Ed, What can you tell us about the epoxy paint and any related comments that resulted from the product safety assessment.
thanks, Brian From: [email protected] [mailto:[email protected]]On Behalf Of Price, Edward Sent: Thursday, December 24, 2009 10:44 AM To: [email protected] Subject: RE: [PSES] Conductive Paint / Grease and bare metal bonding connections Recalling that a “safety ground” is one which is expected to draw a heavy fault current, in order to activate the protective line disconnect device (fuse or circuit breaker), then star washers are not the best choice. Although the tiny points of the star washer probably have sufficient contact pressure to maintain a gas-tight contact (and not oxidize over a long time), those same tiny points do not provide a large conductive path for a fault current. A good safety ground must maintain its conductivity over a long period, and also be able to carry the instantaneous fault current. I would always use a flat washer, with a split-ring lock washer to maintain pressure of a bolted joint. So that means that I would need a conductive “footprint” under the flat washer and cable lug that is being pressed down by the fastening nut. Moving on to how a case should be built, I think it’s important to provide for the safety grounds as an engineered, factory made solution. Safety grounds inside your equipment expose you to too much liability to justify allowing grounding to be done by scrapping the cabinet paint and bolting on a jumper cable. If an installer knows what he’s doing, he can create a long-lasting safety ground. But years later, should your company get sued, you better have explicit documentation of the process, and you will have better evidence of proper work if it was done in the controlled environment of your plant rather than at a jobsite. When I think of a safety ground, I envision a factory welded stud, with the entire panel treated for conductivity and then masked and painted for corrosion protection. And then, after assembly, a nice touch would be painting or RTV over the assembled joint. Since my company uses small aluminum boxes for almost everything, we add a chemical conductive film over the entire box, then mask joints (faying surfaces) and then apply epoxy paint. Lid fasteners, connector bolts and ground studs are usually stainless steel. Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
