Idiot on deck - Page 1
Forum section: electronics / general

author: Keith
author_rank: Contributor
timestamp: October 16, 2011, 06:44:47 pm
content: After the "event" I verified...Circuit ground connected to bnc probe clip ground(on scope), which has low/no resistance path through power cord of scope to neutral/earth ground which is connected to my neutral(mains) back at the service entrance to the house. I was on the wrong side of the transformer and inadvertently connected my Oscope probe to the mains and made my second mistake by connecting my scope ground to the hot line. Its amazing how long you keep seeing the arc after you close your eyes. Thank you Tektronix for a decent scope and thinking about the different ways I can screw up. Scope works fine. Back to work after confession and a good self flogging.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 16, 2011, 07:34:05 pm
content: Most people make this mistake exactly once! It makes enough of an impression that you don't forget. That said, you could get away with it using the old boat anchor scopes that had serious metal connections between probe and ground. Some modern scopes have brought that connection through the PC boards and a good short can blow traces. I don't know if anybody still does this, once the problem was discovered. You should probably check the innards of the scope to be sure it still has good ground integrity.

author: elliott
author_rank: Regular Contributor
timestamp: October 16, 2011, 09:03:17 pm
content: I was lucky when I did this, I was working on the other side of a 0.15A fuse that blew pretty quickly.

author: Lightages
author_rank: Unknown
timestamp: October 16, 2011, 09:35:01 pm
content: I did something really stupid when I was 16. I had just ripped apart a portable record player to use the parts. I wanted to use the amplifier and forgot which wire was which. No problem I thought, I will just hook up some speakers and touch wires together to hear the click and know which wire was which input. All was going well but I could not under stand what two wires were and they were obviously for something I did not recognize. So I touched them together.All I could think of when I went blind was that I couldn't see what was going on and I had just shorted two AC wires together and the bed on which I was working might not be on fire. Well I didn't panic as I could feel no heat, just the smell of the burnt hair on the back of my arms. When my vision started to recover, I was relieved to see that all was well, but the wires I had been holding were melted back to the tips of my fingers and my hands looked like a bad cartoon with black soot all over them. My arm hairs were gone, but no real damage.Now, living in Chile is an adventure in stupidity in wiring. There is a neutral here, but the plugs are not polarized and can be inserted in both ways. So I have checked every wall socket and marked which side is live and which neutral, and have marked all my power cords too. I have read that electrical fires are the number one reason for house fires in Chile.

author: EEVblog
author_rank: Unknown
timestamp: October 16, 2011, 10:20:47 pm
content: Yup, everyone makes this mistake exactly ONCE, and then learns a valuable lesson about mains earth grounded scopes.It's a right of passage, well done!Dave.

author: Unknown
author_rank: Unknown
timestamp: October 17, 2011, 02:52:07 am
content: Quote from: Lightages on October 16, 2011, 09:35:01 pmWhy should it matter? Any equipment that cares is broken and against regulations in many countries, since neutral is treated the same as the live wire in most safety standards. Live and neutral are often reversed in installations since all equipment keeps working fine.

author: Lightages
author_rank: Unknown
timestamp: October 17, 2011, 03:44:59 am
content: Quote from: alm on October 17, 2011, 02:52:07 amIt matters when dealing some equipment where ground loops between interconnected instruments can cause some problems. It also matters when some circuits are connected with Ground Fault Interupters. And yes, many things sold here are not within regulations and random repairs by random idiots makes the danger worse.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 17, 2011, 09:05:17 am
content: Quote from: alm on October 17, 2011, 02:52:07 amIt does matter, although it should never cause a fault like this. Any circuit with electrical connections should be isolated from live and neutral and have safety earth connected to any exposed metal parts of the chassis. In the US for instance, you are only required switch and fuse the live conductor, and this is the standard practice. Also, edison style lamp sockets are not the safest devices in the world to begin with, but using them without polarized line plugs is like juggling dynamite. Any country that uses them and doesn't mandate polarized plugs is just homicidal.

author: david77
author_rank: Frequent Contributor
timestamp: October 17, 2011, 09:58:36 am
content: Ooh, good to know that a very large proportion of European countries are homicidal as we use Edison lamp sockets and unpolarized Schuko (CEE7/F) plugs. I'm sorry, I do not see the issue there and I see no problem with unpolarized mains plugs either.I've now got an old Philips scope that by design has no mains earth connection, very handy for these situations.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: October 17, 2011, 01:46:28 pm
content: Quote from: david77 on October 17, 2011, 09:58:36 amWell if you think about it for 5 seconds you should realize the problems. In a light circuit only 1 side is switched.If the Live conductor is connected to the outer (threaded) part of the lamp holder and the neutral side switched, the outer part is live all the time. It is more than possible to be screwing in a new lamp and touch the metal parts, and if you are earthed you will received an nice electric shock for your trouble. This is especially a problem with the new energy saving type lamps which seem to have more in the way of exposed metal work.

