The Applicability of the EMC Directive to Machinery Safety Assessment

For use in establishing conformity with EHSR 1.5.10 of the MD concerning radiation, very few product safety standards covering particular types of machinery now exist, but these are being written to cover all the EHSRs. Those that have been published do not, in most cases, address the hazards of electromagnetic radiation adequately. In some standards, for example, provisional standard pr EN 1807, Bandsaw Machines, Clause 5.12, Radiation, the clear statement "Not significant" shows that the hazard has been considered. But in others, for example, EN 289, Compression and Transfer Moulding Machines, nowhere does it say that the radiation hazard is not significant. The product safety standard for electrosensitive protective equipment (ESPE), EN 61496-1, on the other hand, covers emissions by cross-reference to the EMC requirements of EN 50081-2, the generic standard for emissions in the industrial electromagnetic environment, and includes tests according to EN 55011.

Where there is no reference to the radiation hazard in the product safety standard for a particular class of machinery, or where no such standard has been published yet, the accepted technique is to use the so-called horizontal generic machinery standards that cover general principles of all machinery. The recently republished EN 60204-1, Electrical Equipment of Machines, covers the radiation hazard adequately in Clause 4.4.2, EMC. The standard does so by referencing not only the requirements of EN 50081, but also the specific requirements of product standards, which brings in the risks of specialty machines that use hazardous RF energy, such as x-rays.

A European horizontal machinery safety standard also exists in draft form—pr EN 12198-1, Assessment and Reduction of Risks Arising from Radiation Emitted by Machinery (95/715738 DC). This provisional standard was intended to give advice to technical committees drafting product standards, but it is the best reference available for use by machinery assessors as well. It classifies machinery according to radiated emission levels and provides advice on exposure times, protective measures, and user instructions. The standard complements MD EHSR 1.1.2, Principles of Safety Integration, and the EN 292 series, and provides information for application at the product design stage. Consequently, it does not address test methods, which will be addressed by pr EN 12198-2, Radiation Emission Measurement Procedure (99/715918 DC), when it is published.

Therefore, in the absence of manufacturers' test results, the use of a handheld radiation hazard monitor to check field strengths in volts per meter at the operator position of the machinery is recommended. Test methods in EN 55011 and EN 55022 are not appropriate for hazard monitoring (remember the mobile phones) for two reasons: first, the test levels, designed for detecting emissions that would interfere with telecommunications, are very much lower than hazardous radiation, and therefore the test instruments and methods are inappropriate for discovering safety risks. Second, the test methods in the standards are restricted to particular frequency bands, so hazardous emissions outside of them would remain undetected.

Another useful reference, especially useful at the higher frequencies up to 300 GHz that are not covered by the EMC Directive, is a U.K. National Radiological Protection Board document, Restrictions on Human Exposures to Time Varying Electromagnetic Fields and Radiation (Documents of the NRPB Vol. 4, No. 5). And finally, there are two relevant European interim standards: DD ENV 50166-1, covering possible effects of human exposure to low-frequency electromagnetic radiation between 0 Hz and 10 kHz, and DD ENV 50166-2, doing the same for 10 kHz to 300 GHz.

Immunity (EHSR 1.5.11 of the MD). The immunity requirements of electrical equipment for conformity with the requirements of the EMC Directive are specified in

• EN 50082-2: Generic immunity standard for the industrial environment.
• EN 55024: Product immunity standard for specific types of equipment.
• EN 50082-1: Generic immunity standard for the light industrial environment.

Naturally, only the electronic components of machinery are vulnerable to interference from electromagnetic radiation, particularly NC/CNC/PLC control systems, safety circuits, and solid-state electronics.

The European machinery safety standard EN 954-1, Safety Related Parts of Control Systems, General Principles for Design (as well as pr EN 954-2, covering particular requirements and tests), treats immunity requirements thoroughly by cross-referencing the test methods of the EN 61000-4 series of EMC test specifications. Worst-case test levels are taken. This satisfies the immunity requirements of the horizontal machinery safety standard EN 60204-1 dealing with electrical equipment, which refers in Clause 4.4.2 to the generic EMC immunity standard for the industrial environment, EN 50082-2, which, in turn, cross-references the same test methods in the EN 61000-4 series of standards.

Similarly, the product safety standard for ESPE (mentioned above under "Emissions") also covers immunity in detail by cross-reference to the same tests of the IEC 61000-4 series of standards. As in the case of radiation, however, not all machinery safety standards adequately cover the immunity requirements of the MD. It is then necessary to select generic machinery standards for control systems and safety circuits, as noted above.

A Pragmatic Approach for Third-Party Assessors

Emissions (EHSR 1.5.10 of the MD). Very few machinery designs result in electromagnetic emissions so strong that they might adversely affect the health of exposed persons. If a radiation hazard does exist, it will be obvious, and the manufacturer will have taken steps to identify the degree of hazard and to eliminate or at least minimize the risk to the operator. In most cases, however, the radiation hazard will not be significant and EHSR 1.5.10 will not be applicable.

Where the radiation hazard may be significant, for example, with microwave heating and high-frequency curing systems such as glass- and carbon-fiber fabrication machinery, it is very likely that the manufacturer will have included test results in the technical file. These test records can be verified by reference to the emission standards and other documents mentioned above. Field strengths can then be checked, if necessary, using a handheld radiation hazard monitor.

Immunity (EHSR 1.5.11 of the MD). Here, too, as with assessment of emissions, machinery can be classified effectively into two groups: those machines whose design includes the intentional use of electromagnetic energy to achieve a process and those—the vast majority of machines—that are unintentional radiators.

