Oscilloscope and no proper grounding - Page 1
Forum section: electronics / beginners

author: Zbig
author_rank: Frequent Contributor
timestamp: July 02, 2013, 07:33:52 pm
content: Hello everyoneTomorrow I'm getting my first DSO. I'm living in an old building with no grounding circuit (230V part of the world). Some of the sockets in my appartment are "zeroed" (have their grounding prongs bridged with Neutral), some don't have GND prongs at all. The socket I intend to connect my DSO to (and the one most of my DUTs will be powered off) is the "no grounding prong" type. Now, I have two options:Install a normal modern socket and then "zero" its GND prong (short it to N)Leave it as it is (essentially "floating" my DSO in the process, right?)What would be your suggestion, guys? Which option is less bad?Regards,Zbig

author: c4757p
author_rank: Super Contributor
timestamp: July 02, 2013, 07:48:42 pm
content: I suggest you find a third option. I won't even give one the credit of being "less bad", but you can extrapolate that from "never ever ever ever use neutral as ground".

author: IanB
author_rank: Super Contributor
timestamp: July 02, 2013, 08:13:10 pm
content: There is no good option here. Every piece of electronic test equipment you buy will come with a three pin grounded plug and will expect the ground pin to be connected to a proper mains ground. An electronics lab needs to be fitted with three terminal mains sockets having a proper ground connection.Some reasons for this, apart from safe use of the equipment so designed, are control of floating voltages and static potentials. Your soldering station will connect the iron tip to ground to stop it damaging sensitive devices. If you are working with sensitive devices (basically anything CMOS), then you should have a static dissipating work mat and wrist strap, both also connected to ground. That keeps everything on your workbench at the same equal potential. Sparks won't fly from your hands and your components will be safe.You should contact an electrician and see if the sockets in your lab can be made proper.

author: Paul Price
author_rank: Super Contributor
timestamp: July 03, 2013, 12:10:25 am
content: Take a look at these posted topics for more insight on the subject:https://www.eevblog.com/forum/beginners/is-it-safe-to-use-a-bench-power-supply-not-grounded/https://www.eevblog.com/forum/beginners/new-lab-no-grounded-outlets/]https://www.eevblog.com/forum/beginners/new-lab-no-grounded-outlets/There is a lot of hysteria about grounding and ungrounded equipment, you should read the rants and reason of the above link.Personally, I use all my equipment without 3-wire grounding, as is the practice in my country, and with caution and common sense, and following the essential rules of handing electrical equipment, I have had no problems.

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 04:24:36 am
content: Ah, no, here we go again. My best suggestion is to completely discount anything Paul Price has said on this topic, as without comment as to whether his advice is correct in *his* situation it's rather dangerous in most contexts, and take the advice above and contact an electrician and see if you can get your lab properly grounded - or at least ask for his/her advice on the topic. Even if you choose not to ground your lab, that zeroing sounds awful dangerous.

author: madshaman
author_rank: Frequent Contributor
timestamp: July 03, 2013, 04:37:06 am
content: Quote from: c4757p on July 02, 2013, 07:48:42 pmYeah, 'cause it's not. For all you know neutral could be +80V relative to local earth, then all you s**t gonna zap you when you touch it. It might even be intermittant just to make it more fun.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 04:53:19 am
content: Paul, there is a difference between an ungrounded toaster and an ungrounded oscilloscope. One connects to external (often faulty) circuits. I don't know about you, but I like to know what potential it's at. Just brushing up against something else could change that. Leaving it floating, where you don't truly know unless you measure between all your test equipment every time you make a measurement, is reckless. I'm glad it's worked for you, but you endanger people who are not necessarily as mistake-proof as you by suggesting they defeat the safety of their equipment and rely on "caution and common sense". No, grounding doesn't make things inherently safe, but it eliminates a large part of the safety decision-making process.The grounding pin wasn't stuck on the plug because three's a lucky number.

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 04:55:55 am
content: Put another way, if you could please stick to giving bad advice wrt questions that couldn't get someone hurt or killed by following it, that would be much appreciated, kthxbye.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 03, 2013, 05:08:54 am
content: Quote from: Zbig on July 02, 2013, 07:33:52 pmDo the sockets which are "zeroed" actually have a link between the Neutral & GND pins inside them,or do they just look like that if you meter them?If the latter,you may have a safety ground after all.If the former,they are basically useless as a GND if your socket is wired correctly,& very dangerous,if your socket has the Active & Neutral connections transposed.Don't kid yourself that Electricians don't make mistakes---they do!Of the two,I would go with the 2 pin unearthed option,as in that case,there is no connection between the metalwork of your instruments & either side of the Mains.It is,in my opinion,not as safe as as a proper 3 pin socket with a protective Earth.As Paul has pointed out,many people use the 2 pin non-earthed option without incident.I do disagree with him when he says it is safer than a 3pin connector with protective Earth connection.

