In industrial automation, the integrity of your signal is everything. A 4-20mA loop drifting by a few milliamps, or a digital communication drop-out, can mean the difference between a process running smoothly and an unplanned shutdown. The shield on your instrumentation cable is the first line of defence against the electromagnetic interference (EMI) that causes these problems. For engineers specifying cables, understanding the difference between Individual Screen (IS), Overall Screen (OS), and the combination of both is critical to ensuring reliable system performance-4.
This guide explains the three primary shielding configurations for instrumentation cables, helping you make the right choice for your application.
1. Individual Screen (IS): Stopping Crosstalk at the Source
What It Is: Individual Screen (IS), often called "分屏" (fēn píng) or "pair screening," involves applying a dedicated shield around each individual twisted pair (or triad) within a multi-pair cable--40.
How It Works: Every signal pair has its own protective layer, typically composed of an aluminium/polyester (AL-PET) foil with a tinned copper drain wire--1. This creates a 100% coverage barrier around each individual circuit-.
What It Protects Against: The primary function of IS is to eliminate crosstalk. When multiple signals are running in the same cable, the electromagnetic field from one pair can induce a voltage in an adjacent pair (crosstalk). Individual screening isolates each pair, preventing these internal signals from interfering with each other--8.
When to Use It: IS is essential for transmitting analogue signals such as 4-20mA, thermocouple/mV signals, and RTD inputs. It is also used for digital communications like RS-485 where signal integrity is paramount-4.
2. Overall Screen (OS): Defending Against External Threats
What It Is: Overall Screen (OS), sometimes called "总屏" (zǒng píng) or collective screen, is a single shield that wraps around the entire cable core, enclosing all the insulated and twisted pairs within--4.
How It Works: The OS is also constructed from an aluminium/polyester (AL-PET) foil with a tinned copper drain wire, providing 100% coverage over the whole cable--1. In some high-performance cables, the overall screen may also be a copper wire braid-4.
What It Protects Against: The OS is designed to combat external electromagnetic interference (EMI) . This can come from Variable Frequency Drives (VFDs), large motors, power cables running in parallel, or other high-energy electrical equipment in the industrial environment--8.
When to Use It: OS is often sufficient for discrete (on/off) signals and general control circuits where the risk of crosstalk is low, but external noise is a concern-4.
3. IS + OS (Individual and Overall Screen): The Gold Standard
What It Is: This is the combination of both an Individual Screen around each pair and an Overall Screen around the entire cable core--1. It is frequently referred to as IS/OS or double shielding--4.
How It Works: This dual-layer system is engineered to provide total protection. The inner layer (IS) prevents internal crosstalk between pairs, while the outer layer (OS) protects the entire cable from external interference--8.
When to Use It: IS+OS is the preferred choice for the most demanding applications where both internal and external noise are significant risks. This includes:
Long-distance analogue signal transmission
DCS (Distributed Control Systems) and PLC (Programmable Logic Controller) I/O connections-1
Intrinsically safe (IS) circuits-
Installations in high-EMI environments like refineries, chemical plants, and power stations-
4. Choosing the Right Shielding Configuration
| Feature | Overall Screen (OS) | Individual Screen (IS) | IS + OS |
|---|---|---|---|
| Primary Purpose | External EMI protection | Internal crosstalk prevention | Both external and internal protection |
| Signal Suitability | Discrete, digital, general control | Analogue (4-20mA, mV), RS-485 | Critical analogue, DCS/PLC, IS circuits |
| Cost | Most economical | Moderate | Highest (due to additional materials) |
| Cable Diameter | Smallest | Moderate | Largest |
| Best For | Cost-sensitive applications, clean environments | Applications with multiple pairs in one cable | Demanding industrial environments (oil & gas, chemical, power) |
Shielding Material Options:
The shielding material itself also affects performance. Common options include:
Aluminium/Polyester (AL-PET) Foil: Provides 100% coverage and is excellent for high-frequency interference, but can be less flexible-1.
Copper Wire Braid: Provides good low-frequency protection and is highly flexible and durable, but typically has lower coverage (e.g., 65-85%)-4.
AL-PET + Braid Combination: Some cables use a foil inner screen and a braided outer screen for maximum performance, balancing both high and low-frequency interference protection-4.
5. Critical Implementation: Shielding Grounding
The best shielding is useless if it is not grounded correctly. A shield that is not properly terminated can act as an antenna, actually attracting interference rather than blocking it.
Analogue Signals (e.g., 4-20mA): The shield should be grounded at one end only (single-point grounding). This prevents the formation of ground loops, which can introduce low-frequency hum and noise into the signal-4.
High-Frequency/Digital Signals (e.g., RS-485): The shield should be grounded at both ends (multi-point grounding) to provide an effective low-impedance path to ground for high-frequency interference-4.
Best Practice: Always ensure the shield is continuous and the drain wire is properly terminated with a dedicated grounding clamp or terminal. Never use the shield as a neutral or protective earth conductor-4.
6. How Tiankang Can Help
For nearly five decades, Anhui Tiankang (Group) Co., Ltd. has been a trusted manufacturer of high-performance instrumentation and control cables-. We understand that every application is unique, and our product range is designed to meet the specific demands of your project.
Expert Configuration: We offer cables in all three shielding configurations—OS (Overall Screen) , IS (Individual Screen) , and IS+OS (Individual and Overall Screen) . Our engineering team can help you determine the optimal shielding strategy for your specific signal types and environmental conditions-40.
Proven Performance: Our cables are designed for the most demanding environments, utilising materials like XLPE insulation for superior electrical properties, and AL-PET foil with tinned copper drain wires for 100% shielding coverage-1. We offer solutions suitable for DCS, PLC, and process instrumentation in industries from oil and gas to power generation-1-.
Quality and Compliance: As a leading instrumentation manufacturer, our products are built to meet rigorous international standards, ensuring reliable signal transmission in your critical applications-40.
Conclusion
Choosing the correct cable shield is a fundamental engineering decision. Selecting OS for low-cost, general-purpose applications, IS for eliminating internal crosstalk on analogue circuits, and the robust IS+OS combination for mission-critical signals in harsh environments will protect your system's performance and reliability-4. A small upfront investment in the right cable prevents costly troubleshooting and downtime in the future.
Contact Us
For assistance with cable selection, technical documentation, or project quotations, please contact:
Yin Shuangjie
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable instrumentation and cable solutions.
