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] <blocked::mailto:[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 From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Thursday, December 24, 2009 9:54 AM To: [email protected] Subject: RE: [PSES] Conductive Paint / Grease and bare metal bonding connections Decided to start a spin-off thread. I have an electrical engineering background, an am not a metallurgist or chemist. I spend a lot of time looking at industrial machinery and their control panels. Often, to get a good protective bond (aka safety earth / safety ground etc.) builders scrape off paint (steel) or anodizing coating (alumin[i]um) on part of the machine or cabinet, before attaching either bonding lugs/staps or another part of the machine. (For example, when attaching a control cabinet to a pedestal and both need to be bonded to each other.) This leaves a safety connection with bare steel (or, less common, aluminum). Especially in an industiral environment, the exposed portion will soon oxidize - but what will happen to the conductivity of the connection? If the mechanical connection was made tight soon after the metal was scraped, can I assume that a good electrical connection will exist say 10 years or more from now? If not, what methods should or could be used: - DO NOTHING: scrap to bare metal, connect tightly, no other precautions ? - leave paint in place, instead penetrate paint with star-washer (but I suspect these come loose due to creep) ? - petroleum jelly [e.g. Vaseline brand] ?? - other NON-conductive grease ? - carbon conductive grease ? - copper conductive grease ? - conductive paint ? I'm looking for something that a machine builder can obtain in reasonable quantities at reasonable cost and delivery time. For example - don't like to mention brand names but what about "DeoxIT ® M260Cp Grease (formerly CaiLube), copper particles", www.caig.com <http://www.caig.com/> and distributors. I've never used it. Data sheet looks promising. But is it needed? PS: This started from the earlier thread on Conductive Paint. In searching for "Galvafroid" (zinc rich coating) on the web, I stumbled across "Zinga" anti-corrosion zinc coating system. They say it's not a paint. I couldn't yet figure out if it would be another option. Regards, Glyn Garside
