A triconex system used for SIS has an issue in the instrument power loops. A 24 DC voltage provider is used to supply the needed voltage to the filed instrument. When all the tags are connected, the positive wire has a +16 VDC and the common has a -8 VDC (where the positive wire should have 24 VDC and the common almost zero). The difference is still 24 VDC! We then started disconnecting one tag at a time since we suspected a shortage in one of the tags, but the voltage potential changes between the positive and the common wire by removing some tags and the difference is still 24 VDC (e.g positive +12 common -12).

It is worth mentioning that the SIS system is connected to 3 refrigeration units A, B and C. When we disconnected the 24 VDC breaker going to the tags of refrig A, the problem disappears.

I suspect that there is a ground reference error between the field and the cabinet where the system is installed. I truly run out of ideas to solve this issue. For now, the system is working, but I am afraid of a possible future effects to the instruments.

I would really appreciate you input in this matter. Any idea is welcome, I am open to try anything to solve this issue.

Please note that we have checked all field instrument for defectiveness but we found nothing wrong.

Best Regards,
Nasser

 

Nasser,

This is not abnormal behaviour for many control systems. As you say, it's working correctly.

It's just not what you would expect, but the best source of information would be Triconex.