Choosing shielded instrumentation cable is only the first decision. In a multi-pair cable, the next question may be whether one shield around the entire cable is enough or whether each pair should have its own shield in addition to the overall cable shield. Both constructions are designed to help manage unwanted electrical interference, but they solve somewhat different problems.
An overall shield primarily helps protect the group of conductors from interference originating outside the cable. Individually shielded pairs can add another level of isolation between the separate signal circuits traveling inside the same cable. That difference can matter in industrial instrumentation and process-control systems carrying multiple sensitive signals.
What Does an Overall Cable Shield Protect Against?
An overall shield surrounds the collection of conductors or pairs beneath the outer jacket. Its primary role is to help limit the effects of electromagnetic or electrostatic interference entering the cable from the surrounding environment.
A cable may travel past motors, drives, switching equipment, transformers, power conductors or other sources of electrical noise. An overall shield places a conductive barrier between those sources and the signal conductors inside the cable.
For many applications, that may provide the level of protection the system requires. This is one reason overall-shielded instrumentation cable remains common in industrial facilities.
The Signals Inside the Cable Can Interact Too
External electrical noise is not the only concern in a multi-pair cable. Each pair carries its own electrical signal. Depending on signal characteristics and system design, one circuit can potentially influence another circuit traveling next to it. This unwanted interaction is often described as crosstalk.
When individual shielding is placed around each pair, the shield helps separate that signal circuit from the neighboring pairs inside the same cable. An additional overall shield can then help protect the entire collection of pairs from interference coming from outside the cable. This creates two levels of isolation serving two different purposes.
When Individual Pair Shielding Becomes More Valuable
Individual pair shielding becomes more relevant as the number of sensitive circuits sharing one cable increases. Consider an industrial process where multiple transmitters report conditions such as pressure, temperature, flow or level back to a control system. Those signals may share a common multi-pair cable for a significant portion of the route.
If the design requires greater separation between those channels, individually shielded pairs can help isolate the circuits while still allowing multiple signals to be carried in one cable. The decision should come from the instrumentation design and project specification rather than from an assumption that every multi-pair cable needs the maximum available shielding.
Sensitive Signals Can Raise the Stakes
Some signals are less tolerant of unwanted electrical energy than others. A small amount of interference may have little practical effect on one circuit while introducing instability or measurement error into another. That makes signal type an important part of the shielding decision.
If accurate analog measurement is critical to a process, the engineering specification may call for greater circuit isolation. Other systems may operate reliably with an overall-shielded construction. The purpose of the instrumentation system, signal levels and receiving equipment all need to be considered.
More Shielding Changes the Physical Cable
There is a practical tradeoff. Adding individual shields around multiple pairs adds material inside the cable. That can increase overall diameter, weight and the amount of work required during termination. This matters in cable trays, conduits, cabinets and junction boxes where space is limited.
A cable with individual and overall shields may also have additional drain wires or shielding components that need to be handled correctly during installation. For that reason, the best construction is not necessarily the cable with the greatest number of shields. It is the one that provides the required electrical performance while fitting the physical and installation requirements of the project.
Termination Planning Matters
Shield performance depends on more than what is inside the cable. The way shields and drain wires are terminated is part of the overall instrumentation design.
Poor termination practices can undermine the reason the shield was specified in the first place. The correct method can depend on the system, equipment manufacturer, grounding design and project engineering requirements. For multi-pair cable, installers also need to identify and manage the individual shield and drain components associated with each circuit. That makes termination requirements worth considering before cable is ordered, especially on high-pair-count constructions.
Pair Count Should Be Selected Carefully
A multi-pair cable can simplify installation by carrying many signal circuits through one route, but adding more pairs is not automatically better. Higher pair counts generally mean a larger cable and more terminations at each end. The instrument schedule should determine how many circuits are required. Project designers may also account for spare capacity where appropriate, but unnecessary pair count can increase cable size and installation work.
Custom Cable Corp. offers thermoset shielded instrumentation cable in multiple conductor and pair configurations, including constructions with different shielding arrangements. That gives engineers and buyers the ability to match pair count and shielding configuration rather than treating “shielded instrumentation cable” as one universal product.
How to Make the Choice
Start by identifying what the shield needs to accomplish. If the primary concern is electrical noise entering from equipment and power wiring around the cable route, an overall shield may meet the requirement. If multiple sensitive circuits within the cable also need greater isolation from one another, individually shielded pairs plus an overall shield may be more appropriate. Then look at the rest of the specification.
Pair count, conductor size, voltage, insulation, jacket, temperature rating, installation environment, outside diameter, required approvals and termination design still matter. Shielding should solve a defined signal-integrity problem rather than being selected in isolation from the rest of the cable system.
Frequently Asked Questions
It depends on the cable construction.
Many individually foil-shielded pair designs include a drain wire associated with each shield so the shielding can be terminated as intended. Other constructions may be designed differently. The cable specification should be reviewed before installation so the termination plan accounts for the exact number and arrangement of shields and drain wires.
They can help keep signal circuits physically and electrically separated, but troubleshooting benefits depend on the system design.
Individual pair construction can make it clearer which conductors and shield components belong to a particular signal circuit when the cable is properly identified and terminated. However, pair numbering, color coding, documentation and panel design remain important. Shielding itself should not be treated as a substitute for good circuit identification.
Sometimes, but compatibility should be confirmed by the system designer.
Signal level, circuit function, equipment requirements, interference sensitivity and project specifications can determine whether different instrumentation circuits should share a common cable. Individually shielded pairs can improve isolation, but they do not automatically make every combination of signals suitable for placement in the same cable.
It can.
Individual pair shields, drain wires and an overall shield add components that must be separated, identified and terminated according to the project design. On cables containing many pairs, this can increase termination time and make enclosure space more important. Installation labor and termination layout are practical considerations when comparing shielding configurations.
Ideally, shielding and routing should be considered together.
A route passing near motors, drives, power conductors or other interference sources may create different shielding requirements from a route through a relatively quiet electrical environment. At the same time, the chosen cable diameter and pair count affect tray and conduit planning. Coordinating the route and cable construction early can prevent a shielding choice from creating unexpected space or installation problems later.
