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14/04/2011
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Tom Shelley reports on a serious problem that arises when users install light curtains without knowing what they are doing.
 Machine light guides can give a false sense of security, if their failure does not result in turning the machine off. Furthermore, even if they are of a failsafe type, they can still be worse than useless if they are either too close to the danger area, so that in the event of an accident involving somebody falling into the machinery, there is insufficient time to stop it, or if there are ways of body parts getting between beams without triggering a shutdown.
Nonetheless, there are too many installations, according to Gary Trewhitt, operations director at Safety Systems Technology, part of TÜV SÜD Product Service, which are 'protected' either by the wrong kind of light curtain or by the right kind of light curtain installed in the wrong kind of way.
Because the beams are infra red and cannot be seen, a malfunctioning or incorrectly selected system can result in workers being at greater risk of serious injury of death than if there were no guards at all, because they may approach dangerous parts of machinery more closely than they would if they were unprotected, believing that the light curtain system is protecting them.
The difference is because there are both safety sensor systems that are SIL – Safety Integrity Level 4, and SIL 2. SIL 4 means that the sensors constantly diagnose themselves, and if they are not working, they send a stop signal to the machine, and then go into a lockout state until they have been put right. A SIL 2 system, on the other hand, only stops the machine if it detects something.
SIL 2 light curtains and other sensing systems, look very much the same as SIL 4 devices, but are, of course, substantially cheaper. A light curtain that does not diagnose itself may be perfectly satisfactory within a manufacturing process, if it is used to trigger actions within the process, or if the hazard is minor, but if it is there to protect humans from serious injury or death, it is inappropriate to the task.
Equally, it is no use purchasing a SIL 4 light curtain or other sensor and trying to wire it into a system where there is no provision for a malfunctioning light curtain preventing the functioning of the main machine. If purchasing a system from a reputable company, they will almost certainly go to some trouble to ensure they are applying an appropriate solution.
The problems usually come, he said, when people purchase replacements from a general catalogue or sources on alibaba.com, without really knowing what they are doing. The trick is, to think through the whole system and ensure that if the safety system fails, the machinery shuts down and cannot be made to go again until the safety system is fixed. And in addition, there should be no easy way that fingers or whole persons should be able to get past the safety system before the machinery can be stopped.
For these reasons, Trewhitt said that the trend is increasingly for customers to ask Safety Systems Technology to, "Project manage everything". The company will then do physical tests on the safety system to ensure that it conforms to all the latest requirements of the Machinery Directive and really does protect operators in the way it should. This includes tests with a plunger on light curtains to simulate fingers or body parts entering the guarding system at appropriate speeds.
Design Pointers
• Light curtains guarding dangerous machinery should always be SIL 4, ensuring that they shut the machine down if they malfunction
• Installing SIL 4 light curtains without the right interlocking circuitry in the machinery can lead to an equally disastrous result
• SIL 2 light curtains have their uses if the hazards are minor and within manufacturing systems to control processes, but should not be relied on to protect human operators from possible serious injury or death
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Author
Tom Shelley
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| | | Added: 20/04/2011 | 09:18 | | |  | I like this article, it is definitely something that manufacturers should be aware of. However, there are as far as I'm aware no SIL 4 light curtains. Although SIL 4 is defined under IEC 61508 it is not recognised under EN 62061 which is the machinery specific standard and only defines up to SIL 3.
Perhaps Type 2 and Type 4 light curtains as defined under EN 61496 are what the author is actually discussing? These types of light curtain certainly align with the light curtain variants described; and it is also true that if used in the wrong applications they could be very dangerous.
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