November/December 2019

32 n MACHINE SAFETY November/December 2019 www.drivesncontrols.com Protective sensors: two beams or three? I n November 2010, a new European standard replaced the earlier EN 999:1998 standard for the light beam safety devices that guard access to machines and systems. The“new”harmonised standard EN ISO 13855 describes the required arrangement for these protective devices. Among other things, this standard defines beamheights designed to prevent persons from either crawling under, or stepping over, a protective field. The bottombeammust not be installed higher than 300mm above a reference plane and the top beammust not be installed lower than 900mm above the reference plane. A table with the predefined combination of number of beams and beam height that appeared in EN 999 was moved to informative appendix E of EN ISO 13855. Technically, the table is no longer part of the standard. According to the latest standard, two-beam safety light barriers are no longer regarded as being state-of-the-art. But can they still be used on newmachines? And, for existingmachines, do two-beam systems have to be replaced by three-beam systems? The document provides recommendations in this regard. Why, according to EN ISO 13855, are two- beam protective devices normally insufficient? A two-beam system has a beam spacing of 500mm. If crawling under the lower beam is to be prevented, as specified in the standard, then the beammust be at a height of ≤ 300mm. The upper beam is then at a height of ≤ 800mm. However, at this height, it is possible to step over a beam. If the device is installed to prevent people stepping over the upper beam, then this beammust be at a height of ≥ 900mm. The lower beam is then at a height of ≥ 400mm, allowing somebody to crawl underneath. By comparison, a three-beam system automatically satisfies both conditions, as shown in the diagram. So are there any areas where it is still possible to use two-beam systems, according to the standard? Appendix E of EN ISO 13855 contains the following note:“A beam height of ≤ 400mm for the bottom beam…may be used only if the risk assessment permits this.” For a specific case, the user must therefore be able to argue, on the basis of a risk assessment, why a two-beam protective device would be sufficient for the location. But don’t expect EN ISO 13855 to provide a supporting argument. EN ISO 13855 is what is known as a Type B standard. Type C standards apply to specific types of machine and are not transferable to other types of machine. They have priority over type B standards. In the case of content- related deviations between type B and type C standards, the content of the type C standard applies, even if the most recent type C standard does not reflect the current state-of- the-art. Type C standards are also revised at regular intervals, which generally means that changes must be expected to the current content of type B standards. Another factor that manufacturers and integrators, in particular, need to consider is any requirements resulting from the European Machinery Directive 2006/42/EC. In Appendix I Sect. 1, the Directive states that:“The manufacturer of a machine, or the manufacturer's authorised representative, shall ensure that a risk assessment is carried out in order to determine the health protection and safety requirements applicable for the machine.”The machine must then be designed and constructed taking the results of the risk assessment into consideration. According to the Directive, therefore, there is technically no obligation for manufacturers or integrators to refer to, or comply with, a standard such as EN ISO 13855. However, compliance with harmonised standards does offer legal benefits. And what do the requirements of the European Use of Work Equipment Directive 2009/104/EC mean for users? According to this Directive, employers are obliged to provide safe work equipment. For this purpose, the hazards posed by the work equipment must be assessed at regular intervals. If the assessment identifies a hazard, the protective measures need to be adapted accordingly. Conversion from a two-beam safety light barrier to a three-beam system is therefore necessary if the result of the hazard assessment indicates this. So, in summary then, two-beam safety light barriers are no longer state-of-the-art. They should be used, or continue to be used, only if a risk assessment, hazard assessment or the most recent type C standard for the used machine type allow this. Safety barriers using multiple beams of light are widely used to protect access to machines. A new standard that came into force a few years ago has changed requirements for these devices. Does it mean that two-beam light barriers are no longer allowed for existing machines? This article from Leuze Electronic explains the current situation. The standard covering light beam safety devices specifies both the height of the top and bottom beams, and the spacing between the beams