Shielded Cable Assembly: All You Need To Know

Shielding is the process of encasing the inner wires of a cable with a conductive wrapper. It helps to increase the cable’s thickness to some extent, leads to more weight, and reduces flexibility. However, all this is of little consequence as the main purpose of shielding is to protect the signals transmitted through the wire from external elements such as EMI corruption. Let us delve further into the process of the shielded cable assembly.

Why Shielding Is Important For Cable Assemblies

Below are the two main reasons shielding is so important for cable assemblies.

1. For Optimal Performance

When working with electrical components like cable assemblies, you must always remember that you are handling electrical conductors responsible for carrying power or signals alongside the circuit. Once the transmission process begins and the current starts to flow, the conductor will create an EM field that needs protection from different kinds of surrounding interference that could affect its performance.

The surrounding environments and the electrical components of a cable require protection from a conductor’s emitted interference, which you can achieve through cable shielding. It is an important process that helps to create a barrier between the conductor and its environment. Therefore, to achieve optimal performance, correct shielding is important.

2. To Avoid the Problem of Electromagnetic Interference

A cable assembly comprises single or several insulated conductors bunched together and wrapped inside a single insulation sleeve, sometimes referred to as a sheath. It is often constructed of vinyl, thermoplastic rubber, or thermoplastic. It protects conductors from external elements such as moisture, cold, or heat. However, it does not do much when it comes to other major threats like EMI (Electromagnetic Interference), which can significantly affect the overall performance of the cable and signal integrity.

For your cable to function at an optimum level, electric currents must flow smoothly along the EM field. If there are multiple EM fields, electromagnetic interference may occur.

If you’ve used a cell phone, TV, or radio, you’re probably familiar with EMI effects or radio frequency interference. EMI occurs when two EM fields interfere, distorting fields resulting in loss of data, low signal quality, or total failure of devices. Some types of Electromagnetic Interference include inductive, galvanic, radiation, and wave. 

Surrounding electrical noise can harm the cable’s EM field. Also, the electromagnetic energy emitted from the conductor may affect the EM fields of sensitive components around. In some cases, cables can affect each other. For instance, a power cable placed next to a signal cable will affect the signal cable when powerline noises mix with the signal lines.

How do Shielded Cables Work?

A shielded cable has a protective metal sheath that functions in two ways. The first part involves reflecting a percentage of the incident EMI. The second part is its redirection to a low-impedance path so that it gets to the electrical ground found at alternative ends of the cables.

Once you expose the cable to EMI, the grounding will allow the currents created within the sheath to flow safely, thereby protecting the signal-carrying cables inside from any interference by EMI.

You shouldn’t ground shielded cables at their opposite ends since this may result in issues such as ground loops, rendering the advantageous effects of sheathing null and void.

A substitute for shielding assembly is the twisted pair cable, which is two insulated wires wrapped around one another. The LVDS (Low-voltage differential signals) for these wires are typically sent over the twisted pair of cables. That is limited protection against more powerful sources.

Caption: Twisted Pair Cable

Types of cable shielding

When making a cable assembly, you must ensure the internal conductors and any external elements do not cross paths with each other. As stated, the best way to do this is with cable shielding. They include the following:

Braided Shielding

Due to the higher labor and material costs of braiding, some users have shifted to newer and better alternatives when it comes to cable shielding. However, braided shielding remains a rugged option for some. It’s achieved through interweaving bare copper or small-tinned strands in a criss-cross manner around the insulated center of the conductors. The level of protection primarily depends on the number of strands, the angle, and just how tightly the braided formation has been woven.

Pros

• Consistent coverage
• Proper mechanical strength
• Great flexibility
• Great versatility and flex life
• Low resistance path to ground
• Easy termination
• Crush resistance

Cons

• Pattern prevents maximum coverage
• Limited effectiveness at high frequencies
• Not suitable for tight spaces
• Limited flexibility compared to foil 
• Costly

Foil Shielding

This shielding is normally paired with a tinned-copper drain to eliminate and ground the shield. It encases a cable using a thin layer of aluminum or other conductive materials attached to polyester to promote better resiliency. You can also tape it with materials such as Mylar for sufficient protection.

Pros

• Total coverage
• Supports high frequencies
• Lightweight design
• Easy to terminate
• Cheaper compared to other options
• Compatibility with other materials allows customization.

Cons

• Less effective at low frequencies
• Poor flex life
• Poor mechanical strength

Spiral Shielding

Typically used with audio components such as speaker cables and microphones, spiral shielding, also known as served shielding, is very similar to braided shielding, with the main distinction being that it wraps the cable core or conductor using single strands of copper wire placed in a spiral pattern as opposed to a woven one.

Pros

• Extremely flexible
• Lightweight design
• Resistance to kinks and pulls
• Easy to terminate
• Cheaper to manufacture

Cons

• Less efficient at low frequencies
• Less tensile strength

Combination Foil/Braided Shielding 

Manufacturers tend to recommend custom solutions for cable wires that involve the combination of different shielding options to ensure their effectiveness for a particular application, especially when working in very noisy environments. 

For superior strength, better flexibility, and good coverage, pairing braided with foil shielding is typically the way to go. You can use the mix in a variety of industrial applications. Also, individual pairs are often shielded with foil to protect against crosstalk, whereas the overall cable is shielded using braid, foil, or both.

Caption: Foil Shielded Cable

Shielded vs. Unshielded Cable: When to use

There are two scenarios where you can use shielded cables.

• The first is when you expect the cable to run near potential sources of EMI. For instance, an industrial facility with huge motors and various other kinds of current-drawing machines.
• The second one involves cases where corruption or degrading of the transmitted signals could lead to costly or damaging results. For instance, equipment in motion can lead to spurious signals prompting unexpected movements with dangerous outcomes. Another example is when manipulating the carried data is inconvenient or could result in inaccurate records and wrong decisions. The fact is that cabling used in high-safety applications has to have shielding features to protect against accidents. 

On the other hand, office spaces, different kinds of commercial premises, and other domestic environments usually don’t have substantial EMI sources. Therefore, the extra costs of having shielded cables would be unnecessary in such environments. A twisted pair of wiring will be enough to do the job. 

Industry Applications Of Shielded Cable Assembly

The following are some industry applications commonly seen with shielded cable assembly.

Communications 

You can use it for control systems, CCTV operations, alarms, video, and equipment instrumentation.

Defense 

Since the defense sector requires high levels of effectiveness and reliability, and it is not always possible to comprehend EMI exposure, you will have industries working in this sector using shielded cables, especially when it comes to military equipment.

Aerospace 

Aircraft can be easily exposed to potential sources of RFI and EMI, making quality shielding a priority. 

Medical

Medical equipment used for monitoring, diagnostics, and imaging can get placed in proximity to both RFI and EMI sources, making shielding a necessity.

Conclusion

Use of incorrect shielding, or worse still, failure to use shielding at all, can result in various issues with electrical equipment. As a result, one has to be keen when acquiring such cables and understand how to utilize them effectively. For all your cable assembly needs, feel free to contact Cloom Tech.