floating oscilloscope - Page 1
Forum section: electronics / testgear

author: yes955
author_rank: Newbie
timestamp: June 14, 2014, 05:00:12 pm
content: Hello I recently saw one of Dave’s older videos about how not to blow up your oscilloscope (). He talks about how the ground wire on the oscilloscope is connected to ground and if you connect it to something that is higher than ground current can flow through it. The problem is just that there are no ground wires installed in my house and I have read that it isn’t the best idea to have your oscilloscope floating like that (not sure why). So my question is: “what I should do”.On the back there is a socket for a banana plug that is supposed to go to ground, do I connect that to something like a water radiator or is there actually know danger and I can just leave it as it is.

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 14, 2014, 05:18:40 pm
content: I would say that if there are no ground wires in your house the least of your worries should be your oscilloscope.As far as I know there should always be earth wires to each plug point. Maybe in your part of the world the wiring standards are different! Sounds potentially very dangerous though.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 14, 2014, 05:22:01 pm
content: Your water system *should* be grounded. I'm not sure about other countries, but in the US, you can test this by sticking a multimeter between the hot side of your outlet (the smaller slot) and a nearby pipe. You should get 120V. I've had to to do a similar thing in my last house where I had no grounded outlets.It's a bad idea to have your scope floating because any exposed metal will be at the same potential as whatever you have your ground clip connected to. In some cases, this can be a high voltage. It's easy to accidentally touch your scope and get a nasty shock.edit: While ungrounded outlets are against NEC, there's a grandfather clause; old installations will pass inspection. You should still get an electrician to replace the wiring in your house, though that can get expensive depending on the layout of your house.

author: edavid
author_rank: Super Contributor
timestamp: June 14, 2014, 05:24:56 pm
content: Quote from: yes955 on June 14, 2014, 05:00:12 pmFirst, you should do some more reading so you understand how power wiring works... what ground, neutral, and hot are.It's usually not a big problem that you don't have grounded outlets. It's usually best to have the ground prongs on all your test equipment connected together, which may require some creative use of power strips. Dave's advice still applies in this case.Again, the most important thing is that you understand what is going on. Maybe you can read one of the extremely long previous threads on the subject, but they are complicated by the fact that posters live in different countries with very different domestic wiring systems.Which reminds me, set the country in your profile!!!!

author: yes955
author_rank: Newbie
timestamp: June 14, 2014, 08:53:58 pm
content: I live in Denmark and here it is not that uncommon that older houses don’t have sockets with ground. To protect the user we have RCD/RCCB/GFCI/GFI/ALCI/ HPFI or whatever you want to call them that shut off the power if the current that goes out dos not come back.I can acutely measure 230V between the hot wire and the radiator so I guess that’s where I am going to connect my oscilloscope.

author: Thor-Arne
author_rank: Unknown
timestamp: June 14, 2014, 09:12:43 pm
content: I'm not a sparkie, but that doesn't sound right...Shouldn't the "ground point" be approx halfway between the live live wires?This is what I measure here in Norway.I suggest checking this with a certified sparkie.

author: yes955
author_rank: Newbie
timestamp: June 14, 2014, 09:39:58 pm
content: Quote from: Thor-Arne on June 14, 2014, 09:12:43 pmI don’t know anything about that but I am measuring 228V AC. As I understand it the multimeter measures the voltage potential (or something like that )so that if you put a resistor over it you get the same effect with 230V AC as with 230V DC even though the AC signal in reality goes higher than that, negative and all that stuff.

author: Thor-Arne
author_rank: Unknown
timestamp: June 14, 2014, 09:58:04 pm
content: This needs more inputs.To me it sounds like something is "leaking" one of the mains phases to the radiator wiring.Everything here is brand new and conforming to the code, which might not be the same as in Denmark.I still suggest checking with a sparkie, he/she could probably tell just from your measurements.Did you measure between both the live wires and your radiator?