author: SiBurning
author_rank: Contributor
timestamp: October 17, 2011, 01:49:49 pm
content: Quote from: Conrad Hoffman on October 16, 2011, 07:34:05 pmThanks for giving me something to look forward to.Can I get dispensation for being electrocuted with a high current source at 600VDC?

author: ejeffrey
author_rank: Super Contributor
timestamp: October 17, 2011, 03:13:17 pm
content: Quote from: david77 on October 17, 2011, 09:58:36 amYes, many european electrical codes are homocidal. The problem with the edison socket is that the outer shell is poorly insulated. In any country that uses the europlug or a non-polarized plug, that shell has a 50-50 chance of being live. This is just stupid. Not only is it stupid, it is unnecessary: there is no reason not to require outlets to be polarized uniformly. I at least hope that fixed installations which cannot be unplugged for bulb changes are required to have hot wired to the center conductor, but given how absurd the rest of their electrical codes are, I wouldn't expect much.The Netherlands (where I currently live) implementation of Schuko plugs is even worse. It is common and evidentially legal to use type C sockets, which are ungrounded, unpolarized sockets that will accept schuko plugs. This means that most dutch homes will have appliances designed and required to be grounded with no ground connection.In principle the general premise of using unpolarized connections with double switching and fusing could be safe. However, it fails to take into account that there is a worldwide market for electronics, and much of it has interchangable power cords. The fact of the matter is that there is a lot of equipment in use in the EU with IEC mains inlets that only fuse and switch the live wire. Unless those countries are going to seriously enforce the regulations, using non-polarized outlets is just reckless.Oh, one more absurd thing that isn't really a safety issue, but is still stupid electrical practice: In our lab most of the three phase power is color coded: black, black, brown, blue, green/yellow. This is in a building less than 15 years old, and wired by professionals. Why go to the trouble to color code your wires if you are going to do something stupid like that? Why would someone even bother manufacturing a 5 conductor cable with those colors? I could understand if all three live wires were black, but this makes no sense. At least I hear there is now a standard for new installations.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 17, 2011, 04:28:29 pm
content: Yes,it seems everybody does this once! not only newcomers.In my case,it was when I had become used to using an isolation transformer.The transformer wasn't available,so I proceeded "with due caution".Obviously,not enough caution as it turned out!the bench RCD/core-balance relay/ earth leakage device tripped,so no harm done.Most times if you are looking at the mains with an Oscilloscope,you just want to see if it is present,so measuring betweenearth& active will be satisfactory.Of course,even then,if you put the earth clip in the wrong place it will still go bang!Another precaution you may already practice,is to clip any test leads for any 'scope,DMM etc, to the mains with the power to the device under test OFF,then turn it on.If you get it wrong,it will go bang,but you won't be holding the probe at the time.Lightages' comment reminds me of something I did when I was about the same age.The place where I worked used to build "Radiograms",which had an AM radio chassis & a record player mounted in wooden cabinet.After they stopped making them, they had a couple of the radio chassis hanging around.The boss decided to sell them off cheap,so asked me to check if they still worked.I grabbed a 12" loudspeaker,set up the radio on a bench,& looked for a speaker connection."Ahh,that must be it" thought I,when I found a length of "figure 8" cable with free ends coming out of the back.I connected them up to the speaker & turned the radio on.There was a sound like a shotgun going off,then dead silence!It turns out that the piece of cable I thought was for the speaker was the ac feed for the record player.240v across a 15 ohm speaker really does make a lot of noise for an instant!The boss was nice about it,though,he didn't sack me,or even make me pay for the speaker!VK6ZGO

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2011, 08:45:04 pm
content: Quote from: ejeffrey on October 17, 2011, 03:13:17 pmIn EU single phase wiring, brown is live and blue is neutral. With three phase wiring you need to be able to identify the neutral (blue), and just one of the phase wires (the brown one in your example). Swapping any pair will reverse the phase rotation and you only need one of the phases uniquely identified to do that reliably. All wires other than neutral are equally live so you don't need specifically to know which is which.The more recent European standard for three phase wiring (the past five years or so) has the three phases coloured brown, black and grey (neutral still blue), so then you can tell each phase apart uniquely.Before it got amalgamated into Europe the UK had a more colourful system where the three phases were red, blue and yellow, with the neutral black. In single phase wiring live was red and neutral black. All of those colours at least were bright and distinctive, and red was appropriately associated with danger.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: October 17, 2011, 09:12:44 pm
content: God (I'm showing my age here) I can remember when the colour coding for 3ph was Red WHITE and Blue. Over time the white discoloured to a pale yellowish colour and hence the change (among other reasons). The change to Brown,Gray, Black has caused a lot of problems with interfacing to older systems as the Blue became neutral and the former neutral (black) became a phase..... Does make a more consistent colour system though as single phase cords are Brown Blue Green/Yellow.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 17, 2011, 09:22:04 pm
content: Quote from: IanB on October 17, 2011, 08:45:04 pmI can flip two wires well enough if even if they are the same color. Maybe there is a slight advantage to having brown/black/black, but it is nowhere near as useful as having all three lines individually colored so I know the phase order without hooking it up to a load. It isn't that it is the end of the world, it is just mind boggling to me that someone would think it a good idea to color code the wires, but leave that ambiguity. If all three phases were black I could imagine that the installer just didn't think about the problem or intended the circuit to feed loads that didn't care about the phase order (thermal loads, rectified loads, or single phase loads). When they go to the trouble of marking one of the three lines it tells me they have thought about it, realized the problem, and were just too pig-headed to do the right thing.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2011, 09:50:11 pm
content: Quote from: ejeffrey on October 17, 2011, 09:22:04 pmIn this case brown/black/black was not the installer's choice, it was the required electrical standard at the time for three phase cables. Which leads to a potentially fascinating debate about how such a peculiar standard came to be... (I have no idea).

