Overhall shield vs single shield multicables.

[adeb]

Hello,
I have to purchase multicore cables for installation in oil refinery site. Cables will be used for trasmission of 4-20 mA only signals in intrinsec safety style. Cable lenght will be 500mt and cables will run in parallel row on rack. Segregated by signal type and far enough from any high voltage supply cable.

I cannot see the reason to provide shielding against electrostatic noise for the single pair of the multicable instead of a common shield on the multicable itself...
Still everybody is asking for.

Bye.

 

[dpc]

Individual shields will definitely provide better EMI protection than an overall shield. Whether or not you need them is another issue.

If the pairs are all running between the same cabinets and are powered from the same source, you can probably get away with an overall shield. But if there are multiple power sources and/or field devices involved, the individual shields provide greater flexibility in shield grounding methods, terminations, etc.

Individual shields are pretty standard for the application you are describing, at least in my experience. Don't underestimate the amount of EMI in a typical industrial facility.

 

[jsummerfield]

Some system manufacturers regard the overall shield as adequate. No code requires the individual shield. Individually shielded pairs provide better noise immunity. Consider the total installation cost of running the cable whether in tray or conduit. What is the increased cost of cable and termination labor of individual shields as compared to bundles with an overall shield? The differential cost is not significant.

I favor segregating 24 vdc digital signals in a separate multicore from 4-20 mAdc even if all are intrinsically safe. I favor segregating shutdown signals from basic process control signals. Separate boxes are even better with color code such as Red for shutdown signal JB's.

What insulation rating is required, i.e. 300 volt vs 600 volt? Unless rigidly following intrinsic safety requirements, I prefer to avoid problems with inspectors, etc. by the 600 volt path. I only employee IS design if required by contract and recommend against in most US installations where most engineers and technicians lack adequate familiarity with the code and standard requirements. Color IS terminals or wire duct or boxes, etc. Blue.

Will the installation work if you only use an overall shield? Normally yes. In fact, at least one operating company uses 19 AWG solid conductor telephone wire for home-run cables. However, the signals may see the spike of a hand-held radio or when starting big motors, spurious stuff from radiographic exams, etc.

John