Trapped-key interlocking system HST: Safety of machinery
On the other hand, the machine must only be re-set into operation, if all openings and doors are closed, and nobody is present inside the machine. Such guard locking devices are for instance key-operated mechanical interlocks, also called trapped-key systems. They are based on the easy-to-understand premise that a key cannot be in two places at the same time - it can for instance be inserted in a switch OR be used for opening a guard. Another safety-relevant feature is that a key can only become free in a safe condition, when there are no hazards - a switch is turned off, a guard interlock is closed and locked. The big advantage of the system is the fact that a certain sequence of steps can be pre-determined when designing the system. It is not possible to deviate from this sequence - thus a high degree of integrity is achieved. So virtually any hazard can be eliminated. The system consists of a central electrical part (for instance in the control panel) and mechanical units at the guards. Therefore, the devices are very often used in areas, where due to space conditions or other environmental conditions and explosion hazards electrical components are not suitable or expensive to integrate. The devices can be supplied with individual codes (engraving up to 5 digits). Thus a higher degree of safety and a better protection against tampering is obtained.
- high degree of safety, individual codes
- only pre-determined secure safety sequences are possible
- no spare actuator 'in the pocket'
- no cables to individual safety doors (cost savings, fault prevention)
- easy retro-fitting
- door interlocks have a locking feature
- particularly suited when there are different energy types on the machine (electric, pneumatic, ...)
- in areas where people can be trapped: personal safety key
- simple option for realizing safety conditions for setup
- time delay due to key transfer between the switch and interlock