Immunity Testing: Examining Requirements and Test
Intertek Testing Services
Atmospheric phenomena, like thunderstorms, are
electromagnetic. Franklin proved this when he tried
to draw electromagnetic energy from the sky with
a kite. It is also commonly known that electrical
and electronic apparatus can be affected by such
electromagnetic phenomena, even from a distance.
This occurs when lightning induces a transient electromagnetic
field, which is picked up by cabling systems through
magnetic coupling into loops and electric coupling
by "cable antennas." Effects may include transients
and surges in telecom and electric lines hanging
above ground, which can then conduct the energy
long distances to where it might damage electrical
apparatus and systems.
The fact that man-made electromagnetic energy
can also interfere with electric/electronic systems
is a common experience. Anyone who has watched television
at the same time that a vacuum cleaner connected
to an adjacent plug is being used or switched off
has realized that there is some kind of electromagnetic
connection between the two.
Immunity is the ability of devices,
equipment, or systems to perform without degradation
in the presence of an electromagnetic disturbance.
Susceptibility is lack of immunity (inability to
perform without degradation).
In many cases, susceptibility is thought of as
a temporary reduction of performance or degradation
of quality. Even so, susceptible electronics may
directly or indirectly concern safety. For example,
a telephone call might be disturbed in such a way
that the quality of the speech signal is affected,
but it may also imply a blocking, and in the case
of an emergency call, this may have disastrous consequences.
Today we can easily find numerous examples of more
or less serious electromagnetic problems:
The magnetic field caused by ground currents
in the water pipe system makes it impossible
to use sensitive electronic instruments in part
of a hospital building.
A patient-coupled infusion pump is
damaged by an electrostatic discharge, but
thankfully the alarm system is not affected,
and a nurse is alerted.
An operation using a plastic welding
machine causes interference with a patient
monitoring and control system; the monitor
fails to detect that the circulation has
stopped in the patient's arm, which later
has to be amputated.
A wheelchair carrying a handicapped
man goes out of control when it comes
close to a radio station antenna mast,
and eventually the occupant is ejected
into the street.
A robot starts running amok due
to a radio control transmitter, smashing
all equipment within reach.
Interference from a passing truck with
a radio transmitter causes a crane to
drop its load on a person.
A passenger's laptop causes a plane's
navigation system to malfunction, causing
the aircraft to go off course.
The severity of electromagnetic interference requires
some kind of risk analysis, taking into account
all possible electromagnetic occurrences. This is,
of course, impossible in practice. Testing for immunity
therefore always represents an idealized mirror
of reality, and it should not be seen as a kind
of guarantee that a tested and approved apparatus
will never be affected by an electromagnetic phenomenon
(see Table I).
1992 Generic StandardResidential, Commercial,
and Light Industrial Environments
3 V/m 27-500 MHz
|EN 50082-1: 1997 (IEC 61000-6-1)
Generic Standard (Revised)
150 kHz80 MHz
1995 Generic StandardIndustrial Environment
10(3) V/m 80 MHz-1 GHz
150 kHz-80 MHz
60601-1-2: 1993 Collateral StandardMedical
3 V/m 261000 MHz
Draft 1998 Collateral Standard (Proposed
3 or 10* V/m
80 MHz2 GHz
150 kHz80 MHz
|*10 V/m for
life-supporting equipment in the frequency range
800 MHz2 GHz
Immunity Standard for Electrical Household
Appliances, Tools, and Similar Apparatus
80/230 MHz-1 GHz
IV/II & III
150 kHZ80 MHz
II & IV
80 MHz-14 Hz
150 kHz-80 MHz
|Table I. Comparison of some
immunity requirements according to commonly
used generic, general, and product-related standards.
Immunity standards describe "typical" electromagnetic
phenomena, severity levels, and test methods. An
important issue to keep in mind when testing to
immunity standards is the achievement of the highest
possible reproducibility. That is why immunity standards
have to be followed as closely as possible when
it comes to compliance testing, which in certain
cases calls for rather complex instrumentation.
