973) and
Watson (1973) follow,
reasonably well, a normal
distribution for currents to
300 mA. All patients are
not equally susceptible to
current-induced ventricular
fibrillation. According to
Figure A.1, there is approximately a 1 percent probability of fibrillation at
30 mA.
6) Current perception is a
function of contact
location, contact pressure,
skin condition, moisture,
and contact area. Experiments report a wide range
of current perception.
Dalziel (1968) reports that
only approximately 1 percent of the population can
perceive 500 mA passing
from the fingers of one
hand to the fingers of the
other hand. Tan and
Johnson (1990) report that
300 mA produces a strong
sensation for electrodes
placed 10 cm apart on the
upper arm. Levin (1991)
reports that nearly all the
population will perceive
500 mA without any reaction for current passing
from the finger on one
hand to the underside of
the wrist on the other hand.
Levin further reports that,
on the underside of the
wrist, the stratum corneum
(layer of dead material on
the skin surface) is not as
thick as that on the forefinger and, therefore, the
sensitivity to current
perception might be higher.
Startle current is that level
of perception current that,
when first perceived, might
result in a nurse or other
clinician's involuntary
reaction to the sudden
sensation of perception
current. This uncontrolled
reaction is of great concern.
7) Worldwide, since the
advent of risk current
standards, concern about
grounding, and use of
better practices in handling
catheters and invasive
cardiac connections, there
have been no reports of
incidents involving risk
current passing through
the patient.
NOTE — A minority of the AAMI
Electrical Safety Committee were
opposed to increasing risk current
limits unless scientific studies
supported higher limits.
Also, in order to harmonize with
IEC 601-1 and to allow for
additional fault conditions as
compared to the current standard,
the test measurement classifications of "normal condition" (NC)
and "single fault condition" (SFC)
were introduced.
Measuring risk current under SFC
is important because:
— components can fail;
— single faults exist that are not
now considered in the
standard;
— faults can occur in accessory
equipment.
A.4.2.1 Apparatus interconnection
The total risk current associated
with a device can be a function of
the modules, accessories, and
interconnections used with the
device. Voltage differences can
occur between different parts of a
device, particularly if current flows
in the grounding circuit. Thus, a
remote accessory powered from the
device or from a separate source is,
for purposes of the standard,
considered part of the device.
Auxiliary power outlets may be
provided for powering additional
devices. The labeling requirements
provide some assurance that the
user has appropriate guidance
about the limitations applying to
equipment or accessories connected to an auxiliary power
receptacle.
A.4.2.2 Cleaning and sterilization
The long-term effects of repeated
disinfection or sterilization of a
device on risk currents must be
considered because of possible
degradation of insulating materials.
A.4.2.3 Environmental conditions
Temperature, humidity, atmospheric pressure, mechanical shock,
and similar environmental constraints can have an effect upon
the risk currents. To protect the
patient, the risk current limits must
also be met in the intended
environment.
Electrical
Safety
Made Easy
IV.
A.4.3 Enclosure risk current
The 100 mA NC values were
selected for cord-connected and
permanently connected apparatus
on theoretical grounds. For a
typical ground resistance of
0.2 ohms, 100 mA of enclosure risk
current, measured as in Figure 3,
requires that 500 mA flow in the
power ground (earth) wire. This
would be a major fault condition.
The 300 mA SFC values for isolated,
nonisolated, or likely-to-contactpatient, cord-connected apparatus
were selected on the basis of
reaction current measurements by
Levin and the low probability of
risk current reaching the distal end
of an invasive heart connection via
another person. Levin reported
that currents of 300 mA will not
produce sensations leading to a
"startle" reaction (Levin, 1991).
The 500 mA limit for SFC for cordconnected, no-patient-contact
equipment was selected because
there is no concern about this
current reaching the patient.
The 5,000 mA limit for SFC for
permanently connected equipment
is basically a power ground (earth)
-29-
Electrical
Safety
Made Easy
IV.
wire current, because the
apparatus is, by definition,
permanently grounded. This is the
current allowed from the enclosure
of permanently connected
equipment if the power ground
(earth) wire were opened. The
permanently connected power
ground (earth) wire is not expected
to open.
that produced ventricular fibrillation in humans are considered,
the probability is approximately
zero.
The 50 mA SFC limit for isolated
equipment was selected because
the probability of causing ventricular fibrillation is low if the data of
Figure A.1 are extrapolated and is
approximately zero if only
observed values of currents
producing ventricular fibrillation in
humans are considered.