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 05:13:54 am
content: Many people do a lot of things out of necessity that are not ideal. Just this afternoon, I was driving to the surplus place, and someone in a white car was dodging through traffic, tailgating, hitting their brakes repeatedly, weaving around, and finally cut across two lanes of traffic. He did so without incident and no one got hurt. The alternative of driving safely would have been much safer, but one could say that many people drive stupidly without incident. That doesn't mean you want to tell people that it's OK to drive stupidly because everybody does it.It's a bad example because driving stupidly is a choice, and proper grounding isn't always, but that's how I see it. Ground if you can, and have a good reason for not doing it. Don't just do it because you think you can get away with it.... because that's when you won't.

author: IanB
author_rank: Super Contributor
timestamp: July 03, 2013, 05:17:31 am
content: Quote from: c4757p on July 03, 2013, 04:53:19 amThere's an even bigger difference. My toaster is fitted with a two conductor power cord and a two pin plug, in common with a vast number of electrical devices in the USA. Since it was designed this way, it is perfectly fine to plug it into a two pin socket. You could not connect it to ground if you wanted to.However, my oscilloscope, my power supplies and my soldering station are all fitted with a three pin grounded plug, and the ground is connected internally. Since the devices were designed this way with a three pin plug, the ground pin must be connected to a proper mains ground at the supply. That's the difference. You need to use equipment as it was designed to be used. In the USA (and the UK and probably much of Europe) it is forbidden to cut off the ground pin or leave it unconnected if the equipment is fitted with a three pin plug.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 03, 2013, 09:10:09 am
content: Quote from: madshaman on July 03, 2013, 04:37:06 amPaul Price most likely has TT grounding so that scenario sounds a bit unlikely.

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 10:00:13 am
content: Quote from: vk6zgo on July 03, 2013, 05:08:54 amI agree. Even if neutral and phase are wired correctly now, an absent-minded electrician (or hobbyist) could accidentally switch them in the future. You wouldn't notice since 99.9% of the modern equipment does not care about polarity (as is evident from the unpolarized plugs used in much of Europe), until you got zapped by touching your scope. If someone switches ground and neutral in a proper three wire grounding scheme, then you'll quickly find out because the GFI (if present) will trip.Neutral can also get cut. It then acts as a voltage divider of all plugged in equipment with the leakage resistance of the cut cable. This means that its potential will be very close to that of the phase.

author: Paul Price
author_rank: Super Contributor
timestamp: July 03, 2013, 10:50:54 am
content: My suggestion is to discount anything Duskglow has to say to limit your information on this topic and peruse the two links that I have inserted into my first comment posted here that have a lot of pro and con information on this subject.Duskglow has no expert authority to censor my remarks.Please leave hysterical or censoring comments to bullies, fascists, and some other self-righteous-enabled blogging sites.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 01:45:01 pm
content: Your comment suggests that you think experience matters. Why, then, do you disregard all the expert criticism that you receive every time you post on this topic?

author: Paul Price
author_rank: Super Contributor
timestamp: July 03, 2013, 02:03:57 pm
content: My comment suggests that people should examine all available information available on an important safety issue, that there is more than one side to this story, and a plea to all to refrain from self-righteous censoring and hysterical and rude comments.

author: madshaman
author_rank: Frequent Contributor
timestamp: July 03, 2013, 02:22:53 pm
content: Quote from: Tepe on July 03, 2013, 09:10:09 amI'm not so sure it's unlikely, neutral will be connected to remote ground, which could, due to many things have a non-trivial potential vs local ground.True, if neutral is tied to ground locally then the worst that will happen is that a DC current will run along neutral. If the current is too large (and I don't remotely claim to know anything about power geberation protection and control ststems) it would probably cause some kind of fault.So yeah, unlikely a chassis connected locally to both N and local ground will zap you is this situation, but my been shocked many times in my youth side doesn't want to discount "something I didn't think of." Would rather *not* have to be in a situation where I'm doing forensics to figure out why I got knocked on the floor; interesting though it might be intellectually.When a crazy man like myself yleans to pragnatism..

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 02:26:53 pm
content: Paul, I am not censoring your replies. I have no power to do that. I am suggesting that they be disregarded. There is a time to insist on being right. That time is not when a newbie is asking for advice on a topic that could cause them personal injury if they get it wrong. I don't care how much experience you claim to have, if you are giving dangerous advice, i will call you out on it. That is the responsible thing to do. i will not stand by and watch people be given dangerous advice by someone who should know better.That said, I acknowledge that you may have a point. There are circumstances where not grounding is appropriate. However, to claim that not grounding is *safer* is not responsible, bad advice, those circumstances are not common, and you really need to understand the concept of "not muddying the waters". For the purposes of this discussion, the correct advice is "yes, you should ground if possible", full stop. This is not a discussion to get into the subtleties, because it simply muddies the waters and confuses the questioner.Please stop putting being right over being responsible. And if a little judicious rudeness gets this point across, so much the better. I have actually constrained myself, your responses actually anger me a lot more than I'm getting across. Intentionally putting someone in harm's way for the purposes of ego satiation is one of my triggers.OP, if you have any doubts at all, please consult an electrician. This is what they're paid forEDIT: I just remembered something I had forgotten. About ten years ago, I moved back to my hometown of Toledo, Ohio, and the only job I could get was a near minimum wage job at a call center for AT&T@Home. I was ridiculously overqualified for that job, and the training was boring. I kept doing pretty much what you're doing, and giving way more information than was needed, just to show that I knew it. Training was taken over by an ex military guy for a day and he basically told me to shut up. I caught up with him and expressed my displeasure, and he basically told me that while I was right, it was confusing the issue, and it's really not appropriate to do that. It's a lesson I haven't forgotten. Sometimes, even if you know something to be true, the best course of action is to shut up and let things be simple.Oh, and if I ever give bad or dangerous advice, I *expect* to get called out on it. That's the responsible thing to do. I may not like it, but I promise I'll think about it. Usually I do the right thing.

