By Dustin Guttadauro, L-Com
For industrial communications cabling, cost and performance specifications are often the most important factors considered when purchasing cabling for a new installation or retrofit. However, there are a wealth of other considerations that are often neglected for the sake of economic expedience. Among these oft-forgotten considerations are mechanical reliability and safety standards. Given that industrial communications cabling is now being relied upon for high-speed communications, control signaling, and video monitoring/surveillance in a wide range of industrial applications, fire safety codes and other safety standards may be the difference between a successful business or project and safety liabilities leading to bankrupting lawsuits. Furthermore, as counterfeit cabling has become so rampant, ensuring that an honest and reputable supplier is directly providing industrial communications cabling that meets all applicable safety standards, can also prevent liability and the risks to human life.
Built to last
Industrial Ethernet has effectively replaced Fieldbus technology in certain manufacturing environments with automation and process control. In the 1960s Fieldbus networks could support the throughput of analog valves, pumps, and motors. As the complexity of instrumentation on the plant floor and the data transmission between master and slave devices increased, Ethernet connections and protocols became a more feasible option. Commercial Ethernet evolved significantly to keep up with information throughput and data rates, form the 10-Mbit/sec thick coax to the twisted shielded pairs and fiber optics today with Gigabit Ethernet. While proprietary Fieldbus technologies are still leveraged today, Ethernet has proliferated in industrial environments due to its cost-effectiveness, reliability, and scalability.
|This is a screenshot from an L-com video in which crush-resistance of an armored cable is tested.|
According to an Aberdeen study, as of 2010, 69 percent of the best-in-class manufacturers were using Industrial Ethernet protocols for communication between industrial control systems and their components. The best-in-class manufacturers (the top 20 percent) achieved 99.97 percent uptime, or approximately 3 hours of downtime per year, while the laggards (the bottom 30 percent) achieved 99.14 percent uptime, or about 75 hours of downtime per year. The 72-hour difference between the best-in-class and the laggards could mean the difference between a company’s long-term success or eventually going under. Still, commercial Ethernet was not a perfect fit for industrial applications as the protocols and physical cables needed some custom-tailoring. While industrial-grade Ethernet protocols were designed for determinism and their electrical performance optimized for factors such as EMI/RFI protection, the physical layer needed to be ruggedized.
As some of the end users of data cables, data networking engineers face these hardware mishaps often enough. To shed light on some of the statistics, in a recent report from NewWorld Telecom, 83 percent of Category 6 manufactured patch cables failed testing, with field-terminated patch cables faring far worse. Stray nails, sharp metal edges, rodents gnawing, sun exposure, backhoes digging, forklifts running over cables, automated machines vibrating constantly, temperature fluctuations, humidity and moisture ingress, fire, and even shark attacks are all realities that a commercial off-the-shelf (COTS) data cable can face in the real world. While some of these circumstances are rare, many are frequent, if not constant, in nature and enough to warrant a design for mechanical toughness.
Points of failure
A robust industrial cable assembly generally requires multiple lines of defense against potentially damaging scenarios. In processing plants, a cable may be flexed frequently and should therefore be protected against the common failures at the fulcrum point between a connector and a cable. Frequent mating and unmating of the connector head could cause the gold plating on the conductors to rub off, reducing the overall connectivity the cable can provide. Harsh environments can expose cables to a host of failures; if a cable is not ruggedized in these circumstances any one potentially degrading scenario can cost an operation their vital connectivity for seconds to hours at a time.
Crushing - Industrial manufacturing environments can expose cabling to excessive shock, vibration, and sometimes crushing. Armored Ethernet cables with interlocked metal sheaths are typically leveraged for crush resistance in scenarios where heavy construction machinery can repeatedly run over these interconnects or for underground routing where the cabling can be repeatedly torqued and pulled on. The interlocking generates cable resistance to cross-sectional deformities under pressure while its semi-rigid nature allows for routing flexibility. Even small changes in the cross-section could heavily increase the attenuation of the cable.
