Ethernet, the undisputed king of networking, has reigned in commercial office environments for several years. Today, the industrial control and automation sector is rapidly moving toward this open, standards-based technology. Yet some challenges exist when it comes to specifying the physical layer -- the cabling system.
Do not assume that the cable acceptable at the office receptionist's desk will suffice in the harsh, real world of industrial process control and automation. Commercial-off-the-shelf (COTS) Category 5e (Cat 5e) unshielded twisted pair (UTP) Ethernet cable is actually quite fragile by industrial standards. And, beyond the obvious physical hazards to the cable are the more insidious, invisible hazards of electromagnetic interference (EMI) and radio frequency interference (RFI). Let's examine some of the issues that would likely arise if one attempted to utilize a non-hardened, commercial grade Cat 5e UTP on the plant floor. (Tough environments demand tough cabling solutions. Ruggedized industrial Ethernet cables, such as the Belden® DataTuff® UTP copper and TrayOptic® fiber optic cables shown here, offer maximum cable protection and reliable signal transmission, even in harsh environments. Several of the UTP cables feature Belden's patented Bonded-Pair technology to provide the strength and balance required to withstand EMI/RFI.)
Installation Challenges
Pulling a commercial grade UTP with excessive force will stretch the cable. Commercial standards specify 25 lb maximum pulling tension. You may start with 100 feet of cable and end up with 100 feet and two inches! This elongation causes excessive signal loss (attenuation) and signal delay, effects that will limit the distance the cable can be run.
Furthermore, as the cable is handled, the twisted pairs may open up, changing the pair conductors' center-to-center spacing and inducing imbalance. This will result in more coupling from pair-to-pair (crosstalk), signal echoing (return loss), and greater susceptibility to ambient EMI/RFI. Any one of these factors could cause a loss of data on the cable, resulting in process downtime or safety issues.
One solution to this problem is to specify cables incorporating Bonded-Pair technology. In these cables, the insulated conductors of each pair are physically joined together along the entire length of the cable, which prevents the pairs from separating or gapping. So, no matter how you handle the cable, it maintains its mechanical stability and robust electrical performance.
Temperature Effects
Temperature also plays a significant role in cable performance in industrial environments. Extreme cold makes cables stiff, brittle, and hard to work with, and extremely elevated temperatures can degrade the plastic used in the construction of the cable. It is important to specify cables designed to withstand the ambient temperatures expected.
The cable's temperature rating is not, however, the only consideration. Typical COTS Cat 5e cable attenuation increases at a rate of 0.4% per degree Celsius above 20°C. At 60°C, the attenuation may be increased by 16%. For these reasons, it's also critical to select a ruggedized cabling system that offers industrial-strength, high-temperature attenuation performance.
Ultraviolet Radiation and Sunlight
Most commercial Cat 5e cables are not designed for outdoor use. When COTS cables are exposed to the ultraviolet (UV) radiation in sunlight, the plastic outer jacket tends to decompose at an accelerated pace. The jacket starts to lose mechanical strength (i.e., fall apart), thus limiting the useful life of the cable. To combat this problem, look for cabling products that provide sunlight-resistant (SUN RES) rated jackets to ensure maximum service life in real-world applications.
Oil Exposure
In the day-to-day world of manufacturing, machinery and its moving parts must be kept well lubricated to run smoothly. The petroleum-based lubricants used pose a real threat -- they will soak into COTS Cat 5e cables, especially under heat. The oil causes the jacket to swell and lose mechanical strength, leaving the inner core susceptible to contamination and degradation. In choosing cables for industrial environments, be sure to specify oil-resistant jackets that can meet and overcome this challenge.
Mechanical Damage: Abrasion, Cut-through and Crush
Industrial workers walking all over Ethernet cables can cause errors or signal delays in the network. Forklifts and other vehicles driving over cables may even take the network down -- not a desirable situation when plant floor processes and productivity depend on timely, reliable data transmission. To ensure optimal network performance in harsh environments, specify cables with industrial-grade jackets. Some super-tough cables available today ensure an extra measure of protection, combining Bonded-Pair technology with interlocked armor to provide the ultimate in mechanical protection against abrasion and cut-through.
EMI/RFI Noise and Shielding
The previously cited issues are visible and obvious -- they can be seen or felt upon cable inspection. However, EMI and RFI are much more subtle. You cannot readily detect when and if EMI/RFI will wreak havoc on your network. Avoiding the potential for EMI/RFI noise sources is the best strategy.
Routing the cable as far away as possible from EMI/RFI noise sources -- such as arc welders, switching relays, or AC drives -- is the first step. But what else can you do to control a noise source such as a robotic welder, for example? One option is to use fiber optic cable, which is absolutely immune to the effects of EMI/RFI. However, if you are concerned about the cost, complexity, or robustness of fiber, consider a shielded Cat 5e copper cable solution. Even a simple aluminum foil shield, when properly grounded, provides significant immunity to EMI/RFI.
Proper Grounding is Key
Proper grounding is the key to effective cable shielding. Missing or inadequate grounding can degrade the shield effectiveness. Ideally, the shield should be grounded at only one point. When interference couples onto the shield, the current is drained to ground, largely before the pairs underneath are affected. Conversely, when both the near end and the far end of the shield are connected to ground, the potential for a ground loop arises. The difference in ground potential from the near end to the far end location may create a significant voltage difference and an associated current flow on the conductive shield. The ground currents flowing on the shield can actually make the shield a source of interference on the signal carrying pairs.
Balance for Noise Immunity
A highly balanced and stable UTP cable will provide significant noise immunity. Each half-twist of a pair may be thought of as a loop antenna. Each consecutive half-twist is opposite in orientation of the conductor on top. In a perfectly balanced cable, the alternating polarity of the loop antennas cancels out any noise coupled on the pair. Bonded-Pair cables, however, provide the closest approximation of perfect balance, especially after the rigors of installation and everyday abuse. The balance and stability of the cable can make or break an industrial Ethernet application in the presence of noise.
Install for Optimum Performance
Ethernet technology brings the advantages of speed and economy of scale to industrial control and automation networking. However, it is critical that the physical layer of the Ethernet network be adapted to the unique challenges of the industrial environment. While COTS cable may work in some industrial applications, in most instances their lack of robustness and protection will be unacceptable.
To minimize the effects of harsh environmental factors and realize consistent, reliable performance, it is critical to install ruggedized UTP cables that offer the highest level of strength and mechanical stability, maximum temperature protection, chemical and abrasion resistance, and EMI/RFI shielding. This will ensure optimum network performance and uptime and avoid data transmission errors that can lead to unsafe conditions or interruption of mission-critical industrial control functions.
As you design and develop or upgrade your plant floor networking and control system, remember to ask the critical question: Is my Ethernet cable tough enough?
