Shielded Cables: An Effective Way to combat EMI
Electrical noise, either radiated or conducted as electromagnetic interference (EMI), can seriously disrupt the proper operation of other equipment. Insulation protects a cable mechanically from scraps and abrasion and environmentally from moisture and spills. But insulation is transparent to electromagnetic energy and offers no protection. Shielding is needed to combat the effects of EMI. Cables can be a main source of transfer for EMI, both as a source and receiver. As a source, the cable can either conduct noise to other equipment or act as an antenna radiating noise. As a receiver, the cable can pick up EMI radiated from other sources. A shield works on both.
The primary way to combat EMI in cables is through the use of shielding. The shield surrounds the inner signal- or power-carrying conductors. The shield can act on EMI in two ways. First, it can reflect the energy. Second, it can pick up the noise and conduct it to ground. In either case, the EMI does not reach the conductors. In either case, some energy still passes through the shield, but it is so highly attenuated that it doesn’t cause interference. Cables come with various degrees of shielding and offer varying degrees of shielding effectiveness. The amount of shielding required depends on several factors, including the electrical environment in which the cable is used, the cost of the cable—why pay for more shielding than you need?—and issues like cable diameter, weight, and flexibility.
There are two types of shielding typically used for cables: foil and braid.
Foil shielding used a thin layer of aluminium, typically attached to a carrier such as polyester to add strength and ruggedness. It provides 100% coverage of the conductors it surrounds, which is good. It is thin, which makes it harder to work with, especially when applying a connector. Usually, rather than attempting to ground the entire shield, the drain wire is used to terminate and ground the
shield. A braid is a woven mesh of bare or tinned copper wires. The braid provides a low-resistance path to ground and is much easier to termination by crimping or soldering when attaching a connector. But braided shields do not provide 100% coverage. They allow small gaps in coverage. Depending on the tightness of the weave, braids typically provide between 70% and 95% coverage. When the cable is stationary, 70% is usually sufficient. In fact, you won’t see an increase in shielding effectiveness with higher percentages of coverage. Because copper has higher conductivity than aluminium and the braid has more bulk for conducting noise, the braid is more effective as a shield. But it adds size and cost to the cable. For very noisy environments, multiple shielding layers are often used. Most common is using both a foil and a braid. In multi conductor cables, individual pairs are sometimes shielded with foil to provide crosstalk protection between the pairs, while the overall cable is shielded with foil, braid, or both. Cables also use two layers of foil or braid.
In practice, the purpose of the shield is to conduct to ground any of the noise it has picked up. The importance of this cannot be overstated—and failure to understand the implications can mean ineffective shielding. The cable shielding and its termination must provide a low-impedance path to ground. A shielded cable that is not grounded does not work effectively. Any disruptions in the path can raise the impedance and lower the shielding effectiveness.