Catalogs > Safety Products Catalog > Principles, Standards and Implementation > System Design According to IEC/EN 62061
System Design According to IEC/EN 62061
| System Design According to IEC/EN 62061 |  |
Subsystem Design: IEC/EN 62061 | |
Transition Methodology for Categories | |
IEC/EN 62061 Terminology Overview | |
Diagnostic Coverage (DC) | |
| Management of Functional Safety | |
Probability of Dangerous Failure (PFHD) | |
Proof Test Interval | |
Safe Failure Fraction (SFF) | |
Systematic Failure | |
The Safe Failure Fraction is similar to Diagnostic Coverage (DC) but also takes account 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 an SFF of less than 60%. But most electronic devices, used for safety, have designed in redundancy and monitoring. Therefore, an SFF of greater than 90% is common. 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. |