Enclosure grounding for board like FeatherS3 - Page 1
Forum section: electronics / projects

author: rkagerer
author_rank: Contributor
timestamp: December 28, 2022, 12:34:01 am
content: I'm using aFeatherS3for a project that will go in an aluminum enclosure. AC mains will run into the same box, to aRAC03-SERAC/DC power converter (either 5V or 3.3V model). I could use some help making sure I connect everything properly and safely.First off, can you confirm that I should bond the AC ground (green) wire to the case?The case will also be in contact with the metal shield of the board's USB port, which is connected to the board's GND pin. When I asked on the FeatherS3 support channel whether it's safe for AC mains ground to be tied to this pin I got the response:QuoteWhat would be the correct way to wire things up, and do I need to be concerned about ground loops if a USB host is plugged in, e.g. a PC (whose USB shield I gatheristied to earth), a laptop running on battery, etc?Or in other words:Is DC and AC ground the same on a circuit, and is it safe to have DC ground from some DC electronics connected directly to AC ground/earth?Is it safe to ground the case and have the USB connected to the ground and also connected to a PCB that has USB grounded, potentially causing a big ground loop?

author: seon
author_rank: Regular Contributor
timestamp: December 28, 2022, 12:44:23 am
content: Quote from: rkagerer on December 28, 2022, 12:34:01 amFor accuracy - my reply of NO was specifically on the "USB connector touching the earthed case and then being used plugged into a PC/source with a grounded USB cable. This sounds like a bad ground loop opportunity here... and I advised to find another way to mount the board and not have the USB connector touching the case.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 28, 2022, 08:33:31 am
content: Bond ground to metal cases.If you need to keep RF from entering or leaving via mains earth put a small choke between the DC ground and the AC ground.Its usual pratice to connect circuit ground to mains ground. As I see it you will breach the DC-DC converter isolation. This may or may not be a safety issue depending on your application.I'm sure Recom will only tell you not to because it will invalidate the isolation specs.In general AC+DC grounding the -Vout side will not affect function. Try it without connecting up the the feather. Check that you measure 3.3V relaive to mains gnd. If you do, I dont see the problem.Remember that connecting any ground referred signals to the device (and you likely are) will join the two gnds anyway. I sense ass covering here. Ask them to tell you precisely why not, not if its just ok.

author: floobydust
author_rank: Super Contributor
timestamp: December 28, 2022, 07:45:00 pm
content: For protection against electrical shock, the enclosure going hazardous live due to internal spacings or insulation failure, it is best to earth-ground the enclosure.Your mains fuse, on/off switch, mains wiring or PC board/mounting hardware/spacers clearances - if anything could cause a mains fault to the metal enclosure, grounding it gives coverage for that.The Recom power supply is two-wire and likely has a Y-capacitor inside, which means there will be stray voltage between the output and earth ground due to the capacitive coupling. You can measure the ACV between earth ground and the output, it usually is around 60VAC which will cause problems with your I/O.The USB shield touching the (earth grounded) enclosure may or may not be a problem. If you have analog signals then the ground loop will cause noise.Otherwise I would earth ground the DC side (3.3 or 5V) to get rid of the stray voltage and then not worry about the USB shield touching the enclosure.Note that getting RF out of a metal box is an issue, it can shield the ESP32 antenna...

author: eugene
author_rank: Frequent Contributor
timestamp: December 28, 2022, 10:19:20 pm
content: Do you need mains voltage in the box at all? Or is it just to power the AC-DC converter?If you don't need mains AC inside the metal box, then I suggest using an external power supply (a.k.a. wall wart) to provide the DC voltage you need. Assuming the wall wart is not junk, the 5V and GND will be completely isolated and can be safely connected to the metal box and USB connector.

author: rkagerer
author_rank: Contributor
timestamp: December 29, 2022, 01:07:26 am
content: Thanks for the responses and tips.QuoteQuoteHow do Class I consumer products that are in metal enclosures with shielded data ports typically handle this?QuoteJust digital signals going through the USB-C connector.QuoteI do need mains voltage inside the box.

author: thm_w
author_rank: Super Contributor
timestamp: December 30, 2022, 01:45:42 am
content: Quote from: rkagerer on December 29, 2022, 01:07:26 amA desktop PC, or an office printer are setup like this. You have your 3 pin earthed power lead, as well as the earthed USB cables.Its not a universal truth that "ground loops are bad and must be avoided in all scenarios".It would probably help if you describe your actual application. Why is 110V AC needed inside this case?