We are in process of installing the ABB 800xA system with AC800M Hi controller & S800 I/O. The system will be used for SIL-2 application. For I/O we have used AI880, AO815, DI880 and DO880. All I/O are proposed with 3-layer TU (Terminal Unit). (AI880 with TU834; AO815/DI880/DO880l with TU830).
Since carrying out maintenance job is not easy with 3-Layer TU; our maintenance department proposed to replace it by single layer TU. But contractor is telling that they do not have any single layer TU for SIL-2 application.
Please advice: "Is the SIL certificate is also required for passive component like Terminal Unit or we can use any non-SIL certified Terminal Unit?"