Taking A Closer Look At Shielded Cable
Several industrial and commercial applications such as traffic control, mechanical and industrial factories, telecommunications networks, powerlines and any other places where the use of machines (such as welding machines, motors, and digital networks) is prevalent, cause significant electrical noise. This noise, either radiated or conducted as Electromagnetic Interference (EMI), can seriously disrupt everyday operation of equipment. Insulation in a cable will mechanically protect it from scraps and abrasion while environmentally protecting it from moisture and water. However, insulation is transparent to electromagnetic energy and does not offer protection against it. Thus, shielded cable is needed to elude the effects of EMI.
Cables are usually the main source and receiver of transmitting EMI. Being the source, cables can either produce noise to other equipment or act as an amplifier radiating noise. But then as a receiver, cables can pick up EMI radiated from other sources. Shielded cable prevents both of these from occurring.
Installing communication cables near or next to power cabling can also allow power-line noise to bridge into the communications lines.
The following chart shows the general guidelines for areas that are exposed to varied levels of noise transmitted over the air. Machines such as inductive heaters, transformers, generators and switching heavy loads can present high levels of conducted and radiated EMI.
Shielding is the primary way to combat EMI in cables. The shield protects the internal signal or power-transmitting conductors. The shield interacts with EMI in 2 different ways.
1) Reflects the energy
2) Picks up the noise and conducts it to the ground
No matter the case, some energy still passes through the shield, but it is so highly attenuated that it doesn’t cause interference nor reach the conductors.
Cables contain various levels of shielding and provide varying degrees of shielding efficiency. The degree of shielded cable required depends on several factors.
Factors That Affect the Required Shielding Degree in Cables
- The Electrical Environment in which the cable is used
- The Cost of the Cable – no need to pay for higher shielding in applications with low EMI
- Cable Diameter, Weight and Flexibility
In many industrial applications, unshielded cabling is used in a controlled environment where it is installed inside a metal cabinet or in conduit protecting it from ambient EMI. The cabinet or conduit metal enclosure shields the electronics and cabling inside.
Shielding Usually Comes in Two Types:
- Thin layer of aluminum, usually attached to a conveyor like polyester to add strength and ruggedness.
- Provides 100% coverage of the conductors it surrounds.
- Because it is a thin layer, it is much more difficult to work with, even more so when using a connector.
- A drain wire is usually used to terminate and ground the shield, rather than trying to ground the entire shield.
- Woven mesh of bare or tinned copper wires.
- Braid provides a low-resistance path to ground.
- Much easier to terminate by soldering or crimping when applying a connector.
- Provides somewhere between 70 to 95% coverage depending on the tightness of the weave.
- If a cable is stationary, then 70% coverage is sufficient.
- Increase in the effectiveness of shielding does not usually occur since copper has better conductivity than aluminum and the braid has more bulk for conducting noise.
- The braid is more effective than foil as a shield, however the braid adds cost and size to the cable.
Multiple shielding layers consisting of foil and braid are often used for very noisy environments. Sometimes, individual pairs are shielded with foil to provide crosstalk protection between the wiring pairs, while the overall cable is shielded with foil, braid or both. Two layers of foil or braid are also used in cabling.
The main purpose and key point when using proper shielding is to correctly ground any noise that has been picked up. Failure to understand this importance of grounding will result in improper shielding and work ineffectively. The shielded cable and its termination point must provide a low-impedance route to the ground. Any disruptions in the path of the cable will increase impedance and decrease the shielding efficiency.
Proper Shielding Guidelines
- Acquiring cabling with the proper shielding for your application needs.
- Make sure to use the proper cabling for your application needs.
- Most importantly, be sure the equipment that is connected via the shielded cable is properly grounded.
- Prevent noise inducing ground loops by grounding the cable at one end.
- Confirm the connector offers shielding that matches that of the shielded cable.
-A foil shield should be adequate protection for an average noisy environment.
-A braid or foil-braid shield should be utilized in very noisy environments.
-Cables that flex use a spiral wrapped shield usually.
-Foil shielding should not be used with flex cables as the repeated flexing can tear the foil shield.
-Wherever available, use an earth ground and check the connection between the ground termination and equipment.
-A low resistance path to the ground will prevent noise.