Flexing and bending - Extensive flexing and bending to a cable in automated manufacturing environments causes tears in the shielding and inner conductor, leading to intermittent signals and even downtime. Each and every component of a cable can be designed toward flexibility including the inner conductors, shielding, insulation, and cable jacket. The friction generated from constant flexure is what causes tearing in the cabling. This heat generation needs to be mitigated in order to qualify as a high-flex cable. Constant bending can also permanently deform the cable strands due to irregular compression and stretching in the cable core. This would not be of consequence in a cable with a uniform cross-sectional area such as a coaxial cable but since Ethernet cables include multiple conductors bundled together, there is an inner radius that compresses during a bend while the outer radius stretches. In a multi-layered bunch of cables each conductor is compressing and stretching to a varying degree. This may not be an issue in a fairly static environment but when a cable is flexed millions of times, the “corkscrewing” of the cable can occur and render it nonfunctional.
|Data cables can be under high duress with automated equipment that flexes thousands to hundreds of thousands of times daily. (Source: bmwblog.com)|
To mitigate the effects of heat generation from friction, certain metal alloys are employed to ensure more flexibility. Some factors that contribute to the flexibility of a material are tensile strength, or the resistance of a material to stretching, and yield point, or the upper limit of the stress applied to a plastic material before it begins to deform permanently. Corkscrewing from excessive bending and flexing can be prevented by limiting the pitch, or distance between conductors, to distribute the tensile stresses on the inner conductors. The core and cable jacket can be specially designed to assist with buffering the forces on the bundled wires.
Oil exposure - Oil rigs, offshore drilling, deepwater drilling, and other gas installations call for data cables resistant to water submersion as well as oil. A standard COTS cable for commercial use typically will not take oil seepage into consideration as specialty plasticizers can be inserted in the PVC material in order to produce cold flexibility, heat or UV resistance, flame retardance, and extraction resistance from chemical exposure.
Oil ingress is one such type of agitation that can weaken the jacket of a cable by dispersing the plasticizers from the material. This can either swell, melt, or crack the jacket and insulating material. These types of deformities in the cable can not only degrade performance, but also can cause complete failures, both of which can cost tens of thousands of dollars per hour for an operation. Generally, when the plastic jacket fails the cable corrodes rapidly thereafter, rendering the cable useless.
UV exposure - Sunlight exposure is another such circumstance that can rapidly decrease the lifecycle of a cable as the UV radiation can leach out the plasticizers in the material. Commonly used jacket materials such as polyethylene (PE) and polyvinyl chloride (PVC) can have melting points as low as 150 degrees Fahrenheit. While a sun exposure may not subject a cable to temperature that high, the constant UV will eventually cause the cable to crack and allow moisture to migrate through the cable and potentially to the connector head conductors, shorting out and permanently damaging electronic industry.
To avoid interconnects occupying operational spaces, wires are routed in inside walls and in plenum spaces, or spaces that facilitate air circulation for heating, ventilation and air conditioning (HVAC) systems. While plenum areas provide a convenient platform for routing, they also provide a path of least resistance in the case of a fire outbreak. Fire can very rapidly spread through an entire building undeterred in plenum spaces. In these circumstances, the plasticizers used in the cable jacket are critical. Overheated wires and cabling can contribute to Class-C fires, where ohmic heating occurs, or a large current flow that overheats the cable due to the resistance in the conductor. This leads to the entire cable overheating, causing an almost instantaneous burst of flames that can spread to nearby combustible materials. In these types of fires, water is not necessarily effective in mitigating the spread due to the continuous power running through the cabling. Either the power source has to be shut down or the oxygen deprived from the space to control the fire. In some instances, these backup plans may not be an option so combustible overheated wires have been the cause of some of the most serious telecommunications fires.
