I will not really trust the gripping action of star washers unless such thing are custom made and fitted for their intended design. In general star washers tend to loosen their grip over time especially if the chassis has to experience some form of vibrations.
This is an element of dogma... Personally I would would expect to find from a cabinet of electronic equipment running from a normal 50 Hz mains supply, a good Earth connection made to the equipment metalwork using a corrosion-resistant technique and situated close to where the mains lead enters the cabinet. All equipment metalwork should be electrically bonded in a manner which does not rely on 'hopeful' electrical conduction through anti-corrosive treatment like anodised aluminium and paint. Careful attention to the assembly process have to be done to weed out things like ball-bearings races, nylon runners and coasters, or other insulating materials. Conduction through painted panels should not be dependent on the gripping action of star washers. The design should be such that no currents flows in any part of the metal work. The objective is to ensure that any part of the metalwork can be relied upon as an effective electrostatic screen and not the reverse, a radiator.... regards :-) Tim Foo |---------+---------------------------------> | | Kazimier_Gawrzyjal@Del| | | l.com | | | Sent by: | | | owner-emc-pstc@majordo| | | mo.ieee.org | | | | | | | | | 03/22/02 08:56 AM | | | Please respond to | | | Kazimier_Gawrzyjal | | | | |---------+---------------------------------> >-------------------------------------------------------------------------------------------------------------------| | | | To: cdwe...@stargate.net, emc-p...@ieee.org | | cc: (bcc: Wan Juang Foo/ece/staff/npnet) | | Subject: RE: chassis bonding - star washers enough? | >-------------------------------------------------------------------------------------------------------------------| 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 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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