The Industrial Internet of Things (IIoT) relies heavily on cables to transfer data, commands, and power between industrial machines, and between the factory floor, IT, and the cloud. However, connectivity on the shop floor requires careful attention to cable selection and routing due to physical, environmental, and electrical hazards: designers must juggle a complex set of often conflicting performance and cost priorities.
This article will review the cabling challenges posed by the industrial environment and the IIoT, and discuss the differences between commercial off-the-shelf (COTS) and industrial cabling solutions. It will then show how specifying the correct cable can increase performance and reduce total cost of ownership (TCO), a key metric in IIoT planning.
Two application examples will be considered: variable frequency drive (VFD) cables for industrial motors; and industrial Ethernet networking. These were chosen because they illustrate different aspects of cable use in the factory: high-power operation and high-speed data networking.
The industrial environment is the enemy of cables
Industrial installations include manufacturing facilities, oil and gas processing plants, coal mines, wastewater treatment facilities, and transportation systems including tunnels and subways. These environments are tough on cables. Hazards include chemical, ultraviolet (UV) light, moisture ingress, impact, temperature extremes, and vibration (Figure 1). At the same time, the cable must ensure signal integrity and minimize signal losses and EMI effects.
Figure 1: Industrial grade cables must maintain signal integrity in the face of numerous factory hazards. (Image source: Belden)
Failures can compromise operator safety, cause quality problems, and be very expensive: every minute of lost production can cost thousands of dollars.
Mechanical hazards to cables in the factory include impact, abrasion, shock, tension, and vibration. In addition, many industrial applications are continuously in motion: multi-axis machine tools, robots, wind turbines, pick-and-place machines, automatic handling systems, and conveyor systems are a few examples. Cables for these applications must be able to withstand repetitive flexing for hundreds of thousands of cycles.
Ingress hazards include moisture, chemicals, and dust. Many industries use harsh chemicals and the cable must survive prolonged exposure without compromising performance. For example, in food and beverage processing, equipment must undergo regular cleanings with high pressure water and caustic chemicals to meet stringent hygiene requirements. A poorly designed cable can allow fluids to wick into the conductors.
Industrial cabling is often exposed to the elements in both indoor and outdoor installations. The resulting hazards include high and low temperature extremes, UV radiation (sunlight), moisture, and even rodent gnawing or invasive tree roots.
The industrial electrical environment includes heavy currents, high voltages, inadequate grounding, and electromagnetic interference from arc welders, furnaces, and HVAC equipment. In factory upgrades, space constraints may cause new cables to be routed close to noise sources such as motors and generators.
Compare the industrial hazards discussed above with those in a typical commercial environment. Commercial installations, including office space, server rooms, and homes, are typically climate controlled with HVAC systems that keep the cables at a constant temperature and humidity level. Much of the infrastructure cabling is in the airspace above ceiling tiles (the plenum), or is run inside walls where it can lie undisturbed for years. Standard commercial environments are generally free from fine particulates, liquids, and excessive temperatures, so commercial cables are not exposed to dust, moisture, chemicals, temperature extremes, or UV radiation.
COTS cables are designed to meet the conditions under which they will be used, so using a commercial cable in an industrial application will lead to a high failure rate, loss of production, increased cost, and compromised safety. A comparison between the two cable types on a series of standard tests shows the superior performance of the industrial grade products (Figure 2).
Figure 2: When subjected to the same tests, industrial grade cables provide superior performance. (Image source: Belden)
Anatomy of a cable
A cable is made up of several key components that contribute to its overall performance as shown in Figure 3. These are:
- Cable conductors
- Insulation for cables and cable jackets
- EMI shielding
Figure 3: A selection of DataTuff industrial Cat 5e Ethernet cables shows some of the design improvements that increase reliability and performance. (Image source: Belden)
There are two types of wires commonly used to transmit power or electrical signals through the cable: solid wire and stranded wire. Each have differentiating characteristics.
As its name implies, the conduction path in a solid cable is a single wire, most often copper. In general, a solid cable is less expensive than a stranded one, and smaller in diameter for an equivalent current. It has superior electrical performance with lower resistance, but is not as flexible, so a solid cable is less suited to use with moving machinery, such as robotics.
In a stranded cable, the conductors are made up of many filaments twisted together to form a larger, thicker wire. The flexibility of stranded cables makes them preferable where vibration is an issue, or in applications that require frequent flexing and bending.
