| Radiocommunications Agency
|Shielding of cables|
What this technique is used for
Cables behaving as ‘unintentional antennas’ are usually the main contributor to emissions and immunity problems at frequencies below 200MHz.
Cable shielding is used to reduce the unwanted emissions (‘leakage’) from the signals carried by a cable, and also to improve the immunity of the signals in the cables to ambient electromagnetic noise.
‘Cable screening’ is another term for cable shielding.
How this technique is used
A conductive layer is applied all around the circumference of a cable, and all along its length. For flexible cables the layer is usually a metal braid or a spirally-wrapped metallised plastic foil, sometimes both together, and sometimes multiple layers of braid and/or foil.
High-permeability metal tape is sometimes wound around the cable, along with one or more braids, to give a very high-performance (and costly) shielded cable known as ‘superscreened’. Cables which do not need to be flexible can be shielded with a solid metal outer shield, giving excellent shielding performance at a low cost.
Key issues in employing this technique
Continuity of the shield
Any holes or gaps in the shield compromise its shielding effectiveness.
The cable shield needs to RF bond to the enclosure shield at both ends of each shielded cable, otherwise there is a gap in the overall shielding and the RF emissions and immunity performance will suffer as a result.
Where an item of equipment is small, it may be possible to RF bond the cable shield to its RF reference plane instead of needing an enclosure shield.
The need for shielded connectors and glands
To be able to use shielded cables effectively, connectors and glands used with them must carry and/or bond to the cables’ circumferential shields without creating holes or gaps in the overall shield. This is sometimes called 360° bonding.
Shielded connectors and glands are sometimes called screened connectors and glands.
Bonding shields at both ends
As the frequencies used for signals, and the frequencies present in the electromagnetic ambient continue to increase, the requirement for having few very small holes or gaps in the overall shielding of a product (including its enclosure, connectors, glands and cables) gets tougher.
Many years ago it used to be normal practice in some industries to only bond cable shields at one end. This was because the poor circuit designs typically used in those industries forced any cable shield noise currents to flow in their circuits. But a cable shield that is only connected at one end provides no useful shielding for signals and noise at wavelengths shorter than six times the length of the cable (for instance: a cable 5 metres long would not provide useful shielding at greater than 10MHz if its shield was only RF bonded at one end).
In some low-frequency applications high-specification filtering can be used instead of cable shielding, but the costs usually favour a mixture of low-specification cable shielding plus low-specification filtering. Since many such applications already use shielded cables, but only connect one end, there is little incremental cost in using the screened cable properly to provide RF shielding.
In some older industrial and other applications where the meshed earth-bonding techniques recommended by IEC 61000-5-2 have not been applied, earth-faults or lightning strikes can cause high levels of currents to flow in the shields of cables that are bonded at both ends, and these can sometimes damage the cables. In such situations the use of a ‘parallel earth conductor’ as described by IEC 61000-5-2 will divert the high currents from the cable shield, allowing good RF shielding properties to be maintained.
Never use pigtails
A length of twisted-braid, foil shield drain wire, other wire or connector pin used to electrically bond a cable shield to an item of equipment is often called a ‘pigtail’.
Cable shields have large amounts of stray capacitance, making any amount of inductance in the bonds at their ends have a large effect at surprisingly low frequencies by creating a series resonant circuit. Close to the resonant frequency the shield will amplify any voltages it picks up from internal cables, or from the ambient, instead of attenuating it.
So it is now very important (and strongly recommended by IEC 61000-5-2) that cable shields are only bonded using RF bonding techniques appropriate to the highest frequency that is to be protected against.
Hybrid shield bonding
Differences in earth potential between items of equipment that are widely separated cause low-frequency shield currents to flow when both ends of a cable’s shield are electrically bonded. This will cause inductive noise coupling with the signals in the cable. But it is rarely a dominant noise contribution and is often much less than the improvement in the overall noise caused by the improved RF immunity.
Where damaging levels of shield current could flow, a parallel earth conductor can be used to divert the current away from the shield. (Note that at frequencies whose wavelength is less than 4 times the cable length, the types of shield bonds at the ends of the cable have little/no effect on the shield currents.)
However, it is possible to electrically bond the shield of a cable at one end, and bond it via a capacitor at the other end – thereby achieving RF bonding at both ends without permitting low-frequency shield currents to flow. This is known as hybrid bonding. If this is done with a discrete capacitor with connecting leads or PCB traces a resonant circuit is created which means the technique only works well over a narrow band of frequencies, usually less than a decade wide. This might be acceptable for fixing a particular interference problem at a given location.
Long shielded cables which only have their shields bonded at one end, or use capacitive bonding at one end, may need to have surge protection devices fitted at the unbonded or capacitively bonded end. These are to protect people and equipment from the high voltage surges that can occur during thunderstorms or faults in the power distribution network, and also to reduce the risk of fire. This particularly applies to cables that pass between buildings.