Myth: Ground Loops - Page 1
Forum section: electronics / general

author: sorbik
author_rank: Newbie
timestamp: May 30, 2013, 04:21:05 am
content: Can you make a Myth video about ground loops? I'm a calibration tech/new electronics hobbyist and I've seen techs who preach that ground loops will fry equipment but others think it's some mythical creature. We have spade lug cables where each end has an attached 6 inch mini spade lug cable for a shield connection. To prevent ground loops we use all the shielding cables possible but leave one off. A lot of us still aren't convinced that this is a real thing though. Granted, most of the time we don't even use half of the spade lug cables since they only help with making measurements cleaner but for the calibration of items like 3458 DMMs or Frequency measurements down to 10Hz, we try to use all of them but that's when the debate of the ground loop comes in.Here's the main cable we use. The picture looks a little strange because they looped it around itself to make the length short for the photo I guess but it's just one straight connection with shield wires. The connections we make are the basic High to high, Low to Low, etc. with the shield wires going to the ground terminalshttp://www.alliedelec.com/search/productdetail.aspx?sku=70198521

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 04:26:08 am
content: Of course they're real. Damn hard to get rid if you don't know enough about electronics to even know that they're real, so I can see why the suspicion develops. A lot of people treat them like mythical demons to be exorcised. A healthy understanding of the current flow in a circuit tends to solve that problem.I can't see a ground loop frying anything, though.

author: sorbik
author_rank: Newbie
timestamp: May 30, 2013, 04:36:26 am
content: I don't know, my supervisor and another tech was trying to monitor some voltages using a 8508 reference multimeter while trying to calibrate a safety analyzer and the new shiny 8508 fried. Management blamed ground loops and the mistake of using that good item to monitor the voltages when something cheaper and less accurate could have been used. The outrage of that tech when management left was hysterical. "Ground Loops?! WTF is that? It's not real!"

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 04:40:11 am
content: Ground loops cause hum in audio equipment. I don't think that was a ground loop. What exactly was being measured?

author: David_AVD
author_rank: Super Contributor
timestamp: May 30, 2013, 04:41:51 am
content: It's not really a ground loop in itself that causes an issue. It's when current flows in that ground loop that causes damage (or signal integrity is compromised) by that current.Of course not everyone is talking about the same thing/issue when they say "ground loop".

author: sorbik
author_rank: Newbie
timestamp: May 30, 2013, 04:43:24 am
content: Quote from: c4757p on May 30, 2013, 04:40:11 amThis was a couple years ago so I don't remember. I just thought it might be a good episode idea I could share with my coworkers

author: c4757p
author_rank: Super Contributor
timestamp: May 30, 2013, 04:44:47 am
content: Quote from: David_AVD on May 30, 2013, 04:41:51 amI'm having a hard time understanding what would cause that much current to flow through a ground loop involving a multimeter and a safety analyzer. What am I missing?

author: David_AVD
author_rank: Super Contributor
timestamp: May 30, 2013, 04:45:34 am
content: Sorry, I wasn't specifically referring to that particular case.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: May 30, 2013, 06:10:42 am
content: Ground loops are common especially when connecting instruments powered from separate circuits. The result may just be noise at the power line frequency or it may be far worse. Isolation transformers are the usual solution for isolating audio frequency systems.I remember one case when I was monitoring a sonar system on a submarine. Our instrumentation was located remotely from the sonar shack, connected by a 50-pin umbilical cable with twisted/shielded pair. We set up the system and tried to run the computer-controlled spectrum analyzer. The program was erring in a way which I had never seen, failing in the gain-setting procedure for the spectrum analyzer. Inspection showed the analyzer was overloaded at the maximum input range of 20 volts. The typical range is somewhere between half a volt and a volt.The analyzer was clearly overloaded even at the maximum range which should never be reached. I disconnected the signal cable and the overload disappeared. Connected it and it was immediately back. When I disconnected it again, I noticed an arc as the shell of the BNC separated from the connector! Measuring between the ground of the cable from the sonar shack and the instrument ground showed about 40 volts! Strange things occur in shipboard power systems and this was a really unusual condition. The immediate solution so testing could commence was to insert a rack of signal isolation transformers between the sonar equipment and test equipment.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 30, 2013, 06:54:14 am
content: Quote from: c4757p on May 30, 2013, 04:44:47 amMaybe the same thing that lets people nearly kill their new oscilloscope: Not taking into account the probe's ground is connected to PE and then probing mains.

author: IanB
author_rank: Super Contributor
timestamp: May 30, 2013, 07:13:53 am
content: Quote from: sorbik on May 30, 2013, 04:21:05 amIt's quite real and no myth at all.Here's the basic theory:Suppose there are two instruments and one is sending a signal to the other. In order to communicate a voltage from sender to receiver, both sender and receiver need to share a common ground reference voltage. Let's call this VGND. So the sender generates a transmission voltage VSENDby offsetting the signal voltage VSIGSENDfrom VGND. Thus, from the sender's point of view we have:VSEND= VGND+ VSIGSENDNow this signal travels down the wire to the receiver, which tries to recover the original signal voltage. Assuming the receiver has the same ground reference, it can recover the original signal by comparing the received voltage against ground:VSIGREC= VSEND- VGND= VSIGSENDHowever, suppose due to some circumstance the receiver does not share the same ground reference that the sender was using. Call the sender's ground reference VGND1and the receiver's ground reference VGND2. What happens now is this:VSEND= VGND1+ VSIGSENDVSIGREC= VSEND- VGND2= VGND1- VGND2+ VSIGSENDThe received signal voltage has now been shifted by an amount VGND1- VGND2. Depending on the magnitude of the difference, this shifted voltage could overload the input circuit of the receiver, or in extreme cases could damage components or cause insulation breakdown.

author: Neilm
author_rank: Super Contributor
timestamp: May 30, 2013, 06:20:41 pm
content: Of course, a well designed, well laid out and well thought out piece of equipment should not have problems with ground loops. I remember reading a story about a company that made audio mixing desks. A new engineer came in and designed a new desk. Because the engineer had paid close attention to the way the "ground" had been done it wasn't affected by mains hum. After about six months the company started getting complaints - not from the people buying the desks but from the installers who set there charges assuming they would have to work lots of overtime to fix these noise problems.Neil

author: plesa
author_rank: Frequent Contributor
timestamp: May 30, 2013, 06:30:45 pm
content: Good example when the grounding loops can fried equipment are the STP and FTP cabling in large buildings.When you connect the PC to this LAN you will make grounding loop. I most cases it will fried the cable itself.Try to measure the voltage and current flow between the STP/FTP and the mains ground.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 30, 2013, 07:38:20 pm
content: http://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/Not sure, but I think this is the episode you want.