From
Immunity Testing for the CE Mark
by Rodger Gensel
Mar 15, 2007
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The European Union (EU)
currently has 25 member countries with 2 additional countries to be
added in 2007. The total population at that time will be nearly a half
billion people. In 2005, the total exports to the EU accounted for 20%
of the total US exports. This number should continue to grow and should
create more business for U.S. exporters in years to come.
Since January 1, 1996, manufacturers of electronic equipment have had
to meet the Electromagnetic Compatibility (EMC) guidelines of EC
Council Directive 89/336/EEC when shipping electrical and electronic
products to the EU. Manufacturers must test and certify that their
equipment meets the directive and they must apply a CE mark as
testimony to this. Current and pending changes to the specifications
that describe the tests to be made place more stringent requirements on
the equipment used for CE mark testing. Indeed, future changes will
require that the testing equipment be able to expand its capabilities
without causing the equipment to become useless.
The legally prescribed test requirements for EMC standards in the EU
are issued by CENELEC, the European Committee for Electrotechnical
Standardization. CENELEC issues both Generic and Product standards. The
generic standards are EN 61000-6-1, which addresses the requirements
for immunity testing in the residential, commercial and light
industrial environment. The industrial environment immunity testing
required is addressed by EN61000-6-2. The generic standard for emission
requirements in the residential, commercial and light industrial
environment is covered in IEC 61000-6-3 and the industrial environment
is addressed in EN 61000-6-4.
The generic standards above apply to products for which no dedicated
product or product-family standard exists. If a product or
product-family standard exists, it takes the place of the generic
standards in prescribing the test requirements. The generic and
product-family standards outline the test requirements. They refer to
what is known as the Basic Standards to define the tests to be
performed, the test methods, the test set-up and the specifications of
the generator used to simulate interference phenomena. The
International Electrotechnical Commisson (IEC) writes the basic
standards to which CENELEC refers in their ENs. This article will focus
on five of the European Standards from CENELEC used in the immunity
portion of the standards, four of which are required by the generic
standards for CE mark certification.
Basic Standards for EMC Immunity
The IEC promotes standardization in the fields of electricity,
electronics and related technologies. IEC 61000 part 3 covers emission
specifications and IEC 61000 part 4 covers immunity specifications. The
specifications are further broken down into sections. CENELEC has also
adopted the same numbering system and refers to the specifications as
ENs or European Norms. They are EN 61000-3 and EN 61000-4. The immunity
portions of the specifications are listed in Table 1.
| EN 61000-6-1 and EN 61000-6-2, Generic Immunity Standards refer to: |
| EN 61000-4-2 | IEC 61000-4-2 | Electrostatic Discharge (ESD) |
| EN 61000-4-3 | IEC 61000-4-3 | Radiated EM Field |
| EN 61000-4-4 | IEC 61000-4-4 | Burst-Electrical Fast Transients (EFT) |
EN 61000-4-5
| IEC 61000-4-5 | Surge |
| EN 61000-4-6 | IEC 61000-4-6 | Conducted Radio Frequency disturbances |
| EN 61000-4-8 | IEC 61000-4-8 | Power frequency magnetic field |
| EN 61000-4-9* | IEC 61000-4-9* | Pulse magnetic field |
| EN 61000-4-11 | IEC 61000-4-11 | Voltage dips, interruptions |
*EN 61000-4-9 and IEC 61000-4-9 are not required by the Generic Standards but are referenced here for later use in this article.
|
Table 1: Basic Standards
Interference Generation
The Basic Standards also specify the generator to be used for
simulating the interfering phenomena. But, since the disturbance
phenomena have to be determined and measured first, the Basic Standards
and the generator requirements usually lag behind the real
environmental conditions. Consequently, the standards are subjected to
constant changes. To reduce the cost of the generator used for testing
to the immunity standards, it must be versatile and capable of covering
as many of the foreseen specification changes as possible.
Testing of one’s own product is a requirement for the CE mark. This
viewpoint is satisfactory; however, immunity testing of a manufactured
product is also valuable as a tool to achieving customer satisfaction
with the end product. If a product is not susceptible to ESD and fast
transients, for instance, it will not fail as readily during normal
use. This is not only important from a performance standpoint, but from
a safety and legal liability standpoint as well. Therefore, it is
useful to use interference generators in the product design and
development stage, as well as in the CE mark certification process.
Burst
The burst pulses, also called electrical fast transients (EFT), are
created on public power lines by electrical arcs across switch contacts
during opening of the switch. The nature of the loads connected to the
power lines causes short bursts of pulses across the switch, which can
generate interference in electrical/electronic equipment connected to
the lines. Figure 1 shows the bursts called out by IEC 61000-4-4. The
most recent version of the burst standard is IEC 61000-4-4:2004 Ed 2.
