In automation‑control engineering projects, the selection of 4‑20 mA signal cables directly determines system anti‑interference performance, signal transmission accuracy and long‑term operational reliability. Combining industry standards and field engineering practices, precise cable selection can be performed from the following five core dimensions.
I. Core Construction and Anti‑Interference Design
The 4‑20 mA signal is a weak analog signal highly susceptible to electromagnetic interference from on‑site sources such as frequency converters, large electric motors and welding machines. Therefore, cable construction must satisfy the following requirements.
1. Twisted‑Pair Structure
Twisted‑pair cables are mandatory. The symmetry of twisted pairs offsets interference induced by external electromagnetic fields. Shorter twist pitches deliver stronger anti‑interference capability.
2. Group‑Based Shielding
Adopt a layered‑shield S/FTP structure: individual aluminium‑foil shielding for each pair plus overall copper‑braid shielding. This effectively mitigates crosstalk between pairs and external common‑mode interference.
3. Pure‑Copper Conductors
High‑purity copper conductors (copper content ≥ 99.9 %) shall be used. Aluminium‑core or copper‑clad‑aluminium conductors are strictly prohibited, to guarantee sound conductivity and prevent oxidation and overheating at joints.
4. Insulation‑Layer Materials
Insulation shall be polyethylene (PE) or polyvinyl chloride (PVC), with insulation resistance ≥ 500 MΩ·km. PE insulation offers a lower dielectric constant and superior high‑frequency performance, suited for high‑speed data‑transmission scenarios.
II. Conductor Cross‑Section vs. Transmission Distance
Loop voltage drop shall be calculated during selection to guarantee that the voltage at the transmitter terminal remains above its minimum operating voltage (typically ≥ 16 V) at maximum transmission distance. Below are reference selections for standard 24 V power‑supply systems.
| Transmission Distance |
Recommended Cross‑Section |
Approximate Loop Resistance |
Remarks |
| Within 100 m |
1.0 mm² |
≤ 3.6 Ω |
Cost‑effective for basic requirements |
| 100 m ~ 300 m |
1.5 mm² |
≤ 2.4 Ω |
Most‑common standard size for automation projects |
| 300 m ~ 800 m |
2.0 mm² or 2.5 mm² |
≤ 1.8 Ω / ≤ 1.2 Ω |
Restrain voltage drop caused by line resistance |
| Above 800 m |
2.5 mm² |
≤ 1.2 Ω |
Strict voltage‑drop verification required; for distances exceeding typical limits (1000‑1500 m), install signal isolators or repeaters |
Voltage‑drop calculation formula:
ΔU = I × R × L × 2
Where: ΔU = loop voltage drop (V); I = loop current (mA); R = resistance per unit length (Ω/km); L = one‑way transmission distance (km).
III. Jacket Material and Environmental Compatibility
Cable outer jackets shall be selected according to actual installation environments.
| Application Scenario |
Recommended Jacket Material |
Properties |
Applicable Standards |
| General indoor / cable‑tray routing |
PVC |
Cost‑effective, flame‑retardant, resistant to mild chemical corrosion |
GB/T 19666 |
| Inside control cabinets / densely‑occupied areas |
LSZH (Low‑Smoke Zero‑Halogen) |
Releases no toxic gas during fire; light transmittance ≥ 60 % |
GB/T 19666, IEC 61034 |
| Harsh industrial environments (oil contamination, chemicals) |
PUR (Polyurethane) or CPE |
Oil‑resistant, abrasion‑resistant, acid‑alkali‑resistant, low‑temperature‑resistant |
HG/T 2006 |
| Frequent movement / drag‑chain applications |
High‑flexibility PUR |
Bending cycles ≥ 1 million, torsion‑resistant |
VDE 0295 |
IV. Key Installation and Grounding Specifications
Proper cable selection is only the first step; correct installation also determines final performance.
