Hello Brian, Let’s start with the easy question first, the order of stacking lugs on a single stud. The equipment’s protective earthing conductor should go on the bottom and have a lock washer and nut on top of it before any other bonding conductors are added. The rationale is that you don’t want service personnel to have to remove the main earthing lug to remove any other parts. If the main earthing lug is bolted below other lugs, there is a lower risk of it being disturbed in any servicing procedure. There is one advantage of placing a lock washer and nut over each additional lug added. If two lugs are under the same nut, there is a chance that they could rotate against each other loosening the nut above them and compromising the connection. I recommend a separate bonding stud for each lug if the equipment is expected to require servicing that removes bonding conductors to remove parts.
If you have a welded, press fit or similarly fixed stud, you likely do not need to mask of the paint or coating around the stud. The flat faces of the nuts will make good electrical contact with the grounding lugs and the threads of the nut will have good electrical contact with the stud. However, I have had at least one NRTL tell me that they wanted the paint masked off in a ring around the stud. That NRTL was in the minority. I’ve had differing opinions on whether split or star-tooth lock washers are better at cutting through paint, coatings or oxidation. I take no position on that, and it shouldn’t matter based on a connection through the threads. Bolts and screws pose a different problem. These may not make as good of electrical connection to the chassis. Lock washers that cut through coatings and oxidation will likely be required. That being said, I have a personal aversion to bolts and screws for bonding. The exception is where there is a threaded insert or otherwise set of well formed threads in the chassis where the bolt will make a good connection through the threads. Screws are too easy to strip out if overtightened, thereby compromising the bonding. I’m sure these rules are written down somewhere, but I don’t know where that would be. I would be interested in knowing, because I assume there are written rules somewhere that would tell me that everything I just wrote is wrong. Ted Eckert Microsoft The opinions expressed are my own and do not necessarily reflect those of my employer. From: Brian Kunde <bkundew...@gmail.com> Sent: Friday, June 7, 2019 6:10 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Stacking Grounds on a Stud My question(s) is about stacking ground lugs on a single stud for the Protective Earth Terminal inside of a piece of electrical equipment (IT or Lab Equipment). Where is it documented how to properly stack ground lungs? I have seen this done several ways: 1. A welded stud or just a bolt (#6 or #8 machine screw) pushed through a hole, with the Power Cord Ground Conductor lug stacked FIRST, followed by additional lugs. I have seen this with and without locking washers between the lugs. 2. I have seen the above with Nuts or Locking Nuts between each Lug. This method seems much more reliable to me but the nuts take up a lot more space limiting the number of lugs that can be stacked. 3. I have seen the above on black oxide, anodized aluminum, or powder coated painted metal with not attempt to scrape off the non-conductive material to ensure a good bond. I just evaluated a 3rd party product that had a #6 machine screw pushed through a hole in a powder coat painted metal plate with 7 ground lugs and one nut holding it all together. With very little effort, the nut became loose and the ground became intermittent. Yet, this unit has passed an NRTL inspection. It is my understanding that PE Ground bonds made with a Screws cannot be stacked. One Lug, One Screw. Is this documented somewhere? Thanks much for any input. The Other Brian. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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