Electrical characteristics of instrument cables

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1. Design principles for instrument cables In petrochemical plants, the selection and design of instrument cables follow these general steps: (1) Determine the applicable design standards. In accordance with the project contract and the requirements of patent holders, etc., international standards, **standards, and industry standards to be followed for cable manufacturing, inspection, and acceptance shall be selected appropriately. These standards must meet the project design requirements without conflicting with relevant **, industry, and local standards. (2) Determine the operating environment of the cable. Factors such as ambient temperature, environmental corrosion, electromagnetic interference, explosion protection rating, and installation methods have an impact on aspects of cables including the material of their conductors, insulation, shielding, armor, and installation radius. (3) Determine the parameter requirements for the cable. On the basis of clarifying the above requirements, the specific parameters of the cable should be selected appropriately in accordance with the design standards and operating environment. (4) Determine the inspection and testing of cables. Specify the manufacturing requirements for cables, as well as the inspection and testing requirements at the time of leaving the factory. 2. Electrical characteristics: The electrical characteristics of instrument cables mainly include rated voltage, insulation strength, insulation resistance, distributed capacitance, and the cable inductance-to-resistance ratio (L/R), among others. The rated voltage is usually denoted by U0/U; for example, 300/500V means that the effective value of the rated voltage between the cable conductors and ground is 300V in AC, while the effective value of the rated voltage between the cable conductors themselves is 500V in AC. Insulation strength refers to the ability of a cable, under specified testing standards and with specific power-frequency or direct-current test voltages applied, to prevent breakdown within 1 minute. Insulation resistance indicates the electrical insulation performance of a cable under normal operating conditions; it is primarily determined by the insulating material used for the conductors, and it is also influenced by the quality of the cable’s manufacturing process. In the application of intrinsically safe safety systems, the distributed capacitance and inductance of cables act as energy storage elements that affect the performance of such systems; therefore, they must meet the requirements of the wiring design for intrinsically safe circuits. The value of L/R is related to A; when A ≤ 1 mm2, L/R < 25 μH/Ω ； When A=1.5mm2, L/R＜40μH/Ω ； When A=2.5mm2, L/R＜60μH/Ω.