A.4.4 Patient-applied risk current
(source current)
The 10 mA NC limit was selected as
the current that may flow directly
into the heart continuously. Figure
A.1 shows that the probability of
inducing ventricular fibrillation is
very small if the data on which the
chart is based are extrapolated. If
only observed values of currents
The 100 mA SFC limit was selected
for nonisolated equipment,
because if this current enters and
exits the surface of the body, then
only a fraction will reach the heart.
Starmer 60 Hz D = 1.25mm
Watson 60 Hz D = 2.0mm
99.9
99
Cumulative Percent
95
+
+
90
50
+
+
+
20
+
+
+
+
+
5
1
0.1
0
50
100 150 200 250 300 350 400 450 500
Microamperes
Figure A.1 — Normal probability plot
-30-
A.4.5 Patient isolation risk current
(sink current)
The 50 mA SFC value for isolated
equipment was allowed because of
the low probability of line voltage
appearing on a patient, and
because of the low probability of
50 mA inducing ventricular
fibrillation. For line voltage to
appear on a patient, a power
ground (earth) wire must be open
and there must also be a fault in
basic insulation. If the data in
Figure A.1 are extrapolated, then
50 mA has a low probability of
inducing ventricular fibrillation. If
only recorded human data are
considered, the probability of
inducing fibrillation at 50 mA is
approximately zero.
A.4.6 Earth risk
(ground risk) current
The original standard of 1978 and
the revised edition of 1985 did not
include earth current as a potential
risk current. This was not
considered to be an issue as most
medical devices of the era utilized
conductive enclosures requiring
grounding. Thus, earth current was
effectively measured as enclosure
(chassis) current under the open
ground condition.
In the last 25 years, however, the
change to nonconductive
enclosures negated this equality.
The enclosure current is now
measured as the capacitivecoupled current to a 200 cm2 foil in
contact with the enclosure. This
current bears little resemblance to
the earth risk current due to the
current-limiting characteristics of
the capacitive coupling of the
nonconductive enclosure.
The earth risk current does not
pose a direct risk to the patient or
medical personnel. However,
excessive earth risk current, either
by design or internal breakdown,
will raise the potential of the
device's ground with respect to
true power ground (earth) as
represented by structural elements,
modular wall units, cold water
pipes, and other installed piping, or
by an adjacent receptacle power
ground (earth). Contact with such
elements and a second device
under test will result in current
flow. Thus, the committee felt that
leaving the earth risk current
unmeasured and unlimited constituted a potential hazard that
should be avoided.
The allowable values for earth risk
current detailed in this standard
are not thought to pose a direct
hazard as the current is safely
returned to earth. The values
selected were chosen to avoid any
significant increase in the current
flowing through the protective
grounding system of the installation and to be consistent with the
limits of IEC 601-1 for power
ground (earth) and nonconducting
enclosures. Further, the 2.5 mA
limit in normal mode is within the
limit for isolation monitors set by
the National Fire Protection
Association (NFPA, 1993, Section
3.4.3.3), which specifies that
isolation monitors should not
alarm at 3.75 mA.
Electrical
Safety
Made Easy
IV.
A.4.7 Risk current limits
versus frequency
Figure 2 of the standard was
derived from strength/frequency
data for perceptible and lethal
currents (Geddes and Baker, 1971).
The flat portion between 100 kHz
and 1 MHz does not reflect
physiological data obtained with
purely sinusoidal currents.
Stimulation has been observed
with complex waveforms at high
frequencies, but little data are
available. In the absence of data, it
was deemed prudent not to
extrapolate beyond 100 kHz.
A.5 Tests
The test procedures documented in
Section 5 of the standard provide
referee test methods for verifying
compliance with the requirements
of Section 4. These referee tests are
not necessarily intended for
purposes of manufacturing or
quality control (although these
applications are not precluded), as
equivalent measurements may be
obtainable by other means.
-31-
ASSOCIATION FOR THE ADVANCEMENT OF MEDICAL
INSTRUMENTATION. Safe current limits for electromedical apparatus.
ANSI/AAMI ES1—1985. Arlington (Vir.): AAMI, 1985. ISBN 0-910275-50-5.
Electrical
Safety
Made Easy
DALZIEL, CF. Reevaluation of lethal electric currents. IEEE Trans Indus
Gen Appl, GA-4, 1968, vol. 1, no. 5, p. 467-476.
