Understanding Cable Shielding

Foil, braid and combination screens

Aluminium foil gives 100% coverage and is highly effective against high-frequency electrostatic interference, but it is thin and its transfer impedance rises at low frequency. Braid gives 70–95% coverage with far lower DC resistance, so it handles low-frequency magnetic interference and fault current better.

Combination foil-plus-braid constructions cover both regimes and are the standard choice for variable-speed drive cables and instrumentation in electrically noisy plants.

Individual versus overall screens

Individually screened pairs (ISOS) prevent crosstalk between signals inside the same cable — essential when mixing 4–20 mA loops, thermocouples and RTDs in one multipair. An overall screen only protects against interference entering from outside.

Most instrumentation specifications end up as individually screened pairs with an overall screen, so both mechanisms are covered.

Earthing practice

Earth analogue instrumentation screens at one end only, normally the control system end, to avoid circulating currents in an earth loop. Earth data and power screens at both ends so the screen presents a low impedance to high-frequency noise.

• Never leave a screen floating — an unearthed screen becomes an antenna
• Terminate through a 360° gland, not a pigtail, wherever frequency matters
• Keep drain wire length inside the enclosure as short as practical
• Maintain screen continuity through every junction box