Cable Shielding
A method of keeping external electric fields away from the signal with a braid or foil layer wrapped around the signal conductors. The shield offers the noise current a low-impedance return path and stops it reaching the signal conductor. Its effectiveness depends far more on how it is terminated than on the quality of the screen; a badly terminated shield can do harm instead of good.
A shield comes in two kinds of construction: aluminium foil gives almost full coverage at high frequency but is mechanically fragile; copper braid is flexible and more effective against low-frequency magnetic interference. Cables that use both together protect over the widest band.
For the shield to do its job, the noise current must be able to return to its source through the screen. If this return path is high impedance, the shield is useless.
- 360-degree termination: the screen must be pressed all the way around against the connector body or the gland. Connecting it with a thin pigtail wire raises the impedance at high frequency and reduces the effect.
- Where it connects: the shield is connected to chassis, not to circuit ground, with the lowest possible impedance.
- One end or two: on low-frequency signal cables one end is generally preferred to avoid a ground loop, while two ends are preferred where high-frequency noise is involved.
- Coverage ratio: braided shields typically show 70–95% coverage; as the ratio falls, high-frequency leakage rises.
- Continuity: every point where the shield is broken is a leakage gate; continuity must be maintained in intermediate junction boxes.
- Common mistake: cutting the shield and leaving it insulated. In that case the shield does not protect and in some arrangements even turns into an antenna.
A similar rule applies on the enclosure side: a closed conductive box blocks RF. This is a desirable property for keeping noise in, but it is a problem for the receiver antenna; that is why in IP67 sets the antenna is supplied so that it can be taken outside the panel through a cable gland.
On low-level signals such as crane remote control antenna cables, proportional valve feedback lines and tilt sensor signals, using screened cable and terminating the shield correctly is the precondition for being able to work on the same vehicle as high-current motor cables.
The context in which this term is used: Installation, redundancy and field continuity