Catalogs > Safety Products Catalog > Principles, Standards and Implementation > System Design According to IEC/EN 62061
System Design According to IEC/EN 62061
| Introduction |  |
Subsystem Design: IEC/EN 62061 | |
Affect of the Proof Test Interval | |
Affect of Common Cause Failure Analysis | |
Common Cause Failure (CCF) | |
Diagnostic Coverage (DC) | |
| Hardware Fault Tolerance | |
Management of Functional Safety | |
Proof Test Interval | |
Safe Failure Fraction (SFF) | |
Systematic Failure | |
The safe failure fraction is similar to diagnostic coverage (DC) but also takes account of any inherent tendency to fail towards a safe state. For example, when a fuse blows, there is a failure but it is highly probable that the failure will be to an open circuit which, in most cases, would be a “safe” failure. SFF is (the sum of the rate of “safe” failures plus the rate of detected dangerous failures) divided by (the sum of the rate of “safe” failures plus the rate of detected and undetected dangerous failures). It is important to realize that the only types of failures to be considered are those which could have some affect on the safety function.
Most low complexity mechanical devices such as E-stop buttons and interlock switches will (on their own) have a relatively low SFF. Most electronic devices for safety have designed in redundancy and monitoring therefore an SFF of greater than 90% is common although this is usually completely due to the Diagnostic Coverage capability.
The SFF value will normally be supplied by the manufacturer.
The Safe Failure Fraction (SFF) can be calculated using the following equation:
SFF = (Sl S + Sl DD) / (Sl S + Sl D)
where
| l S | = | the rate of safe failure, |
| Sl S + Sl D | = | the overall failure rate, |
| l DD | = | the rate of detected dangerous failure |
| l D | = | the rate of dangerous failure. |