Many industries use shielded cables to keep data and power signals clean and provide dependable transmission. Shields keep electromagnetic interference (EMI) at bay, especially in sensitive or high-noise environments like airports and industrial facilities. However, using a shielded cable can make the installation process a little different.
For a shield to work as effectively as possible, it must be properly terminated. Shield termination methods might involve connectors, jumper wires or direct installations. Let’s explore what shielded cables do, when to use one and how to terminate shielded cables.
Table of Contents
What Is a Shielded Cable?
Cables often have many layers of protection. Insulation can protect against mechanical threats like abrasion and bending, while jackets protect against environmental hazards such as chemicals and moisture. A shield is another conductive layer between the jacket and insulation that offers an extra layer of protection against EMI. Electromagnetic interference can cause a range of problems, from a subtle hum on an audio signal to a sudden loss of power.
EMI can occur naturally in the environment, such as during storms and solar radiation. The operation of any electrical equipment also produces it as a byproduct of how electrons flow through the circuit. In areas like industrial facilities with heavy electrical equipment use, the proximity of the devices may call for powerful EMI shielding solutions.
A cable can function as both a source and a receiver for EMI. As a source, it sends out noise to other equipment and circuits. As a receiver, it picks up on the EMI that other sources radiate. A shield reflects energy and conducts received noise to the ground, protecting a cable from both receiving and emitting EMI. Any energy that does pass through the shield becomes highly attenuated, so it causes negligible interference.
A cable shield typically uses one of the following designs:
- Foil: A foil shield is a thin layer of aluminum wrapped completely around the conductors. It is usually attached to a carrier material, such as polyester, that adds strength. The shield’s thinness can make it somewhat harder to work with, and high-torsion applications can cause it to break down faster. Still, it’s often affordable and effective for static environments.
- Braid: Braided shields use a woven mesh layer of metal wires, usually copper or aluminum. The weave allows the shield to flex and create gaps when the cable bends. While these gaps can reduce coverage to about 70%-95%, the coverage is still plenty for many applications and allows for much more movement. Braided cables offer a low-resistance path to grounding and can be easier to terminate with a connector.
- Spiral winding: Some shields will arrange metal strands in a spiralized pattern around the conductors. This design works well for installations with high torsion.
You can also combine these types of cable shields for more protection.
When Should You Use a Shielded Cable?
Many environments call for shielded cables to protect sensitive equipment or ensure performance in settings with significant EMI. Using a shield is critical whenever power and data transfer paths are at risk of EMI exposure.
Shielding frequently prevents adverse effects from EMI in harsh, busy workplaces. Particularly noisy settings may require multiple layers of shielding, such as a layer of foil surrounded by a braid. You can also shield individual pairs of conductors to prevent crosstalk.
Shielded cables are necessary for consumer electronics and settings such as industrial facilities, radio stations and airports. Many regulations require certain devices to meet EMI standards and shielded cables can help accomplish this goal. Some applications, like medical devices, have strict specifications that will require shielded cables to meet.
The following components and any wiring near them often require shielded cables:
- Heavy or medium-sized motors
- Generators and transformers
- Power lines
- Electrolytic processors
- Control relays
Unshielded cables may be appropriate in controlled environments or those with little EMI risk. For example, you might use an unshielded cable for light wiring in a small office or a local area network. You could also use one for cables within a metal cabinet or conduit that offers protection from EMI.
What Is a Cable Shield Termination?
Like other cables, you can terminate a shielded cable into another component, such as equipment or another cable. Electrical termination ends the line of transmission at another device and prevents signals from reflecting.
Terminating a cable usually requires stripping back all layers except for the conductor, including the jacket, insulation and shielding. As with other protective components, the shielding must be in place up to the connector or termination point to ensure it can continuously shield the line from EMI. In other words, the termination point for a shielded cable extends EMI protection throughout the circuit’s path.
Shielded cable terminations can require specialized tools and components to link the cable and the termination point, such as:
- Connectors: Connectors are the structures that link the conductors to other components. They come in many styles, such as plugs and jacks, that you can attach in different ways, including crimping and soldering.
- Backshells: A connector’s backshell attaches to the back of the connector and guides the wires and cables into place. It can provide protection from environmental or mechanical stress like cable strain and help reduce EMI.
- Jumper wires: Jumper wires have connector pins on either end. You can use them to connect two points without any soldering.
- Termination tools: Some kinds of connectors call for special termination tools. For example, a termination tool for an RJ11 or RJ45 connector — common in telecommunications — will fit these units precisely. A quick clamp will terminate the cables in place and attach them to the prongs in the connector.
Shield Termination Methods
You can terminate a shield in several ways, including using jumper wires, floating your cable or adding heat shrink tubing or solder sleeves.
In most cases, you can start by attaching the shield to a jumper wire and solder sleeve. Then, do one of the following:
- Connect the jumper wire to the connector’s backshell.
- “Daisy chain” the jumper wire to another shield’s jumper wire.
- Route the jumper wire to the connector’s contact cavity.