Insulation for conductors and cable jackets
In addition to providing insulation, the plastics used to coat the conductors act as dielectrics. Their dielectric constant and the dissipation factor affect signal transmission (Figure 4). Specifically, the dielectric constant measures the ability of the cable to store electrical energy: it is a function of the velocity at which the energy travels through the insulation. The dissipation factor measures the rate at which energy is lost to (absorbed by) the dielectric. Reducing the value of either of these parameters gives better signal transmission.
Figure 4: Comparison of the properties for common cable insulation materials. A lower dielectric constant and dissipation factor allows better signal transmission. (Image source: Texas Instruments)
Many industrial cables include shielding around the conductors to reduce electrical noise and protect against interference. Braided shielding and foil shielding are the two most common types. Foil shielding provides 100% coverage but is hard to terminate, and its relatively high resistance offers a poor path to ground. Braided copper shielding offers only 60% to 85% shielding coverage, but its greater mass provides better conductivity and easier termination with a good connection to ground.
In harsh EMI environments, a combination shield (foil/braid) can give the highest level of protection. For example, Alpha Wire has cables that include both a triple layer aluminum/polyester/aluminum foil shield and a tinned copper braid shield (Figure 5).
Figure 5: Shielding can comprise foil (bottom), copper braid (middle), or for better performance a combination of aluminum/polyester/aluminum foil shield plus a tinned copper braid shield (top). (Image source: Alpha Wire)
The cable jacket protects the underlying conductors from mechanical, moisture and chemical damage during the installation and service life of the cable. The jacket also can enhance flame resistance, protect against UV radiation, and facilitate installation.
Cables with armored jackets protect the wires and shield against crushing; their construction techniques include aluminum interlocked armor and galvanized steel wire sheathing. An armored cable can also include a jacket of PVC or similar material around the metal, sealing the cable and armor from corrosive vapors and moisture.
Standard cable options include aluminum or steel interlocked armor, and cables optimized for burial or outdoor use, gasoline resistance, high flex capability, or high and low temperature operation.
Many manufacturers include additional proprietary features in high performance cables. Belden’s DataTuff Cat 5e industrial Ethernet cables, for example, use their patented Bonded-Pair construction that eliminates gaps between the conductor pairs for consistently reliable electrical performance.
Application example: Ethernet networking
Ethernet has been on the factory floor for many years, but recently its use has expanded into high voltage (>600 volts) machine control applications where safety is a primary design consideration. Although they don’t carry high voltages, control cables intended for these applications are still subject to the requirements of the National Electrical Code (NEC), leading to an increase in the availability of 600 volt Ethernet cables.
The Flamar series of cables from Molex is a good example. Designed for industrial automation, these cables feature 600 volt capability and are offered in versions for general control applications, servo motor control, and networking. The cables are weld slag and oil resistant, have achieved compatibility with the Ecolab standards for food hygiene, and meet the Oil Resistance II Compliance standard from Underwriters Laboratories (UL).
Telecommunication related cables have their own environmental standard (ANSI/TIA-1005-A) that covers industrial premises. The standard defines four levels of environmental classifications for mechanical, ingress, climatic/chemical, and electromagnetic (MICE) robustness. The classifications are rated by severity for each category: 1, 2 or 3. The environmental classification for a commercial building is typically M1I1C1E1; the harshest environmental classification in the standard is M3I3C3E3.
When selecting between cables that nominally satisfy the top level requirements (such as Cat 5e Ethernet), the designer should compare cable specifications carefully. There are often multiple cables at different price points that appear to satisfy the top level application requirements, but a closer examination will reveal differences in the specifications.
Belden’s 7928 and 7939, for example, are both 8 conductor DataTuff cables rated for industrial Cat 5e operation, but there are differences in their recommended applications. The reason is that subtle variations in construction give the 7928 performance advantages over the 7939, but also make it more expensive to produce (Table 1).
Table 1: The 7939 and 7928 from Belden are both 8 conductor Cat 5e DataTuff cables but subtle variations in construction give the 7928 some performance advantages. (Data source: Belden)
The FEP insulation of the 7928 allows it to operate at a higher temperature than the PVC of the 7939. The solid copper conductor of the 7928’s cable results in better direct current resistance (DCR) per foot and much lower maximum capacitance per foot compared to the stranded conductor of the 7939. These electrical differences give the 7928 a lower delay and a higher propagation velocity.
