— A Practical Guide for Engineers, EPCs, and Plant Operators
In process plants—refineries, chemical facilities, power stations, and offshore platforms—instrumentation cables are the nervous system of the control loop. They carry low-level analogue signals from field sensors, digital communications between control systems, and commands to final control elements. A poorly selected cable can introduce noise, degrade signal integrity, compromise safety, and create troubleshooting nightmares that persist for the life of the plant.
Unlike power cables that simply deliver energy, instrumentation cables must preserve signal fidelity in environments filled with electromagnetic interference (EMI), temperature extremes, moisture, and corrosive chemicals-. Selecting the right cable is not a trivial exercise—it requires understanding signal types, shielding requirements, environmental conditions, and applicable standards.
This guide provides a practical framework for selecting instrumentation cables for process plant applications, covering cable construction, shielding configurations, insulation materials, armouring options, and key selection criteria.
1. What Is an Instrumentation Cable?
An instrumentation cable is a specialised cable designed to transmit low-level analogue or digital signals from sensors, transmitters, and monitoring devices to control systems-. These signals are typically low voltage (often 24V DC) and low current (typically 4–20 mA), making them highly susceptible to electrical noise and interference-.
1.1 Instrumentation Cable vs Control Cable
| Feature | Instrumentation Cable | Control Cable |
|---|---|---|
| Primary function | Signal transmission (measurement data) | Command transmission (on/off, start/stop) |
| Signal type | Low-level analogue (4-20mA, mV) or digital | Discrete (24V DC, 110/220V AC) |
| Conductor size | Typically 0.5–1.5 mm² (16–20 AWG) | Typically 1.5–4.0 mm² |
| Key requirement | Low capacitance, noise reduction | Mechanical strength, durability |
| Shielding | Almost always required | Often not required |
Control cables execute operational commands (motor start/stop, valve open/close), while instrument cables deliver precise measurement data that must remain accurate despite environmental interference-. The conductor size of instrument cables is smaller because they only need to carry low-current signals-.
2. Cable Construction: The Building Blocks
Understanding cable construction is essential for making informed selections. A typical instrumentation cable consists of:
| Component | Function | Key Considerations |
|---|---|---|
| Conductor | Carries the electrical signal | Material (copper/tinned copper), stranding (solid/stranded), size (AWG/mm²) |
| Insulation | Electrically isolates conductors | Material (PE, XLPE, PVC), temperature rating, dielectric properties |
| Twisted pairs | Reduces crosstalk between signals | Pair count, lay length, pair identification |
| Shielding | Protects against EMI/RFI | Type (foil, braid, composite), coverage percentage, drain wire |
| Inner sheath | Binds and protects the core | Material compatibility with shield and armour |
| Armour | Provides mechanical protection | Material (steel tape, steel wire, aluminium), application |
| Outer sheath | Protects against environment | Material (PVC, LSZH, PE), chemical resistance, UV resistance |
2.1 Conductors
Material: Copper is the standard conductor material for instrumentation cables. For corrosive environments or offshore applications, tinned copper is preferred because the tin coating provides corrosion resistance.
Stranding: Stranded conductors (Class 2 or 5) offer greater flexibility than solid conductors and are better suited for installations involving vibration or tight bends. Solid conductors are used in fixed installations where flexibility is not required.
Sizing: Conductor size is typically specified in AWG or mm². Common sizes for instrumentation are 0.5 mm², 0.75 mm², 1.0 mm², and 1.5 mm². The appropriate size depends on signal type, transmission distance, and loop resistance requirements.
2.2 Insulation Materials
| Material | Temperature Rating | Key Properties | Best For |
|---|---|---|---|
| PE (Polyethylene) | -40°C to 80°C | Low capacitance, excellent dielectric properties | Analogue signals, long-distance transmission |
| XLPE (Cross-linked Polyethylene) | -40°C to 90°C | Better temperature rating than PE, good electrical properties | General industrial, higher temperature areas- |
| PVC (Polyvinyl Chloride) | -15°C to 70°C | Low cost, adequate electrical properties | General-purpose, indoor applications- |
| LSZH (Low Smoke Zero Halogen) | -30°C to 90°C | Fire-safe, low smoke, no toxic gases | Confined spaces, control rooms, offshore platforms- |
PE insulation is often preferred for instrumentation cables because its low dielectric constant minimises capacitance, which is critical for preserving signal quality over long distances.
3. Shielding: Protecting Signal Integrity
Shielding is the most critical feature of an instrumentation cable. It protects the signal conductors from electromagnetic interference (EMI) and radio frequency interference (RFI) that can corrupt measurements-.
3.1 Shielding Types
| Shield Type | Coverage | Best For | Characteristics |
|---|---|---|---|
| Foil (Aluminium/Polyester) | 100% | High-frequency noise | Lightweight, flexible, requires drain wire- |
| Braid (Copper Wire) | 70–95% | Low-frequency noise, mechanical protection | Durable, flexible, lower DC resistance- |
| Composite (Foil + Braid) | 100% + braid | Both high and low frequencies | Maximum protection, higher cost- |
Foil shielding provides 100% coverage and excellent protection against high-frequency noise. It is lightweight and flexible but requires a drain wire for termination-. Foil is preferred when 100% coverage is required-.