Creating an electromagnetic field for checking the
susceptibility of an electronic circuit need not
be very complex, but immunity testing with homogenous
electromagnetic RF fields, according to IEC 61000-4-3,
requires quite extensive facilities.
Some tests may seem simple to perform, like
using an ESD pistol to test for immunity against
electrostatic discharges. It should be noted, however,
that the discharge pulse represents a very fast
transient, which is easily affected by cabling,
ground planes, how the gun is held by the operator,
etc. Full compliance testing therefore, unfortunately,
tends to be a bit expensive.
That is why precompliance testing is recommended
in order to find the most susceptible parts of electronic
devices at an early stage of the development process.
Simplified standard tests may be used for such pretesting.
For example, if a computer-controlled apparatus
does not cope with an electrostatic discharge toward
an adjacent ground plane, the apparatus is probably
not tight enough to pass a test with radiated electromagnetic
fields. Immunity testing by injecting a fast transient
on a cable can also be performed as a fairly simple
precompliance test with any suitable pulse generator
(for example, an ESD pistol instead of a burst generator).
And if you don't have an ESD pistol, you might use
your imaginationwhy not stroke an Angora cat!
Immunity test methods are described in detail
in basic standards. My intention is therefore not
to rewrite the standards in this article, but to
highlight practical circumstances that I believe
are worth noting. Furthermore, the focus will be
on the commonly used immunity test methods according
to the basic standards (Table II), to which generic
and product-oriented standards refer.
|Electrostatic discharges (ESD)
IEC 801-2, ed. 1:1984
IEC 801-2, ed. 2:1991
|Radiated RF electromagnetic fields
|RF field with keyed carrier (amends 61000-4-3)
|Electrical fast transients/burst (EFT)
ISO 7637 (for dc power lines)
|Surge (1.2 µs/50 µs)
|Conducted disturbances induced by RF fields
|Power frequency magnetic fields
|Voltage dips, short interruptions, and
|Table III. Basic immunity test methods
and related basic standards.
EUT and Test Performance
When planning an immunity test, it is important
to define the equipment under test (EUT), and its
ports (see Figure 1). Input and output ports may
be process measurement and control ports as well
as certain data buses involved in control, such
as a field bus. Signal ports may be signal lines
and data buses. Power ports may be ac or dc. The
enclosure port is defined by chassis and screening
cables. Immunity testing means that typical electromagnetic
disturbances are applied to all relevant ports while
the apparatus is in operation.
|Figure 1. The EUT may have many ports that
might pick up disturbances.
The configuration, as well as operational modes
of the EUT, should be carefully described in a test
plan. The installation and the operational test
conditions should be in accordance with product
specifications. Certain tests have to be performed
with the EUT placed on a reference ground plane.
Test setups, with necessary insulating supports
and ground connections, are usually well described
in standards and should be followed in detail to
obtain reproducible results.
Immunity testing is to be conducted in a well-planned
and reproducible manner, as specified in a test
plan. If the test object, in its normal operation,
is to be connected to auxiliary devices, it should
be tested with a minimum representative configuration
of such devices. The most susceptible modes of operation
should be selected and the configuration varied
to achieve maximum susceptibility. Severity levels
as well as criteria have to be defined. It is, of
course, important to define the test setup in the
test plan as well as to analyze what parameters
have to be monitored during testing. Television
cameras are often used to guard screens and control
lamps and to monitor movements. Electrical measuring
instruments, like oscilloscopes and bit analyzers,
must be arranged in such a way that they cannot
affect the test results. Television cameras must
be shielded from radiated electromagnetic fields.
When illuminating the EUT with electromagnetic fields,
fiber-optic transmission techniques are often used
for transferring the monitored signals into the
control room. This reduces the possibility that
the field is coupled onto the cabling.
The required severity test level/class for
an apparatus or system depends on the environment
and installation conditions where it will be used.