If you want to float the cable — keep it disconnected from a conductor or ground point — you can forego the solder sleeve and jumper wire. Instead, place shrink tubing over the shield.
If your cable has a drain wire, commonly called a pigtail, you’ll need to use a method that retains the connection’s low impedance. You can use shielding tape to extend the shield onto the connector’s backshell. You can also use copper overbraiding to adjust the circumference of a shield that would otherwise be too large or small to fit into the backshell.
In many cases, terminating shielded cables is similar to terminating unshielded ones. You may simply need to take some extra steps to remove the shielding where you strip the wire. As with jackets and insulation, the cable’s shielding ends at the connector. Any protection from EMI and environmental or mechanical stress will occur on the other side, whether that’s within an electronic device or another cable.
How Do You Terminate Shielded Cables?
While there’s no one-size-fits-all solution, the following steps will walk you through how to terminate shielded cables into a connector. You’ll need a cable stripper, a pair of work gloves, a flush cutter, a termination tool and a screwdriver or a similar tool with a round metal rod.
1. Strip the Cable Jacket
Start by grabbing your cable stripper and running it around the cable a few times to score the jacket. The distance from the end is usually a few inches and may vary depending on your application. After scoring, remove the tool and bend the jacket a few times to separate it.
If the cable has ripcord or waterproof tape, remove that as well, up to the point where your cable jacket ends. Be careful not to nick any conductors.
2. Remove the Shield
You can simply cut a foil shield and fold the drain wire out of the way. For a braided shield, you’ll need a pricking awl. Place the awl between the weave and pull up to unravel the wires. Pull the braids out of the way and twist them to create a contact.
3. Arrange the Conductors
Once you have the conductor pairs revealed, bend them outward in a star-like pattern around the cable. If the cable has a plastic spline in the middle, make a downward cut in each edge with your flush cutters. Don’t cut straight across. After making your four cuts, simply twist the spline for even removal along the jacket’s edge.
Untwist the conductor pairs, then put on gloves and grab your screwdriver or another metal rod to remove the kinks. Start with the screwdriver at the base of the conductor and gently pull it up to straighten it out. Place the conductors into the sequence or arrangement that will fit your connector. Keep them lined up and place the connector or backshell onto the conductors. Slide it down to the appropriate distance from the cable jacket and flush-cut the conductors.
4. Reshape and Terminate the Cable
Depending on your connector, you may need to adjust the shape of the cable jacket. For example, you may need to use the oval cutout in a pair of pliers to reshape it to minimize twists and better fit a square connector. Ensure that the conductors make contact with the end of the connector.
If necessary, use a termination tool. Place the plug into the tool and press the lever down to terminate the conductors with the connector’s metal prongs.
You may also need to use the termination tool to crimp a ground tab. For example, when connecting an RJ45, you’ll need to wrap the drain wire around the cable jacket and keep it in contact with the ground tab. Bend the ground tab so that the wings are facing up. Use a screwdriver tip to push just the wings downward. Then, use the crimp cavity on your termination tool to crimp the ground tab firmly on the cable jacket.
5. Check Your Work
Ensure all contacts connect with the conductors and the connector is snugly in place.
Shield Termination FAQ
Cable termination can get tricky, especially when shielding is involved. Here are some answers to common questions about terminating shielded cables.
Where Should You Terminate the Shield of a Shielded Cable?
You should terminate a cable shield at the same place as the cable jacket. The distance from the end of the cable can vary by application and connector type. When terminating a cable directly into a shielded enclosure, the entire circumference of the cable shield must connect to the enclosure.
It’s essential not to terminate a shield too far away from the point at which the cable enters a protective conduit or the backshell. The shielding must extend to the termination point to ensure it continues protecting the line from EMI.
Do You Ground Both Ends of a Shielded Cable?
In the past, some electricians only grounded the shield on one end. They did so in an attempt to eliminate ground loops, which would create noise and disrupt the signal, particularly on audio circuits. A preferable solution is to prevent energy from leaking to ground at all. We now know that a cable shield is most effective if it creates a conductive “tunnel” around the signal lines.
To create this tunnel, you should ground the line at both ends, giving them the same ground potential to prevent current from flowing along the shield. Without grounding on both sides, one end might radiate noise like an antenna at some frequencies. If needed, add high-quality ground straps to your installation.
What Happens if You Don’t Ground Shielded Cable?
Cable shielding depends on having a low-impedance path to ground. If you do not provide this path, the shielded cable won’t work as effectively and could lead to EMI along the circuit. Similarly, a disruption to the path will increase impedance and limit the cable’s ability to resist EMI.
Master Cable Termination With AerosUSA
Working with shielded cables is a necessary part of many electrical installations. Whether you’re working with simple unshielded cables or complex systems with multiple conductors and connectors, AerosUSA can help you simplify the job. We’re your one-stop shop for cable and wire protection products, including cable entry systems and conduits.
If you want to protect devices from EMI, proper termination is essential. With a wide range of products and electrical expertise, we can help you find the right solution for your next installation. Browse our blog to learn more about cable termination and other topics, or reach out to us with any questions!
