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EMC Assessment of Programmable Controllers: Choosing the Appropriate Standards | Interference Technology
May 3, 2005 Item Media Articles, EMC Directory & Design Guide, Standards, Test Instrumentation, Testing
Using out-dated standards may result in erroneous conclusions regarding the true electromagnetic compatibility of the device under test.
Nikolai Panteleev and Borislava Medzhidieva
EMC Testing Laboratory, Sofia, Bulgaria
Programmable logic controllers (PLCs) are widely used in industrial control systems for several reasons. Using specialized languages, they are easy to program, a facility that means designers from varying disciplines can use them in widely different applications. Complex, cost-effective control systems can be constructed quickly using these modular devices. Moreover, their compact dimensions and low power consumption make PLCs an attractive design option.
At present, among At present, among the many standards referenced within Directive 89/336/ECC, there are three applicable to the EMC evaluation of PLCs. They are:
EN 61131-2: 1994, along with Amendments A11 (1996) and A12 (2000)
EN 61326: 1997, along with Amendments A1 (1998) and A2 (2001)
According to the Official Journal of the European Communities, both EN 61131-2 (1994) and EN 61131 (2000) are applicable to the EMC testing of PLCs. In EN 61326 (1997) the term scope includes “industrial process measurement and control (IPMC) equipment, including devices such as process controllers and regulators and programmable controllers (PCs).” Additionally, EN 61326 includes this proviso, “Where a relevant dedicated standard exists, it shall take precedence over all aspects of this product-family standard.”
Obviously, some difficulties and questions arise. In performing a conformity assessment of equipment electrical control devices or equipment (which could well include one or more programmable controllers), which of the referenced standards is applicable? In dealing with these standards, can contemporary trends in EMC assessment be discerned? What are the most significant differences among these three standards?
EMISSION AND IMMUNITY REQUIREMENTS
Tables 1, 2, and 3 show the requirements of the three standards in regard to emission and immunity. Listed are ports to be tested—enclosure, AC power, DC power, digital and analog I/Os, data communication, and earthing—as well as the limits and test levels.
Several conclusions can be drawn from the careful examination of these tables. First, EN 61131-2: 1994 fails to include requirements on either radiated or conducted emissions. Thus, a tester evaluating for compliance with the EMC Directive—viz. 89/336/EEC—might reasonably turn to EN55011: 1998 for guidance on testing for electromagnetic emissions. Similarly, EN61131-2: 1994 fails to include any requirement for a PLC’s immunity to surge, conducted disturbances, induced radio frequency fields, or power frequency magnetic fields as set forth in EN 61000-4-5: 1995; EN 6100-4-6; 1996, and
EN 61000-4-8:1993, respectively. (See first column, Table 1.)
Similarly, EN 61131-2: 1994 is not entirely adequate in stipulations for testing the various PLC ports for immunity to electrical fast transient/burst. Testing is carried out with an amplitude of 2 kV with frequency impulses within the package of 2.5 kHz. Also, the minimum test duration is 10 seconds, a considerably shorter period than the commonly accepted 60 seconds. Finally, the recommended amplitude for testing digital and analog ports (250 V) is lower than that provided for in EN 61000-4-4: 1995. Given these inadequacies, it becomes evident that erroneous conclusions about the actual immunity of a PLC to the EMT/burst phenomenon could be made.
Another provision of EN 61131-2:1994 provides for the testing of dumped oscillatory waves. The basic standard EN 61000-4-12 describes, “repetitive oscillatory waves occurring mainly in power, control, and signal cables installed in high voltage and medium voltage (HV/MV) stations.” Switching isolators in HV/MV open-air stations often exhibit such waves, particularly when the switching of HV bus bars is involved. Background disturbance in industrial plants is another contributing factor. Compliance with this standard, especially when applied to digital and analog ports, is often very difficult because of the high amplitude of the interfering signal and the fast rise times of the dumped oscillatory waves. Thus, using this standard to determine PLC immunity in a typical industrial environment could be quite problematic.