author: Paul Price
author_rank: Super Contributor
timestamp: July 03, 2013, 02:42:34 pm
content: Darkglow, you say, " And if a little judicious rudeness gets this point across, so much the better. "IMHO, this is a fascist, bullying attitude that is totally sophomoric.Darkglow, you say, "Sometimes, even if you know something to be true, the best course of action is to shut up and let things be simple."I totally disagree, what if the millions of people in Egypt demonstrating today for freedom from oppression took your advice?

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 02:48:52 pm
content: My nick is duskglow, not darkglow. Please take the minimum amount of effort to get that right. If you are intentionally getting it wrong, that's way more sophomoric than anything I've said.Typically I would just let it slide. But as I said, your advice is dangerous and not helping. If that's "fascist, bullying, and sophomoric", I accept that as a risk of protecting a newbie from your irresponsible advice. If you find it that offensive, please feel free to bring a moderator into the discussion and I will abide by his decision. But until then, I will continue to call you out, and that's just something you'll have to live with. I judge the safety of the original poster to be more important than your ego.

author: richcj10
author_rank: Unknown
timestamp: July 03, 2013, 02:56:41 pm
content: In the US, The neutral is tied to ground at the breaker panel in the house. So, They are at the "same potential".

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 02:58:35 pm
content: That, of course, assumes that the wiring was installed to code and that there isn't a fault that has disrupted that bonding.

author: richcj10
author_rank: Unknown
timestamp: July 03, 2013, 03:02:47 pm
content: Quote from: duskglow on July 03, 2013, 02:58:35 pmI would agree. That is why, I never plug anything into a outlet that I haven't checked. I carry a outlet wire checker with me at all times.

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 03:03:53 pm
content: The equivalent of a surgical mask for an engineer.

author: edavid
author_rank: Super Contributor
timestamp: July 03, 2013, 04:14:06 pm
content: Quote from: IanB on July 03, 2013, 05:17:31 amNot true in the US, at least if you are talking about meeting code. The ground can be left unconnected if the circuit has a GFCI installed, and the outlet is labeled.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 04:54:07 pm
content: Quote from: richcj10 on July 03, 2013, 02:56:41 pmThis assumes that you live in an inductance-free world with wires that never fail.I just measured almost half a volt here, and the only things running on this circuit are my laptop, the multimeter, and a small fan. You'll get some nice, big spikes when motors stop and start.

author: richcj10
author_rank: Unknown
timestamp: July 03, 2013, 05:55:45 pm
content: Quote from: c4757p on July 03, 2013, 04:54:07 pmAcross what?

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 05:58:42 pm
content: Neutral to ground.

author: SeanB
author_rank: Super Contributor
timestamp: July 03, 2013, 06:26:09 pm
content: Quote from: duskglow on July 03, 2013, 05:13:54 amYou mean like half the drivers and all the taxi drivers do here........I have measured 10V between neutral and earth, and this is often the case when there are large loads on the home line.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 06:30:31 pm
content: In before "but 10V won't shock you!!!!"

author: SeanB
author_rank: Super Contributor
timestamp: July 03, 2013, 06:43:17 pm
content: No, but it can deliver around 100A or more into a short circuit, and the current is NOT limited by anything other than wire resistance, as there are no protective devices in the neutral or earth conductors, other than a double pole disconnector on the main supply.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 07:08:16 pm
content: I'm not sure I follow you. Wouldn't it ultimately come from the live conductor anyway, where it can trip the breaker?

author: SeanB
author_rank: Super Contributor
timestamp: July 03, 2013, 07:15:37 pm
content: Comes from the sum of all the live feeders fed from the substation transformer, so while it theoretically is fused it would be a 400A fuse typically.

author: c4757p
author_rank: Super Contributor
timestamp: July 03, 2013, 07:20:54 pm
content: ...yes, the rest of the world isn't wired like the U.S., one transformer per 1-4 houses...

author: SeanB
author_rank: Super Contributor
timestamp: July 03, 2013, 07:28:48 pm
content: Here it is more like one minisub for around 100 houses, or more for the older suburbs. At work I share a 250kVA minisub with another 5 businesses, about 30 flats and a hotel.

author: madires
author_rank: Super Contributor
timestamp: July 03, 2013, 08:31:32 pm
content: Quote from: richcj10 on July 03, 2013, 02:56:41 pmBut only at the point where they are connected together. Since neutral is the return path for hot the current flowing through neutral causes a voltage drop. That voltage drop increases with the wire length and the load attached to it.