Protective Devices: How to calculate safe distance

Protective Devices

Protective devices (electrosensitive protective equipment, abbreviated to ESPE below) are always used when access to the corresponding hazard zone is to be particularly easy to achieve and there are no hazardous repercussions to be anticipated from the machine itself (example: welding or grinding processes). To ensure that a potential hazard can be shut down quickly enough, the protective device must be installed at an appropriate distance.

This distance or safety distance (S) is defined in EN 999 and depends in particular on the following factors:

·         t1 = Response time of the protective device itself.

·         t2 = Response time of the machine, i. e. the machine's stopping performance in response to the signal from the protective device

·         C = Potential approach towards a danger zone undetected by the protective device, e. g. reaching through two beams of a light curtain undetected, depending on the distance of these beams

·         K = Anticipated approach speed of the human body or parts of the human body. This factor is defined in EN 999 as 1.6 m/sec for walking speed and 2 m/sec for hand speed

The distance to be implemented is therefore

S = K* (t1 + t2) + C

If the ESPEs form horizontal or inclined protected fields above an accessible area which requires safeguarding, the fields must be positioned at a minimum height, as predetermined by the application and ESPE. Here too, the safety distance between the outer edge of the protected field and the danger point to be safeguarded should be such that the possibility of injuries resulting from the hazardous movement in the danger zone is excluded, bearing in mind the machine’s stopping performance.