Equipment to be used in a well-protected environment
(Levels 1 and 2), like a computer room, may be tested
at a lower immunity level than equipment to be used
in an unprotected environment (Levels 3 and 4).
Severity levels are defined in each basic standard.
The following are generally used for testing against
radiated RF fields (IEC 61000-4-3) and conducted
disturbances induced by radiated fields (IEC 61000-4-6),
However, these figures should not be viewed as
though an immunity test with an RF field of 3V/m
will give identical results as an induced test of
3V if performed on the same object at the same frequency
(see Figure 2). Different test methods are not directly
convertible and should not be regarded as kind of
an absolute measure of immunity. Standardized test
methods may not simulate a real situation at all,
but as previously mentioned, the test methods are
well defined to achieve the highest possible reproducibility.
|Figure 2. An example of comparative testing
performed on the same test object with two different
immunity test methods, RF radiation and injection
respectively (IEC 61000-4-3 and 61000-4-6).
The marked frequency intervals indicate interference.
The induced RF method may be used from the kHz
region up to 230 MHz, while the RF field illumination
method is used from 26 or 27 MHz up to GHz. In recent
generic and product standards, testing with radiated
fields is commonly used above 80 MHz, while the
induced RF test method is used below 80 MHz. The
reason is simple: the induced RF test method is
much cheaper. Testing of immunity against radiated
RF fields at 30 MHz (wavelength l = 300/f MHz =
10 m) requires a fairly large (length ~ 20 m) and
expensive anechoic chamber. The wavelength of the
radiated field as compared to the mechanical dimensions
of the EUT must be taken into consideration with
respect to the size of cabinets as well as the lengths
of connected wires.
The uncertainty of the actual value of the
ultimate disturbance level creates the need for
margins. All instruments in the chain needed to
define a specified level of disturbance must be
calibrated (for example, oscilloscopes for measuring
pulse characteristics and probes and/or power meters
used for defining field strength). If, for example,
the total uncertainty when reproducing an RF field
is about 30% (including monitoring), the field strength
control should be set on 13 V/m if 10 V/m is to
be given as the minimum test level.
Most basic immunity standards define tolerances
for disturbance characteristics like pulse and rise
times, and pulse width. Besides calibration of instruments
used for reproducing disturbances, regular checks
are recommended to find out (at an early stage)
if a generator or a measuring instrument tends to
go out of specifications. Transient pulses can be
monitored with an oscilloscope. Note, however, that
to study a pulse with a rise time shorter than 1
ns, you need an oscilloscope having a real-time
bandwidth of at least one GHz, since a rise time
tr ~0.7 ns corresponds to
a 3-dB bandwidth B ~500 MHz (B = 1/
Environmental conditions may affect some of the
immunity tests. The physical properties of an electrostatic
discharge, for example, depend to a great extent
on humidity. The performance of transient generators
may also, to some extent, depend on humidity, particularly
for an old burst generator using a spark gap. Requirements
concerning environmental conditions therefore apply
to some of the immunity standards. According to
relevant standards, the air temperature usually
has to be controlled to within 15°35°C,
air pressure within 86106 kPa, and humidity
according to the following:
|IEC 61000-4-2 (IEC 801-2)
|IEC 61000-4-4 (IEC 801-4)
|EC 61000-4-5 (IEC 801-5)
Test results are classified as pass or no-pass
on the basis of the operating conditions and functional
specifications of the EUT, according to the following
Criterion A: Normal performance with respect
to specifications given by the manufacturers and/or
as defined in the standard.
Criterion B: Temporary degradation or loss of
function or performance, which is self-recoverable.
Criterion C: Temporary degradation or loss of
function of performance, which requires operator
intervention or system reset.
These general criteria are applied according to
A: Phenomena of a continuous nature (RF disturbances).
B: Phenomena of a transient nature (transient
C: Power supply failure phenomena (tripouts).