Braid shielding provides mechanical strength and flexibility, making it suitable for dynamic applications where cables may be subject to movement. Braid offers lower DC resistance than foil but typically provides 70–95% coverage-.
Composite shielding combines foil and braid for maximum protection across both high and low frequencies. This is the most robust option for critical signals in high-EMI environments-.
3.2 Shielding Configurations
| Configuration | Description | Best For |
|---|---|---|
| Individual Screen (IS) | Each twisted pair has its own shield | Eliminating crosstalk between pairs in multi-pair cables |
| Overall Screen (OS) | One shield around the entire cable core | Protection against external EMI |
| IS + OS (Dual Screen) | Individual screens on each pair + overall screen | Maximum protection for critical analogue signals |
The choice between individual and overall screening depends on the application. For multi-pair cables carrying multiple analogue signals, individual screening prevents crosstalk between pairs. Overall screening protects the entire cable from external EMI.
3.3 Shielding Grounding Best Practices
Proper grounding is essential for shielding to work effectively. An incorrectly grounded shield can act as an antenna, actually attracting interference rather than blocking it.
General rule: For most instrumentation applications, the shield should be grounded at one end only—preferably at the control room or power source end-. Grounding at both ends can create ground loops that introduce noise into the signal-.
Exception: For high-frequency interference (>1 MHz), grounding the shield at both ends with 360° circumferential bonds may be more effective-.
Key practice: The shield should never be used as a signal conductor. A drain wire (bare conductor in continuous contact with the shield) should be provided for termination-.
4. Sheath Materials and Environmental Protection
The outer sheath is the cable's first line of defence against the environment. Selection depends on the installation conditions.
| Sheath Material | Temperature Range | Key Properties | Applications |
|---|---|---|---|
| PVC | -15°C to 70°C | General-purpose, moderate chemical resistance | Indoor, dry environments |
| Oil-resistant PVC | -20°C to 90°C | Resists hydrocarbons and oils | Refineries, chemical plants |
| LSZH | -30°C to 90°C | Low smoke, halogen-free, fire-safe | Control rooms, offshore, confined spaces- |
| PE | -40°C to 80°C | Excellent moisture resistance | Direct burial, outdoor |
| SHF2 | -40°C to 90°C | Mud-resistant, halogen-free, low smoke | Drilling platforms, offshore- |
LSZH (Low Smoke Zero Halogen) materials are used in place of PVC where there is a significant risk to life and equipment in fire situations-. In a fire, LSZH cables emit minimal smoke and no toxic halogen gases, improving visibility for evacuation and protecting sensitive equipment from corrosion.
SHF2 sheaths are specifically designed for offshore drilling platforms, providing resistance to drilling mud and chemicals while maintaining halogen-free, low-smoke properties-.
5. Armour: Mechanical Protection
Armour provides mechanical protection against impact, abrasion, and rodent attack. It is specified when cables are subject to physical stress.
| Armour Type | Protection | Applications |
|---|---|---|
| Steel Tape Armour (STA) | Crush resistance, rodent protection | Direct burial, exposed runs |
| Steel Wire Armour (SWA) | Tensile strength, crush resistance | Vertical runs, offshore platforms |
| Aluminium Armour | Lightweight protection | Weight-sensitive applications |
Armoured cables are typically required for direct burial, installations subject to mechanical damage, and hazardous area installations where cable glands must maintain flameproof integrity.
6. Key Standards for Instrumentation Cables
Instrumentation cables must comply with various international and national standards. Understanding these standards is essential for specifying cables that meet project requirements.
6.1 Flame Retardance Standards
| Standard | Test | Application |
|---|---|---|
| IEC 60332-1 | Single vertical burn | Basic flame retardance for all cables |
| IEC 60332-3-22 (Cat A) | Bunched cable burn (most stringent) | Cable-dense areas, offshore platforms- |
| IEC 60332-3-24 | Bunched cable burn | General industrial applications- |
6.2 Low Smoke and Halogen-Free Standards
| Standard | Test | Application |
|---|---|---|
| IEC 61034-1/2 | Smoke density measurement | LSZH cable qualification- |
| IEC 60754-1/2 | Halogen gas emission | LSZH cable qualification- |
6.3 Other Key Standards
| Standard | Scope |
|---|---|
| IEC 60228 | Conductor sizes and resistance- |
| IEC 60092-376 | Marine instrumentation cables- |
| NEK 606 | Offshore instrumentation cables (Norwegian standard)- |
| BS 5308 | UK standard for instrumentation cables- |
7. Selection Criteria for Process Plants
7.1 Signal Type
| Signal Type | Cable Requirements |
|---|---|
| 4-20 mA analogue | Twisted pair + individual shielding + overall shielding (IS+OS) for critical loops |
| Thermocouple / mV | Thermocouple extension wire (matching type), low noise |
| RTD | 3-wire or 4-wire configuration, low resistance |
| RS-485 / Modbus | Twisted pair, 120Ω characteristic impedance, overall shield |
| Discrete (on/off) | Overall shield only (or unshielded if EMI is low) |
For most process instrumentation, twisted pair is the preferred construction because it minimises crosstalk and provides more reliable signals-. For 4-20 mA and RS-485 signals, twisted pair cables are the standard choice-.
