Download EMC For Medical Devices: A Review Of IEC 601-1

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EMC For Medical Devices:
A Review Of
IEC 601-1-2, 2nd Edition
Significant changes from the earlier version of this standard
make testing more complex
I
n the European Union (EU), medical equipment
marches to a different beat than other types of
equipment in the areas of EMC and product safety.
The reasons lie in several areas: regulatory,
philosophical, and in the standards used. The first step
in understanding the contrasts between the requirements imposed on medical products versus other
types such as telecom and information technology
equipment (ITE) is to look at the governing directives.
Both ITE and medical equipment are governed by
so-called “New Approach” directives. Under the New
Approach, equipment must meet whatever directives
apply to it to gain entry to the EU’s market;
compliance is indicated by the application of the CE
mark and the documentation of the steps taken to
meet the applicable directives. The steps taken, of
course, vary with each directive, and will include a
combination of testing, documentation and, in some
cases, notified body supervision of ongoing
production and manufacturer quality systems.
Most product directives set overall goals and methods
for assessing responsibility, but leave detailed
technical requirements to standards which are
generated by international groups such as the IEC,
CEN/CENELEC, and the ISO (a few directives
contain the technical requirements without reference
to external standards).
These standards gain regulatory force—that is,
become suitable for demonstrating conformity to a
given directive - upon publication by the European
Commission in the Official Journal of the European
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Isidor Straus
Union. An up-to-date posting, organized by directive,
is maintained on the Internet can be found at the
following URL: http://europa.eu.int/comm/enterprise/
newapproach/standardization/harmstds/reflist.html
Most of our readers are familiar with the EMC
Directive (89/336/EEC) and the Low Voltage
Directive (73/23/EEC). These govern the
electromagnetic performance and safety of most
electrical and electronic equipment. Both directives
are generally applicable unless superseded by a
“specific directive.” The Directive on Medical
Equipment (93/42/EEC) and the related Directive on
in-vitro medical devices (98/72/EC) are just such
specific directives.
Standards Overview For
Medical Equipment And Devices
A large number of standards have been designated
and published for assessing compliance under these
directives. These include a number that are highly
specific to the construction and operation of particular
device types, such as ophthalmic, respiratory, and
x-ray equipment. In addition, there is a large family of
standards, grouped under the EN 60601-x-x designation, which address the issues of EMC and safety that,
in the absence of these overriding directives, would be
addressed by the EMC and LVD requirements.
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The 60601 family is concerned with the
safety of medical electrical equipment.
The standards are grouped in two parts.
Those numbered “–1-x” are considered
relevant to the general requirements for
safety, and are labeled “collateral
standards.” Those numbered “-2-x”
contain requirements for specific kinds
of medical equipment. Some examples
should make this clear:
Table 1: Table 201 from the standard
● EN 60601-1-1 is entitled “Medical
electrical equipment – Part 1-1:
General requirements for safety.
Collateral standard: safety
requirements for medical electrical
systems.” This is the primary safety
standard containing requirements
applicable to most types of
equipment; the “Part 2” standards are
far more specific.
● EN 60601-1-2 is entitled “Medical
electrical equipment – Part 1-2:
General requirements for safety.
Collateral standard: Electromagnetic
compatibility – Requirements and
Tests.” This is the primary focus of
this article.
● EN 60601-2-2 is entitled “Medical
electrical equipment – Part 2-2:
Particular requirements for the safety
of high frequency surgical
equipment.” This is an example of a
product-specific standard. There are
several dozen of these.
From this, we see that EMC comes to
be regulated under the Medical
Directive(s) as part of a family of safety
standards. This influences the
requirements of EN 60101-2-2. EMC is
viewed in the context of safety and
clinical function.
Table 2: Table 202 from the standard
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The demonstration of EMC compliance
for medical equipment, which is
covered by a “specific” directive, differs
from the more familiar procedure used
under the EMC and LVD Directives.
For those, self-certification is the
method of compliance; a test report is
generated, and on that basis, the
manufacturer CE marks his product
and asserts compliance with a
declaration of what directives and
standards were applied.
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Most medical products do not undergo
self-certification. Rather, a Notified
Body reviews medical, quality, and
other data, including the EMC and
product safety reports and generates a
certificate of compliance, which forms
the basis for CE marking.