Detailed specifications are left open to
the manufacturer and the parties involved in testing,
who therefore must analyze relevant parameters carefully
before testing. It is not, for example, a major
concern if a PC screen flickers during a static
discharge, as long as the PC is running normally
after the disturbance has ceased. The needle of
a sewing machine should not, on the other hand,
be allowed to move due to a discharge. A measuring
instrument or a gauge might go out of specs during
a disturbance with short duration, but should stay
within its specified error band when a continuous
disturbance source, like a radio transmitter, is
used in its vicinity. A household machine with an
automatic power control should not be affected by
a mobile phone in such a way that it may cause harm,
but a power variation of 10% is probably harmless.
Degradation or loss of function, which is
not recoverable due to damage of equipment (components
and/or software), or loss of data, should never
The collateral standard for medical devices,
IEC 601-1-2 (EN 60601-1-2), states that the EUT
shall perform its intended function as specified
by the manufacturer or fail without creating a safety
hazard. One could say, therefore, that a medical
device is allowed to break during a test, provided
that the alarm system is still in operation! This,
of course, is not a recommended practice, and this
paragraph will be replaced by detailed criteria
in the forthcoming revised collateral standard.
Product-related standards usually specify
operating conditions and criteria for specific products.
In some cases, such criteria might seem to concern
product quality rather than safety. For example,
the immunity performance criteria for a television
according to CISPR 20 is merely a perceptible degradation
of the picture.
No deviations are allowed when harmonized
standards are used for compliance, but in certain
cases the test requirements may be reduced! As an
example, the generic immunity standards state that:
"It may be determined from consideration of the
electrical characteristics and usage of a particular
apparatus that some of the tests are inappropriate
and therefore unnecessary. In such a case it is
required that the decision not to test be recorded
in the test report."
Note, on the other hand, that the generic
standard may also require enhanced severity levels:
"In special cases situations will arise where the
level of disturbance may exceed the levels specified
in this standard, e.g. where a handheld transmitter
is used in proximity to an apparatus. In this instance
special mitigation measures may have to be employed."
Standard Tests and Their Mirrored Reality
Standard immunity tests concern continuous
or transient disturbances, low- or high-frequency
phenomena, and conducted and radiated electromagnetic
effects. The goal of testing is to reveal eventual
susceptibility against common disturbances, but
observe that in some cases the equipment may be
damaged. This might be the case when it comes to
testing of immunity against surges and electrostatic
Testing of immunity against radiated disturbances
is usually performed by illuminating the EUT with
an electromagnetic field. Interference might also
be simulated by directly inducing disturbances on
cables connected to the equipment.
Conducted disturbances are usually injected
directly on an EUT port or injected/induced through
an impedance, a clamp, or a coupling network (decoupling
networks are not used to influence auxiliary equipment).
Generic, general, and product family standards may
stipulate that certain tests are unnecessary if
the cables connected to the EUT are short; therefore,
injected RF disturbances and fast transients are
required only if the total length according to the
manufacturer's functional specification may exceed
Continuous disturbances have to be applied
as long as it takes for the EUT to fulfill a certain
operation cycle. Concerning transient tests, interference
usually depends on when the pulse occurs in respect
to the operational mode or a clock cycle. It is
therefore necessary to repeat the test a number
of times in order to catch the worst case.
Testing with Continuous Disturbances
Testing with radiated electromagnetic fields
according to IEC 61000-4-3 simulates possible radio
communications threats. Testing is usually done
within the frequency range up to 1 GHz with a field
strength of 3 V/m (Level 2) or 10 V/m (Level 3).
However, if one wants to be sure that an apparatus
will not be affected by, for example, a mobile phone
in close vicinity, a test with fields of at least
30 V/m is recommended (100 V/m for life-supporting
equipment and critical apparatus).
