The word “screen” covers several different constructions, and “screen” and “armour” are often confused. A screen protects the signal or the surroundings from electrical interference. Armour protects the cable from mechanical damage. This chart sets out the screen types met in industrial cables and where each one belongs.
Screen against armour
| Feature | Screen | Armour |
|---|---|---|
| Main purpose | Limits electrical interference, in or out | Resists crushing, impact and, for wire armour, pulling force |
| Typical material | Copper braid, aluminium or copper foil tape | Galvanised steel wire, steel strip or aluminium |
| Position | Over the cores, or over each pair or core | Over the bedding, beneath the outer sheath |
| Earthing | Connected to earth at a defined point or points | Often bonded to earth as well |
Screen types
| Type | Construction | Strength | Limit | Typical use |
|---|---|---|---|---|
| Foil (tape) | Thin aluminium or copper foil on a plastic carrier, often with a drain wire | Full coverage of the surface, light and low cost | Weaker at low frequencies, and can crack if flexed repeatedly | Fixed instrumentation and data cables |
| Braid | Woven copper wires over the core | Low resistance, good mechanical durability, good at lower frequencies | Gaps between the wires, and heavier and costlier | Flexible cables and power-frequency noise |
| Foil plus braid | Foil layer and braid together | Broad frequency coverage | Costliest, with a larger diameter | Harsh electrical environments |
| Individual screen | Screen over each pair or core | Reduces crosstalk between pairs | Larger diameter | Multi-pair instrumentation |
| Overall (collective) screen | One screen over all pairs or cores | Protects the group from outside interference | Does not reduce crosstalk between pairs | General multi-core control and data |
Reading the APAR instrumentation cable description
APAR’s screened and instrumentation cables are described by their screening arrangement. CAM refers to a collective screen, and IAM/CAM or ICAT refers to individual and collective screens together. Each arrangement is available unarmoured (Type 1) or with single wire armour (Type 2). The instrumentation cable selection guide explains how to choose between them.
Termination rules that apply to every type
- A screen only works if it is connected. A screen left floating at both ends gives little benefit.
- For low-frequency instrument signals, earth the screen at one end only, normally the control room end, to avoid earth loop currents. For high-frequency noise, such as from drives, a connection at both ends is common. Follow the system designer’s instructions.
- Keep the screen connection short. A long pigtail wire adds inductance and reduces the screen’s performance at high frequencies.
- Use glands made for the cable type, as set out in the cable gland selection guide.
- Do not use a screen as the protective earth conductor unless the cable standard and design allow it.