Instrumentation Cable Specifications Follow the Physics of the Sensor
Instrumentation cable is specified from the sensor backwards. The governing standard is EN 50288-7, the sectional specification for multi-element metallic cables carrying analogue or digital signals in industrial processes: conductors from 0.5 to 2.5 mm², a 300/500 V rating, and an explicit instruction that the cable is not to be connected directly to a low-impedance supply network. What the sensor outputs decides the construction. A 4–20 mA loop rejects induced noise because it is a current, but conductor resistance still consumes transmitter voltage, so the loop budget is calculated across the whole run. A thermocouple outputs microvolts — roughly 41 µV/°C for type K — so extension cable must reproduce that same electromotive-force-to-temperature characteristic per IEC 584-3; otherwise every connection point becomes a second measuring junction and adds error. Compensating cables use cheaper alloys and stay valid only over a limited temperature range. A Pt100 resistance thermometer under IEC 60751 changes about 0.385 Ω per degree, meaning 0.1 Ω of extra lead resistance costs roughly 0.26 °C — the arithmetic behind three-wire terminations. Distributed parameters become safety data in hazardous areas. Cable capacitance and inductance add to the external capacitance and inductance allowance of an intrinsically safe circuit, so per-metre figures — EN 50288-7 sets maxima around 250 pF/m mutual capacitance and 1 mH/km inductance — must be recorded and checked against the barrier certificate. A blue outer sheath identifying IS circuits is a project convention, not a requirement of the standard. Order pair count and screen configuration from the loop schedule, not the catalogue, and verify the extension cable's own temperature limit before routing it beside a hot line. A measurement is only as accurate as the copper carrying it.
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