author: ciccio
author_rank: Frequent Contributor
timestamp: October 19, 2011, 10:11:18 pm
content: The European Union bureaucracy is so busy in writing and enforcing stupid and useless regulations that she will never find the time to write down the specification for a common standard for plugs and sockets.I'm sure there are commercial reasons behind the chaos, but at least some European countries use polarized plugs:- Switzerland (not in the EU, in effect) whose plug was copied by Brazil some years ago- UK (whose plug is very nice, fused, but way too big)- France, who has a version of the German Plug with the ground pin protruding from the socket, not the from plug as the rest of the world does... I know, my fellow French engineers aim is to do anything "the other way".I'm in fact baffled when I see that in some countries (here in Italy, but also in Spain), there is a "de facto" dual standard in new buildings, with Italian sockets mixed with German sockets (to avoid changing the plug in new equipment and voiding the warranty). Why not French sockets? They are compatible with most of the dual standard German-French plugs, but no French sockets are available for sale in Italy, only the German ones...In any case, I know how electricians work: even with polarized sockets, there is a chance of 1/6 of having the work done correctly: they still have 3 wires to screw-in..

author: ivan747
author_rank: Super Contributor
timestamp: October 19, 2011, 10:20:44 pm
content: Quote from: Lightages on October 17, 2011, 03:44:59 amYep, same here. It's a shame you find this sort of stuff that causes all sorts of nasty things, like shocking you when you touch the shield of your printer's USB cable.I still have no decent scope, just a battery powered Velleman (I am a hobbyist). I hope I find a bunch analog scopes being tossed out around here some day.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 20, 2011, 07:02:18 am
content: Quote from: ciccio on October 19, 2011, 10:11:18 pmThe standard is there since a long time. IEC 60906-1. There is just no incentive for member state to put pressure on the commission to write a directive to mandate it.QuoteYou know, there aren't huge piles of electrocuted bodies piling up in those countries where they use non-polarized plugs.And yes, there are commercial reasons. But not the way you think. E.g. Denmark allows a second standard for some years now. This is because they want to break free of a monopoly manufacturer who was and is producing the old danish standard. However, adaption is slow.People know the different national standards are bad. There would be huge cost savings having a single plug standard in the EU. However, the potential cost of harmonizing that, the volume of sockets and plugs that need to be changed is something the politicians fear. Further, many national electrical codes require to upgrade all or larger parts of an installation if you upgrade the sockets. These regulations are a compromise between wanting to have a high electrical safety standard and avoiding the cost from forcing people to always upgrade all installations when there are new regulations. Would the EU enforce new sockets, they would enforce a huge installation upgrade all over Europe through the backdoor. And there are for sure not even remotely enough qualified electrician who could perform this huge wave of upgrades.Instead, what seems to happen is a silent move to a de-facto standard. The German standard probably winning the race here. Although Denmark's second standard isn't the German one. The pressure comes from the manufacturers of equipment who are getting tired of the many standards.QuoteAFAIK Brazil started with the above mentioned IEC 60906-1 and modified it.QuoteAnd the UK is another reason why there will be no single plug in the EU for a long time. Their ring-type installations require the fuse. And anyhow, they won't touch anything coming from the EU, because it is coming from the EU. They want to be different for the sake of being different.QuoteAnd because it is possible to manufacture plugs that both fit the German and the French socket this clever idea probably helped more to make the German sockets popular, than the proprietary French one.QuoteFor practical reasons? There is much more equipment on the market having German plugs than French plugs. Having a German socket therefore means you can use equipment with the German plugs plus French equipment with the dual German/Frensh plugs. While having a French socket means you can only use French equipment. (And in both cases equipment with the Europlug).QuoteAnother reason fort his might be the Italian electrical code (which I don't know). Maybe it allows German but not French sockets.QuoteI think this is the real point. It's not polarized or unpolarized sockets that make an electrical installation at home unsafe. It is the electricians working at them. I am an EE, but I do call a professional electrician when I need to get some work done in and around the house. However, I supervise them like a hawk. And boy, did I see things ... Professional standards ain't what they used to be.