I. Core Limiting Factors
Signal Attenuation: Long-distance transmission can cause a 4-20mA signal voltage drop and response delay. When the cable length exceeds 300 meters, it is necessary to verify whether the total circuit resistance is within the transmitter's drive capability range; otherwise, signal distortion or communication failure may occur.
Intrinsically Safe System Parameter Constraints: Intrinsically safe circuits have strict requirements on the distributed capacitance (C₀) and inductance (L₀) of the cable. Excessively long cables can accumulate C/L values exceeding the allowable values of safety barriers or associated equipment, compromising intrinsic safety performance and posing an ignition risk. Therefore, the total C/L of the entire circuit must be ensured to be within the certified range.
Electromagnetic Interference Accumulation: Longer cables are more prone to coupling with external electromagnetic noise (such as from frequency converters and motors). Even with shielding, improper grounding or incomplete shielding can lead to increased interference.
II. Solutions for Extended Transmission
Using Intrinsically Safe Signal Isolators/Repeaters: For distances exceeding 300 meters, an isolation safety barrier or intrinsically safe signal repeater module can be installed at the intermediate end to achieve signal regeneration and electrical isolation, overcoming the limitations of single-segment length.
Selecting Low-Capacitance, Low-Inductance Dedicated Cables: Instrument-grade shielded twisted-pair cables with capacitance ≤ 100 pF/m and inductance ≤ 0.5 μH/m are recommended. This helps extend the effective transmission distance without affecting intrinsically safe certification compliance.
Segmented Laying + Junction Box Connections: Explosion-proof junction boxes are installed in long-distance cabling, using short jumper connections for easy maintenance and troubleshooting, while controlling the parameters of each cable segment within safe ranges.