EMC Under The Medical Directive
At present, EMC compliance can be
demonstrated using two versions of
EN 60601-1-2. Either the 1993 version
or the 2001 version may be employed.
EN 60601-1-2:2001 was officially
designated for the Medical Directive on
December 13, 2002. There is a period
of overlap, until November 1, 2004,
when either version of the standard may
be used for assessing conformity.
It is important to note that there is no
grandfathering for equipment assessed
to the old standard after that date; the
presumption of conformity ceases,
which would require retesting to the
new standard for equipment imported
into the EU after than date. Thus, for
equipment with any reasonable
expected life, the later standard should
be used.
EN 60601-1-2:2001, draws on well
known EMC standards used by the
EMC Directive, but adds a lot of
interpretive information and documentation requirements. It differs from
the 1993 standard, and from most EMC
Directive standards in two key ways:
● Relative to the 1993 standard, there
has been a shift to current EMC
standards and methods. The 1993
standard was based on the older IEC
801 series of EMC standards; the new
one uses the IEC/EN 61000-4-x series
that has matured in the intervening
years, as well as the harmonic and
flicker requirements, IEC 1000-3-2
and IEC 1000-3-3.
● Documentation requirements—the
information that must be supplied to
the user—are greatly increased in
quantity and detail. Described in
Section 6.8.201 of the standard, these
include:
A series of warnings and markings.
These include general information
on EMC, the effects that RF
communications can have on
medical equipment, markings of
ESD sensitive connectors (which
have been excluded from pin
discharges) and series of warnings
concerning them, and information
on required physiological signal
levels and warnings on the effect of
low levels on accuracy
Detailed listing of all accessories
which can affect compliance,
including cables and their maximum
allowed lengths, as well as allowed
transducers
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FEATURE
Table 3: Comprehensive list of EN 60601-1-2 EMC requirements
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FEATURE
A detailed, standardized table
running some 10 pages, which lists
the types and levels of EMC
performance the product is capable
of, and gives instruction on the
proper environments for operation.
The tables follow the qualification
test plan. Tables 1 and 2 show a
portion of the information required.
Table 1 is an emissions performance
table that is filled in for the
particular equipment with the aid of
the flowchart in Figure 1. Similarly,
Table 2 is a table covering some of
the immunity modalities, while
Figure 2 is a graphical guide to
completing it.
Note that for equipment which does not
meet the suggested levels, instructions
on what steps must be taken to control
environmental EMC to achieve the
same level of product functionality
must be included. For example, for
equipment with low RFI immunity, a
shielded room might be suggested.
A Closer Look At The Requirements
The basic technical requirements are
shown in Table 3. There are actually
two tables—one set of requirements for
life-supporting equipment, and another,
with lower RFI immunity requirements,
for non-life supporting equipment. At
first glance, the table should look quite
familiar to those used to the generic
EMC standards, or the product specific
telecom and information technology
(ITE) requirements. The standard tests
are applied. However, EN 60101-1-2
incorporates important differences in
the definition of what constitutes
acceptable performance and in a
number of test application conditions.
The issue of acceptable performance is
both important, yet potentially tricky in
interpretation. The general requirements
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(section 3.201) contain the usual limits
on controlling electromagnetic
interference that could affect radio
communications or other equipment,
along with an immunity requirement
that is more carefully drawn than in the
default EMC Directive. This
requirement is that the “essential
performance” of the equipment shall
have adequate immunity to
electromagnetic disturbances. The term
“essential performance” is discussed in
the following paragraph (3.201.2) and
in the definitions (Paragraph 2). The
“essential performance” is defined as
those “characteristics necessary to
maintain the residual risk within
acceptable limits.”
Prior to EMC immunity testing, it is
strongly suggested that an EMC risk
analysis be performed. All functions –
defined as those operating modes of
clinical significance – should be
considered. If this risk analysis is
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performed, the equipment is to be
tested in the mode (and with operating
settings and configuration) that is most
critical from the perspective of the
patient. If this risk analysis is not
performed, all modes must be tested.
Documentation of the test must include
details of the test methods, equipment
configuration, operating mode and the
state of controls, the use of any
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physiological or other signal simulators
and accessory equipment, and when
necessary, justification for any
deviations from prescribed methods or
levels of performance.
