The electromagnetic noise environment in industrial applications is often very complex, and the radiated or conducted (EMI) of electromagnetic noise can
seriously interfere with the normal operation of electromechanical equipment. In this process, an important carrier for electromagnetic noise propagation
is the various cables used in production line equipment. Some of them are noise sources, and some are disturbed objects. As a source of noise, the cable
propagates noise to surrounding lines and equipment like a radio transmitting antenna; as a victim, the cable absorbs noise from other sources like the
receiving antenna. It should be noted that large power switches, induction heaters, large transformers, etc., are likely to emit higher levels of conducted
noise and radiated interference; placing signal cables near the power line also couples noise to the signal lines Up.
A very important way to combat electromagnetic noise interference on electrical lines is to use shielded cables.
A shielded cable is a cable in which one or more (insulated) wires are wrapped by a common conductive layer. This common conductive layer is the
shielding layer of the cable, which is generally composed of a braided strand of copper (or other metal such as aluminum), a non-woven spiral wound of
copper strip, or a conductive polymer layer.
The inside of the shielding layer can wrap the signal or the power wire, and the electromagnetic noise interference can be prevented by two ways.
On the one hand, it can simply reflect noise energy as an isolation layer;
On the other hand, it can absorb noise and conduct it to the ground, which becomes the return path of the noise signal.
And in either case, electromagnetic noise is not transmitted directly to the line conductor. Although noise energy is still likely to pass through the shield,
it is usually attenuated to a large extent and it is difficult to cause interference. Therefore, regardless of whether the cable itself is a source of interference
or a victim, it is very effective to use shielding for it.
Cable shields have different performance levels and offer different levels of shielding. At the same time, there are many factors that determine the amount
of demand for shielding, such as electrical application environment, cost (ie why pay higher cost for more shielding?), and cable diameter, weight and flexibility...
There are two types of general cable shielding: metal foil and woven mesh.
Metal foil shields typically use a thin layer of aluminum attached to a support such as a polyester material to increase strength and make it stronger.
The metal foil provides a 100% wrap coverage for the wire and provides better shielding performance. But at the same time, because it is very thin,
it is more difficult to use, especially when it is connected to the connector. Therefore, it is usually not the case that the entire metal foil shield is grounded,
but a drain wire is used to connect the shield.
Braided mesh shields are usually woven from bare copper wire or tinned copper wire. It provides a low-impedance ground path for electromagnetic noise
and can be docked by crimping or soldering when using connectors.
Woven mesh shielding does not provide 100% coverage because they always have some small gaps in the surface of the wire. Depending on the tightness
of the weave, the woven mesh usually provides 70% to 95% coverage. However, in general, for fixedly laid cables, 70% of the shield coverage is sufficient.
In fact, despite the high shielding coverage of metal foil, this does not mean that its shielding effect is even better. The reason is that copper has a stronger
electrical conductivity than aluminum, which allows the woven mesh to conduct electromagnetic noise better. Therefore, as a shielding layer, the woven mesh
tends to have a better shielding effect. Of course, it is not difficult to see that it also increases the size and cost of the cable to some extent.
In applications where the noise environment is harsh, multiple layers of shielding are often required. The most common is the use of both metal foil and
woven mesh. Sometimes in a multi-core cable, each pair of wires will have a metal foil shield to prevent crosstalk between the twisted wires, while the entire
cable will also be shielded with metal foil or woven mesh or both. This combination of metal foil and woven mesh allows the two shielding technologies to
support each other and overcome the other limitation with one technology advantage, thus providing the cable with shielding beyond any single technology
In practical applications, the purpose of shielding is to conduct the induced electromagnetic noise to the ground and become the return path of the noise signal.
Failure to understand its meaning may mean invalid shielding. The cable shield and its termination must be able to provide a low impedance ground path for
electromagnetic noise. Ungrounded shielded cables will not work effectively, and any breakpoints or excessive nodes in the ground path will reduce the shielding
performance of the cable by increasing the ground impedance.
Finally, summarize some practical suggestions about cable shielding:
1. Make sure the cable is adequately shielded to meet your application needs. In a common electromagnetic interference environment, the use of metal foil alone
should provide sufficient noise protection; in a harsh noise environment, it is necessary to consider the use of shielded cables that combine both woven mesh and metal foil;
2. Use a suitable shielded cable based on the specific application environment. For example, a cable that needs to be repeatedly bent during use usually
uses a spirally wound shielding layer instead of a woven mesh. At the same time, flexible cables tend to avoid using only metal foil shielding because the
continuous bending of the cable may tear. Cracked foil layer
3. Make sure that the equipment to which the cable is connected is effectively grounded. Use the earth as much as possible and check the connection
between the ground point and the device; the elimination of electromagnetic noise depends on the low impedance of its ground path;
4. Many devices and connectors are designed to allow 360° omnidirectional shield connections to ensure reliable engagement with the cable shield.
For example, many common connectors are equipped with metal-coated plastic, cast zinc or aluminum housings; connectors that match the cable
shielding performance should be selected to avoid shielding effects or excessive specifications due to low standards. The cost is increased.