Ted
Just a very minor clarification regarding appliance couplers – non-filtered ones don’t provide direct Ground connections to the chassis. Obvious I know, but could be overlooked. John E Allen W. London, UK From: Ted Eckert [mailto:[email protected]] Sent: 24 September 2019 14:55 To: [email protected] Subject: Re: [PSES] Grounding of metal cabinet The rationale I remember learning is that the main grounding/earthing connection must be made in a way such that any repairs would not need to disconnect that main grounding connection. Let me use Mr. Woodgate’s examples. If you have an insulating terminal block, replacing the terminal block could disrupt the grounding connection. If the repair person fails to connect the ground on the load side, the device will likely still function, but it will have lost its safety ground. If you connect incoming ground to the chassis first, and then to the terminal block, replacing the terminal block would not disrupt the grounding connection even if the wiring on the terminal block is connected incorrectly. However, if you have a grounding terminal block, where the ground connection in the block is a direct metal connection to the chassis, replacing the terminal block would reconnect the incoming ground to the chassis. Of course the ground wring to the terminal block could be done incorrectly, but you have at least removed some of the risk of a missing grounding connection. When using appliance couplers, filtered couplers can serve multiple purposes. They can not only provide EMC filtering, they make a direct bond between incoming ground and the chassis. This would serve a similar safety purpose as a grounding terminal block. Of course, for cord connected equipment, there is the repair of replacing the cord that requires disconnecting the ground wire. All we can do is to make the design fool-resistant. Nothing can be foolproof in this world of talented fools. Best regards, Ted Eckert Microsoft Corporation The opinions experessed are my own and do not necessarily reflect those of my employer. From: Bill Owsley <[email protected]> Sent: Monday, September 23, 2019 9:45 PM To: [email protected] Subject: Re: [PSES] Grounding of metal cabinet Our safety regulators have insisted that power cord safety ground/earth go first to the metal chassis, with rare exceptions. Exception, the majority of chassis is plastic, with internal metal structures to tie it all together. Only the screw heads are exposed and tied to some of the internal metal chassis. A complex system of wires, washers, terminals, etc. were needed to ensure it was all one complex earth grounded structure. I AM NOT a safety engineer, but I do know a few ! - Bill On Monday, September 23, 2019, 07:42:11 PM EDT, Pete Perkins <[email protected]> wrote: This is a great discussion. It took some work to get most of the world to use a solid, direct earth/ground connection in cabinets and equipment. CSA published detailed connection diagrams and pushed direct connection into many product standards with good results. The struggle has been since the modularization of components and units has come into play. This raises questions as to how to interconnect everything together in a way that preserves the integrity of the earthing/grounding connection when it is a principle safeguard against electric shock. As an OF (= Old Fellow, in polite company) I prefer that the earth/ground wire be tied to the mechanical structure of the cabinet or equipment first then carried to the distribution components. It is very difficult to destroy the integrity of this earthing first philosophy. There are many scenarios where the quick install/quick connect terminal block can be disconnected from the earthed rail and leave the cabinet/equipment without the needed earthing/grounding connection. Let’s keep the focus on providing the highest integrity protection scheme in place (and common practice). :>) br, Pete Peter E Perkins, PE Principal Product Safety & Regulatory Affairs Consultant PO Box 1067 Albany, Ore 97321-0413 503/452-1201 IEEE Life Fellow <mailto:[email protected]> [email protected] Entropy ain’t what it used to be From: John Woodgate <[email protected]> Sent: Monday, September 23, 2019 4:19 AM To: [email protected] Subject: Re: [PSES] Grounding of metal cabinet The question is how the cabinet is earthed if the PE wire goes to a terminal block. If a wire comes out of the terminal block to a stud on the cabinet, that is less reliable than a solid connection. I suggest you take the advice. It doesn't apply if the 'terminal block' is not an insulating block with metal inserts but a metal block firmly attached to the cabinet. Best wishes John Woodgate OOO-Own Opinions Only J M Woodgate and Associates www.woodjohn.uk <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.woodjohn.uk&data=02%7C01%7Cted.eckert%40microsoft.com%7Cc5d624bb977847beec2408d740ab67fd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637048977245765905&sdata=spnE6aXfHr7UAFO31TVLojkdhSSuidZEmmVEemhnn2s%3D&reserved=0> Rayleigh, Essex UK On 2019-09-23 11:33, Amund Westin wrote: A flexible 1-phase AC cable is entering a metal cabinet and the PE-wire is directly connected to an Earth terminal block (green/yellow). Some years ago, a safety engineer told me that the earth wire should first be connected to a ground stud inside the cabinet via a lug terminal climp and fastened with a nut. From the stud, a green/yellow wire should connect to the Earth terminal block. I thought it’s OK to go directly to the Earth terminal block, right? … BR Amund Westin - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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