Fire-retardant polymers can be specifically engineered for plenum spaces allowing cables to be “CMP” rated. Similarly, CMR-rated, or cables designed for vertical installations such as between floors and in elevator shafts, are often required by local and national building codes for fire safety. Both CMR- and CMP-rated cables are fire-retardant and self-extinguish. The CMP-rated cable has the highest fire resistance level and so it goes through the most stringent tests. Counterfeit cabling from unreliable vendors can lead to these types of hazardous scenarios where a PVC cable jacket allows for nearly spontaneous combustion of the surrounding cables, providing a path for fire to spread rapidly. Fire-retardant, or self-extinguishing, jackets are a must in industrial standards for multiple wire runs in plenum spaces.
|Shown here is a screenshot from a video that captures the difference between CMP-rated communications cable and a counterfeit cable, by performing a burn test on each set of cables. The counterfeit cable is a pathway for fire to spread rapidly in a plenum space, and is a serious fire hazard. (Source: Communications Cable and Connectivity Association)|
CMP- and CMR-rated cables are a good fit for spaces where smoke is designed to be ventilated out, but this may not be an option in confined spaces in buildings, cars, trains, submarines, or in aircrafts where there may not be any straightforward way to exit. Industrial communications cables can often employ halogens such as fluorine, chlorine, bromine, and iodine in their cable jacketing as they can be effective fire retardants when burned, these halogens release poisonous gases. For instance, a chlorine-containing plastic material. When burnt, releases hydrogen chloride, a poisonous gas that forms hydrochloric acid when it comes in contact with water. With the increase in cabling in industrial applications, it is often necessary to employ low-smoke/zero-halogen (LSZH) rated cables for their ability to self-extinguish rapidly to stifle the flow of toxic gases.
Approximately 72 percent of network faults can be attributed to failure at the OSI Layer 1 (Physical Media), Layer 2 (Data Link), and/or Layer 3 (Network). The mechanical reliability of these cables can not only directly affect the electrical properties of cables, but poor cable construction also can eventually render them nonfunctional, inevitably impacting the long-term bottom line of both manufacturer and customer. The ramifications that come with counterfeit cabling can be enormous in the case of a fire. True CMR- and CMP-rated cables can be ensured from reputable manufacturers. Failures for large-scale cable installations will either lead back to a poor installation, failure of moving parts (i.e. power supplies, fans, etc.), or subpar cable qualities.
From cables bent beyond their prespecified bend radius to extended exposure in outdoor spaces, the heavy burden of connectivity for massive companies falls on the hardware and its interconnections - intermittent connectivity and latency may be tolerable on a small-scale, but can cost millions in large-scale enterprises. According to Ponemon Institute, the average cost of a data center outage has steadily increased from $505,502 in 2010 to $740,357 in 2016 - a 38-percent net change in a matter of six years. From shock and vibration to sun, moisture, and oil exposure, industrial communications cables must be robustly designed to prevent failures as the interconnections between equipment are cornerstone to the functionality of an operation.
Dustin Guttadauro is product manager with L-Com.
Understanding cable flammability ratings
Plenum (CMP)-rated cable: Complies with NFPA-262 and UL-910. It is the only cable allowed in spaces defined as air plenums such as raised flooring systems and air-handling ducts. Plenum cables must self-extinguish and not re-ignite.
Low-smoke zero-halogen (LSZH)-rated cable: Used in shipboard applications and computer networking rooms where toxic or acidic smoke and fumes can injure people and/or equipment. Examples of halogens include fluorine, chlorine, bromine, and iodine. These materials, when burned, produce acidic smoke that can cause harm. These cables will self-extinguish.
Riser (CMR)-rated cable: Complies with UL-1666. Defined for usage in vertical tray applications such as cable runs between floors through cable risers or in elevator shafts. These cables must self-extinguish.
General purpose (CM, CMG, CMx) cable: Complies with UL-1581 testing. They will burn and partially self-extinguish. Not for use between building floors or in air plenum spaces. Often these cables are used for workstation cables and patch cords.