Conformity Magazine - July 2008 - (Page 48)

of fasteners - continuous welds are ideal, but admittedly not usually feasible. The key is to have a substantial wide surface. You could use the metallic base as a start, with metal risers bolted down as needed. Mechanical engineers like to use various metal stock like Tinker Toys; unfortunately, they make poor ground paths. Better to have larger sheet metal stampings. Movable metal members are unacceptable for any ground, even if wired down with a green wire. Screw threads, latches and bearings are unacceptable as ground. Anodized, paint or other nonconductive coatings are unacceptable for mating surfaces or ground contact, but permitted elsewhere. The bottom line is, to get a good ground, use planar metal members and bolt them together at every opportunity. Do not use wires for RF grounds. If you need to use a flexible metal interconnect, use a flat metal strap, with length to width radio no greater than 5:1. Shielding Assuming you are going with some number of shielded modules, these modules should be bolted to the ground plane. Do not use green wire for this purpose. If the module cannot be mounted onto a ground plane, bring the ground plane up to the module. Safety grounds are always permitted, but they cannot be relied upon to provide high frequency grounding. Wire Routing We strongly recommend putting your wire harnesses and cables under control drawing. When we open an enclosure and see a rat’s nest of wires, we immediately draw two conclusions: first, that we have significant parasitic antennas, suitable for both radiated emissions and susceptibility; and, second, that there will be poor repeatability EMI performance between any two similar boxes. You will get better and more repeatable performance by following these basic rules: • Route all wires, including signal and power cables, along the ground surface to minimize loop areas. Avoid hanging them in midair. If you are using part of the metallic outer skin as a ground surface, make sure it is very well grounded to the above mentioned ground structure. • Group cable types—Noisy power or data lines should be separated from vulnerable data lines. Avoid long parallel runs, or shield one or both cables. • Use twisted pair to route power lines, switches and indicators. • Avoid daisy chaining power to various modules. It is particularly important to separate noisy and sensitive loads. Power and Data Entry Mount connectors and power entry at the enclosure shield boundary (this would also normally be in intimate contact with the ground mentioned above). This includes any filters, 8 Conformity JULy 2008 transient protection and connector mounts. This could be to the ground plane, or to a plate directly connected to ground. From a systems standpoint, all external data cables (those destined to another piece of equipment) and power cables should enter the enclosure in contiguous locations, to minimize facility ground currents passing through the enclosure. This would be to the ground plane or to the plate mentioned above. Avoid having any enclosure seams in the immediate proximity of the connectors. Don’t skimp on the shield here, of all places. Shielded Cables Shielded cables, whether internal or external to the enclosure, need to be designed appropriately, or they won’t work as intended. The cable shield needs to be circumferential to the connector. Pigtail wire grounds and drain wire grounds are not acceptable. Foil shielded cables are difficult to terminate adequately, and we avoid using them. Internal cable shields should be clamped directly to the ground plane as close to the cable end as possible. Do not use pigtails to ground the shield. If the cable connection is too far from the ground plane, peel the cable sheathing back so you can make a direct connection, and leave the cable shield extended to the end of the cable. Cable shields may be grounded up to 1/20 wavelength from the end. Except for low frequency electrostatic shields, the cable shield needs to be grounded at both ends. Single point grounded shields are unacceptable for cable lengths longer than 1/20 wavelength. External patient connected cables cannot be grounded at the patient end, and the shield termination at the equipment end will need to go to isolated ground. This places additional burden on signal filtering. Summary EMI shielding for medical electronic devices poses some unique problems. Patient isolation requirements make it difficult to effectively shield or filter signal lines. Complex equipment can be difficult to shield effectively. A combination of selective shielding, module grounding, cable routing and shielding will help minimize the demands placed on the enclosure shield. William D. Kimmel, PE and Daryl D Gerke, PE, are principals of Kimmel Gerke Associates, and can be reached through their web site at www.emiguru.com. FAST Link www.conformity.com/1960 http://www.emiguru.com http://www.conformity.com/1960

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Conformity Magazine - July 2008 Contents Editor's Note FCC Adopts 6 Million Dollars in DTV Enforcement Orders Commission Enforcement Bureau Adopts V-Chip Consent Decree FDA Seeks 2 Million Dollars from Medical Device Manufacturer FDA Answers Questions About CFLs and UV Radiation EPA Issues Energy Star Specs for TV, Video Boxes Canada Unveils Sweeping Product Safety Legislation How the Evolution of CISPR Standards Continues to Shape the Requirements for Anechoic Chambers ESD Open Forum Reach Compliance for Non-EU Article Manufacturers The iNARTE Informer Preamplifier Considerations for Emission Measurement Systems Effective Shielding in Medical Electronics Applications Product News Focus On...Power Components Buyer's Guide Updated Standards List for the EU's ATEX Directive EOS/ESD Symposium Set for September CPSC Actions in the News Researchers Work to Develop Non-Flammable Batteries Safety Link Launches Redesigned, Enhanced Web Site IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - July 2008

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