The pulses within the 15-millisecond burst period are defined by the
specification as having a frequency of 5 KHz. The latest edition of the
standard added 100 KHz as a new frequency and the burst duration for
this pulse will be 0.75 millisecond. The reduction in the burst
duration is due to the energy content having to remain the same as the
5 KHz pulse. The burst pulses are applied in peak voltage levels from
500 volts to 4 KV on the power supply lines of the equipment under test
(EUT). Because the burst pulses can be radiated into equipment signal
lines (from nearby power leads), the burst pulses are also required to
be capacitively coupled to the signal leads.
Figure 1: Fast transient/burst pulses
The burst test is a low energy test and, as such, it is not
destructive. The burst test is particularly hard on complex digital
equipment with high clock frequencies. The test can cause degradation
of performance, loss of function, uncontrolled process sequences,
failures in programmable equipment, loss of information stored in
memory and incorrect data processing. For these reasons, the burst test
should be the first test performed on an EUT. In order to take into
account constantly increasing clock frequencies of microprocessor
applications, higher burst pulse frequencies exceeding the 5 KHz
specified frequency and even higher than the 100 KHz proposed frequency
should be used in developmental testing. If the burst testing were done
at 5 KHz, it would take many tests to find and isolate a burst
susceptibility design problem. If, for example, the EUT were to have a
clock frequency of 10 MHz, the 5 KHz burst pulse tests only every
2,000th function state. The test piece’s critical states are likely to
be among the 1,999 function states not tested. If the testing is done
at 100 KHz, the design problems can be more quickly discovered because
20 times as many function states are tested in any given testing time.
CENELEC has also added new requirements related to pulse verification
and calibration of the burst generator. These are included in Ed 2 of
the IEC 61000-4-4 standard. According to Amendment A2:2001 to the IEC
61000-4-4:1995 standard, test generators must meet the verification and
calibration requirements effective July 1, 2004. The changes made
require that the burst pulse be verified at the generator coaxial
output into an open circuit (1000 ohms) and into a 50-ohm load. Newly
added to the Ed 2 standard, the burst verification also needs to be
performed at the generator coupling/decoupling output with a 50 ohm
load. Because of the high frequency character of the burst pulses, to
reduce stray capacitance, a special coaxial adapter (see Figure 2) and
a minimum 400 MHz bandwidth oscilloscope are required to make the pulse
verification. The specification changes were made to achieve more
uniform pulse characteristics among the various transient generator
manufacturers and to provide repeatable test results.
Figure 2: Coaxial adapter for burst pulse observation
Since the burst test is relatively easy to set up and its
reproducibility is high, the test is suitable for in-house
developmental testing.
Surge
IEC 61000-4-5 prescribes tests for simulating the effects of lightning
discharges as well as voltage surges caused by switching disturbances
in power stations. The surge waveforms are defined by the specification
as shown in Figures 3 and 4. The peak voltage for the voltage waveform
in Figure 3 varies between 500V and 4.0KV. The peak current in Figure 4
is between 250A and 2.0KA.
Figure 3: Waveform of open circuit voltage (1.2/50 ms)
Figure 4: Waveform of short circuit current (8/20 ms)
IEC 61000-4-5 requires that the generator be capable of applying surge
pulses at a rate of “at least one per minute.” The surge pulse must be
applied at 0, 90, 180, and 270 degrees of the power input waveform.
This requires that the surge generator be synchronized to the AC input.
The surge pulses must be coupled from line to line and from the line(s)
to earth of the power leads. The peak voltage level of the surge pulse
is adjusted in steps starting at 500V to 4000V. In total, 600 pulses
are required. The simulator must have sufficient energy available to
complete this test in a timely matter because, if a generator just
meets the bare minimum requirement of one pulse per minute, the test
will take 10 hours total. If the generator is capable of providing
surge pulses faster than one per minute, the test time can be reduced.
This assumes the EUT itself is designed to handle the faster surge
pulse repetition rate. The surge test can be destructive to the EUT if
adequate protection is not incorporated into the design. Therefore,
this test should be carried out only after a successful burst test so
that, if the EUT is destroyed, the tester at least has some useful test
data.
The coupling and decoupling networks for applying the burst and surge
pulses are usually contained within the surge generator. For coupling
to the signal leads for both burst and surge pulses, external coupling
clamps or networks are required.
Power Fail Simulation
IEC 61000-4-11 specifies the levels for testing supply line dips,
interruptions and variations. Faults in the power distribution
networks, brownouts or sudden large changes in the load on the lines
cause voltage dips and short interruptions. The line voltage variations
are caused by continuously varying loads connected to the network.
These phenomena occur randomly. When large rotating machines are
connected to the network, they can act as generators on the power lines
during interruptions resulting in gradual variations in the line
voltage rather than abrupt interruptions. The line voltage dips and
interruptions are required tests of IEC 61000-4-11, but the voltage
variations test in the IEC standard are optional dependent on the EUT
product specification and the product’s sensitivity to voltage
variations.
