"THERE'S MORE focus on EMI today than there was five or 10 years ago due to the sheer volume of electronic devices being designed in, says Wolfgang Berner, technical director, R. Stahl. The operator interface has become more complex, and the addition of industrial Ethernet cabling has increased the requirements.
EMI includes any type of radiation that can affect circuit performance adversely. Devices such as electric motors can influence the performance of sensitive electronic equipment that isnt properly shielded. To protect integrity of electronic equipment, electromagnetic emissions must not exceed certain levels, typically defined and governed by regulatory agencies. These levels arent described at the device level, but rather apply to the completed unit.
Basically, the degree of EMI/RFI shielding required depends on the application. EMI/RFI shielding might not be needed in every application, and the shielding of a standard enclosure might be adequate for less sensitive applications, says Mike Jackson, engineering manager, Stahlin. While the enclosure itself isnt covered by regulatory requirements, once the electronic equipment is installed in the enclosure, the package must comply with applicable EMI/RFI directives set by the FCC and other organizations.
While the enclosure isnt covered by regulations, once electronics are installed in it, that package must comply with applicable EMI/RFI directives set by the FCC and others.
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Carbon-based or stainless steel sheet-metal enclosures with special gaskets are effective ways to provide EMI protection. Fiberglass enclosures are an ideal choice in harshly corrosive environments, but they provide no EMI/RFI shielding because theyre nonmetallic, adds Jackson. This feature is especially desirable in new wireless/data acquisition systems, where the antenna and all sensitive controls are housed in the enclosure, reducing the number of entries. Fiberglass enclosures can be coated internally with a highly conductive material that provides EMI/RFI shielding.
Sprayable, metal-based paints commonly coat the inside of fiberglass enclosures. Other methods include making the systems components out of glass fibers coated with aluminum or graphite, or using carbon fibers that can serve as a matrix reinforcement and conductive path. Openings in enclosures such as doors, cable ports and vents also can be pathways for radiated EMI, says Jackson. "Conductive EMI gaskets with a wire mesh can be used to preserve the current continuity of the enclosure.
The effectiveness of shielding material typically is measured in decibels. When a product has a rating of 30 dB, it means the EMI can be reduced 1,000 times (103). For example, a 40 dB reduction in interference would be 10,000 fold (104).
Conductive components must be grounded to protect users from electric shock. The separate parts of an enclosure must be electrically bonded together and grounded for the shielding to work. Disruption in the conductive continuity between parts adversely affects shielding performance, says Ron Zeitler, quality assurance manager, Saginaw Control and Engineering.
Most standard industrial systems have two grounds: the electrical, or power, ground and the instrument ground. These systems serve different purposes. A power grounds primary purpose is safety. All metallic or conducting equipment should connect to this ground. Code requires that a ground grid, ground rod, building steel and piping all tie into the power grounding system, if they are present.
The instrument grounds primary purpose is protecting instrumentation from EMI. To do this, any part of the protective shielding system must connect to ground at one point only. "An instrument ground is sometimes referred to as an isolated ground," explains Jackson. If the shielding system becomes grounded at two points, a ground loop is formed in which current can flow due to the potential between separate grounds. A ground loop can transmit noise or interference along the very shielding system that should be protecting the sensitive instrument signals, thus defeating its purpose. This interference can cause signal disruption in sensitive electronic instruments. By grounding one end, the current has no path so it eliminates the ground-loop potential.
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