Modern standards, like the European generic
standard EN 50082-1: 1992, may still refer to old
"basic" standards like IEC 801-3, which actually
restricts testing up to a maximum of 500 MHz with
an unmodulated carrier. It should be noted that
500 MHz is far too low to cover today's mobile threats.
The generic standard, however, has added a requirement
for amplitude modulation of the electromagnetic
field. This is usually far more severe than testing
with an unmodulated carrier, since it is mainly
the envelope of the picked-up electric signal that
causes the interference in electronic circuits.
Most modern standards therefore require AM and/or
PM modulation (FM has little effect and is therefore
The high-frequency carrier couples the electromagnetic
wave into electronic equipment with the help of
different "antennas," such as cables and enclosure
parts of suitable length. For example, a cable of
1 m length can act as a monopole antenna with a
maximum sensitivity around 75 MHz (l/4 = 1 m). After
being picked up via antenna effects, a radio frequency
signal can itself be a threat to an electronic circuit.
For example, a selective HF receiver, as in a navigation
system, is sensitive to RF frequencies within its
passband. An RF signal may also cause resonances
of a few hundred MHz, due to lead inductances and
Nonlinear effects in the electronics, however, usually
rectify or demodulate the RF signal. After the high-frequency
carrier is eliminated, the remaining disturbance
may represent an offset or a low-frequency signal,
which might affect a sensitive analog amplifier
or an integrating A/D convertor. The reason a mobile
phone of type TDMA can affect audio equipment is
because the carrier is keyed by an LF signal (217
Hz in the case of GSM), which together with harmonics
causes disturbances in the acoustic range (up to
Besides modulation, IEC 61000-4-3 also requires
that testing be performed with a homogenous EM field.
This implies high-performance test facilities, with
a need for a high-power amplifier, since the antenna
has to be placed at a relevant distance from the
Accurate RF immunity testing requires a shielded
anechoic chamber, but can also be performed in a
large GTEM cell. For testing according to CISPR
20/EN 55020, an open stripline is prescribed, a
simpler form of a TEM cell that is used for generating
fields within the frequency range 150 kHz150
MHz. Because this is a rather specific test method
for immunity testing of radio, television, and audio
equipment, it will not be addressed here.
Testing with induced RF disturbances simulates
how electromagnetic fields may be picked up by cables,
which in turn feed the disturbances into the connected
equipment. Induced RF immunity testing according
to IEC 61000-4-6 has been introduced as a less expensive
alternative to RF field immunity testing in the
lower frequency range. Standards primarily refer
to testing with a modulated RF carrier from 150
kHz up to 80 MHz, in some cases up to 230 MHz. The
current into the EUT is injected through a coupling
network or by a clamp.
The major advantage of this method compared
to RF field testing is that there are no requirements
for using an anechoic chamber. Testing is usually
performed by injecting one cable at a time. This
technique is subject to criticism, however, because
in a real situation when there are several cables
connected to the EUT, the whole system will be affected
by an electromagnetic field and all cables may carry
RF currents simultaneously. Immunity testing requires
that all ports be subjected to different threats,
such as injection of transient disturbances on ports
and cables, or illuminating chassis ports and cables
with electromagnetic fields.
Testing with LF magnetic fields is defined in the
basic standard IEC 61000-4-8. The test simulates
magnetic fields resulting from currents running
in power-line systems (there are also two basic
standards for testing of immunity against pulsed
magnetic fields, IEC 61000-4-9 and -10).
Earlier generic and product family standards do
not refer to this fairly new basic standard, but
may have general requirements concerning immunity
against power-linefrequency magnetic fields
of from 3 A/m up to 300 A/m. However, testing is
only required when it comes to equipment, including
a component or a subunit, which is known to be sensitive
to LF magnetic fields, for example, a Hall element,
a coil, or a monitor. Immunity testing against LF
magnetic fields up to around 100 kHz is also stipulated
in some other standards, like MIL-STD 462, RS-101
(30 Hz100 kHz).
Immunity Against Transients and Other Disturbances
of Short Duration
ESD tests are performed with an ESD pistol.
This is a pulse generator with a capacitor that
can be charged to about 15 kV and which is discharged
via a known impedance. The test level is often 46
kV, contact, and/or 8 kV, air discharge. However,
discharges up to 1020 kV may occur in real
life, especially during the winter in northern parts
of the world when the air humidity drops to very
low levels due to indoor heating; this can also
occur in other dry parts of the world like desert
areas. Note that the humidity may also be very low
close to a surface that is heated by electronic
Note that there are two ESD standards, IEC 801-2:
1984 and IEC 801-2: 1991 (equivalent to IEC 61000-4-2).
The first, edition 1, defines the discharge pulse
with rise time to 5 ns, while the rise time according
to the latter, edition 2, is only 0.7 ns, which
is far quicker and usually represents a more severe
threat. It should be noted, however, that both pulses
exist in reality. The slower air discharge is typical
for a finger discharge against an object (human
body model), while the quicker pulse is typical
for a discharge through a sharp conductive object,
like a screwdriver touching a metallic enclosure
(charged device model) (see Figures 3 and 4).
|Figure 3. An electrostatic discharge in
real life may have a rise time of about 5 ns,
as defined in the old ³basic² standard IEC 801-2:
1984, modeled on a discharge through a human
|Figure 4. If the discharge is caused by
a charged metallic object like a screwdriver,
the rise time may be as quick as 0.7 ns, as
defined by the new standard IEC 801-2:1991 (IEC
It is generally the case that the higher the frequency,
the shorter the rise time of a disturbance, and
therefore the easier it is to couple into an electronic
circuit. It happens, however, that equipment can
be immune to the shorter pulse but not to the slower
pulse. This can be explained by the fact that faster
pulses are more easily bypassed by stray capacitances.
Even though a short ESD pulse may have low energy,
it may still damage sensitive integrated circuits
(ICs). Device failure may occur some time after
the original damage was sustained; for example,
a pn-channel in an IC may be affected in such a
way that it will be damaged after being in operation
for some time. Because the IC may leave the factory
in this undiscovered damaged state, it is said to
carry a latent defect, known to industry as "the
Burst or electromagnetic fast transients
(EFT) represent transients generated from circuit
breakers, nonprotected relays, etc. IEC 61000-4-4
defines an idealized burst with relatively long
duration (15 ms) repeated at a rate of a few kHz.
The pulse amplitude of the transients during the
burst is usually 0.54 kV peak, but since the
duration of each transient is fairly short, the
bursts have fairly low energy. The pulse rise time
is typically 5 ns and the pulse width, 50 ns. They
are generated with a burst generator and injected
into cables through a coupling network or a capacitive
The bursts are injected into both power lines (both
common-mode [line to ground] and differential-mode
[line to line]) and I/O lines (if longer than 3
meters). Power-line injection is performed using
a discrete capacitor, while I/O injection is performed
using a long capacitive clamp (with the capacitance
distributed along the cable).
Surge is a transient voltage propagating
along a line. Surges can be induced in cables by
lightning, but might also pop up on the power line
when connecting a phase compensating capacitor into
the distribution net or when a fuse breaks. The
double exponential pulse, according to IEC 61000-4-5,
is defined by both open-circuit and short-circuit
characteristics. It is usually generated with a
hybrid generator, capable of giving a peak voltage
of 14 kV and a maximum peak current of some
kA. The surge pulse has a fairly long pulse-width,
20 or 50 µs, and therefore represents a fairly
high energy pulse that may easily damage unprotected
electronic circuits and components.
Surges are to be applied on power lines with a
defined timing referred to the power-line phase.
They may also be imposed on some signal cables,
like long telecom cables. The pulse is injected
through an impedance or a network.
Voltage variations, dips, and short interruptions
are caused by faults in the distribution system
or by a sudden large change of load. IEC standard
61000-4-11 defines simulation of typical power tripouts
with a duration of a few cycles of the power-line
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