7.2 Environmental Conditions
| Condition | Cable Requirement |
|---|---|
| High temperature | XLPE or fluoroplastic insulation, appropriate temperature rating- |
| Chemical exposure | Oil-resistant or chemically resistant sheath- |
| Moisture / outdoor | PE sheath for moisture resistance |
| Fire risk | LSZH sheath, flame retardant (IEC 60332) |
| Offshore / salt spray | Tinned conductors, SHF2 sheath, LSZH- |
| Hazardous area | Intrinsically safe cable with low capacitance/inductance, blue sheath |
7.3 Electrical Parameters
| Parameter | Why It Matters |
|---|---|
| Capacitance | High capacitance causes signal attenuation and slow response |
| Characteristic impedance | Critical for digital bus cables (RS-485 requires 120Ω) |
| Loop resistance | Affects maximum transmission distance |
| Insulation resistance | Affects signal leakage and accuracy |
7.4 Routing and Installation
Instrumentation cables should be operated without electrical power cables and should avoid noise-generating equipment as much as possible-. Cable crossings should always be done at right angles-.
8. Intrinsically Safe Cables for Hazardous Areas
For installations in hazardous areas (Zone 0, 1, 2), intrinsically safe (IS) cables are required. These cables are designed to limit electrical energy to levels incapable of ignition under fault conditions.
Key requirements for IS cables:
Low capacitance and inductance (must match safety barrier parameters)
Physically isolated from non-IS circuits-
Shielded (foil or braid) to prevent EMI coupling-
Light blue outer sheath for identification (per IEC 60079-14)
Full Ex-ia or Ex-ib certification
9. Common Selection Mistakes to Avoid
| Mistake | Consequence | Correct Practice |
|---|---|---|
| Using unshielded cable in high-EMI environment | Signal noise, erratic readings | Select shielded cable appropriate for the EMI environment- |
| Grounding shield at both ends | Ground loops, signal noise | Ground shield at one end only (control room end)- |
| Using PVC sheath in fire-risk area | Toxic smoke, equipment corrosion | Use LSZH sheath- |
| Ignoring temperature rating | Insulation degradation, short circuits | Select cable rated for the maximum ambient temperature |
| Using non-IS cable in hazardous area | Safety incident, regulatory violation | Use certified IS cable with appropriate Ex rating |
| Oversizing conductor for signal only | Unnecessary cost | Select conductor size based on signal and distance requirements |
10. Why Choose Anhui Tiankang for Instrumentation Cables?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial cables for nearly five decades. Our instrumentation cables are trusted by major oil and gas companies, chemical plants, and power generation facilities worldwide.
Complete product portfolio:
Instrumentation cables: IS, OS, and IS+OS shielding configurations
Intrinsically safe cables: Low capacitance, blue LSZH sheath, Ex-ia certified
Armoured cables: Steel tape and steel wire armour options
Fire-resistant cables: IEC 60331 compliant, hydrocarbon fire-resistant
LSZH cables: Low smoke, halogen-free for fire-safe installations
Bus cables: RS-485/Modbus with 120Ω characteristic impedance
Core advantages:
Full certifications: CCC Ex, ATEX, IECEx, CCS marine
CNAS-accredited laboratory: Full electrical, mechanical, and fire performance testing
Proven track record: Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
Complete package: From instrumentation to cables to Ex cable glands—one-stop supply
11. Conclusion
Selecting instrumentation cables for process plants requires a systematic approach:
Identify the signal type – Analogue, digital, or discrete?
Assess the environment – Temperature, chemicals, moisture, EMI, fire risk?
Select the shielding – Foil, braid, composite; individual, overall, or both?
Choose the sheath – PVC, oil-resistant PVC, LSZH, PE, or SHF2?
Consider armour – Is mechanical protection required?
Verify standards compliance – IEC 60332, IEC 61034, IEC 60754, etc.
Confirm hazardous area requirements – IS cable with appropriate Ex certification.
Remember: The cost of a poorly selected cable is not just the cable itself—it is the troubleshooting time, the production lost to signal errors, and the safety risks that could have been avoided.
With nearly five decades of experience and a complete range of instrumentation cables, Anhui Tiankang is your trusted partner for reliable signal transmission in process plants.
Contact Us
For instrumentation cable selection advice, technical documentation, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable instrumentation cable solutions.
