Constantin:
The military practice is to never use a star washer or ribbed underside bolt to cut into a painted, plated or unknown surface to achieve a ground bond. I would show you a nice view of the fastener stack from the very old MIL-B-5087 Bonding standard, but the server does not allow graphics within posts, so we’ll just have to imagine it. MIL-HDBK-1857 is much more current; it has a number of examples of bonding and says essentially the same thing: 3.2.6 Grounding requirements. A ground stud shall be provided on equipment. The ground stud shall provide the electrical ground connection to the chassis or frame and shall be mechanically secured to insure low resistance joints by soldering to a spot welded terminal lug or to a portion of the chassis or frame that has been formed into a soldering lug, or by use of a terminal by a screw, nut and lock washer. The ground stud shall be of a size to allow electrical connection of size AWG 10 wire. All hardware used for grounding or other electrical connections shall be made from copper or copper alloys. Terminal luge shall be tin plated or hot tin dipped. Paint, varnish, lacquer, etc., shall be removed from the vicinity of the fastening point to insure metallic contact of the two surfaces. Corrosion protection shall be provided for all ground connections. Internal or external lock washers shall not be used on any grounding or other screw type electrical connections. Lock washers shall not be located between the metal plate and terminal lug or other part being grounded, so as not to interfere with the full and direct contact between these two members. Neither locking terminal lugs nor self—locking nuts shall be used for grounding. Flat washers shall be inserted next to any part having insufficient contact area with its adjacent part. The way I read all this is that the military does not consider any ground or bond proper if it doesn’t start out with surfaces prepared for good conductivity. If you have a painted chassis or box, you should either mask the intended ground location before painting, or remove that paint in a separate operation before assembly of the fastener stack. Maybe the commercial codes (NEC?) allow for fasteners which cut their own bond path (I see that a lot in appliances and residential wiring). Remember that I’m thinking of a bond as doing two things; first, providing a low-impedance RF path, and second, providing a low-impedance and high current path, so as to pass heavy fault current to allow protective devices to activate. I agree that multi-toothed star washers, when really torqued down, seem to do a good job of cutting through coatings and oxides; it’s just that the military doesn’t see that as good enough. Maybe another thing the military has against star washers is that, by cutting their own bond path, they are actually doing a small machining operation, and the paint and base metal micro-debris could be considered FOD. I can’t cite any prohibitions, but I can’t recall military products using any lock washers other than the split-ring style (not getting into locknuts here J). Hope this helps! Ed Price WB6WSN Chula Vista, CA USA From: Bolintineanu, Constantin [mailto:[email protected]] Sent: Friday, February 27, 2015 5:59 AM To: Ed Price Subject: RE: [PSES] Serrated head screws for grounding/bonding Dear Ed, I red your posting and I have a question: you specified “The washer is placed between the nut and the top face of the flat lug, never between the lug and the ground surface.” My question regards the correctness of the above underlined and highlighted statement in the light of penetrating the paint in a such design solution that you stated. In my opinion, the washer plays this role in almost each and every situation. Sure, I know that there are situations when it is not necessary to penetrate the paint etc. but generally speaking I think it shall. Could you please be so kind and comment on it? As well, there are situations when a lock washer shall be used in conjunction with a tooth washer and maybe you are talking about it within the above statement. Do you have a drawing showing the GROUND (PE) connection within a Class I appliance structure? Please accept in advance my many thanks for the clarifications. Sincerely, Constantin Bolintineanu P.Eng. iNARTE CERTIFIED ENGINEER Tyco Security Products Tel: +1 905 760 3000 / 2568 3301 Langstaff Road / Concord, Ontario, L4K 4L2 / Canada <mailto:[email protected]> [email protected] / <http://www.tycosecurityproducts.com> www.tycosecurityproducts.com From: Ed Price [mailto:[email protected]] Sent: February-26-15 12:25 PM To: [email protected] Subject: Re: [PSES] Serrated head screws for grounding/bonding Peter: The military doesn’t like that method. I think the primary reason is that the serrations form small point-contact connections, and, under heavy fault current, these little points will melt and/or vaporize. The military prefers a strap which terminates in a flat ring-lug, with the lug being clamped against the physical ground structure with a heavy nut and split-ring compression washer. The washer is placed between the nut and the top face of the flat lug, never between the lug and the ground surface. The purpose of the compresion washer is to maintain pressure of the lug against the ground structure. Ed Price WB6WSN Chula Vista, CA USA Good morning. I am reviewing the suitability of serrated head screws in grounding and bonding applications. I am aware that these screws are good at resisting vibration, but I've not seen them used for grounding and bonding purposes. I question this application since, while the serrations oppose loosening of the screw, they do not bite into the metal beneath the head and also seem unlikely to form a gas-tight connection, allowing degradation of the grounding/bonding interface over time. What are your opinions? Are you aware of any evidence of the reliability of an grounding/bonding connection using such screws? The screw will secure a wire, possibly with a crimp-on ring connector. (I also question the value of using a single toothed washer in these applications.) Regards, Peter L. 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