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FAQ of the Day

Question:
How common are Category 4 safety system requirements and how can they be satisfied?

Answer:
Category 4 safety system requirements are typically associated with extremely high-risk applications in which:

• The severity of a potential injury is extremely high (eg. amputation or death).
• The employee/operator is exposed to the hazard highly frequently or continuously.
• There is little possibility of the employee/operator avoiding the hazard.

Classic safety hierarchy states that dangers should be:

• designed out;
• guarded against, if they cannot be designed out; and then
• (as a last resort) warned against.

Since this classic safety hierarchy reflects general machine design practice, few machines present Category 4 risk conditions.

When Category 4 safety requirements are encountered (that is, when the safety control system must be able to detect any single fault, or provide multiple fault tolerance, without loss of the safety function), it is important to remember these define the performance requirements of the overall safety system? ... not of the individual components. (This, of course, is true for all safety categories ... not only Category 4.)

In this "system" context, it is clear that safety system component selection and design for equipment assessed as a Category 4 risk will be dictated by the number of faults the system can tolerate without loss of the safety function. Hence the appropriate safety system components are application-specific, requiring a thorough understanding of the operation of the machinery and its control system.

Use of a safety circuit monitor or safety relay module rated at Category 4 does not, in itself, assure the overall safety system meets this level of performance requirements.

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