1. Single‑Point Shield Grounding
The cable shield must be reliably grounded only at the control‑cabinet (PLC/DCS card‑module) end. The shield at the field‑instrument end shall be insulated and floating. Grounding both ends is forbidden, as ground‑potential differences create ground loops that introduce severe interference and signal drift.
2. Segregation of Low‑Signal and Power Cables
4‑20 mA signal cables shall never share conduits or cable trays with power cables (e.g., 380 V supply cables, frequency‑converter output cables). For parallel routing, maintain a minimum clear separation of 30 cm. Where crossing is unavoidable, route cables at a 90‑degree perpendicular angle.
3. Minimum Bending‑Radius Control
Avoid sharp bends or kinks during installation. The minimum cable bending radius shall be no less than 6‑7.5 × the outer cable diameter, to prevent internal‑insulation damage or broken shield braid wires.
4. Conduit Protection
Route cables inside galvanised‑metal conduits or cable trays; both ends of metal conduits shall be reliably grounded. Conduit fill factor shall not exceed 40 % to ensure heat dissipation and maintenance space.
V. Common Selection Pitfalls and Mitigations
| Common Pitfall |
Potential Risk |
Correct Practice |
| Copper‑clad‑aluminium conductors |
Joint oxidation, higher contact resistance, signal attenuation |
Specify high‑purity oxygen‑free copper conductors |
| Shield grounded at both ends |
Ground‑loop interference, signal drift |
Ground shield only at the control‑cabinet end |
| Shared trays with power cables |
Electromagnetic‑coupling interference, signal distortion |
Separate trays; maintain ≥ 30 cm clearance |
| Neglecting voltage‑drop calculation |
Insufficient power supply for remote instruments, abnormal signals |
Calculate voltage drop per formula; add repeaters if necessary |
| Jacket mismatched to operating environment |
Jacket cracking and ageing, insulation failure |
Select jacket material matched to site conditions |
VI. Quick‑Reference Selection Table
| Application Scenario |
Conductor Material |
Shielding Structure |
Jacket Material |
Conductor Size |
| Standard DCS / PLC control cabinet |
Oxygen‑free copper |
Aluminium‑foil + copper‑braid double shield |
PVC |
1.5 mm² |
| Near frequency converters / high‑interference zones |
Oxygen‑free copper |
S/FTP double‑layer shield |
PVC |
1.5‑2.5 mm² |
| Petrochemical / hazardous‑explosion areas |
Oxygen‑free copper |
Aluminium‑foil + copper‑braid double shield |
LSZH |
1.5‑2.5 mm² |
| Outdoor / harsh environments |
Oxygen‑free copper |
Aluminium‑foil + copper‑braid double shield |
PUR/CPE |
2.5 mm² |
| Long‑distance transmission (> 800 m) |
Oxygen‑free copper |
S/FTP double‑layer shield |
PVC / LSZH |
2.5 mm² plus repeater |
Closing Remarks
Selecting 4‑20 mA signal cables is a systematic engineering task. Signal integrity, electromagnetic compatibility, environmental durability and installation specifications must all be considered comprehensively. Engineers shall fully investigate site operating conditions before selection, strictly follow relevant national standards, and enforce quality control over critical steps such as shield grounding during installation, to guarantee stable long‑term operation of automation‑control systems.
Reference Standards
‑ GB/T 19666‑2019 General Rules for Flame‑Retardant and Fire‑Resistant Electric Cables and Optical Cables
‑ GB/T 5023‑2008 Polyvinyl‑Chloride‑Insulated Cables of Rated Voltages up to and Including 450/750 V
‑ HG/T 2006‑2022 Thermoplastic Polyurethane Elastomers
‑ IEC 61034‑2 Measurement of Smoke Density of Cables Burning under Specified Conditions
‑ GB 50057‑2010 Code for Design of Building Lightning Protection
‑ SH/T 3019‑2016 Specification for Design of Instrument Piping and Cabling in Petrochemical Engineering
Mesajınız 20-3.000 karakter arasında olmalıdır!
