Chris, Admittedly I'm not expert on materials or on grounding principles/issues but the relevance of a rock solid bond to ground is usually beyond dispute. In a previous life, I've found star washers had a varied level of difficulty in penetrating painted materials ...it all depended on the type of "paint" being used. For example, the powder coat type of "paint" found on numerous enclosures and frames is extremely durable and you might find that star washers will barely make a dent much less provide a good bond connection. Due to the power levels involved we would make sure that bond connections were masked off as opposed to relying on a star washer for adequate bonding. At times we found they were a point of failure when conducting the Earthing test to the CSA C22.2 No. 0.4 standard as invoked by the binat....could get them glow a nice shade of red. I don't have them handy but I do recall reading some studies indicating ground connections due to the use of star washers or the like were a likely source of ignition under extreme transient events....IEEE I think. My personal opinion and not that of my employer. Regards, Kaz Gawrzyjal Dell -----Original Message----- From: Chris Wells [mailto:cdwe...@stargate.net] Sent: Thursday, March 21, 2002 6:40 PM To: 'emc-pstc' Subject: chassis bonding - star washers enough?
Looking for some feedback on bonding various parts of a painted metal chassis together for safety and EMC performance issues. I know we have a lot of experts out there on this topic and can remember a number of threads on the topic. Please help me pull it together: * When screwing the cover on the base will star washers cut through paint, even epoxy paint and provide a reliable connection? * Are there special versions of electrical star washers that work better than others? * What about oxidation of the mating surface? I understand star washers can make a gas tight seal. * Should there be a special treatment to the painted area where the screw/washer come together? * Do PEM nuts act as a good mating mechanism for the screw? There are many types are some better than others? What about the self clenching types? * One can get screws with captive washers. Are there any issues with these? Preferred Vendors and types? * There are various chemical materials to keep screws from backing out and are applied to the threads of a screw such as those from Loctite. Do these interfere or help? Are some formulas better than others? * Under the paint if the base metal is steel are there any treatments or plating that help? For Steel, for Aluminum? * If the chassis is plated steel instead like zinc chromate yellow. What changes? * Proper plating on screw and star washer? (assume painted steel) Any advice on the topic welcome! Thanks in advance! Chris Wells Senior Design Engineer Cutler-Hammer christopherdwe...@eaton.com <mailto:christopherdwe...@eaton.com>
