- By Matt Hou
- Automation Basics
To ensure that the electrical interfaces between panels and field wiring perform reliably for the long haul, panel designers need to consider the best connectivity options.
Terminal blocks, connectors/cordsets, and interface modules connect electrical and control panels to the outside world.
Much of the focus of electrical and control panel designers is on selecting, arranging, and protecting internal components. After all, locating power and automation equipment in the field, close to the loads and devices they serve, is the main purpose.
However, it is also important to properly select, design, and install components to make associated electrical connections. Even though wireless is becoming popular for certain types of communications, it is often still necessary to connect electrical wires using terminal blocks or other connectors.
Whether for commercial or industrial use, these panels and the associated connections can be in harsh environments, with extremes of temperature, liquids, chemicals, vibration, and more. To ensure that the electrical interfaces between panels and field wiring perform reliably for the long haul, panel designers need to consider the best connectivity options. Terminal blocks, connectors/cordsets, and interface modules are the basics worthy of attention.
Classic connectivity: Terminal blocks
Terminal blocks have long been the standard way for landing field wires—both individual conductors and multiconductor cables—into a panel. But that does not mean there has been a lack of innovation for this termination style. The major terminal block connection technologies (shown in figure 1) are:
Screw terminal. This traditional technology uses screws, with or without pressure plates, to terminate wires directly or via a crimp lug. A wiring cage can compress on bare wire or a ferrule, or a ring or fork lug can be connected to the wire and inserted under the screw. Users must torque the connection properly to ensure a long-lasting, positive connection. To prevent the wires from loosening over time, some manufacturers have designed the structure to lock the screws in place and be maintenance-free. The screw terminal method is universal, recognized globally, and can handle the widest range of wire sizes.
Spring clamp. This technology uses spring force on the conductor to provide maximum contact reliability, even in high-vibration environments. A tool, usually a screwdriver, is needed to push open the terminal points as the wire or ferrule is inserted, but completing a termination is faster and easier than it is with a screw terminal. This type of connection is beneficial to use in high-vibration environments such as railway and marine applications.
Push-in design (PID). PID terminals are also spring loaded and allow users to directly push in solid wires or stranded wires with ferrules without a tool. It is the fastest and most efficient connection possible. This technology reduces wiring labor by at least 50 percent compared with traditional screw terminals, and it provides a durable connection, similar to spring cage technology. A tool is needed to remove a wire.
Larger-size conductors and higher-power connections typically use screw terminals. Smaller conductor sizes for lower power and signaling applications work well with either spring cage or PID terminals. Many terminal blocks click on to standard DIN rails for easy and secure installation, but some traditional or larger sizes must be mounted to a backpanel.
Users must also consider what functional arrangements are needed. These could include:
- Feed through, the most common and simple type of terminal block. It is single level and provides one wire-to-wire connection. Some types have multiple connections for one-to-two or two-to-two connectivity.
- Multilevel, which is like the feed-through style, but with two, three, or more isolated levels stacked together for substantial space savings.
- Grounding, where the terminals are electrically connected to the DIN rail or panel on which the terminal is mounted. This provides a ground connection, without having to purchase and install a separate ground wire.
- Disconnect, which is like the feed-through style but incorporates a knife switch to easily open (disconnect) the circuit without removing wires.
- Fused, which is like the disconnect style, but the switch houses a fuse for downstream circuit overcurrent protection. It may also include an LED blown fuse indicator for troubleshooting convenience.
- Sensor/actuator, which is like the multilevel style but may also include a grounding connection. This is especially suitable for common multiconductor cable wiring that is often used for connections with sensors and actuators.
Flexible connectivity: Circular connectors
Circular panel casing-mount connectors, and the associated molded-connector cordsets, have become the industry standard in many industries for connecting devices to electrical and control panels (figure 2). They reliably transmit signals, data, and power, while providing the physical flexibility desirable for many installations. Furthermore, they offer a simple way to quickly disconnect circuits for service, making work safe and convenient for maintenance technicians.
Device connectors are designed to mount on the exterior casings of equipment and panels, via threaded or precut holes, while maintaining the environmental rating. There are also PCB-mount versions in straight or right-angle configurations.
The associated cordsets have matching fittings—commonly in metric sizes M8 or M12—and insert into the connector. The knurled fitting is then tightened to secure and seal the connection. Cordsets are available with straight or right-angle ends, in male or female, as needed.
Designers should ensure the materials are rated for the environment. For instance, nickel-plated die-cast fittings can withstand high humidity, large temperature changes, and vibration. PVC cables are best for solvent resistance and washdown environments, while PUR cables are better for the abrasion resistance often required in high-frequency movements, such as for robotic arms.
High density connectivity: Interface modules
Interface modules combine terminal blocks and connectorized cables to provide the highest possible connection density, and they are typically used for programmable logic controller and similar signaling and I/O module applications (figure 3). One wire harness connects between an I/O module and the interface module, with the interface module installed on a DIN rail. The harness may use industry-standard IDC, D-Sub, or MDR connectors as needed.
This connection method transfers anywhere from two to 40 signals as individual points, and it provides compact terminals for the field wiring connection. Installers save significant time on the panel side connections and avoid installation errors.
Effective panel connectivity
Regardless of what combination of terminal blocks, circular connectors, and interface modules designers choose for panel connectivity, there is more to consider. All components should be recognized or rated by the proper agencies with regard to standards such as UL508, UL1059, and IEC 60947. These standards ensure the materials and component construction are sufficient for industrial use. Furthermore, designers should think about usability features such as color coding, along with easy provisions for marking and tagging.
Electrical and control panels are necessary for almost any application using electrical power or automation. By using modern connection options, designers can save significant panel space and create equipment that is easy to fabricate, install, and maintain. Labor costs are reduced, and the right connection products provide improved reliability, functionality, and safety throughout their lifecycles.
Figures courtesy of Dinkle International
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