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 14, 2014, 10:32:55 pm
content: A point to be aware is that many properties now have plastic pipes throughout for water and whilst internally gas pipes typically copper or steel the various gas boards are replacing their distribution metal pipes with plastic.This makes good electric earthing via your supplier (and given the circumstances a local earth spike) even more important.

author: yes955
author_rank: Newbie
timestamp: June 14, 2014, 10:35:12 pm
content: Quote from: Thor-Arne on June 14, 2014, 09:58:04 pmmmmh… I do not understand what you mean by this. Don’t we agree if I say that there are two wires? One of them is hot and has the Alternating signal. The other one is null and is basically the same as ground.What I have done is that I have taken one probe and plug it into the hot wire and the other touched the radiator.

author: rexxar
author_rank: Frequent Contributor
timestamp: June 14, 2014, 10:50:52 pm
content: Unless Denmark uses a split-phase system like US 240V, you should indeed see your full mains voltage between hot and ground.I can't seem to find any information on how your AC system is set up, but AFAIK, most (all?) 230V countries use a single-phase system.You can probably test Thor-Arne's leakage theory by measuring from your radiator to the neutral connector. You should see something like 10 or 20V if everything's normal. This small voltage is a 'ghost' voltage, it's pretty much meaningless.

author: VK5RC
author_rank: Unknown
timestamp: June 15, 2014, 01:55:31 am
content: Australia 230v has 3 full phases, 120 degrees apart, about half of houses are on just one phase, most new houses have 3 phase connections for 3 phase airconditioner electric motors. All houses the neutral is earthed at the power distribution board. ?

author: edavid
author_rank: Super Contributor
timestamp: June 15, 2014, 04:08:09 am
content: Quote from: yes955 on June 14, 2014, 08:53:58 pmIt's really not such a great idea to connect your oscilloscope to the radiator. All you need for your test equipment is a common ground, not an earth ground.

author: Zad
author_rank: Super Contributor
timestamp: June 15, 2014, 04:58:46 am
content: If you are doing quite a lot of testing and development, it is worth buying a mains isolation transformer to connect your test equipment to.

author: JOERGG
author_rank: Contributor
timestamp: June 15, 2014, 05:03:06 am
content: If your Oscilloscope for example has a Schuko plug , what i supposehttp://en.wikipedia.org/wiki/Schukoand you plug it into an older danish wall socket, for euro-plugs, with only two wires (earth is connected to neutral somwhere in the installation)http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#Danish_Section_107-2-D1_earthed.28Type_K.29then you don´t have a connection between the earth wire (green/yellow) on the oscilloscope chassis (bnc ground, metal housing etc.) and your earth wire on your wall socket.I believe that would be the same as if in a proper modern house-installation the earth wire inside the oscilloscope was broken.In that case you will get 115V (half the mains voltage) on exposed metal parts of the oscilloscope (for example the bnc connector ground).Now comes it to the part where i have to say that i can´t explain why, but i have read that this voltage comes through the anti-interference capacitor.I also could not find out, if it depends on which direction you plug it in.But i know that 115V is enough to give you a nasty schock.So i would highly recommend to find an electrician, who knows exactly what he is doing, and at least get one proper earthed wall socket installed to plug your scope into.

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 15, 2014, 05:04:28 am
content: Quote from: Thor-Arne on June 14, 2014, 09:12:43 pmNope, your system is wired in a way that's almost unique to Norway, a so called IT network. (I=isolated, T=terra, earth) In this system, both the power carrying wires are isolated from the earth. Essentially it's an ungrounded system, and connecting either of the conductors to earth should not cause a problem. It's not until the isolation of both wires is compromised (a double fault) that a real problem occurs. The reason you're still measuring half of the voltage with your meter is because of stray capacitance between the wires and the earth.This system is different from most of the other world, which has a neutral and a live wire, where the neutral wire is nonimally at earth potential, plus any voltage drop over the wire.http://en.wikipedia.org/wiki/Earthing_system

