118V between USB shield and mains earth. What's happening? - Page 1
Forum section: electronics / beginners

author: aeberbach
author_rank: Regular Contributor
timestamp: November 11, 2020, 01:01:07 am
content: Having watched Dave's "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!" (because I did trip the RCD a few hours after getting a scope some years ago) I understand there's direct connection between scope shield and mains earth. What I did not expect was to feel a shock when touching the shield of the USB-type B plug at the same time as the oscilloscope shield clip. Measuring with a multimeter shows 118V AC between these two points...I'm using a FPGA development board that is programmed by USB but am not using the scope on it with USB connected. Instead I program it with a .pof file so that it will configure itself on boot from flash. I can program it, disconnect it from USB, and then power it from a bench power supply, red and black terminals only, no clip across to green so no earth connection. No problems there and safe scope probing. But in making the change, pulling wires out and reconnecting, I felt the voltage mentioned above.The USB lead is connected to an Anker USB hub powered by a 12V power adapter. The USB upstream is connected to a Dell monitor. That connects to two PCs - I use its KVM feature to switch between those machines and USB connections at the same time. If I take the hub and the monitor out of the equation and just connect the USB cable to the PC that does the programming I still see 88V AC between USB shield and scope shield.How does this happen? Do I have some electrical fault somewhere?

author: aeberbach
author_rank: Regular Contributor
timestamp: November 11, 2020, 01:46:51 am
content: I guess that means any device in the house could have the bad Y capacitor :-(

author: radiolistener
author_rank: Super Contributor
timestamp: November 11, 2020, 02:40:10 am
content: Quote from: aeberbach on November 11, 2020, 01:46:51 amno, it just means that your mains connector doesn't have Ground on the Ground terminal. Usually mains connector has a 3 terminals:N - neutral,P - phase,GND - grounding.In your case GND terminal is not connected and it leads to such issue.

author: amyk
author_rank: Super Contributor
timestamp: November 11, 2020, 02:44:01 am
content: That's just the coupling thorugh the EMI suppression capacitor. Maximum current to fit within safety standards is something like 300uA. Classic web page on this subject:http://www.aplomb.nl/SMPS_leakage/Doc_ie.html

author: Berni
author_rank: Super Contributor
timestamp: November 11, 2020, 06:48:20 am
content: Your earth connection is missing or interrupted somewhere. This can be a dangerous situation!This is leakage from mains filter capacitors in the PSU and this is normally diverted into earth so you don't get any voltage. But when earth is interrupted the voltage sticks around and since USB shields are connected to earth you also get this voltage appear on those.Often this voltage can be felt too if you touch it just right. But if you have multiple things plugged into a power strip with a missing earth the currents can add together from say the PC, 2 monitors, speakers etc... and then the current can actually become large enough to be unpleasant to touch. I seen it happen myself due to a faulty extension cord at a coworkers desk, also seen a youtuber reviewing a 3D printer get shocked by it after having multiple printers plugged into a similarly faulty extension cord (And blamed the 3d printer manufacturer until he found out what the problem actually was).Even when multiple devices combine there leakage currents on a common extension cord the currents are still too low to actually kill you, it just hurts a bit to touch. But if any of the devices fails and develops an internal short then live 230V might end up on this unconnected earth, this means that touching your computer case is the same as touching a live wire and that can definitely be FATAL!Get an electrician to look at it or at least get one of those power outlet testers with LEDs that show if the power socket is wired correctly and that earth works.

author: Nerull
author_rank: Frequent Contributor
timestamp: November 11, 2020, 07:14:18 am
content: Quote from: Berni on November 11, 2020, 06:48:20 amYou realize that a large percentage of modern electronic devices are double insulated and have no ground connection, right?This almost certainly includes the adapter powering the OPs USB hub.

author: Berni
author_rank: Super Contributor
timestamp: November 11, 2020, 07:34:04 am
content: Quote from: Nerull on November 11, 2020, 07:14:18 amYes a lot do not have ground, but desktop PCs and monitors pretty much always have IEC C13 mains connectors with earth in them. So if any of those are earthed the voltage should have went away once the hub is plugged in.

author: aeberbach
author_rank: Regular Contributor
timestamp: November 11, 2020, 07:32:33 pm
content: Thanks all. I will need to unplug everything and check what appears to be grounded properly and what is not. I have two quite long power boards (Rack mount units repurposed) to check.I do see the voltage in two situations. One is when a desktop PC is connected via USB - 88V AC from USB shield to ground. The other is when the USB cable connects to an 8-port hub (powered by big laptop-style adapter, with earthed IEC cord) - 118V AC. That hub connects to the monitor's USB hub, the monitor connects the first mentioned PC and a Mac mini. All are powered off one long power board.

author: aeberbach
author_rank: Regular Contributor
timestamp: November 16, 2020, 01:52:54 am
content: Disconnected and reconnected everything in the office one by one - it seems the problem was an 8-outlet power board where the brass earth contacts had become loose from many years of use. It was a Jackson unit, good quality, and so possible to open with a 2mm triangle security bit and repair. I didn't know they were still sold, maybe will buy more.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 16, 2020, 02:51:56 am
content: and I thought my -5V wrt ground was a problem.

author: Berni
author_rank: Super Contributor
timestamp: November 16, 2020, 06:25:34 am
content: Good thing you found it.Earth faults in extension cords are not all that rare. If the live or neutral breaks then the extension cord stops working and people stop using it (most people throwing it away) but if the earth connection breaks then the user does not even know it happened so they keep using it for years.