Safety system (SIL 2) solenoid (ASCO 327) is connected with SIS (Himax) through an I/P relay (Phoenix safety relay). The question is how will SIS know if solenoid has failed.
The diagnostic coverage of SIS will stop at I/P relay. As I know that Himax superimposes and AC over the DC to keep loop status in check. But this will stop at the relay (btw relay has its own hardwired DI health status feedback into SIS) and beyond that SIS has no visibility about loop health. Am i missing something, kindly guide how can SIS make sure that loop beyond relay is also healthy. Thanks
We have all sorts of clever devices making integrity checks which come to nothing.
"how will SIS know if solenoid has failed"
Even if the relay was connected directly to the Himax, it would only check the solenoid electrically. Electrical failures on solenoids are rare, it is more likely to stick or jam through wear or lack of maintenance.
If you really want to check whether the solenoid has operated within a process, check it's functionality whilst operating. It wouldn't be the first time a solenoid appears to have failed when the compressed air supply to it has rapidly dropped due to another piece of equipment's demand.
Partial or full stroke testing is part of the SIS protocol. Check out a valve stroke testing paper here: