I am working on a project to improve the fault tolerance of several compressor PLC's. What we have planned for our shutdown outputs is this: Two racks with CPU, two relay output cards in each. Outputs in each rack are series together, then the two racks are paralleled. We are now being told by an outside consultant that this setup with not meet the required SIL 2 level for these shutdowns. I would welcome any comments or opinions, as we have already purchased most of the hardware for several of these PLC's.