Logix SIS Safety Data
Use the following information to determine probability of a dangerous failure on demand (PFD) and average frequency of a dangerous failure per hour (PFH) values. These values apply to
GuardLogix®
5580 controllers in a Logix SIS 1oo2 SIL 3 system.Useful Life
The useful life of safety controllers is 20 years.
Safety Data
For safety I/O devices safety data, including PFD and PFH values, see the manuals for those products.
Product Failure Rates
For a mean repair time (MRT) of 72 hours, use the safety calculations in the following table.
Attribute | MRT = 72 hr |
|---|---|
PFH | 1.367E-10 |
PFD avg (20 yr) | 1.241E-05 |
Assumptions for safety calculations:
- Component failure rates are constant over the life of the product.
- Within the specified useful life (20 years), no proof test is needed.
IMPORTANT:
To minimize system degradation during repair time, use the shortest MRT possible for your application. Exceeding an MRT of 72 hours is not recommended. If you can justify an MRT longer than 72 hours for your application, then you must use the formulas below to compute PFD
avg
and PFH.For an MRT other than 72 hours, use the following PFD and PFH formulas to calculate PFD
avg
or PFH. Repair time is included in the calculations.
IMPORTANT:
These calculations use assumptions that become invalid when the MRT value gets too large. The maximum MRT value permitted for SIL 3 safety functions in Logix SIS is 730 hours.
Parameter | Value | Description |
|---|---|---|
λ DU | 6.40 x 10 -9 hr -1 | Dangerous undetected failure rate of a single controller |
λ DD | 6.54 x 10 -7 hr -1 | Dangerous detected failure rate of a single controller |
λ S | 6.61 x 10 -7 hr -1 | Safe failure rate of a single controller |
λ NPED | 2.58 x 10 -6 hr -1 | No part/effect detected failure rate of a single controller |
λ Det | 3.895 x 10 -6 hr -1 | Total detected failure rate of a single controller (λ DD S NPED |
T | ≤ 20 years (17,5200 hours) | Mission time Mission time is the length of time over which the device maintains the stated PFD, PFH, and λ ratings before replacement is required. |
T D | < SRT | Diagnostic test interval |
β | 2% | Common cause percentage |
HFT | 11 | Hardware fault tolerance The hardware fault tolerance equals n , where n +1 faults could cause the loss of the safety function. An HFT of 1 means that 2 faults are required before safety is lost. |
LORF | 7.61 x 10 -7 | Loss of redundancy factor |
MRT | [User-defined value] | Mean repair time for a failed controller |
SC | 3 | Systematic capability Systematic capability is defined in IEC 61508-4 as the confidence that the systematic safety integrity meets the requirements of the specified SIL rating. |
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