This standard has been revised recently and the most current edition is
IEC 61000-4-11:2004. It was published March 1, 2005 and has a Date of
Withdrawal (DOW) 6/01/2007. A new test level of 80% has been added to
the Voltage Dips and Interruptions tests. The Voltage Variation Test
also changed with the transition time to the dip voltage being changed
from 2 seconds to an abrupt time. This fast switching time requirement
could lead to a design change in some simulators which perform this
test. Also, the time for the increasing voltage has changed to 0.5
seconds from 2 seconds. The specifications for the voltage dips and
short interruptions are given in Tables 2 and 3. Table 4 shows the
optional power supply variations test levels.
| Classesa | Test level and durations for voltage dips (50Hz/60Hz) |
| Class 1 | Case-by-case according to the equipment requirements |
| Class 2 | 0% during 1/2 cycle | 0% during 1 cycle | 70% during 25/30 cycles | 80% during 250/300 cycles |
| Class 3 | 0% during 1/2 cycle | 0% during 1 cycle | 40% during 10/12 cycles | 70% during 25/30 cycles | 80% during 250/300 cycles |
| Class Xb | X | X | X | X | X |
a Classes as per 61000-2-4, see Annex B in this present document
b To be defined by product committee. For equipment connected directly
or indirectly to public network, the levels must not be below class 2 |
Table 2: Test levels for voltage dips and interruptions
| Classesa | Test level and durations for short interruptions (ts) (50Hz/60Hz) |
| Class 1 | Case-by-case according to the equipment requirements |
| Class 2 | 0% during 250/300 cycles |
| Class 3 | 0% during 250/300 cycles |
| Class Xb | X |
a Classes as per 61000-2-4, see Annex B in this present document
b To be defined by product committee. For equipment connected directly
or indirectly to public network, the levels must not be below class 2 |
Table 3: Test level and durations for short interruptions
| Voltage test level | Time for decreasing voltage (td) | Time at reduced voltage (ts) | Time for increasing voltage (ti) (50Hz/60Hz) |
| 70% | Abrupt | 1 cycle | 25-30 cycles |
| Xa | a | a | a |
| a To be defined by product committee. |
Table 4: Timing of short-term supply voltage variations
For dips and interruptions, the transient generator usually contains
built-in electronic switches to switch in the appropriate voltage dip
or interruption. The generator controls must allow adjustment of the
phase, synchronization and duration of the dips and interruptions.
Power Frequency Magnetic Field and Pulse Magnetic Field Immunity Testing
IEC 61000-4-8 describes the tests and levels for power frequency
magnetic fields and IEC 61000-4-9 defines the tests and levels used for
pulsed magnetic field immunity testing. These two specifications are
grouped together because they are very similar. Magnetic fields can
affect the reliable operation of electrical and electronic equipment.
The power frequency magnetic field simulates power frequency current in
conductors or leakage from transformers or other current carrying
conductors in the area. Pulse magnetic fields are generated by
lightning strikes on buildings and other metal structures near
electrical or electronic equipment.
The generic immunity standards now require that IEC 61000-4-8 testing
be completed for the CE mark. IEC 61000-4-9 is only a requirement of
some product specifications at this time. The products usually tested
to IEC 61000-4-9 are railroad equipment and those products associated
with power sub-stations. One way to test for the power frequency
magnetic field requires an external one meter square coil and a current
transformer to drive the square coil. One config-uration using this
method for power frequency magnetic field immunity testing is shown in
Figure 5.
Figure 5: Power frequency magnetic field testing
Connecting the surge current output of the transient generator directly
to the external one-meter square coil generates the pulsed magnetic
field. The generator controls are used to adjust the level of current
through the coil to the proper magnetic field level. Figure 6 shows the
test diagram.
Figure 6: Pulsed magnetic field testing
Electrostatic Discharge Immunity Testing
The pulse waveform generated for ESD simulation is a very fast
transient pulse up to 15KV in amplitude and containing high impulse
current. Because of this, the test can be very destructive in nature.
ESD testing should be completed after EFT testing to insure that the
EUT has a solid board layout, satisfactory lead dress and a proper
grounding system to reduce the possibility of EUT damage from the ESD
pulse.
The set-up and application of ESD testing is critical. Because of this
testing complexity, a specific description of ESD testing is out of the
scope of this article (refer to Figure 5 of IEC 61000-4-2). In fact,
the subject of ESD testing is worthy of an entire article of its own.
Conclusion
CENELEC refers to basic standards issued by the IEC. CE Marking
requires emissions and immunity testing. This article has addressed
some of the immunity tests required by the generic standards. The basic
standards describe the transient phenomena waveforms characteristics
and the generator characteristics used to simulate the transient
waveforms in the test environment. Testing electrical and electronic
products to achieve the CE Mark requires using test generators that
simulate the transient phenomenon occurring on power lines. Transient
generators are available which incorporate the ability to test burst,
surge, magnetic field and voltage dips and drops in one package to
simplify the test engineer’s task. Recent changes in the generic and
basic standards require that the generator be chosen with the ability
to meet the new specification requirements. n
Rodger Gensel is a product-line specialist for transient generators at AR Worldwide, and can be reached at RGensel@amplifiers.com.
© Copyright 2007 Conformity