Considering first equipment that radiates electromagnetic energy unintentionally, the machines must continue to operate safely despite the effects of externally generated radiation. The MD here strongly resembles the EMC Directive, which requires the machinery to continue to operate as intended. Technical committees have chosen test levels deemed appropriate for standards covering the industrial environment and providing a presumption of conformity with the EMC Directive. The question is whether or not these levels are sufficient to meet EHSR 1.5.11 of the MD. Here lies the only difference in the immunity requirements of the two directives: the field strengths to be withstood. Naturally, there is a chance the machine is installed near a transmitting station or that a mobile radio transmitter is being used immediately alongside the machine's PLC. In these situations, the field strengths could be higher than the EMC test levels. However, means of preventing such situations other than by providing immunity—for example, instructions and marking—are available under the MD. It should therefore be acceptable to a third-party assessor that the parts of a machine that are vulnerable to malfunction through interference have been tested to the requirements of the EMC Directive and that instructions and procedures designed to ensure control of the electromagnetic environment are in use. The manufacturer's declaration of conformity with the EMC Directive and associated conformity assessment records in the technical construction file do therefore provide evidence of conformity with EHSR 1.5.11 of the MD, when supplemented by control procedures.

On the other hand, equipment designed to use RF energy will need to be immune from its own interference, and the field strength would be higher than expected in the normal industrial environment. EMC immunity testing, however, would already have covered, among other frequencies, the single frequency being generated by the machine by virtue of the fact that it operated as intended despite the potential interference from its own RF energy. Consequently, the effect of the RF energy used by the machine on conformity with MD EHSR 1.5.11 is no different from its influence on conformity with the immunity requirements of the EMC Directive; where the latter exists, it proves the former.

In many cases in which the relevance of EHSR 1.5.11 is not immediately apparent, it may be unnecessary to prove conformity directly with that essential requirement. A thorough engineering design analysis of the electrical circuits may reveal that external radiation cannot cause an unsafe situation, thus showing that the EHSR is not applicable. The designer may have deliberately decided not to provide immunity but rather to use electromechanical devices not susceptible to interference. To complete the risk analysis of the interference hazard, the designer would have had to complete a single fault analysis to investigate the outcomes of failures (failure mode effects analysis). This should be recorded in the technical file along with any measures taken to eliminate the risk by design, such as employing electromechanical rather than electronic or semiconductor devices.

A number of other EHSRs are closely linked with EHSR 1.5.11, including EHSRs 1.2.6 and 1.2.7 concerning reinstatement of lost power and failure of the control circuit. These address the symptoms of faults that could have been caused by interference. They both include the detailed requirement that the machine must not start unexpectedly, which is a requirement one might expect to have been repeated under EHSR 1.5.11 concerning immunity. In most cases, the design will provide conformity under more than one EHSR, including 1.5.11, and so conformity with EHSR 1.5.11 will not need to be reestablished. For example, the use of a latching-in or key-operated stop-control device specified as one of the options in another European machinery safety standard, EN 1037, Prevention of Unexpected Start-Up, contributes to conformity with both EHSR 1.2.7 and 1.5.11.

There are other examples of the applicability of EHSR 1.5.11 being diminished through its logical links with other EHSRs. A particular instance is EHSR 1.2.1, Safety and Reliability of Control Systems, with its focus on the systems' ability to withstand the rigors of normal use and external factors. Electromagnetic interference is one of these external factors, thereby creating the link between EHSRs. Furthermore, if the control system is required to be fail-safe, and this capability is incorporated in the design, why should the cause of the control failure matter, whether or not it is electromagnetic interference?

More often than not, it is possible to show conformity with EHSR 1.5.11 without reference to immunity test results by demonstrating that the requirement is not applicable as a result of the control system being designed and the components selected in such a way that radiation effects could not lead to hazardous malfunctions.  

Conclusion

The requirements of the Machinery Directive and the EMC Directive differ for some types of machinery and installations. In the case of machines that do not emit hazardous radiation, low-level emissions that might interfere with other apparatus are irrelevant to the safety requirements included in the EHSRs of the MD. On the other hand, in the rare case of a design that involves intentional use of RF energy to achieve a process, conformity with the EMC Directive may not satisfy the safety requirements of EHSR 1.5.10 of the MD concerning emitted radiation levels for parts of the spectrum not considered by EMC test standards (e.g., microwaves, x-rays, and ionizing radiation). Any obvious risk associated with a radiation hazard will undoubtedly have been considered by the designer during the risk analysis, and the manufacturer will have included conformity assessment records in the technical file. The immunity requirements of EHSR 1.5.11 of the MD will have been proven for these uncommon machines by the fact that the sample operated as intended during EMC testing despite the intramachine effects.

The main difference in the immunity requirements of the two directives concerns the possible field strengths that might have to be withstood. The possibility that handheld transmitters might be used alongside the machine controller cannot be ignored; then, the field strengths would be much higher than the immunity limits of the EMC Directive test standards. However, local control procedures, warning notices, training, and user instructions can be provided to minimize the risk. Conformity with EHSR 1.5.11 can usually be achieved pragmatically without test results by fault analysis of the control system and safety circuits and their components. Conformity is argued in the technical file from the evidence of conformity to other EHSRs. If the control system is fail-safe, it does not matter under the MD whether electromagnetic interference causes it to fail, despite noncompliance with other legislation, such as the EMC Directive and that pertaining to product liability.  

Tony Leathart is product-safety manager of Technology International (Europe) Ltd. (Shrivenham, Wilts, UK), a notified body under the Low Voltage and Machinery Directives, and a competent body under the EMC Directive. He may be contacted at tony.leathart@iti.co.uk.