Safety and reliability

Safety professionals are normally aware of what is safe and unsafe, but we also need to consider the reliability of the safety measure.  For example, a moveable guard over a hazardous area is unsafe if there is no interlock with the control system.  If there is such an interlock, it is safe, but what about the reliability of the interlock?

Let’s just look at safety and reliability. When I started to drive, cars had a single circuit braking system.  Leakage from anywhere in the circuit would cause loss of braking on all four wheels and the only recourse was use of the cable-operated handbrake.  Nowadays, cars have dual circuit brakes with cross-checking and a dashboard light that shows if there is a problem, long before you lose the ability to brake.  A properly maintained single circuit system was not unsafe, but the modern systems give a high level of reliability to that safety.

The requirements for reliability of safety systems is covered in EN 13849-1.  This is not particularly readable, so for my own use and the benefit of others, I provide a summary of this in http://www.strategicsafety.co.uk/pdf/Technical-Paper-4-EN13849.pdf .

Using the steps in this standard, you can take into account the severity of the outcome (S), the frequency of exposure (F) and possibility of avoiding the hazard (P) to arrive at a performance level for the interlock.  This standard is required for new machines, but this analysis is invaluable when deciding what to do about existing risk controls.

One error I often see is employers failing to check that interlocks continue to work and the EN13849 approach can help you decide a checking regime for these.  Note that, in my opinion, it is better to carry out such checks thoroughly less frequently, rather than have people carry out perfunctory checks every day.  However, it does depend on the S+F+P outcome.

One word of caution when assessing reliability. If the interlock is wired as an input to the PLC, then you have to take into account the reliability of the PLC and its software.  Interlocking independent of the PLC is normally far better.

Therefore, the actions you need to take are:

• Carry out the S+F+P analysis.
• Decide if existing safety provisions are reliable enough
• Put into place suitable checking systems or upgrade the interlock system if necessary.

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