Functional Safety Interfaces - SIL ratings

The following table states the maximum ‘SIL capability’ that can be claimed for Crouse-Hinds series MTL4500 (backplane-mounted) and MTL5500 (DIN-rail mounted) intrinsic safety isolator modules, in accordance with IEC 61508: 2010 (Edition 2.0).

The table includes columns for Hardware Fault Tolerance (HFT), where HFT = 0 indicates the use of a module in a simplex, non-redundant arrangement, and HFT = 1 indicates the use of a module in a redundant arrangement. Simplex architectures may be referred to as 1oo1 (One out of One); whereas redundant architectures include 1oo2, 2oo3 etc. In redundant, voting architectures the logic solver must resolve the status.

In the case of MTL intrinsic safety isolators, a redundant arrangement would typically consist of parallel ‘arms’ within the Safety Instrumented Function (SIF), where both the MTL isolator and its associated field instrument are duplicated or triplicated.

The Safety Integrity Level (SIL) correctly applies to the entire safety function. However, a SIL ‘capability’, or SIL ‘attainment’ is often declared for individual elements, such as the MTL intrinsic safety modules. These indicate the highest SIL that is achievable for the overall safety function if the element is used within it. Note, however, that assembling multiple SILx capable elements does not ensure that the overall safety function will achieve SILx.

The SIL numbers given in the table relate to the IEC 61508: 2010 standard. For process applications, the sector-specific standard, IEC 61511, may require additional levels of hardware redundancy. In particular, Table 6 (“Minimum HFT requirements according to SIL”) of IEC 61511: 2016 (Edition 2.0) requires HFT = 1 for SIL 3 safety functions.