Entrapment risk: better to prevent than to cure
Every year in the Netherlands there are approximately 280 accidents involving machines, 73% of which result in permanent injury. In almost all cases there is something wrong with the physical shielding. This immediately demonstrates the importance of physically protecting hazardous areas and safely regulating access to them.
Work often requires operators to get close to or into machine parts with body parts, it is of course essential in these cases to prevent unintentional restart, especially as in 10% of all accidents the machine is initially switched off. Preventing unintentional restarts can therefore prevent 28 cases of permanent injury per year.
ISO 102018-2: “Robots and Robotic Devices” pays attention to manual start/restart and unexpected restart in section 184.108.40.206. The standard describes the measures to be taken to prevent start/restart in case of full body access as follows:
“To ensure that no one is in the protected area before starting/resetting, use the first feasible option from the following list”.
- Provide a clear and unobstructed view of the protected area from the start/reset device.
- Use presence detection devices to detect operators in the protected area.
Provide other protective measures:
- Means to isolate and interlock the hazardous equipment
- Measures to lock movable guards in the open position
- Additional time-limited reset devices located in the protected room
- Audiovisual pre-start warning signal that can be heard inside and gives sufficient time to get out
Unobstructed view from the restart or start position is desirable where possible, but often unfeasible. The industrial equipment in the protected area (e.g. robot lines, conveyors, etc.) often covers a large area with multiple access doors, making visual inspection of the entire area impossible from a single point.
Integrating start/reset into the safety interlock at the access doors does contribute to a better inspection possibility.
Presence detection of the restricted areas such as; safety laser scanners, pressure mats or safety radar like the safest solution. However, implementing safe presence detection is difficult in many cases, for example due to the layout of the area, external environmental factors or the fact that optical safety sensors do not meet the required safety level.
In addition, it must be taken into account that operators must be able to be detected from every angle, which entails high investment, failure and difficult adaptability.
Isolation and interlocking of energy sources and residual energy, or Lockout-Tagout-Tryout (LOTOTO), can certainly prevent unintentional restart, but instead of only switching the safety contacts, the entire main power supply must be isolated, and locked with a safety padlock by every operator in the zone. LOTOTO is therefore mainly used during maintenance work, since the impact on tool life and thus on production is often too great for short-term inspection and cleaning work. Another disadvantage of LOTO is that it is not a compulsory but a completely procedural way of ensuring safety, in which mistakes can easily be made.
Locking in the open position of access doors ensures that the safety contacts of the installed safety switch cannot be switched and that the machine cannot be restarted. It must therefore only be possible to deactivate this lock-out protection by the operator carrying out the work in the protected zone. A simple way to achieve this is by providing safety switches with a coded safety key or by retro-fitting a fully mechanical interlock blocking device as an addition to an existing safety switch.
“Entrapment safety devices are a cost-effective low-threshold solution for preventing containment and unintentional start/restart”.
Time-delayed reset devices, this means a pre-reset in the protected zone that prevents resetting from outside the zone as long as the pre-reset button in the zone is not activated. This pre-reset allows the operator to leave the protected zone with a programmed delay.
Audiovisual pre-resets, such as a signal light with siren, inform the operator of an upcoming machine restart, giving him time to leave the zone in time. However, this is only a supporting tool that is ineffective when the operator is, for example, unwell or wearing hearing protection.
Pro-Active or Re-Active Protection ISO 12100 (2010) describes in section 220.127.116.11 the measures to be taken for escape and rescue of trapped persons:
If the risk analysis shows that there is a risk that an operator could become trapped in a restricted area when using controlled access (lockable safety switches), the operator would leave the area at all times in the event of a calamity or unintended machine start-up. It could also be the case that the employees have to leave the area via a different access point than when they entered, such as in case of fire.
When looking at the ISO 12100, providing an escape route by using the examples mentioned in ISO 14119 (2013) section 5.7.5 such as an emergency release or escape route release is a common measure to prevent entrapment, however, we are only talking about re-active protection that allows the space to be escaped or gives third parties the opportunity to offer help. A pro-active solution, on the other hand, prevents the possibility of unintended machine start-up/re-start-up.
The American ANSI B11 Standard better clarifies the distinction between pr-active and re-active and even offers practical solutions:
“Preventing a machine from being reset, guaranteeing safety within a safeguarded area”.
Proactive Inhibit Function
- Interlock Blocking
- Latching Buttons
Reactive Inhibit Function
- Escape Release
- Emergency Stop
In summary, we can conclude that the state of the art certainly offers solutions to prevent containment and unintentional restart. The choice for the right solution depends on several factors and should be based on a risk analysis.
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