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I am looking for general advice, rules of thumb, or best practices but I also have a specific project I am looking for guidance on.
Overview of my project: I need to install instrumentation and control cable from an existing DCS cabinet to a new junction box. Cable distance will end up being 700 feet or so. From the junction box others will installing cable to instrumentation and valves which I believe may be another 50 feet or so of cable length. The instruments are loop powered 4-20mA devices. The other party will be installing shielded twisted pair cable for the 4-20mA devices. From the junction box to the DCS cabinet we will install a multiconductor twisted pair cable with individual and overall shields. There are a few solenoids valves that we will be controlling from the DCS. The position of the valves are signals that are being brought back to the DCS. These discrete input and output signals are 120Vac. From each valve to the junction box the other party is proposing to use an untwisted multiconductor cable with an overall shield (cable will contain wires for valve open command, valve close command, common for valve open and close commands, valve opened indication, valve closed indication, and common for valve opened and closed indication). The cables from the junction box to the valves will not be in close proximity to "power" cables. The cables from the DCS area to the junction box area will be installed in a new underground duct bank. This duct bank will also have (pretty low) power cables in other conduits.
Where I am unsure what to do: Unlike the seemingly universally accepted practices for analog signals, I can't seem to find any hard-and-fast rules about what kind of cable to use for discrete I/O nor any methods to make this determination based on specific circumstances. Anecdotally, at my first job I was doing some work at a newly built plant. That particular installation could not have been worse: control and power cables in a shared cable tray; unshielded untwisted multiconductor cables for discrete I/O; among the power cables were random lay single core cables providing power to relatively large 480V motors from VFDs. There were several valves that had position indication brought in to the DCS (frequently appearing as "ZSC" and "ZSO" on P&IDs in the industry), just as I am dealing with now on my project. At that plant the position indicators never worked due to the noise - the valves were always shown as opened and closed simultaneously. This is the issue I wish to avoid plus I have concerns about being to open and close the valves. Also anecdotally, future plant builds used individually shielded twisted pairs for discrete I/O and power and control/instrumentation cable was kept in separate trays, and we never experienced similar issues.
Following is a list a related and semi-related threads, some of which contain similar issues. (Mainly providing for future researchers on this topic that may come across this thread)
There are several mitigating techniques available but I don't know what is necessary or how to justify their expense. The following all seem like prudent things to do but with varying additional cost to the project:
1. Separate power and control/instrumentation duct banks
2. Use only steel conduits (rather than PVC as intended)
3. Do not use "common" wires for the solenoids or valve position signals
4. Use individually shielded and twisted pair cable for the discrete I/O
This is not my area of expertise although as mentioned above I have experience with things gone wrong. My coworker whose area of expertise includes this topic does not seem concerned with doing what the other party is doing - just use a cable with an overall shield. The purpose of this thread is to seek some additional opinions.
So, what kind of cable to you use for discrete I/O?
Other considerations: Given the distances and control voltage, would you expect capacitive coupling to be a problem? Would a DC control voltage help (or be necessary) and/or be advised in this situation?
Also, I don't have IEEE 518 and it is a withdrawn standard, but is it still worth owning/purchasing?
Edit:
Reading for future researchers:
Overview of my project: I need to install instrumentation and control cable from an existing DCS cabinet to a new junction box. Cable distance will end up being 700 feet or so. From the junction box others will installing cable to instrumentation and valves which I believe may be another 50 feet or so of cable length. The instruments are loop powered 4-20mA devices. The other party will be installing shielded twisted pair cable for the 4-20mA devices. From the junction box to the DCS cabinet we will install a multiconductor twisted pair cable with individual and overall shields. There are a few solenoids valves that we will be controlling from the DCS. The position of the valves are signals that are being brought back to the DCS. These discrete input and output signals are 120Vac. From each valve to the junction box the other party is proposing to use an untwisted multiconductor cable with an overall shield (cable will contain wires for valve open command, valve close command, common for valve open and close commands, valve opened indication, valve closed indication, and common for valve opened and closed indication). The cables from the junction box to the valves will not be in close proximity to "power" cables. The cables from the DCS area to the junction box area will be installed in a new underground duct bank. This duct bank will also have (pretty low) power cables in other conduits.
Where I am unsure what to do: Unlike the seemingly universally accepted practices for analog signals, I can't seem to find any hard-and-fast rules about what kind of cable to use for discrete I/O nor any methods to make this determination based on specific circumstances. Anecdotally, at my first job I was doing some work at a newly built plant. That particular installation could not have been worse: control and power cables in a shared cable tray; unshielded untwisted multiconductor cables for discrete I/O; among the power cables were random lay single core cables providing power to relatively large 480V motors from VFDs. There were several valves that had position indication brought in to the DCS (frequently appearing as "ZSC" and "ZSO" on P&IDs in the industry), just as I am dealing with now on my project. At that plant the position indicators never worked due to the noise - the valves were always shown as opened and closed simultaneously. This is the issue I wish to avoid plus I have concerns about being to open and close the valves. Also anecdotally, future plant builds used individually shielded twisted pairs for discrete I/O and power and control/instrumentation cable was kept in separate trays, and we never experienced similar issues.
Following is a list a related and semi-related threads, some of which contain similar issues. (Mainly providing for future researchers on this topic that may come across this thread)
There are several mitigating techniques available but I don't know what is necessary or how to justify their expense. The following all seem like prudent things to do but with varying additional cost to the project:
1. Separate power and control/instrumentation duct banks
2. Use only steel conduits (rather than PVC as intended)
3. Do not use "common" wires for the solenoids or valve position signals
4. Use individually shielded and twisted pair cable for the discrete I/O
This is not my area of expertise although as mentioned above I have experience with things gone wrong. My coworker whose area of expertise includes this topic does not seem concerned with doing what the other party is doing - just use a cable with an overall shield. The purpose of this thread is to seek some additional opinions.
So, what kind of cable to you use for discrete I/O?
Other considerations: Given the distances and control voltage, would you expect capacitive coupling to be a problem? Would a DC control voltage help (or be necessary) and/or be advised in this situation?
Also, I don't have IEEE 518 and it is a withdrawn standard, but is it still worth owning/purchasing?
Edit:
Reading for future researchers:
