What does the term 'screen' mean in a cable? Does it serve the purpose as a shield or is it more towards grounding?

 

A "screen" is a shield--the foil (usually; sometimes it's a braided, stainless steel mesh surrounding the twisted conductors). The screen, or shield, should ALWAYS be grounded/earthed at one end--and one end only. (This topic has been covered MANY times on Control.com; and there were some very good (at least in my opinion--I "drew" some of them using ASCII text characters before the site had the ability to attach photographs and electronic images) "drawings" of acceptable and unacceptable grounding practices for different types of interconnecting cables. Use the 'Search' feature to find them; I almost never used the term "screen," preferring to use "drain wire" to describe the bare wire which is in constant contact with the shield (or "screen") and which is usually used to ground/earth the "shield" ("screen").

This is a very simple concept that is VERY misunderstood by electricians (unless they are experienced, industrial electricians) and quite often by engineers who have had very little, if any, practical, hands-on experience with field wiring of instruments and sensors. BUT, if not properly implemented, it can cause all manner of nuisance and intermittent problems, up to, and including, equipment failure(s) (expensive process equipment, not just instruments and sensors!).

To sum up, "screen" and "shield" when referring to twisted pair instrument cables (or three-wire twisted instrument cables, often referred to as "triads") usually mean the same thing: the "wrapping" which surrounds the twisted conductors (cores) to capture any induced electrical signals and prevent these induced signals from adversely affecting the instrument or sensor wiring and functionality.

It MUST be said here that one CANNOT use a twisted, shielded instrument cable for control and/or protection which is running in the same conduit, cable tray or cable vault as high-voltage, high-current wiring (such as 440 VAC motor leads, or even 220 VAC or 125 VDC motor leads; or 4160 VAC power wiring; etc.) and expect the shield (screen) to eliminate all induced electrical signals which can adversely affect the instrument/sensor circuit. Control wiring should be run in separate conduits (groups of control wiring, included)--but NEVER in the same conduit/cable tray/cable vault is higher voltage, higher current wiring of any type. (I have seen yard lighting (220 VAC) wiring cause serious problems with instrument circuits run in the same cable vault...!)

There is ONE accepted exception to mixing high- and low voltage wiring--and that is when entering and exiting a control panel (which shouldn't have 440 VAC wiring in it to begin with!). It's generally acceptable to mix/mingle cores/cables/conductors over short distances, such as when entering a control panel--but efforts should be made to keep the distance they are near each other to a minimum. You will generally find a well-configured control panel will have high- and low-voltage compartments or terminal boards separated from each other to try to help prevent mixing/mingling the wires.

Hope this helps! Again, it's a simple concept--once it's understood--but it can mean the difference between smooth operation of equipment or a process and incessant and intermittent and unexplained problems. I have been to several power plants that failed to observe proper cable separation practices during construction that had LOTS of problems (which, of course, are ALWAYS (falsely) blamed on the control system!!!). Some were easier to find and fix than others; some were so bad they couldn't be fixed. BUT ALWAYS, it required a plant shutdown to investigate and resolve the issues, usually more than one plant shutdown, too. And, the bean-counters (the people who THINK they run the plant) get VERY upset when the plant has to be or is shut down. And, that's not good for anyone. Good on you for taking the time to try to understand this concept--and hopefully avoid problems in the future!