Trapped Key Interlocks for isolating multiple energy sources
Isolating dangerous energy to safely operate a machine during maintenance or production operations can be complex and difficult to enforce, especially when different energy sources need to be isolated.
Electrical hazards can often be quickly identified, but what about hydraulic and pneumatic energy? Is switching off these energy sources dangerous and are residual energy sources such as gravity and pressure taken into account?
Lockout-Tagout procedures are designed to reduce these risks, but there is no guarantee that employees will isolate and lock the energy sources in the right positions. Lockout-Tagout is an administrative procedural task, which must always rely on the correct actions of those involved. When properly implemented and maintained this can be effective, but it depends on different factors.
Hierarchy of Controls
Almost 50 years ago, the British Standard BS5304, Safeguarding of Machinery, was created, which was the start of many guidelines in the machine safety standard. One of the examples mentioned in this standard is the rubber mixer application.
In the vast majority of machine safety applications there are more access points to lock than that there are energy sources, but in case of the rubber mixer this is the other way around.
This makes it an excellent example for isolating energy sources, which consist of: two electrical circuits, pneumatics, hydraulics and stored energy. It demonstrates the complexity of ensuring that all energy sources are locked in the correct position and it ensures safe access for all tasks.
Nearly 50 years later the same example is being used in the revision of ISO 14119 Safety of Machinery – Safety Interlocking devices, safety interlocks in combination with shields, principles for design and selection.
Rubber mixer with overview of access points and energy sources.
The schematic shows a trapped key interlock system for a rubber mixer application
Keys are released from an interlock system only when the respective energy source is isolated. A hazard is created by removing the hydraulic energy source to the ram and it must be held and locked in position before entry to the machine.
When there is stored energy, a time delay ensures that the key only can be released after the energy has been dissipated.
All the keys associated with energy sources must be entered into the key exchange unit before the 4 keys are released to allow entry to the safeguarded area.
Unlike lockout-tagout procedures, a key interlock system requires all energy sources to be isolated and locked before it is possible to access the secured area. This makes this system a technical check instead of an administrative check.