Trapped key interlocking ensures that a process is followed and cannot be circumvented or short cut. The transfer of a key ensures that wherever personnel find themselves, in either starting or shutting down operations, they can be assured that they are safe.
There are three simple steps in using trapped key components in an integrated safety system, what is being isolated, how many access points are there and what type of access is required.
A key is used to start the process and remains trapped whilst the machine is running. The only way to remove the key is to isolate the hazard. This (isolation) key is then trapped in either a key exchange box for release of multiple keys or directly in an access lock which retains the key and allows safe access to the dangerous area. This key remains trapped in position while the gate or door is opened and can only be removed when the gate or door has been shut. In this way the key is either trapped when the machine is running and access cannot be gained, or the key is trapped while access is gained and the machine cannot be started.
To design interlock components into an integrated safety system there are a number of key questions that need to be addressed.
• The operational flow to start and stop equipment
• What is being isolated
• The number of systems that need to be isolated to make access safe
• Time delay requirements for safe access
• The number of access points
• The type of access (full body or part body)
• The severity of the possible injuries
• The possibility of avoiding the hazard
• The nature of the hazard
• The present energy sources
• The operating environment
• Use of risk assessments as a guide to examine how the integrated safety system functions