It all comes together in the high frequency performance specification. The operation of the 7928 cable is specified up to 350 megahertz (MHz) versus 100 MHz for the 7939. The 7928 also has better performance at lower frequencies.
Although there are clear performance differences, there are also differences in cost. Either one may meet the basic specification for a particular application, but the higher quality version will provide an extra margin of performance and confidence in operation.
Application example: VFD cable
A motor converts electrical energy to mechanical motion, and motors have been a key component in industrial operations for decades. Motor types include brushed and brushless direct current (BLDC) designs, alternating current (AC) designs, and stepper motors, each with their own characteristic performance and drive characteristics.
Characteristics of a VFD pulse train
A VFD provides precise speed and torque control of an AC motor via pulse width modulation (PWM). VFDs are widely used in manufacturing processes, but the characteristics of the switched drive signal make it critical to pick the right cable for the best performance and a long operating life. Some of these characteristics are:
- Standing waves: A VFD cable has an impedance of around 85 to 120 ohms (Ω).The impedance of a VFD motor is higher, typically several hundred ohms. When the PWM pulse train meets the higher impedance of the motor, a significant portion of the energy is reflected. This standing wave increases the voltage on the cable by a factor of two or three, leading to degradation of the insulation and eventual failure.
- Corona discharge: The intense electrical field surrounding the conductors ionizes the air between them, causing a discharge of energy. Corona discharges degrade the cable insulation material and damage the shield. They can also damage the drive electronics, waste power, and even generate enough heat to melt the insulation.
- Harmonic distortion: Any signal contains energy at its operating frequency plus energy at multiples of that frequency (harmonics), resulting in wave distortion. The energy at higher harmonics increases the joule losses in the cable and causes heating.
- In-rush current: At start-up, a motor can draw very high current. Most VFD controllers limit the maximum starting current by ramping up motor speed slowly, but the cable must still be designed to handle an initial surge.
- EMI: The fast switching of digital pulses creates electromagnetic interference. This energy can transfer to other circuits causing signal degradation, false signals, and other problems.
When selecting a VFD cable, it’s critical to understand the entire drive system and the required current capacity, perhaps with an allowance for future increase. High performance VFD cables have superior grounding and shielding capabilities than construction grade cables and provide more reliable and stable connections. Here are some tips and key recommendations for improved VFD performance:
- The grounding system should be designed for the lowest possible ground path impedance. A cable with extra copper in the ground path (called a 300% ground design) ensures that potentially harmful common-mode current (CMC) is contained and returned to the drive without adverse effects.
- Choose a conductor designed for high frequency operation with tinned copper conductors to protect against corrosion, and a high strand count for increased surface area.
- Choose a cable with low capacitance and high dielectric strength. Thermoplastic high heat nylon coated (THHN) construction grade VFD cables have higher cable charging losses and will build reflected wave voltages faster. Such cables have about one-third the insulation strength of a high grade conductor with a thermosetting insulation such as cross-linked polyethylene (XLP): XLP also provides much better resistance to corona discharges than THHN.
- The shielding material greatly influences the noise performance. Low impedance shielding results in less current reflection and greater system reliability. Conversely, an unshielded cable can act as an antenna and be a source of radiated emissions. The shield should have maximum surface area for best high frequency performance. As mentioned earlier, dual copper tapes or braid will provide the best shielding performance.
Alpha Wire’s V-Flex cables are designed specifically for high performance VFD applications on robots, conveyers, and other machines with repetitive or continuous movement. The family features seven cable designs, stranded tinned copper conductors from 4 AWG to 16 AWG, a TPE jacket, and enhanced flexibility for easier routing and handling. The VF16006 BK005 cable, for example, is a four conductor cable with 6 AWG wire and a foil/braid shield (Figure 6).
Figure 6: The VF16006 BK005 cable from Alpha Wire is designed for VFD applications. It can handle up to 52 amps per conductor and uses a foil/braid shield. (Image source: Alpha Wire)
The cable is oil and UV resistant, has an operating temperature of -40°C to +90°C, and can handle drives up to 50 horsepower (HP) with a full-load current of 52 amps per conductor.
For a given application, the designer usually has a choice of several cables at different price points that all appear to satisfy the top level specifications. However, the commercial and industrial operating environments are vastly different. This article has reviewed the construction differences between commercial and industrial cables and examined the cabling requirements for two common industrial applications.
As shown, industrial cables include subtle and not-so-subtle improvements over commercial cables that more than pay for themselves over a lifetime on the factory floor.