GEDDES, LA. and BAKER, LE. Response to the passage of electric currents
through the body. J Assn Adv Med Instrum, 1971, vol. 5, p. 13-18.
INTERNATIONAL ELECTROTECHNICAL COMMISSION. Medical electrical
equipment—Part 1: General requirements for safety, 2nd ed. IEC 601-1.
Geneva: IEC, 1988.
LEVIN, M. Perception of chassis leakage current. Biomed Instrumentation
and Technology, 1991, vol. 25, no. 2, p.135-140.
IV.
NATIONAL FIRE PROTECTION ASSOCIATION. Standard for health care
facilities. NFPA 99-1993. Quincy (Mass.): NFPA, 1993.
RAFTERY, EB., GREEN, HL., and YACOUB, MH. Disturbances of heart
rhythm produced by 50 Hz leakage currents in human subjects.
Cardiovascular Research, March 1975, vol. 9, no. 2, p. 263-265.
STARMER, CF. and WHALEN, RE. Current density and electrically induced
ventricular fibrillation. J Assn Adv Med Instrum, 1973, vol. 7, no. 1, p. 3-6.
TAN, KS. and JOHNSON, DL. Threshold of sensation for 60 Hz leakage
current: Results of a survey. Biomed Instrumentation and Technology, 1990,
vol. 24, no. 3, p. 207-211.
Bibliography
WATSON, AB., WRIGHT, JS., and LAUGHMAN, J. Electrical thresholds for
ventricular fibrillation in man. Med J Australia, 1973, vol. 1, p.1179-1182.
You can contact BAPCO:
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-32-
Glossary
Of Terms
AMPACITY: Current-carrying capacity of electrical
conductors expressed in amperes.
ANESTHETIZING LOCATION: Any area of the
facility that has been designated for the administration of
any flammable or nonflammable inhalation anesthetic
agents in the course of examination or treatment,
including the use of such agents for relative analgesia.
CONDUCTIVE: Materials, such as metals, that are
commonly considered electrically conductive, and
materials that, when tested, have a resistance not
exceeding 1,000,000 ohms. Such materials are required
where electrostatic interconnection is necessary.
CRITICAL EQUIPMENT:
Equipment that is
essential to the safety of the occupants in the facility.
CRITICAL SYSTEM: A system of feeders and branch
circuits in nursing homes and custodial care facilities
arranged for connection to the alternate power source to
restore service to critical receptacles, task illumination,
and equipment.
DIRECT ELECTRICAL PATHWAY TO THE
HEART: An external conductive pathway, insulated
except at its ends, one end of which is in direct contact
with the heart muscle and the other outside the body,
that is accessible for inadvertent or intentional contact
with grounded objects or energized, ground-referenced
sources. Catheters filled with conductive fluids and
electrodes, such as may be used for pacing of the heart,
are examples of direct electrical pathways to the heart.
DOUBLE-INSULATED APPLIANCES:
Appliances having an insulation system comprising both
basic insulation necessary for the functioning of the
appliance and for basic protection against electrical
shock and supplementary insulation. The supplementary
insulation is independent insulation provided in addition
to the basic insulation to ensure protection against
electric shock in case of failure to the basic insulation.
ELECTRODE: A device intended to probe an
electrically conductive connection through a cable to a
patient. There are several types:
Active Electrode: An electrode intended to generate a
surgical effect at its point of application to the patient.
Dispersive Electrode: An electrode intended to complete the electrical path between patient and appliance, and at which no surgical effect is intended. It is
often called the "indifferent electrode", the "return
electrode", the "patient plate", or the "neutral
electrode".
EXPOSED CONDUCTIVE SURFACES: Those
surfaces that are capable of carrying electric current and
that are unprotected, uninsulated, unenclosed, or
unguarded, permitting personal contact.
FAULT CURRENT: A current in an accidental
connection between an energized and a grounded or
other conductive element resulting from a failure of
insulation, spacing, or containment of conductors.
FREQUENCE: The number of oscillations, per unit of
time, of a particular current or voltage waveform. The
unit of frequency is the Hertz (Hz). (The unit of frequency
used to be "cycles per second", a term no longer
preferred). Note : The waveform may consist of
components having many different frequencies , in which
case it is called a complex or nonsinusoidal waveform.
GROUND-FAULT CIRCUIT INTERRUPTER:
A device whose function is to interrupt the electric circuit
to the load when a fault current to ground exceeds some
predetermined value that is less than that required to
operate the overcurrent protective device of the supply
circuit.
