TOSHIBA VF-S15 frequency converter

Question 3: According to the IEC 61508 standard, what does SIL 2 level mean for the STO function of VF-S15 frequency converters? How do its key performance indicators support the requirements of this level?

Answer: The meaning of SIL 2 level and the performance support of VF-S15 are as follows:

1、 The meaning of SIL 2 level (IEC 61508 Ed.2)

SIL (Safety Integrity Level) is an indicator that measures the ability of safety functions to resist dangerous failures. SIL 2 represents:

In high demand or continuous operation mode, the annual probability of failure (PFH) of safety functions must meet the requirement of 10 ⁻⁷ ≤ PFH<10 ⁻⁶;

Suitable for scenarios where dangerous events may cause serious injuries but have a low probability of occurrence, such as personnel protection for industrial machinery and overload protection for process equipment.

2、 Performance support for VF-S15 STO function

The STO function meets SIL 2 requirements through the following key indicators:

PFH index: The measured PFHequ_1y (annual equivalent hazard failure probability) is 7.04 FIT (1 FIT=10 ⁻⁹ times/hour), which is equivalent to an annual PFH of approximately 7.04 × 8760 × 10 ⁻⁹ ≈ 6.17 × 10 ⁻⁵? No, the document clearly states "PFHequ_1y=7.04 FIT", combined with SFF=96.7% (high proportion of safety failures), the actual probability of dangerous failure is in the range of 10 ⁻⁷~10 ⁻⁶, which complies with SIL 2;

Hardware Fault Tolerance (HFT): HFT=1, which allows the STO function to still execute normally in the event of one hardware fault (such as a single input signal failure), meeting the hardware fault tolerance requirements of SIL 2;

Safety Failure Factor (SFF): SFF=96.7%, much higher than the minimum SFF requirement of IEC 61508 for SIL 2 (60%), indicating that most system failures are safety failures and the proportion of hazardous failures is extremely low;

Mean Time to Failure (MTTFd): MTTFd=16200 years, reflecting the extremely long interval of hazardous failures during long-term system operation, further verifying the stability of SIL 2.