author: JOERGG
author_rank: Contributor
timestamp: June 15, 2014, 05:15:04 am
content: Quote from: Zad on June 15, 2014, 04:58:46 amNever isolate the ground of your testequipment!The mains isolation transformer is for the device under test.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 15, 2014, 05:48:01 am
content: Quote from: JOERGG on June 15, 2014, 05:03:06 amThe half-voltage thing comes from RF filter capacitors which are connected between both legs of the incoming Mains & Earth.The Earth/Neutral connection at the point of entry to your house does mean much at RF,so both sides are just as prone to pickup,hence a symmetrical capacitor connection Active to Earth & Neutral to Earth.In the North American 240v system,both legs are "hot" w.r.t. Earth,so chassis/metalwork which is connected in this way is at a "virtual Earth" & will in theory at least,have no potential w.r.t. true Earth.When the identical circuit is used in the usual European or Australian system,the capacitors become a voltage divider,so that, lacking a proper Earth connection,the metalwork will be 120v w.r.t Neutral & hence Earth.The reactance of the RF filter caps is too high at 50Hz to allow a dangerous current to flow,but it will give you a bite.Of course,if the capacitor from Active to chassis fails short circuited, all bets are off,& you can get a dangerous shock.Another point which arises,is that,with a wired Earth, in the NA 240v system,both capacitors only need to withstand a peak voltage of around 170v,whereas in the more common 230v systems,one cap has to handle a peak of around 325 v,& the other one has,in theory,zero volts across it.I very much doubt that NA manufacturers have neglected to address this problem in recent equipment expressly meant for export to Europe,but it may be worth checking in older equipment made in that region.

author: leppie
author_rank: Frequent Contributor
timestamp: June 15, 2014, 05:50:26 am
content: Quote from: JOERGG on June 15, 2014, 05:03:06 amNot a bad shock at all. Much less 'painful' than say sticking your tounge on a 9V battery.

author: jahonen
author_rank: Super Contributor
timestamp: June 15, 2014, 09:24:48 am
content: 115 VAC voltage comes from capacitive divider formed by two Y-class capacitors in a mains filter.Typical short circuit current to ground per device is in order of 0.3 mA. While that is not lethal, it still feels nasty. But of course nastiness depends how well it connects to the body.Problem is worse if there are several devices connected together by some means and they are all left floating without connecting to the ground. Then that 0.3 mA figure is simply multiplied by the number of connected devices. So PE wire may carry significant currents in a building.In that respect, having neutral and phase in antiphase configuration, this current is reduced very significantly since currents cancel themselves in the mains filter instead of going to PE wire. Only unbalance due to the Y-capacitor tolerance will create current to PE wire, or residual voltage to the ungrounded chassis.Regards,Janne

author: JOERGG
author_rank: Contributor
timestamp: June 15, 2014, 10:17:30 am
content: Thank you vk6zgoThe capacitors as a voltage divider was the missing part for me to understand how it became half the mains voltage.I was also not aware of the difference between NA and European systems.And i learned that the current is limited by the reactance and that leppie rather would touch it than licking a 9V battery.Thank you jahonenpointing out that the current can add up is important.I think these are all the same:JOERGG: anti-interference capacitor ( looked it up at dict.leo.org )vk6zgo: RF filter capacitorsjahonen: Y-class capacitors in a mains filter

author: Thor-Arne
author_rank: Unknown
timestamp: June 15, 2014, 10:34:03 am
content: All these replies clarifies it quite a bit.I assumed that Denmark had the same system as here (phase/antiphase?), that's why I expected to see half the voltage between the phase and ground.So, if one have isolated neutral (IT-network) one would measure half the voltage between P or N to ground.And if not one would measure only low voltage between N and ground, and full voltage between P and ground.Regardless, I would recommend to get a certified electrician to fix the grounding issue. Especially for the lab area.

author: cs.dk
author_rank: Unknown
timestamp: June 15, 2014, 10:46:23 am
content: In Denmark the null is the transformer-stations earth point. We have 3x400 Volts between the phases and 230/240V between one phase and null.It's quite normal to see non-grounded outlets in older buildings here - There should be 230/240V between a phase and earth.Don't use the radiator as a earthpoint, it can go wrong.http://ing.dk/indhold/152704