GROUNDING SYSTEM: a system of conductors
that provides a low-impedance return path for leakage
and fault currents. It coordinates with, but may be locally
more extensive than, the grounding system described in
Article 250 of NFPA 70, National Electric Code.
HAZARD CURRENT: for a given set of connections
in an isolated power system, the total current that would
flow through a low-impedance if it were connected
between either isolated conductor and ground. The
various hazard currents are:
Fault Hazard Current: The hazard current of a given
isolated power system with all devices connected
except the line isolation monitor.
Monitor Hazard Current: The hazard current of the
line isolation monitor alone.
Total Hazard Current: The hazard current of a given
isolated system with all devices, including the line
isolation monitor, connected.
IMPEDANCE: Impedance is the ratio of the voltage
drop across a circuit element to the current flowing
through the same circuit element. The circuit element
may consist of any combination of resistance, capacitance, or inductance. The unit of impedance is the Ohm.
INTRINSICALLY SAFE: As applied to equipment
and wiring, equipment and wiring that are incapable of
releasing sufficient electrical energy under normal or
abnormal conditions to cause ignition of a specific
hazardous atmospheric mixture. Abnormal conditions
may include accidental damage to any part of the
equipment or wiring, insulation or other failure of
electrical components, application of overvoltage,
adjustment and maintenance operations, and other
similar conditions.
ISOLATED PATIENT LEAD: A patient lead whose
impedance to ground or to a power line is sufficiently
high that connecting the lead to ground or to either
conductor of the power line results in a current flow
below a hazardous limit in the lead.
ISOLATED POWER SYSTEM: A system
comprising an isolating transformer or its equivalent,
a line isolation monitor, and ungrounded circuit
conductors.
ISOLATION TRANSFORMER: A transformer of
the multiple-winding type, with the primary and
secondary windings physically separated, that inductively
couples its ungrounded secondary winding to the
grounded feeder system that energizes its primary
winding.
LEAKAGE CURRENT: Any current, including
capacitively coupled current, not intended to be applied
to a patient, but may be conveyed from exposed metal
parts of an appliance to ground or to other accessible
parts of an appliance.
LINE ISOLATION MONITOR: An instrument that
continually checks the hazard current from an isolated
surface to ground.
MACROSHOCK: The effect of large electric current
(milliamperes or larger) on the body.
MICROSHOCK: The effect of small electric currents
(as low as 10 microamperes) on the body. To be
hazardous, such currents must be applied to a conductor
inside or very near the heart.
mV:
Millivolt.
mA:
Milliampere.
PATIENT CARE AREA: Any portion of a health care
facility where patients are examined or treated.
Note : Business offices, corridors, lounges, day rooms,
dining rooms, or similar areas are not classified as patient
care areas.
PATIENT-CARE-RELATED ELECTRICAL
APPLIANCE: An electrical appliance that is intended
to be used for diagnostic, therapeutic, or monitoring
purposes in a patient care area.
PATIENT EQUIPMENT GROUNDING
POINT: A jack or terminal that serves as a collection
point for redundant grounding of electrical appliances
serving a patient vicinity or for grounding other items in
order to eliminate electromagnetic problems.
PATIENT LEAD: Any deliberate electrical connection
that may carry between an appliance and a patient. This
may be a surface contact (such as an ECG electrode), an
invasive connection (such as an implanted wire or
catheter), or an incidental long-term connection (such as
conductive tubing). Adventitious or casual contacts such
as a push button, bed surface, lamp, hand-held
appliance, and so fourth, are not considered patient
leads.
PATIENT VICINITY: In an area in which patients are
normally cared for, the patient vicinity is the space with
surfaces likely to be touched by the patient or an attendant who can touch the patient. Typically in a patient
room, this is a space within the room 6 ft. (1.8m) beyond
the perimeter of the bed in its normal location and
extending vertically within 7 ft. 6 in. (2.3m) of the floor.
REACTANCE: The component of impedance
contributed by inductance or capacitance. The unit of
reactance is the Ohm.
REFERENCE GROUNDING POINT: A terminal
bus that is the equipment grounding bus, or an extension
of the equipment grounding bus, and is a convenient
collection point for installed grounding wires or other
bonding wires where used.
WET LOCATIONS: Those patient care areas that are
normally subject to wet conditions, including standing
water on the floor, or routine dousing or drenching of the
work area. Routine housekeeping procedures and
incidental spillage of liquids do not define a wet location.
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