Requirements of Ethernet Cable for Industrial Environments
Is an extra layer of protection all you need to make an Ethernet cable industrial-ready? Here, Lu Li of Alpha Wire explains the requirements of Ethernet cable for industrial environments.
Moving Ethernet from an office environment into the industrial world is not as easy as one might think. The office environment offers its cabling system a relatively safe harbor, whereas industrial applications are often subject to harsh and hazardous environments such as oil, gas, and petrochemical plants; water and wastewater facilities; wind farms; and mining. Such environments require the Ethernet cable to resist temperature extremes, humidity and moisture, dust and mud, oil and solvents, corrosive chemicals, mechanical vibration, EMI interference, and more.
In order to meet Industrial Ethernet cabling demands, some cable manufacturers have introduced so-called “Industrial Ethernet” cables that are essentially commercial off-the-shelf (COTS) Ethernet cable covered by an extra jacket. This kind of Ethernet cable has certain rugged qualities, but is usually oversized due to the dual-jacket construction. This larger cable is not convenient for installation and is extremely inflexible when compared to the common COTS cable with the same construction. Another huge disadvantage of such cable is that the temperature rating is still limited by the COTS cable itself. To provide a complete solution to the above-mentioned requirements, a special cable customized to the industrial environment must be designed using the
current available technologies.
A typical Ethernet cable construction consists of either two or four twisted pairs surrounded by a metal shielding (for shielded cable only) and covered by a jacket. For Cat6 and above, a cross-filler is often used as well.
The Telecommunications Industry Association (TIA) has developed voluntary, consensus-based industry standards for a wide variety of ICT (information and communication technologies) products, and currently represents nearly 400 companies. Its TIA-568-C.2 specifies the standards for cable categories such as Cat5e, Cat6, Cat6a, and Cat7. Let’s use Cat5e Industrial Ethernet cable as the example in the design factors discussed below.
Typically, oxygen-free high conductivity (OFHC) copper is selected as the conductor material for Ethernet cables. This kind of material has 99.95% minimum copper content and an average annealed conductivity of 101%. Depending on the Ethernet cable type, bare copper and tinned copper are often used for category cables. The copper conductor size and stranding construction determine the DC resistance, insertion loss (previously called attenuation), and other related electrical parameters. Generally speaking, 26AWG to 22AWG conductors are used for this type of design. A slight variation on the size is usually adopted
to meet the insertion loss while keep the cable light and flexible.
Depending on the flame resistance requirements, polyolefin (polyethylene and polypropylene) and fluorinated ethylene propylene (FEP) are commonly used as insulation material. These materials have good dielectric constant and dissipation factors which are critical to Ethernet cables’ electrical performance including impedance and insertion loss. It should be noted that both dielectric constant and dissipation factors are frequency-dependent, meaning that to make the cable Cat5e-, Cat6-, Cat6a-, or Cat7-compliant, a different grade of the aforementioned materials should be considered since the operating frequency spans of these category cables are different.
Eccentricity is a critical parameter at the extrusion stage. High eccentricity is the root cause of impedance irregularities by means of the unstable mutual capacitance when the conductors are twisted into a pair. A desired eccentricity should be kept within 3% for a
Pair twisting is a very important process during the cable’s production because almost all of the major electrical characteristics are determined at this stage.
Design-wise, the twist lay length is the main concern. Each pair’s lay length affects not only each individual pair’s impedance, signal transmission delay, and time skew, but also the crosstalk between pairs. Process-wise, the twisting not only determines how balanced the insulated conductors are (electrically determines the resistance unbalance, capacitance unbalance, differential, and common mode performances such as TCL and TCTL), but also influences cabling since an imperfect twisting can exaggerate the flaw caused by the cabling process itself. Because the signals with different frequencies travel in twisted pair, a certain signal will have the same (or harmonic) frequency as the pair’s intrinsic frequency determined by the twist lay length. So the energy coupled between pairs at this frequency will be accumulated, causing a spike showing on the crosstalk graph.
Cabling can be as simple as bunch-bundling four pairs in sequence for lower category cables or as complicated as adding filler with all kinds of shapes, isolation tapes (usually clear Mylar tape), and drain wires (usually stranded tinned copper wire) all together as in the higher category cables. Various filler designs have been seen in the network cable industry, but the goal stays the same: minimize the crosstalk between pairs. Although all kinds of material can be used as the filler, a rule of thumb is to use a material that has similar or better characteristics than the insulation material. The major purpose of using an isolation tape is to segregate the pairs from the drain wire and shielding tape/braid to avoid hipot failure. At the same time, it pushes the drain wire and shielding tape/braid away from the pairs to help electrical performance.
Another design variable to help the overall electrical performance is using variable cabling lay length. Occasionally the intrinsic frequency of cable lay length will inter-modulate with the pairs’ frequency to cause a cabling spike. By using the variable cabling lay, one can spread out the cabling spike into a range of frequencies. In other words, the energy with higher amplitude at one frequency can be redistributed to many frequencies with lower amplitude.
Braiding is not common for typical Ethernet cables, but for industrial-grade Ethernet cables, braiding is an excellent way to minimize low frequency EMI and add extra mechanical protection to the cable. With the possibility for cable abuse on the factory floor, a braid will also assure the grounding continuity for the low frequency machine noise and high frequency electromagnetic influence.
That being said, the braiding is an extremely difficult process with a high potential to jeopardize electrical performance. An uncontrolled braider tends to introduce variable tensions when the pay-off bobbins have less and less material left. Thus, the braiding process is of utmost importance in manufacturing a successful Industrial Ethernet product.
Jacket extrusion involves covering the cable core with thermoplastic material for mechanical protection and adding shielding material for the EMI protection required for shielded cables. For a foil-shielded cable, Aluminum/Mylar tape is usually applied with drain wire in this process rather than in the cabling process; if the drain wire is spiral-applied or foil tape faces out, other equipment that’s not convenient to operate at a jacket extrusion line is needed. A tape folding guide and a series of closing dies are arranged in sequence to apply the shielding tape with 100% coverage while keeping the desired tightness. Improper tightness can vary the impedance by three ohms or more.
For a foil/braid shielded cable, jacket extrusion involves only one process – jacketing – because the foil/braid process has to be done separately. However, a paper tissue might be applied at this stage to keep the jacket material from intruding into the braid interstices. Using paper tissue will give the jacket a tube or semi-tube configuration versus a pressure-extruded configuration.
As the demands for reliable, real-time automation and process control in industrial environments increase, an Industrial Ethernet cabling system quickly becomes the most attractive solution. Cables used in industrial environments must be designed to work in harsh environments of temperature extremes, humidity, and vibration. Many times this can exceed the ranges for information technology equipment intended for installation in controlled environments.
This article is excerpted from a white paper originally published on Alpha Wire’s site. To read the paper in its entirety, click here.
Dr. Lu Li is senior product development engineer at Alpha Wire. He has extensive development experience in networking cable for industrial and commercial environments, transmission lines for power systems, and communication, control, and instrumentation cables for other specific applications.