Proper oscilloscope connection to prevent ground looping - Page 1
Forum section: electronics / testgear

author: DMoss
author_rank: Newbie
timestamp: February 13, 2019, 05:27:50 pm
content: I have a question about proper oscilloscope connection for doing work on vintage stereo equipment (120v AC input to +-76v DC board voltage). Our house wiring is US 1950’s 2 wire system with no ground wire to the outlets. The only earth ground in the house is a bare copper wire from the breaker box to water main. There is also an earth ground at the service transformer in the alley. I need to know how to properly connect an oscilloscope and DUT in this environment to prevent ground looping. Obviously, neither the scope nor DUT is earth grounded when plugged in? What’s the potential difference between the two (are they both floating)? If I’m probing, am I the only earth ground in this situation? I don’t know how isolating the DUT would accomplish anything? Would a battery operated scope be an option (still needs an earth ground) and what about probe isolation? Is a differential probe the only real option? I want to be able to do thissafely, but reasonably too. Thanks

author: TheNewLab
author_rank: Frequent Contributor
timestamp: February 13, 2019, 06:15:17 pm
content: My best understanding is. use an isolation transformer. Some older consumer electronics only use two prongs (USA) everything I have in audio and video/TV are two pronged only. Also, if the repair unit has a transformer to rectify AC to DC. then the the PC board is already isolated. just do not probe around before the transformerSwitch-mode power supplies,3-wire (computers, computer monitors commercial video editing machines (SONY editing equipment), all need an external isolation transformer. Good piece of equipment to have anyhow

author: Fungus
author_rank: Super Contributor
timestamp: February 13, 2019, 06:21:38 pm
content: The danger with floating 'scopes is that there's bare metal on the front panel that gets connected to whatever the ground clip on the probe is connected to. If that's a live wire then the other BNCs on a multi-channel scope are then live.If you live in a two-wire house then the neutral wire can be connected to ground at the transformer and/or breaker box.Ifthis is the case then the correct thing to do is to connect the earth wire of the 'scope to neutral inside the plug. This connectsthe BNCs on the front to ground and therefore also the ground clip of your probe.If your neutral isn't connected to ground and/or if you have reversible two-pin plugs then this is obviously very dangerous - neutral wires can carry current relative to ground and if you put the plug in backwards your BNCs are now connected directly to the live wire.Your call...

author: DMoss
author_rank: Newbie
timestamp: February 13, 2019, 06:39:25 pm
content: Right, that's my concern. There is no "earth" ground, only live and neutral flow. So, effectively the chassis on the DUT and scope are in-line? The neutral house wire is not connected to the ground at the breaker box, I believe its for the house system (lightening, etc.) I'm not sure isolating the DUT would make a difference? Is there potential between the two?

author: Fungus
author_rank: Super Contributor
timestamp: February 13, 2019, 06:58:47 pm
content: Quote from: DMoss on February 13, 2019, 06:39:25 pmThe only thing floating the DUT achieves is that you can put the ground clip anywhere and not blow it up.It does nothing for personal safety - as soon as you connect the ground clip of your probe to the DUT then it's no longer isolated.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 13, 2019, 07:22:49 pm
content: Quote from: DMoss on February 13, 2019, 06:39:25 pmWouldn't make a difference. If your outlets only have two connections each, you are already floating relative to the earth (I'm sure it's referenced somewhere, though), and effectively on the same circuit (the scope is not isolated from the DUT). The only risk for ground fault related stuff is if you manage to become the ground reference, or if something else manages to... but unless you're working outside and barefoot or on a metal floor that's bolted into the dirt, it's not a particularly likely thing.Is this a concern with safety, or signal, though? When you say "ground loop" in audio gear, I'd expect this to be about noise and feedback, whereas if you talk about outlets not being grounded, I'd expect you to be talking about ground isolation and safety. If you're worried about ground loop noise, it's not as big of a deal thanks to your isolated outputs. Each analog ground in your gear is floating relative to earth, even if the circuit is designed otherwise, so you don't run into the classic problem of differing impedances in the ground paths which give you some potential (and picked up noise) on a given circuit's ground. That said, you run into a bit of the same problem on circuit grounds between devices, so just make sure the ground connection that goes along with the signal between devices is good, so the two circuit grounds are connected with a very low impedance path and their local potentials don't drift. If you're worried about the scope injecting noise................ don't? The scope's resolution and noise floor likely won't even pick it up, and you're using the scope to measure signals for diagnostic work, so you can sort of ignore any picked up noise while testing, then when you're satisfied, disconnect the scope and any of that noise goes away.

author: DMoss
author_rank: Newbie
timestamp: February 13, 2019, 08:24:37 pm
content: Thanks DaJMasta...that's good info. In this case I'm referencing "ground loop" from a power perspective. Mostly, I want to make sure I connect correctly for safety.

