| | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | |
| | | Interface DIN-Rail Housing Catalogue What is new? What is important? | | |
| | | |
| | | If the operator of the safety line extends the test cycle from 1 to 2 years, then the PFD values also double and with them the probability that a device forming a component of the complete safety mechanism will fail when it is needed. If the operator reduces the test cycle for the safety device from 1 year to six months, then the PFD values are correspondingly halved. The PFD value of a device is a quantitative statement of the reliability of the unit within a safety-aligned application. Therefore the only part of the failure value that counts is that which is detrimental to the safety function. Failures, which do not endanger the safety function and the number of "dangerous" failures, which are however detected by the system and are therefore taken into consideration, do not contribute to the PFD value. | | |
| | | |
| | | SFF | | The SFF (safe failure fraction) is the proportion of "safe" failures, which does not endanger the safety function (Comprising "Xsd" and "A,su"). This includes the "dangerous" failures, which however are detected by the system and are therefore accounted for ("^dd"). It is merely those dangerous failures ("A,du") that are undetected by the system, which are detrimental to the safety function. | | |
| | | |
| | | |
| | | A SIL evaluation by Pepperl+Fuchs - all the facts about functional safety at a glance | | By considering SFF and Tproof the following values are obtained for PFD (see table). Example for the report (full version) on the KFD2-STC4-... transmitter power supply | | |
| | | |
| | | | | | | | | | | | | Failure categories | Tproof = 1 year | Tproof = 2 years | Tproof = 5 years | SFF | | | | Fail low (L)= safe Fail high (H) = safe | PFDavg = 1.6 x 10"4 | PFDavg = 3.2 x 10"4 | PFDavg = 8.0 x 10"4 | > 91 % | | | | Fail low (L)= safe Fail high (H) = dangerous | PFDavg = 2.2 x IO"4 | PFDavg = 4.5 x 10"4 | PFDavg = 1.1 x 10"3 | > 87 % | | | | Fail low (L)= dangerous Fail high (H) = safe | PFDavg = 7.9 x 10"4 | PFDavg = 1.6 x 10"3 | PFDavg = 3.9 x 10"3 | > 56 % | | | | Fail low (L)= dangerous Fail high (H) = dangerous | PFDavg = 8.6 x 10"4 | PFDavg = 1.7 x 10"3 | PFDavg = 4.3 x 10"3 | > 52 % | | | | | | | | | | | | |
| | | |
| | | |
| | | Transmitter power sources of the type KFD2-STC4-... are used as professional interfaces between the (intrinsically safe) professional transmitters and the process governor system inputs. The signal level is 4 mA ... 20 mA. If the transfer is in accordance with NE 43, then a difference occurs between the case when the current is less than the measuring range (4.0 mA ... 3.8 mA) and a lead breakage (less than 3.6 mA). Likewise, a distinction can be made between a value above the measuring range (20 mA ... 20.5 mA) and a lead short-circuit (21 mA or higher). The conclusions to be drawn from the diagnostic options are of great significance for safety assessment. The key factor is the diagnostic capability of the safety control. If the safety control system is unable either to detect a lead breakage or a lead short-circuit, then only 52 % of the theoretically possible failures will be detected. In other words: Every second failure could lead to the failure of the safety function, because it goes unnoticed. If both lead fault types - lead breakage (Fail low) and lead short-circuit (Fail high) - are detected by the safety control and processed, then over 90 % of the theoretically possible failures result in a safe condition of the control. Or, in other words, this means that only about 10 % of the possible failures of the safety function would be detrimental. In order to achieve SIL2, IEC/EN 61511 requires a SFF (safe failure fraction, proportion of failures, which lead to a safe condition) of at least 60 %. If this value is not achieved, then although the same transmitter and transmitter supply device are used - a SIL classification higher than SIL1 is not permitted. | | |
| | | E to CO | | |
| | | |
| | | CD g o CO | | |
| | | |
| | | |
| | | CD O O o | | |
| | | |
| | | o | | |
| | | |
| | | Subject to reasonable modifications due to technical advances. | | Copyright Pepperl+Fuchs, Printed In Germany | | |
| | | |
| | | 11 | | |
| | | Pepperl+Fuchs Group • Tel.: Germany +49 621 776-0 • USA +1 330 4253555 • Singapore +65 67799091 • Internet http://www.pepperl-fuchs.com | | |
| | | |
| | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | |