Example of a Trapped Key Interlock System in the Control of Multiple Energy Sources

Lockout Tagout programs are designed and implemented to mitigate risk. However there is no guarantee that employees de-energise and lock energy sources off.

Moreover, Lockout Tagout is an administrative control, so there is a reliance on human behaviour. Whilst it can be functional if managed effectively, procedures can still be bypassed where multiple energy sources exist.

Almost 50 years ago work began on a British Standard, BS5304, Safeguarding of Machinery, which was to lead the way in machinery safety standards, and one of the examples featured was the internal rubber mixer. In the vast majority of applications there are typically more access doors than energy sources, but in the case of the internal rubber mixer example this was reversed.

It is an excellent example for the control of hazardous energy because with two electrical, a hydraulic, pneumatic, and stored energy sources, it demonstrates the complexity of ensuring energy is being used in the revision of ISO14119 – Safety of Machinery – Safety Interlocking devices associated with guards.

Keys are released from trapped key interlocking devices when each energy/power source is reduced to zero. 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. Where stored energy is present a time delay ensures the release of a key only after the energy has dissipated.

All of the keys associated with energy sources must be inserted into the key exchange device before the four keys are released to allow entry to the safeguarded area. A trapped key interlock system enforces the isolation sequence and guarantees energy sources are locked off before access to the safeguarded space is granted. This is an Engineering Control and does not rely on user remembering to carry out lockout procedures.