26 Nov 2019

Light curtains: a deceptive feeling of security

They are used in every industrial production plant and most people appreciate them very much. And rightly so. Light curtains and other optical protective devices from the category of active opto-electronic protective devices (AOPD) offer many advantages. They often reliably secure the so-called machine operator interfaces, guaranteeing good process times and simple handling.

Light curtain

However, AOPDs give a false sense of security when incorrectly selected, designed or dimensioned. Machine operators see a modern protective device and weigh themselves on the safe side, thinking the light curtain will switch off the machine already. Experience has shown us that a great deal is done wrong in practice, especially with optical protective devices. Whether an AOPD may be used at all must be clarified in the course of the risk assessment (ANSI B11.0) and the identified hazards. First, the alignment, the possibility of bypassing and the height of the protective field of the protective device must be determined. In the next step, the stop time of the hazardous machine movement must be calculated in accordance with ANSI B11.19 and then the resulting distance of the protective device from the hazard must be determined. An optical protective device may only be used if this minimum distance is maintained, bypassing of the protective device can be ruled out and standing undetected between the AOPD and the hazard is not possible. In all other cases, additional measures are required.

The question of who is responsible for the professional design of the optical protective devices and for permanent safety over the entire lifespan of a machine cannot be answered so easily. In Europe, for example, the manufacturer declares that his machine conforms to all applicable legal requirements and complies with the state of the art, when placing the machine on the market for the first time. So far so good.

With increasing machine age, however, the switch-off time of the machine can also change and as a result the originally calculated safety distance may no longer be sufficient. This can and must be avoided, but how? The employer must ensure a safe workplace, this applies in the USA and in Germany. With AOPDs, a follow-up time measurement during machine acceptance and follow-up time measurements recurring at regular intervals are absolutely essential. Only this way the permanent effectiveness of the protective device and the protection of the employees can be guaranteed permanently.

The use of active opto-electronic protective devices is and will remain a user-friendly and safe solution if correctly designed. However, operating companies should be aware of their responsibility and should always be critically questioning the choice of AOPDs.

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