If you’ve specified or installed tray cable, you’ve likely seen the thin layer of foil or braided metal surrounding the wire pairs or all the conductors.
This layer is called shielding. Its purpose is to collect and drain off electromagnetic interference (EMI) and radio frequency interference (RFI) caused by common mode currents. When common mode current is generated through a copper conductor, EMI is created naturally by the copper’s electrical properties.
The noise and interference can affect other wires, cables, and electronics in the local system. Shielding helps offset those effects in power and communication cables, sensitive electronics, and network systems near the cabled electrical system.
Placing a layer of foil or braided metal between the tray cable’s jacket and conductors substantially reduces EMI effects. The shielding, through its natural electrical properties, attracts, collects, and effectively (when properly grounded) drains off the EMI. This will help prevent cable crosstalk and other negative effects within a variable frequency drive (VFD), network switch, or other electronic devices.
Depending on your needs and budget, there are several types of shielding available, including:
Aluminum foil is one of the most common shielding options, though copper and tinned copper are other popular choices. It’s important to know that although shielding is good for keeping EMI interference to a minimum, it is not meant to protect conductors from impact, crush, or abrasion damage. That falls on the tray cable’s jacketing and insulation, which offer mechanical protection.
Variety is the spice of life, and that applies to tray cable. Similar to picking your insulation, jacketing, and conductor type, you can also choose how the shielding is applied to your tray cable.
Some application methods achieve better coverage than others, which provides better overall shielding protection. Others may have less coverage but are much more flexible options. When deciding what direction to go, consider the environment where the cable will be installed and what possible interference could occur in the system.
Remember, EMI is inversely proportional to wire gauge size; the larger the wire size, the less EMI will be transmitted from the current. When many smaller gauge sizes are collected in a system, more EMI will be present.
Foil is a low-cost option to get the most coverage possible without sacrificing weight.
The most common type is aluminum foil with a polyester backing for commercial and industrial applications, but copper foil can also be used. Copper shielding is used in substation and power generation applications. Both are excellent choices for high-frequency situations and locations.
Whether aluminum or copper, foil shielding is applied the same way. To wrap the cable, shielding is applied around the conductors with a 25% overlap. The slight overlap ensures the entire tray cable is covered, allowing it to block out 100% of EMI, but limits the cable’s flexibility. It’s also worth noting that foil isn’t the strongest shield, making it less durable than braided or spiraled options.
No matter which metal is used, foil shielding needs a grounded drain wire to operate effectively.
With a braided shield, metal strands are wrapped and braided around the conductors to create a flexible tube.
This shield type is usually made from copper and offers about 92% EMI protection. Despite offering less EMI protection than foil tape because it can’t cover the entire cable, a braided shield provides some impact resistance, has better mechanical strength, and is a bit more flexible. The braiding also helps extend the life of the cable itself.
Unlike low-cost and low-weight foil, braided shields are more costly to create, make the cable heavier, and may be more difficult to terminate.
The best use for this type of tray cable shielding is locations where low-frequency noise is prevalent.
For those who seemingly want it all, combination tray cable shielding is there for you.
To create a combination shield, foil is wrapped about the conductors and then a braided shield is layered on top of it. The resulting cable completely blocks EMI and has low-frequency noise protection. This type of wrapping is used mainly for signal and communication cabling, thanks to its high EMI blocking and serviceable flexibility.
Like the foil wrapping, a combination shield needs a grounded drain wire.
Take the stranded metal used in the braided shield but wrap it around the conductors the same way foil would be applied, and you’ve got spiral shielding.
The most flexible shield type, a spiral will cover about 95% of the cable and block almost all signal noise. However, in the small portion of areas where there is no protection, interference may impact the cable’s performance and create problems for nearby wires when bent.
Spiral shielding has great mechanical strength. It’s typically used with smaller gauge wires and large conductor counts, providing excellent protection for audio and low-frequency sources.
With several options available, deciding what type of shielded tray cable you need can be tough.
Tray cable is an indispensable tool capable of fitting many industrial, commercial, and retail situations, but it’s only as good as the materials used to create it. Understanding how and where the wire will be installed can determine everything from the shielding used to prevent EMI interference to the jacketing and insulation used to protect the cable from other threats.
One way to decide is to see where the wire will eventually be installed and what cables, wires, machines, and other appliances may be nearby. It also helps to ask a few questions, like:
Once you have the answers to these questions, you’ll be better prepared to select your tray cable shielding, jacketing, insulation, and any other protections you may need to get the most out of your cable.
Subscribe to the Kris-Tech Blog
