Safety - Earthing/Bonding of a metal enclosure - Page 1
Forum section: electronics / projects

author: Northy
author_rank: Regular Contributor
timestamp: September 30, 2020, 03:35:43 pm
content: Hi all,I'm a bit confused by the terminology used within the safety standards (62368 in this case) when it comes to Earthing/Bonding of a metal enclosure.If you've got a metal box with an IEC inlet is the 'protective earthing conductor' the internal earth wire from the back of the IEC connector to the metal box?If that is the case, then what is a 'protective bonding conductor'?If you've got a metal enclose made up of several pieces of metal that are screwed together does there need to be a physical wire connecting them together or is it ok (regulation wise) to rely on face to face unpainted surfaces?Thanks,G

author: Neilm
author_rank: Super Contributor
timestamp: September 30, 2020, 07:15:46 pm
content: "Protective earth" is a catch all statement for the Earthing wire. So this would be the earth wire from the IEC inlet. Also - some connectors have a metal surround on them that will contact a metal chassis and earth it so you don't need a separate wire.If you are screwing a box together, then I would look at screws and star washers to bite through any protective coating to bare metal.I don't know the standard, but I imagine there is a test you can do to check the interity of the bonding. I know IEC61010 requires 25 A bonding test with a resistance from the earth terminal to exposed metal at 0.1 ohms (from memory)

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 30, 2020, 07:38:59 pm
content: I'm also somewhat working off memory here. Not sure if this is general industry practice or local to my experience, but I tend to use the terms like this:Earthing Conductor: Conductor from the assembly "somewhat more directly" connected to the eventual system ground. For instance, a green/yellow wire from the box to a ground spike, the wire from the IEC inlet, etc.Bonding Conductor: Conductor between two pieces of metal. For instance, the wire between the box and the door of the box.I have EN 62368-1:2014.See definition 3.3.1.1.8, "protective bonding conductor":QuoteI just noticed theMarantzMcIntosh (sorry - my bad) amplifier in 62368!

author: bjbb
author_rank: Contributor
timestamp: September 30, 2020, 09:12:46 pm
content: Tomorokoshi is correct. The green wire in the power cord that is connected to the round pin is a 'protective earthing connector'. And the internal connection(s) to the PE pin are 'protective bonding conductor'. IEC62368-1 is "hazards-based safety engineering", so you must mitigate the energy transfer mechanism, or build and test per the prescriptive part of the standard (5.6).Parts that must be grounded must meet construction requirements and exposed metal parts shall pass the ground bond test; which varies per the national differences. The definition of safety-critical is essentially exposed metal in class I construction that does not meet the material or construction or spacing requirements for class II construction. You will also have to meet touch current limits to exposed conductive parts(5.7).

author: Northy
author_rank: Regular Contributor
timestamp: September 30, 2020, 09:30:32 pm
content: Quote from: Neilm on September 30, 2020, 07:15:46 pmThanks, yes there are some tests to prove the effectiveness of the earth, I'm just trying to get the terminology straight.Screws and star washers won't cut it I'm afraid on the outside of the enclosure, it needs to look nice.Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: September 30, 2020, 09:32:37 pm
content: Quote from: Tomorokoshi on September 30, 2020, 07:38:59 pmThank you. This is starting to make more sense.Where is the Marantz amplifier?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: September 30, 2020, 09:39:48 pm
content: Quote from: bjbb on September 30, 2020, 09:12:46 pmThanks.So is this correct?Yellow/green wire in the IEC lead - 'protective earthing conductor'Yellow/green wire from the back of the IEC inlet socket to the chassis stud - 'protective bonding conductor' - or is this 'protective earthing conductor'?Yellow/green wire(s) from the internal chassis stud to other things (PSU, other bits of metal) - 'protective bonding conductor'If you have one piece of metal that is folded to create the enclosure bottom, both sides and back (which incorporates the IEC inlet and chassis stud), to which you screw a separate front panel and a lid - do the front panel and lid need to be connected to the main chassis with wires? Or if all the faces are masked of paint would that be sufficient?Thanks,G

author: Tomorokoshi
author_rank: Super Contributor
timestamp: September 30, 2020, 10:11:55 pm
content: Quote from: Northy on September 30, 2020, 09:32:37 pmSorry, it's McIntosh! I crossed my vintage Hi-Fi equipment starting with "M" again.In my version it's on page 33, in section 0.10.1.

author: Northy
author_rank: Regular Contributor
timestamp: October 01, 2020, 09:32:34 am
content: Ah yes, I see it. Page 33 here too, in my cheap and cheerful Estonian bargain basement editionG

author: Northy
author_rank: Regular Contributor
timestamp: October 01, 2020, 10:25:06 am
content: OK, next question, I'm a bit shocked () I can't find this info in the standard tbh, but it is a bit crypticIf I plan on mounting an open frame power supply into the metal enclosure, can I just use distance between it and the chassis as a safeguard between electrical injury?What section should I be looking at to try and work this out?Thanks,G

author: tkamiya
author_rank: Super Contributor
timestamp: October 03, 2020, 10:08:41 pm
content: I just wanted to leave a note here that ground prong on mains connector is one of the most neglected part of house wiring. A lot of electricians are guilty of this. Often, they just "hook" the wire and doesn't give the same respect as other two wires. I've seen so many awful work or lack of, completely.

author: themadhippy
author_rank: Super Contributor
timestamp: October 03, 2020, 11:55:02 pm
content: Quotesounds like a job for the BS finger, IEC 61032

author: Northy
author_rank: Regular Contributor
timestamp: October 04, 2020, 05:26:56 pm
content: Quote from: tkamiya on October 03, 2020, 10:08:41 pmThat is true, and I believe the safeguard to cover that is to put within the installation instructions that the electrical system should be checked before installation.G

author: Northy
author_rank: Regular Contributor
timestamp: October 04, 2020, 05:29:01 pm
content: Quote from: themadhippy on October 03, 2020, 11:55:02 pmI do plan to 3d print a BS finger just so I've got one, but in this case the finger will get nowhere near the power supply once it is screwed into the chassis.I think my worry is more a 'clearances' issue?G

author: Northy
author_rank: Regular Contributor
timestamp: November 11, 2020, 01:32:23 pm
content: I've had some support from the test house. Here's some help for others that might be interested:Q: Do metal parts of the enclosure (with touching bare metal edges contacting each other) need to be connected with a wire?A: Not if they pass the test! The test being: 2 x the building (installation) protective current rating (in my case for the UK that's 2 x 16A = 32A) is passed from the Earth input pin to any accessible metal part. If the resistance is greater than 0.1ohm the unit fails.Q: Can you rely on air (distance) as a protective measure?A: Yes, refer to Table 11, 15 & 18 for the requirements.G