SLYY242 November   2024

 

  1.   1
  2.   Overview
  3.   At a glance
  4.   Introduction
  5.   Safety considerations and potential failures in power-supply designs
  6.   Introduction to functional safety and standards in industrial systems
  7.   Voltage monitoring using voltage supervisor ICs
  8.   How voltage supervision affects functional safety ratings
  9.   Safe torque-off design example
  10.   Conclusion

Introduction to functional safety and standards in industrial systems

Defined functional safety standards help determine whether or not a system is safe. The most popular standards are IEC 61508 and International Organization for Standardization (ISO) 13849. Both standards look at the failure mode diagnostic coverage or safe failure fraction, as well as the hardware fault tolerance, to determine the safety integrity level (SIL) or performance level (PL) that a system meets. Table 2 summarizes these ratings.

Table 2 IEC 61508 vs. ISO 13849 safety standards.
Hardware fault tolerance (HFT) Category
IEC 61508 ISO 13849
0 1 2 SFF DC 1 2 3 4
- SIL1 SIL2 <60% None
SIL1 SIL2 SIL3 60% to <90% Low c c d
SIL2 SIL3 SIL4 90% to <99% Medium d e
SIL4 SIL4 ≤99% High e
Type B

Using Table 2 as a guide, you can see that there are multiple ways to obtain each IEC 61508 SIL or ISO 13849 PL. By designing a system with the appropriate safe failure fraction or diagnostic coverage and hardware fault tolerance, you can reach one of these levels. In particular, monitoring the voltage of your power supply can increase your diagnostic coverage. The implementation of voltage monitoring can also increase your hardware fault tolerance.

Table 3 provides more information on each of these safety parameters.

As you can see, you must account for not just the number of failures possible but the likelihood of failures occurring. You can also see that by increasing your diagnostic coverage or safe failure fraction, you can move up in SIL or PL without changing your hardware fault tolerance, and vice versa. Voltage monitoring is an essential aspect of determining the diagnostic coverage or safe failure fraction of your system and reducing your residual FIT of the system solution.

Table 3 Important functional safety rating terms.
Measurement Definition
Hardware fault tolerance Minimum number of tolerable failures for a system while also retaining safety functionality
Safe failure fraction
Equation 1. T o t a l   s a f e   f a i l u r e s   +   T o t a l   d e t e c t e d   d a n g e r o u s   f a i l u r e s T o t a l   s a f e   f a i l u r e s   +   T o t a l   d e t e c t e d   d a n g e r o u s   f a i l u r e s   +   T o t a l   u n d e t e c t e d   d a n g e r o u s   f a i l u r e s
Diagnostic coverage
Equation 2. T o t a l   d e t e c t e d   d a n g e r o u s   f a i l u r e s T o t a l   d e t e c t e d   d a n g e r o u s   f a i l u r e s   +   T o t a l   u n d e t e c t e d   d a n g e r o u s   f a i l u r e s
SIL Functional safety rating system