There is an explicit list of performance
degradations associated with essential
performance and safety that are
proscribed (“compliance criteria,”
paragraph 36.202.1(j)):
● component failures
● changes in programmed parameters
● resetting to factory defaults
● changes of operating mode
● false alarms
● cessation or interruption of intended
operation, even if accompanied by
alarm
● inaccuracy (error of displayed
numerical data) large enough to affect
diagnosis or treatment
● noise, signal artifact, or distortion
interfering with interpretation of
physiologically produced signals
● failure of automatic diagnosis or
treatment equipment to diagnose or
treat, even if accompanied by alarm
Clearly, the emphasis is on the
maintenance of clinical function in the
anticipated electromagnetic
environment.
EN 60601-1-2:2001 replaces a prior
edition dating back to 1993. In the
world of EMC standards, a lot has
happened, with many EMC standards
having been developed and/or revised.
We estimate that testing to the new
edition takes 50% to 75% more time
than testing to the 1993 edition, due to
the increased in the number of tests
required and the more careful
specification of acceptable function.
Let’s look at some specifics.
In the area of tests required and test
levels, many changes have occurred.
The IEC 801-x immunity standards
were used in 1993. They have been
replaced by the IEC/EN 61000-4-x and
IEC/EN 61000-3-x requirements. This
brings in several new tests that were not
performed under the 1993 edition:
Figure 1: Instruction flowchart matching table 201
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● Harmonics and flicker (IEC 61000-32 and –3)
● Conducted RFI (IEC 61000-4-6),
from 0.15 to 80 MHz
● Power Frequency Magnetic Field
(IEC 61000-4-8)
● Voltage dips and interrupts (IEC
61000-4-11)
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In addition, for several of the tests that
were previously performed, there have
been some changes in the levels that
equipment is required to meet.
● ESD: Levels have been increased for
contact discharge from 3 kV to 6 kV.
Also, connector pins are only
exempted from discharge if they are
marked with an ESD sensitivity
symbol
● RFI radiated: The frequency range
for radio frequency susceptibility
testing has changed. It was 27 – 1000
MHz; now, it is 80 – 2500 MHz. The
change at the lower end is due to the
introduction of conducted RFI
requirements. At the high end, the
frequency range has been extended, in
part because of the expanding use of
equipment at higher frequencies,
especially in the cellular and
networking arenas.
Figure 2: Instruction flowchart matching table 202
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● EFT (electrical fast transient): The
test levels for non-permanent AC
wiring (i.e., the wall plug) have been
doubled from 1 to 2 kV; DC power
leads, formerly exempt, are now
subject to a 2 kV level, and the test
level for I/O leads has been doubled
from 0.5 to 1 kV. For equipment
capable of operating at multiple
supply voltages, including
autoranging, the test is repeated at
minimum and maximum voltages.
● Medical equipment has been divided
into two categories—life supporting
and non-life supporting. Life
supporting equipment is subject to
more stringent requirements in several
areas, including RF immunity (see
Table 3) and voltage dips and
interrupt performance. The
categorization of life- and non- lifesupporting equipment existed in the
previous edition, but the differences in
required performance are now more
marked.
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Perhaps the most important changes
concern the test methodology. Because
medical equipment is specialized, and
because clinical function is of
paramount concern, a number of
changes have been introduced in the
way several tests are performed. Here
are the most noteworthy:
● Equipment Configuration
Guidelines: The general guide is that
equipment is to be configured for
immunity testing in the manner which
is likely to make it most susceptible
to interference. Within the normal
parameters of operation, gain should
be maximized, and any input signals,
such as simulated physiological
signals, should be at the minimum
levels.
Consideration should be given to
monitoring essential functions,
whether in software or hardware, that
are not observable in normal
operation. The Annex gives an
example for a dialysis machine. These
machines contain bubble detector
alarms, which are operated by
viscosity or acoustic detection. These
internal measurements could be
affected during immunity testing,
inhibiting bubble detection and,
hence, some method of ensuring
proper function must be devised.
Care has to be taken in termination of
equipment leads and connection to
simulation that the operating
conditions under test are
representative of normal operation.
