By Robert Ourt
EMI-protected rectangular connectors contribute to improved plant EMC.
Electromagnetic interference (EMI) can distort or destroy intricately developed electrical controls on the plant floor. Because interconnected electrical signals and power supplies are used in virtually every facet of plant engineering, protecting these connections from "noise" is crucial to the function of the system as a whole. Protecting or "shielding" individual connections from EMI has spawned new methods for electromagnetic compatibility (EMC). Ultimately, from field to panel and signal to power, EMC should secure entire networks from EMI.
Defined as interference in signal transmission or reception caused by the radiation of electrical and magnetic fields, EMI may be battled in several ways. Shields, glands, gaskets, hoods and housings made of a variety of materials may be used.
The European Commission originally established EMC directives in 1986, with the assumption that the standards would be adopted worldwide in an effort to harmonize all electrical standards for import and export purposes. The directives outlined the acceptable limits of EMI emitted from electrical products primarily for the protection of public telecommunication networks.
In the United States, electrical components must also adhere to the Code of Federal Regulations set up by the Federal Communications Commission (FCC). The regulations are broken down by type of products, bandwidth usage and power limitations, among other things.
Numerous testing agencies exist in the United States and around the world to meet both European EMC directives and FCC regulations. While these standards are necessary for the safety and marketing of products, EMC is also a crucial factor in the cohesion of plant engineering.
Sources of EMI
In many cases, EMI can be avoided during the design phase by minimizing radio frequency emission, crosstalk, insertion loss and corrosion. Typically, EMI effects result from inductive, capacitive or antenna effects. Capacitive or electrostatic noise is coupled into a circuit via a voltage-based capacitive effect. The capacitive effect is the difference between two conductors separated by air and another insulating material.
Inductive noise is current-based within the circuit and will induce noise into the current of another circuit. If the wire or cable leaves the confines of the system, any EMI will be emitted and may affect other equipment in the area. Conversely, if there is any unwanted source outside of the system that is emitting EMI, this can be "conducted" into the system just like an antenna.
Sliding contacts, high contact pressures and precious metal plating negate many sources for EMI by insertion loss. Each time a connector is mechanically joined to the wire, a junction is formed. If this connection is not made properly and a gastight seal is not formed, arcing will occur. This arcing will emit a broadband radiated interference in addition to promoting corrosion and intermittent system functioning problems.
Multipole Connectors
A major source of EMI is high-voltage, high-amperage connections commonly seen in industrial applications, usually 480 V, 50 to 100 A. Typically, these connections are handled by heavy-duty industrial connectors called rectangular or multipole connectors. These connectors have a high propensity for discharging electrical interference and, therefore, play an important role in providing EMC.
Multipole connectors have gone through many changes in recent years. Initially, they were used as power connections that made disconnection for maintenance or replacement simple. The plug-and-play design enabled engineers to terminate input and output "inserts" with multiple wires (both power and signal) and mate the connectors together in a pluggable fashion. These robust, modular connectors presented a major advantage over hard-wired connections coming in and out of panels and enclosures, and were a cost-effective alternative to circular connectors. However, the integration of power and signal contacts within the same footprint (in close proximity) raises the issue of EMI.
Because multipole connectors included hoods and housings equipped with optional gaskets and strain reliefs, engineers could protect termination points from environmental conditions such as dust, spray or chemicals. These same hoods and housings make EMI protection a natural addition. With the use of EMC-rated gaskets, locking clamp mechanisms, sealed wire glands and special conductive platings, EMI-protected connectors are easy to specify and purchase. Ideally, the EMC-rated components of the connector, such as gaskets and glands, are factory-installed for convenience and proper installation.
EMC Construction
These special features are what separate standard multipole connectors from EMC connectors. Engineers should note that just because a connector is equipped with a hood and housing and gaskets does not necessarily mean that it is EMC-rated. Each of these components is individually tested for EMI protection and should be specified as such.
Hoods and housings constructed of a highly conductive, silicone-free, corrosion-resistant alloy eliminate the introduction of EMI. Locking levers and pins constructed of chromated steel offer strength for providing a gastight seal. A silver-plated lamellar shield provides a 360° shield encompassing the hood or housing, which provides up to 94 dB of protection, depending on frequency (see Figure 1). In addition, specially designed 360° shielded glands with grounding capabilities are available for each cable entry/exit.
To ensure that a system does not transmit or receive EMI, these requirements must be in place for the shielded EMC rectangular connector to work correctly. In addition, shields must be properly grounded on both ends of the cable. In other words, the ground connections must be of the same potential and be maintained throughout the entire circuit to be effective. This grounding is crucial to shielding. If the ground connections are not of the same potential, EMI protection will not be achieved. Also, if the shield comes in contact with the connector or the connector comes in contact with the housing of the connector, then the shield becomes ineffective.
Conclusion
Every circuit contains noise, interference and crosstalk. When the signal-to-noise ratio becomes too low, the operation of the circuit becomes adversely affected. Design engineers must consider the effects of EMI in the design and layout of all equipment and systems. The optimal result is that all connections operate within design specifications without unwanted noise. Engineers should specify rectangular connectors with EMC-rated features to ensure a clear and protected controls network.
ROBERT OURT is Product Manager for Multipoles, Wieland Electric Inc., 49 International Rd., Burgaw, NC 28425; (910) 259-5050; Fax: (910) 259-3691; E-mail: rourt@wielandinc.com; Web site: www.wielandinc.com.
SPEC SHEET
End Applications:Industrial controls networks
Related Products:Rectangular connectors, gaskets, glands, hoods and housings, shields
Main Point:Design engineers must consider the effects of EMI in the design and layout of equipment and systems in industrial networks. The optimal result is that all connections operate within specifications without unwanted noise. Engineers should specify rectangular connectors with EMC-rated features to ensure a clear and protected controls network.
 Figure 1. A graph showing EMI protection. The #1 represents a 16-pole EMC rectangular connector and the #2 represents a standard multipole connector. |
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