author: Fungus
author_rank: Super Contributor
timestamp: February 13, 2019, 08:33:18 pm
content: Talk to your power company and ask if they can put a ground wire in your distribution box (or at least ground the neutral wire there). That way you have a chance of doing it properly.nb. It's probably illegal to try and do it yourself.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 14, 2019, 01:17:10 am
content: You could use a 1:1 isolation transformer to provide a TN-S AC distribution supply for your audio equipment. Provide an incoming 'earth', and a link from that earth and neutral at the isolation transformer secondary, and use earthed distribution sockets/cables/plugs to your audio equipment. That would avoid neutral-earth voltages from other building equipment from polluting your local 'audio equipment' mains AC distribution.However, all your audio equipment should then each have the distributed protective earth wire connected to chassis, and should not have any death caps, and should have only the active wire switched and fused and meet all standard safety requirements for that type of AC powering. That would allow you to connect earthed test equipment such as your scope, and know that all your audio equipment has the same protective earth. You could even use earth leakage/residual current type protection devices. But of course you should also be a registered electrician, or have such a friend.

author: openloop
author_rank: Regular Contributor
timestamp: February 14, 2019, 03:15:13 am
content: For one-off repair I've used my computer's true-sine UPS (unplugged).Modified sine might work too, depending on what you're working on.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: February 14, 2019, 03:43:14 am
content: Make sure you plug the device you are testing in to the same power strip as the scope. Before plugging it in, you can assure that the device you are testing has the proper power pins connecting to its ground by continuity testing. If you haven't before, you should make sure your mains outlets are properly grounded with an equipment ground and a "Neutral" that is properly connected to ground as well (this varies with where you live) but you should familiarize yourself with local electrical codes.Use a DVM to measure voltage and current between the ground of what you are testing and the ground of the scope. If there is any - you have a problem to solve.There is no need for the expense or risks of an isolation transformer in most cases. Just make sure all grounds are at the same potential.

author: DMoss
author_rank: Newbie
timestamp: February 14, 2019, 06:41:45 pm
content: Thanks GeoffreyF. When you say use a DVM to measure voltage and current between the ground of the DUT and scope, what points do you measure? In my case, with the 2 wire house system I have line and neutral. The DUT has a two prong plug (no earth ground). The scope has a 3 prong plug but is effectively un-grounded. I'm thinking to earth ground the scope to the #8 bare copper coming from the water main in the basement (that's the house electrical equipment earth ground). Would that setup be safe? Where do you put your probes to test for potential between the scope and DUT then? Thanks

author: tautech
author_rank: Super Contributor
timestamp: February 14, 2019, 07:09:14 pm
content: Engage different thinking.Thinkreference, as in a signal/voltage or whatever need be referenced against something, that in many cases is mains ground or circuit groundandthey are not always the same.In the case of a test instrument with BNC's the BNC shells are most often at mains ground potential but in Dmoss's case, floating.So is there any real potential between a preferred reference point on a DUT and the test equipment, this is where the DMM comes into play.Watch Dave's video again and apply this ^ way of thinking and it will all fall into place.

author: DMoss
author_rank: Newbie
timestamp: February 14, 2019, 09:28:18 pm
content: Thanks...can I ask what Dave's video is?

author: tautech
author_rank: Super Contributor
timestamp: February 14, 2019, 09:49:59 pm
content: Quote from: DMoss on February 14, 2019, 09:28:18 pm

author: DMoss
author_rank: Newbie
timestamp: February 15, 2019, 12:21:14 am
content: Right...that one. Thanks

author: GeoffreyF
author_rank: Regular Contributor
timestamp: February 15, 2019, 01:12:10 am
content: Quote from: DMoss on February 14, 2019, 06:41:45 pmMeasure between the ground of the DUT and the scope ground. You are concerned for grounds at differing potentials, separated by significant resistance, capable of current flow between them. If they are close to the same potential with very minimal current, it's safe. If the potential is quite different there is a safety hazard and a risk of damaging the scope probe or the scope itself. That's the nature of a "Ground Loop" - where the grounds are not in common.

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2019, 08:55:56 am
content: Quote from: GeoffreyF on February 15, 2019, 01:12:10 amThere are other concerns. You could put a scope probe shield on an inverting opamp amplifier's virtual earth. But that would add a large capacitance to that node, and the circuit may well "misbehave".

author: trobbins
author_rank: Frequent Contributor
timestamp: February 15, 2019, 09:17:56 am
content: Quote from: DMoss on February 13, 2019, 05:27:50 pmI think it will depend on how much of this 'work' you intend to do, and what you are aiming to achieve - now and in the future.If you ever aim to connect more than one item of audio equipment, or use other test equipment than a scope and make connections to audio gear, then it won't be galvanic earth connections that cause concern, but trying to mitigate the many and varied ways for hum and noise to circulate.It's not that a local earthed AC distribution is needed for advanced testing, but it can simplify how you assess your set up each time it changes, and provide an accepted level of safety.