Hi Brian:
As you know, my opinions are engineering-based, not standards-based. The rule (or principle) is that the main protective grounding (of the power cord) need not be disturbed when servicing the other stacked ground circuits or assemblies on a stud or stud-equivalent. This means that the power cord grounding wire is at the bottom of the stack with its own nut, such that it is not disturbed when the others are removed. This often means the power cord grounding is double-nutted so as to prevent loosening the ground securement when removing the other ground wires. Or any equivalent construction. Screw cannot be smaller than #8. This is a mechanical requirement, not an electrical requirement. Documentation of the grounding securement is in older standards, not so much in newer standards. I believe this is because many products today use an appliance coupler rather than an attached power cord. As for making contact with metal, a star lock-washer is sufficient. Studies have shown that a star lock-washer will break through the oxide or other coating and make a gas-tight connection. Paint masking gets rid of the paint, but not the oxide, so the star lock-washer is still needed. If a screw head fastens the grounding wire, then no other grounding wires can be stacked to the screw. Best regards, Rich Ps: Thanks, Doug! 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. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
