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High safety demands are stated in the standards EN 61 496-1 and prEN 61 496-2 which deal with optical protective devices. The main demands are on a safe stopping function and that light from light sources other than the transmitter or other disturbances do not affect the safety function.
Depending on how the safety function is built up there are safety components of type 2 and 4 according to EN 954-1.
Type 4 which has the highest safety level, states that a fault is not allowed to affect the safety function and that the fault shall be detected by the outputs opening immediately or that they do not reconnect after being disconnected. Maximum allowed aperture angle for the light is ± 2°.
Type 2 states that a simple but monitored safety function is required, which means that the safety function shall be monitored through periodic tests which open the output when a fault occurs. Between the testing times there can be faults which result in the safety component malfunctioning. The test function can either be built into the safety device or an external unit (e.g. the machine’s control system) can initiate a test. Maximum allowed aperture angle for the light is ± 4°.
Light curtains and light grids are included among the products in the machine directive’s appendix 4, which means that an external certifying procedure with an officially recognized institution is called for.
Light Grids for Passable Protection
Both light curtains and light grids can often be used as passable protection into a risk area. This is called passable protection because it is possible to get in behind the safety device. Common applications are robot installations, openings for in and out passage of material, etc. The choice between light curtain or light grid is often a question of available safety distance, reach and price. Light curtains are often chosen for short safety distances light grids are chosen for long reach - up to 50 m - and for a low price.
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