Principles, Standards and Implementation

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Safety Strategy

Protective Systems and Measures

If access is required, then life becomes a little more difficult. It will be necessary to ensure that access can only be gained while the machine is safe. Protective measures such as interlocked guard doors and/or trip systems will be required. The choice of protective device or system should be heavily influenced by the operating characteristics of the machine. This is extremely important as a system that impairs machine efficiency will render itself liable to unauthorized removal or bypassing.

The safety of the machine in this case will depend on the proper application and correct operation of the protective system even under fault conditions.

The correct operation of the system must now be considered. Within each type there is likely to be a choice of technologies with varying degrees of performance of fault monitoring, detection or prevention.

In an ideal world every protective system would be perfect with absolutely no possibility of failing to a dangerous condition. In the real world, however, we are constrained by the current limits of knowledge and materials. Another very real constraint is cost. Based on these factors it becomes obvious that a sense of proportion is required. Common sense tells us that it would be ridiculous to insist that the integrity of a safety system on a machine that may, at the worst case, cause mild bruising to be the same as that required to keep a jumbo jet in the air. The consequences of failure are drastically different and therefore we need to have some way of relating the extent of the protective measures to the level of risk obtained at the risk estimation stage.

Whichever type of protective device is chosen it must be remembered that a "safety related system" may contain many elements including the protective device, wiring, power switching device and sometimes parts of the machine’s operational control system. All these elements of the system (including guards, mounting, wiring etc.) should have suitable performance characteristics relevant to their design principle and technology. IEC/EN 62061 and EN ISO 13849-1 classify hierarchical levels of performance for safety related parts of control systems and they provide risk assessment methods in their annexes to determine the integrity requirements for a protective system.

ISO 13849-1:2006 provides an enhanced risk graph in its Annex A. This graph is shown in Figure 19.

Figure 19: Risk Graph for Determining the Required Performance Level for a Safety Function—from ISO 13849-1:2006

IEC 62061 also provides a method in its Annex A, it takes the form shown in Figure 20.

The use of either of the above methods should provide equivalent results. Each method is intended to take account of the detailed content of the standard to which it belongs.

In both cases it is extremely important that the guidance provided in the text of the standard is used. The Risk Graph or Table must not be used in isolation or in an overly simplistic manner.

Figure 20: Table for Determining the Required Safety Integrity Level for a Safety Function—from IEC 62061