Enclosure Design Guidelines

This chapter contains sections titled:

• Metal Portions of Enclosures Should Be Connected to Chassis Ground (If this isn't possible, see III)

• Portions of Enclosures That Are Metal, and Are Not Insulated,and Are Connected to Chassis Ground, Should Be Kept 2.2 mm from Exposed Electronic Components or Lines

• Metal Portions of Enclosures That Aren't Connected to Chassis Ground and Are Not Insulated Must Be Kept At Least 2 cm Away From Exposed Electronic Components and Lines

• Avoid Sharp Edges on Metal Components Because They Encourage Secondary Arcs

• Design System Enclosures So an Operator Cannot Approach Closer Than 2 cm to Exposed Nongrounded Metal, Electronic Components, or Electronic Lines; or So a Chassis Grounded Object Is between the Operator and the Electronics (Ungrounded Screws Are an Especially Troublesome Path for ESD Entry Into a System)

• The Enclosures Should Not Constrict PWB Designs So Much That It Is Impossible to Include Sufficient Logic Ground Grid or Chassis Grounding

• Enclosure Designs Should Allow the I/O Devices to Remain Close to the I/O Connector and Each Other

• Try to Locate the I/O Cable Entry Point in a Central Location on Each Enclosure

• There Must Be a Provision for a Short, Low Corrosion Connection of Each Cable Shield to the Chassis; This Connection Must Be Within 4 cm of the Cable Entry Point(s)

• Connectors Must Be Within 4 cm of the Cable Entry and Chassis Connection Point(s), So the Unshielded Portion of the Cable Doesn't Exceed 4 cm

• If a Cable Has a Ferrite Added, this Ferrite Must Also Be Near the Cable Entry Point

• No Slot or Hole Should Have a Long Dimension Greater Than 2cm

• Use Several Small Openings Instead of One Large Opening

• The Space between Openings Must Equal the Largest Dimension of the Opening

• Don't Place a Slot Near a Chassis Cable Ground Connection Point, or Near Sensitive Signal Lines or Devices

• Electrically Fasten the Shielding Seams at Several Points to Reduce the Slot Length That Can Exist (Screws, Clips, etc.)

• If Necessary, Use Conductive Gaskets to Fill Gaps That Remain in Seams

• Gaskets and Fasteners Both Should Be Chosen to Keep Corrosion to a Minimum (See the corrosion discussion in Chapter 5)

• If Foil Tape Is Used, It Must Make Electrical Contact With the Rest of the Shield

• A Shield Seam Should Be Overlapped, and the Overlap Should Be At Least Five Times the Gap Width, and Equal to the Distance between Contact Points

• Bonding Straps Used to Connect Various Chassis, or Enclosure Sections, Must Be Kept Short and Kept Away from Sensitive Electronics. Bonding Straps Should Also Be Wide; It Is Recommended That Bonding Straps Be No More Than Five Times Longer Than They Are Wide

This chapter contains sections titled:

• Summary of Enclosure Design Guidelines

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