Will changing the state of DO's from AUTO to MANUAL state on a SG400 system, trigger Degradation timer?
Will changing the state of DO's from AUTO to MANUAL state on a SG400 system, trigger Degradation timer? If yes after how long or if no please consider the following scenario.
Degradation timer was triggered, with the Safety matrix showing the following errors.
1. Dual start up sync ended with errors
2. Blocked objects in system
3. I/O error detected
4. DO in manaul mode (XXX)
5. DO in manaul mode (XXX)
6. DO in manaul mode (XXX)
On investigation it was found 3 valves were found to be in error (PEC) i.e. the limit switches for this valves were broken in the field. Hence there were solidly opened in the field but on the faceplate they were showing as travelling. And also the DO's for these valves was in manual (rather than AUTO, which is the normal state) may be in order not to cause any trip.
On tricking the system by toggling the limits in marshalling cabinet and software, manage to make all the valves look healthy. Then changed the DO's state back to AUTO from Manual. This fixed the issue and degradation timer disappeared.
The errors 2, 4,5,6 disappeared too from the SCU01.
I am wondering what caused the issue and what fixed it?
1. Putting the DO's is manual caused it? Reverting them back fixed it? Is there anything in SG400 that the DO's should never be blocked or put into manual? Or it can be done only for a certain amount of time?
2. It was caused by the broken limit switches in the field? And the valves being unhealthy for some time? by making the valves healthy we fixed the issue?
Any advice is greatly appreciated. Thank you.
Voted best answer
In general, changing Digital Output's (DO) state from AUTO to MANUAL doesn't trigger Degradation timer but it again depends on the controller engineering.
Error message 3 says "I/O error" that is nothing but the limit switches of 3 valves were broken.
I guess safety point of view there might be SG400 engineering has done when I/O error has occurred, automatically DO's will change from Automatic to Manual mode so that next command won't go from DCS to field to open or close the valve.
I would say just have a offline look at the engineering (CBA FCB) of these 3 valves and 3 DO's conditions when and how change their state from Auto to manual.