If the safe failure rate of a piece of equipment is 3 per year and the total failure rate is 1 per year, which statement is true?

• A. The dangerous failure rate is 2 per year.
• B. With an annual testing program, the PFD of the equipment is 75%.
• C. There is an error in the failure rate data provided.
• D. The safe failure rate does not matter.

Answer: (C)

To determine the correct statement regarding the provided failure rates, it is essential to understand the relationship between safe failure rates, dangerous failure rates, and total failure rates in the context of functional safety.

In this scenario, the total failure rate of the equipment is given as 1 per year, which means this is the sum of safe and dangerous failures. If the safe failure rate is 3 per year, it is inconsistent with the total failure rate of 1 per year. The safe failures should contribute positively to the total, but in this instance, the safe failure rate is greater than the total failure rate, indicating a contradiction.

This discrepancy suggests that there may be an error in the failure rate data provided, as you cannot have a situation where safe failures are higher than the total failures when looking at a simplistic model. Therefore, this reinforces the notion that the conclusion about an error in the failure rate data is indeed valid, making it the true statement.

Understanding this relationship is crucial in functional safety assessments, as it ensures that systems can be analyzed accurately and maintain reliability over time through proper risk management.