Patient connected leads are given
special attention: they are not to be
connected to CDN devices, and
should use the standardized RC
termination (including, where
applicable, the “artificial hand”) of
CISPR 16-1.
● Radiated and Conducted
Immunity: These tests are usually
carried out with 1 KHz amplitude
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modulation. Physiological signals,
such as those associated with pulse
and respiration, have significant
amplitude at lower frequencies. The
standard mandates that a different
modulation rate be used for these
signals. Specifically, where
physiological functions are measured
or controlled these immunity tests
should be performed with a 2 Hz
amplitude modulation.
Shifting to a lower modulation rate
also influences (lengthens) the rate at
which the test is performed. Both the
radiated and conducted tests span a
wide frequency range. This range can
either be scanned digitally, in
increments of no more than 1%, or
continuously. With the normal 1 kHz
modulation rate, the minimum dwell
time for discrete frequency steps is
the longer of either settling time of
EUT plus settling time of test setup,
or 1 second. With 2 Hz modulation,
the minimum dwell time is at the
greater of the settling time or 3
seconds. With continuous modulation,
the sweep time must be no greater
than 4.5 x 10 -3 /X decades per
second, where X is the dwell time for
a discrete 1% step. In other words, the
speed at which the tests can be
conducted is related to the types of
signals processed by the equipment.
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Another test issue concerns how to
deal with frequencies within the range
of those tested where the equipment is
intentionally designed to be sensitive.
Obviously, a strong RF signal would
produce a response. The solution is to
establish an exclusion band around
the sensitive frequency or frequency
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band. For signals below 80 MHz
(which corresponds to the conducted
immunity test), the exclusion extends
+5% and –5% above and below the
affected frequency range. For signals
above 80 MHz (which corresponds to
the radiated immunity test) the
exclusion band is widened to +10%
above and –10% below the sensitive
frequency range.
● Dips and Interrupts: The voltage
dips and interrupts of EN 61000-4-11
are applied, but the performance
criteria vary depending on the power
drawn by the equipment and whether
Test and Equipment Mains Power or Current
Non-Life Supporting
Life Supporting
Dips, <1kVA
Must function normally
Must function normally
Dips, >1 kVA, <16 Amps/phase
No damage, operator resettable
(IEC criterion C)
Must function normally
Dips, >16 A/phase
No Requirement for
testing power dips
No damage, operator resettable,
alarm if function affected
(IEC Criterion C plus alarm)
Interrupts (All Powers)
No damage, operator resettable.
(IEC Criterion C)
No damage, operator resettable,
alarm if function affected
(IEC Criterion C plus alarm)
Table 4: Dips and interrupts (EN 61000-4-11) requirements for medical equipment
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the equipment is considered to be life
supporting, as shown in Table 4.
For dips, all equipment drawing less
than 1 kVA must be unaffected and
continue to operate normally. Between
that power level and 16 Amps of mains
current, life supporting equipment must
still be unaffected, but non-life
supporting equipment need only be
undamaged and restartable with
operator intervention. Higher power
equipment that is life-supporting must
either continue operating, or signal
problems with an alarm; if non-lifesupporting, there is no requirement.
For 5 second interrupts there is a
similar division in requirements. Nonlife-supporting equipment need only be
undamaged and restartable with
operator intervention; life-supporting
equipment must signal an alarm if any
essential function is affected.
Summing Up
If you are involved in the design or
compliance aspects of medical
equipment, you need to be familiar with
EN 60601-1-2. The 2001 edition is now
in effect, and the 1993 version will not
be valid after November of 2004. A
considerable number of differences
exist between the requirements of the
new and the old editions of the
standard. For those familiar with
compliance testing of other equipment,
but less familiar with the special
requirements for medical devices, there
are a lot of wrinkles in test
methodology, test planning, and
required documentation, which we’ve
reviewed in this article. THE SHOW
IS COMING!
THE SHOW
IS COMING!
And Conformity is
ready for it!
Watch for your August
issue containing the special
IEEE EMC Symposium
Show Supplement.
About The Author
Isidor Straus is the Executive Editor of
Conformity. He can be reached at
[email protected].
Don’t forget to visit us
at Booths 528 & 530
AUGUST 18-22, 2003
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