Hi. I hope everybody is doing well. I have been experiencing a strange phenomenon at my site which involves the fuses on several DCS analog input loops blowing from time to time. We had a few isolated incidents here and there when we first commissioned the system but the issue has become more frequent over the past few days.
Before I start detailing what the issue is, what I have done so far and what I am planning on doing, here are some details about the system.
DCS - Honeywell Experion PKS
I/O Family - Series C, Universal I/O (UIO-2)
Fuse Rating - 50 mA, Quick-Blow or Fast-Acting
The Problem:
The issue normally involves analog inputs whereby a particular transmitter becomes NaN on DCS out of the blue. Technicians find the transmitter in question powered off in the field i.e. loop power becomes unavailable. Upon checking, they are greeted with a blown fuse so they go ahead and replace it to resolve the issue. They observed only 2 or 3 failures of this nature in a span of 11 months so they did not feel the need to escalate the matter to me. However, we had three such failures in the past 4 days, one of which involved a safety critical device so I have been asked to check things out. It must be noted that all the transmitters that have failed so far have been loop powered in nature and belong to separate multicores/trunks and I/O cards so this is not a common-cause failure. The only thing common between these transmitters is that they are all Fuji FCX series differential pressure or pressure transmitters.
What I Have Done So Far:
I had a feeling some of the shield wires were not properly terminated so I had it verified. The shield wires are properly connected on one end only. Similarly, the transmitters themselves seem okay based on tests we carried out in our workshop with a tested power supply and measurement setup.
What I Plan On Doing:
* Check the AC and DC voltage on each individual core of the loop with respect to ground to see if there are any stray voltages / ground loops.
* Check the health of the instrumentation ground.
* Check the insulation resistance of the loops in question.
Another thing that I am suspecting is perhaps that the fuses we are using here are rated too conservatively (50 mA for an operating range of 4-20 mA). The analog inputs are all loop powered in our case and use a non-incendive power supply. We generally use 250 mA to 1 A fuses for analog inputs at our site but the problem with the higher rated fuses is that sometimes the channel ends up getting compromised before the fuse gets a chance to blow. So, for this particular installation, we decided to go with a much smaller value to ensure channels are not compromised in the event of a short. If the tests I have outlined above don't yield any fruitful results, I am planning on replacing these 50 mA fuses with 250 mA ones.
So, at this point, I am suspecting three things,
* Ground Loops
* Stray/Induced Voltages
* Fuse Rating Too Conservative
Please let me know if this is the right approach and if you have any more ideas as to what could be wrong here and what else needs to be checked, please feel free to share. Thanks in advance.
Before I start detailing what the issue is, what I have done so far and what I am planning on doing, here are some details about the system.
DCS - Honeywell Experion PKS
I/O Family - Series C, Universal I/O (UIO-2)
Fuse Rating - 50 mA, Quick-Blow or Fast-Acting
The Problem:
The issue normally involves analog inputs whereby a particular transmitter becomes NaN on DCS out of the blue. Technicians find the transmitter in question powered off in the field i.e. loop power becomes unavailable. Upon checking, they are greeted with a blown fuse so they go ahead and replace it to resolve the issue. They observed only 2 or 3 failures of this nature in a span of 11 months so they did not feel the need to escalate the matter to me. However, we had three such failures in the past 4 days, one of which involved a safety critical device so I have been asked to check things out. It must be noted that all the transmitters that have failed so far have been loop powered in nature and belong to separate multicores/trunks and I/O cards so this is not a common-cause failure. The only thing common between these transmitters is that they are all Fuji FCX series differential pressure or pressure transmitters.
What I Have Done So Far:
I had a feeling some of the shield wires were not properly terminated so I had it verified. The shield wires are properly connected on one end only. Similarly, the transmitters themselves seem okay based on tests we carried out in our workshop with a tested power supply and measurement setup.
What I Plan On Doing:
* Check the AC and DC voltage on each individual core of the loop with respect to ground to see if there are any stray voltages / ground loops.
* Check the health of the instrumentation ground.
* Check the insulation resistance of the loops in question.
Another thing that I am suspecting is perhaps that the fuses we are using here are rated too conservatively (50 mA for an operating range of 4-20 mA). The analog inputs are all loop powered in our case and use a non-incendive power supply. We generally use 250 mA to 1 A fuses for analog inputs at our site but the problem with the higher rated fuses is that sometimes the channel ends up getting compromised before the fuse gets a chance to blow. So, for this particular installation, we decided to go with a much smaller value to ensure channels are not compromised in the event of a short. If the tests I have outlined above don't yield any fruitful results, I am planning on replacing these 50 mA fuses with 250 mA ones.
So, at this point, I am suspecting three things,
* Ground Loops
* Stray/Induced Voltages
* Fuse Rating Too Conservative
Please let me know if this is the right approach and if you have any more ideas as to what could be wrong here and what else needs to be checked, please feel free to share. Thanks in advance.