The later standard EN 61326: 1997 reflects the trend toward heightened precision in assessing the electromagnetic compatibility of electronic systems. Nevertheless, the specific ports to be tested are described very generally. The standard does not take into account the several discrete ports found in a PLC, and such lack of precision could result in errors during the application of this standard. Also EN 61326: 1997, as well as EN 61131:1994, fails to include separate categories of industrial environments.
In contrast, the standards for immunity set forth in the EN 61000-4-X series define two types of industrial environments:
A typical industrial environment (e.g., area of industrial process equipment, power plant or HV substation, portable transceivers)
A severe industrial environment (power stations with industrial process equipment located outdoors, an open-air HV substation or switchyard)
Moreover, the determination of applicable assessment levels is based on the type of environment encountered in a particular situation.
EN 61131-2: 2003 is definitely the most modern of the three standards analyzed in terms of assessing PLCs in terms of EMC. It includes precise definitions of different types of electromagnetic conditions in which a PLC might be used, as well as a precise listing of the varying discrete ports of a PLC. Specific test levels are specified for the varying electromagnetic conditions.
Specifically, the industrial environmentment is divided into three zones:
Zone C—Factory mains (isolated from public mains by a dedicated transformer), primary surge protection, and severe interference coupling.
Zone B—Dedicated power distribution, secondary surge protection, and moderate industrial interference coupling
Zone A—Local Power distribution, protected, I/O impedance limiting, and low interference coupling.
The specified disturbance levels listed in the standard for Zone B correspond to recommended testing levels for a typical industrial environment (EN 61000-4-X), while those for Zone C correspond to those appropriate for a severe industrial environment. This differentiation avoids the pitfall of requiring unrealistic EMC standards for a PLC to be used only in Zone B. Also, requirements are adjusted further by factoring in such variables as the types of cables (shielded/unshielded) connecting various ports of the PLC to sensors and peripheral devices.
A significant advantage of the EN 61000-X standards is that they reflect the impact of mobile communications functioning in the frequency range up to 2 GHz. Clearly, contemporary electronic systems must meet immunity requirements so as to withstand radiated RF fields in the frequency range of 80 MHz to 2 GHz. (See Table 2.)
Standard EN 61131-2: 1994 is clearly outdated, and its use in the EMC assessment of PLCs per the EMC Directive 89/336/EEC may result in erroneous conclusions regarding the true electromagnetic compatibility of the device under test. Standard EN 61326: 1997 does not take into account the specific structural characteristics of PLCs and should not be used in assessing devices that must function in severe industrial environments.
EN 61131-2:2003 is the contemporary standard with sufficient specificity to deal with PLCs operating in varying electromagnetic environments. It takes into account the many sources of EMI that can impact a PLC. Its use in the development phase of new PLCs is preferable as it will likely forestall any need for redesign after January 1, 2006.
EN 61131-2:1994+A11:1996+A12:2000 Programmable controllers Part 2: Equipment requirements and tests
EN 61131-2:2003 Programmable controllers Part 2: Equipment requirements and tests
EN 61326:1997+A1:1998+A2:2001 Electrical equipment for measurement, control and laboratory use – EMC requirements.
EN 55011:1998 Limits and methods of measurement of radio disturbance characteristics of industrial, scientific and medical (ISM) radio-frequency equipment
EN 61000-4-2 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques Section 2: Electrostatic discharge immunity test
EN 61000-4-4 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques Section 4: Electrical fast transient /burst immunity test
EN 61000-4-5 Electromagnetic compatibility (EMC). Part : Testing and measurement techniques Section 5: Surge immunity test
EN 61000-4-6 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques. Section 6: Immunity to conducted disturbances, induced by radio-frequency field
EN 61000-4-8 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques Section 8: Power frequency magnetic field immunity test
EN 61000-4-11 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques. Section 11: Voltage dips, short interruptions and voltage variations immunity tests
EN 61000-4-12 Electromagnetic compatibility (EMC). Part 4: Testing and measurement techniques Section 12: Oscillatory waves immunity test