Signal, power wiring technologies drive changes in best practices

A creative tension typically exists between accepted practices that have served an industry well and the need for new practices that take full benefit of innovative new technologies. The current state of signal and power wiring is a good example of this truism.

08/28/2008


A creative tension typically exists between accepted practices that have served an industry well and the need for new practices that take full benefit of innovative new technologies. The current state of signal and power wiring is a good example of this truism.

Since separation between power and signal wires is a critical electrical engineering function, companies tend to rely on traditional, field-proven practices. However, engineers are evaluating new Ethernet and fieldbus-based wiring technologies and methods that are, in some cases, changing the traditional approach to power and signal separation by combining power and signal wires in one cable. It’s important for engineers to understand the traditional approaches, the change drivers and how to leverage new approaches into recognized best practices.

Traditional approaches

The traditional practice of placing power and signal wires in separate cables is based on various established industry standards. The two most common standards are IEEE-518, Guide for the Installation of Electrical Equipment to Minimize Electrical Noise , and NFPA-70 National Electrical Code . IEEE-518 establishes four major wiring classes or noise susceptibility levels. Common practices usually take the four major classes and convert them into three key levels.

Level 1: High susceptibility with analog signals of less than 50 V and discrete instrument signals of less than 30 V. Examples of these signals include:

  • Various fieldbus systems such as DeviceNet, Profibus and Foundation Fieldbus

  • 4-20 mA signals

  • Discrete input and output signals such as proximity switches, limit switches and indicating lights.

    • Level 2: Low susceptibility with switching signals greater than 30 V, analog signals greater than 50 V and 120 to 240 Vac lines less than 20 A. Examples of this level include:

      • Discrete input and output ac signals %%MDASSML%% including pressure switches, limit switches, indicating lights, relays and solenoids

      • 120 to 240 Vac lines of less than 20 A.

        • Level 3: Power ac and dc buses of 0 to 1,000 V with current of 20 to 800 A. Examples of this level include:

          • Power lines for motors

          • Power lines for welders and robots.

            • Spacing distances for the different levels are outlined in Table 1.

              If a signal cable must cross a power line (Level 3), it should do so at right angles.

              The susceptibility level guidelines outlined in Table 1 are very effective and have been standard industry practice for many years.

              New technologies, standards

              However, new technologies are driving a re-examination of these susceptibility levels. These technologies include Power-over-Ethernet (PoE) and various fieldbus technologies such as DeviceNet, Profibus and Foundation Fieldbus. These technologies combine power and signal wires in one cable.

              Moving from point-to-point wiring to consolidated wiring methods that combine power and signal wires, such as trunk and drop topology, can provide cost savings and the ability to perform enhanced troubleshooting with diagnostics. This alone is an important reason to re-examine traditional susceptibility level standards.

              For example, existing standards set the maximum current power for PoE at 15.4 W at 48 V, as specified by IEEE 802.3af . This provides levels up to just 31 mA to one device. The current draft of a new standard, IEEE 802.3at PoE plus, specifies up to 30 W, which would provide a tenfold increase in current levels %%MDASSML%% up to 350 mA %%MDASSML%% to one device. This change would ease current limitations in the existing PoE standard that restrict the types of new devices %%MDASSML%% such as IP Phones, wireless access points, pan-tilt-zoom cameras, RFID readers and building automation %%MDASSML%% that can use PoE. A new standard would enable creative development of new and useful technologies.

              New standards will also allow for greater system operating efficiency %%MDASSML%% a key goal for advanced technology networks. For example, one of the advantages of fieldbus technologies such as DeviceNet is that many devices can draw power from the same cable since the communication pair is part of the same cable going to the device module. Currently, DeviceNet cables are capable of carrying 8 A for Class 1 cable or 4 A for Class 2 cable. The current capability is much greater than can be used in PoE under existing standards. A new standard would allow more devices to be connected directly to the network for improved diagnostics and troubleshooting.

              There are other avenues that lead to higher efficiency. For higher-current devices (greater than 1 A), most fieldbus systems use auxiliary power networks to supply current directly to the devices instead of drawing current from fieldbus network cables. These auxiliary power networks can also be used as power sources for networks that normally do not have power, such as Ethernet networks.

              System designers should also take advantage of information available for powering new information networks. ODVA, an international association comprised of members from the world’s leading automation companies, has a planning and installation guide for both DeviceNet and EtherNet/IP. The guide, accessible on ODVA’s Web site (odva.org), states the common separation distances for these network cables from other cables installed in an industrial facility.

              Also, several standards bodies are implementing into their Ethernet standards a concept called Mechanical, Ingress, Climatic/Chemical and Electromagnetic (MICE). MICE classifies the type of environment in which cables will be installed. The MICE classification is currently in ISO/IEC 24702, Generic cabling for industrial premises , and TIA-1005, Telecommunications standard for industrial premises . The MICE classification can help designers select proper cabling for individual environments. For example, designers may need to select a hardened connector/cabling solution or use local mitigation or isolation techniques to protect the connector/cabling solution.

              Moving toward best practices

              While the change process is not complete, the industry is moving toward innovative solutions and best practices that use new technology that is more flexible and can also reduce installation and operating costs. For example, changes to NFPA-79 and NFPA-70 in allowing factory-applied connectors molded to TC-ER cable types are leading to more flexible solutions for industrial premises. These changes allow a more flexible solution than the traditional wire and conduit solution.

              Other standards bodies are making substantial changes as well. ODVA and ProfiNet, among others, are moving toward defining a standard auxiliary power network that will simplify and streamline system design. Currently, ODVA is finalizing a common 24 Vdc auxiliary power network for all CIP networks. This network standard can be used for either DeviceNet or EtherNet/IP networks, and covers: