How to mesure 240V AC with an oscilloscope - Page 1
Forum section: electronics / beginners

author: kiwiiscraig
author_rank: Newbie
timestamp: July 20, 2014, 09:45:02 pm
content: Hi AllI have some thing that I want to work on but the very fist thing that I need to do is use an oscilloscope to view 240 Volts AC at 50 Hz. I have a hitachi oscilloscope v-422 but I need to do this with out killing myself and blowing up the scope.Any suggestions on how to do this safely would be very much appreciated.Thanks for the time and effortCraig

author: tautech
author_rank: Super Contributor
timestamp: July 20, 2014, 09:48:12 pm
content: Recommend you acquire a 100:1 probe for any mains work.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2014, 11:40:33 pm
content: When I have to measure the AC main voltage directly with an oscilloscope, my first choice is to use a normal power transformer and measure the output at its secondary.

author: edy
author_rank: Super Contributor
timestamp: July 21, 2014, 01:43:52 am
content: Assuming the OP is working on a mains-powered appliance, the scope usually has 400 V max rating, so using a 10:1 should show the AC fine but RMS is 240 V so peak will be 240 V x root 2 (1.414) so closer to 320-340 I believe (which on 10:1 probe will be -34 V to +34 V showing up on the scope)? Wouldn't the more dangerous part of all this is to ensure proper grounding and measurement technique? I am not so worried about the probe as I would be about how you plan to place your ground lead and especially if using both channels... since they share the ground back to common earth as would your appliance earth. You would want to ground both probes on the same ground as your appliance ground...can check if there is continuity to your mains plug ground pin... if not, the appliance may be using an isolating transformer but you need to ensure that you are not creating a grounding path for mains current. If isolated, I believe you would just be setting that portion of the circuit at grounds reference level and everything else is relative to it.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 03:41:59 am
content: The ground connection is a pretty big deal and safety hazard.Neutral is connected to ground at the distribution point so if there is an AC load between hot and neutral on the wires which the probe is connected to, the voltage drop across neutral may allow considerable ground current through the oscilloscope probe ground.

author: mij59
author_rank: Frequent Contributor
timestamp: July 21, 2014, 03:47:45 am
content: Hi,The safes way is to use a high voltage differential probe.You could use a (medical) isolation transformer.Take a look at Dave's video "how not to blow up your scope".

author: G7PSK
author_rank: Super Contributor
timestamp: July 21, 2014, 07:21:12 am
content: I do it frequently, I have a 3 pin plug wired with BNC socket fitted and a probe to bnc adapter and use a 100 X probe of good quality (RS). I use this rig to check alternators so in most cases the high voltage is a different source than that powering the scope although I have run the scope from the alternator under test. This would not be a good idea if the plug is reversible in its socket, the UK and CEE form plugs I use are not by design so as long as the sockets are all wired correctly all is safe and to date in nigh on 40 years I have never come across an incorrectly wired socket.

author: Psi
author_rank: Super Contributor
timestamp: July 21, 2014, 07:27:34 am
content: I've viewed 240VAC on rigol 1052 with 10x probe quite often with no problems.I even set the probe to x1 by accident once and the scope just clipped the waveform. I disconnected after a few seconds when i realized what i had done. The scope was undamaged.

author: tautech
author_rank: Super Contributor
timestamp: July 21, 2014, 07:43:09 am
content: Quote from: Psi on July 21, 2014, 07:27:34 amThis is exactly the reason why it is necessary to have at least one 100:1 probe.They are not switchable and this simple mistake is not possible.Also the reason why many years ago that I chose to market only those scopes with 400 V inputs.

author: Psi
author_rank: Super Contributor
timestamp: July 21, 2014, 07:47:47 am
content: Yeah, a fixed x10 or x100 probe is definitly the way to go when working with 230V

author: kripton2035
author_rank: Super Contributor
timestamp: July 21, 2014, 07:54:38 am
content: you will blow up the mains differential if earth and ground are connectedso either use a 10x or 100x probe and cut the earth ( use a 2 prongs wire or an adaptor)or buy a differential probe (around 100€ you can find on the bay)see dave's video as said earlier in this thread is a good start anyway.

author: tautech
author_rank: Super Contributor
timestamp: July 21, 2014, 08:10:05 am
content: I have been looking at a Siglent accessory ISFE (isolated front end)http://www.siglent.com/en/product/detail5.aspx?id=100000013462552&nodecode=119008006Think I will get one with my next shipment to see how user friendly they are.They do not have a very high frequency spec, but I think they might be a good option for added safety and negate the need for differential probes for simple applications.Anybody had a play with anything similar?

author: kripton2035
author_rank: Super Contributor
timestamp: July 21, 2014, 09:58:58 am
content: it really looks like a differential probe ... if it's powered (and there seems to be a little plug at the side) it IS a differential probe ...

author: tautech
author_rank: Super Contributor
timestamp: July 21, 2014, 10:42:48 am
content: Quote from: kripton2035 on July 21, 2014, 09:58:58 amThese are differential probes:http://www.siglent.com/en/product/detail5.aspx?id=100000008535191&nodecode=119008006http://www.siglent.com/en/product/detail5.aspx?id=100000009274253&nodecode=119008006

author: Riotpack
author_rank: Regular Contributor
timestamp: July 21, 2014, 02:24:27 pm
content: Maybe use a 240-12V TX as someone has mentioned. Load the secondary with a highish value resistor (calculate to flow 100ma or so through it) to lower the impedence and help stop induced noise.If you need to get the exact voltage, you can get a ELV AC source (like another 240-12 TX), then connect it to the transformer you will be using and measure the voltage on the primary and secondary of your TX with a true RMS meter to get the transformers ratio.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 02:34:38 pm
content: Quote from: Riotpack on July 21, 2014, 02:24:27 pmI do not think the load resistance will matter. The output impedance of the transformer is already low.It may be calibrated in situ by measuring the input side with an AC voltmeter.If higher bandwidth measurements are desired which a standard power transformer will not pass effectively, then an audio transformer designed for a tube output stage could be used with the primary and secondary swapped.For really high bandwidth measurements, 10s of kHz to 10s of MHz, I would use an oscilloscope current clamp and small resistive load like an incandescent light bulb. Multiple turns can be run through the clamp to increase its sensitivity.

author: David Hess
author_rank: Super Contributor
timestamp: July 21, 2014, 02:50:27 pm
content: Quote from: tautech on July 21, 2014, 08:10:05 amSome oscilloscopes provide the same function but at full bandwidth. Each vertical input is galvanically isolated so each single ended probe may be used to make an independent floating measurement simultaneously including direct off-line AC measurements.Quote from: kripton2035 on July 21, 2014, 09:58:58 amIt is not a differential probe. It is an isolation amplifier of some type similar to but much lower performance than a Tektronix A6902B. This sort of thing has been replaced by oscilloscopes which include galvanically isolated vertical inputs like the Tektronix THS700 and TPS2000 series.These do not have symmetrical loading like a differential probe so one side of the circuit to be measured needs to be low impedance but that is not normally a problem with off-line circuits. The probe ground may be connected to neutral or hot without problems. With two channels of isolation, both probe grounds are isolated from each other as well and do not even need to connect to the same point.

author: geek
author_rank: Unknown
timestamp: July 27, 2014, 03:23:09 am
content: Quote from: kripton2035 on July 21, 2014, 07:54:38 amcut the earth from scope or DUT?is it safe?

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 27, 2014, 04:14:15 am
content: Can anyone explain what is the great fascination people around here have with looking at the mains power on their oscilloscopes?I mean, you can safely measure the voltage, with most any decent DMM. And you can look at the waveform through any old wall-wart transformer.

author: kripton2035
author_rank: Super Contributor
timestamp: July 27, 2014, 10:13:23 am
content: Quote from: geek on July 27, 2014, 03:23:09 amit is safe as soon as you don't touch the wires ...the earth is here to cut the mains power if someone touches the mains wiresno earth = no automatic cut offthe differential probe is far more safer, if you touch the wires the main differential will cut the power

author: Bored@Work
author_rank: Super Contributor
timestamp: July 27, 2014, 10:15:04 am
content: Quote from: geek on July 27, 2014, 03:23:09 amNo, it it not. But unfortunately, not enough idiots die doing it, so it gets suggested again and again.

author: tautech
author_rank: Super Contributor
timestamp: July 27, 2014, 10:17:23 am
content: Quote from: geek on July 27, 2014, 03:23:09 amIt is not considered safe.Read the attached document ex Tektronix.

author: DoDaMaffs
author_rank: Contributor
timestamp: July 27, 2014, 11:21:56 am
content: I am going to put my two bobs worth in and concurwith bored@wok & richard c.I am suprised that nobody asked kiwicraig what he is tryingto acheive and what safer methods may be employed ifhe is not versed in this type of measurment.You all got side tracked as to whether the DSO or CROcan, can't or should.Pete

author: vk6zgo
author_rank: Super Contributor
timestamp: July 27, 2014, 11:32:53 am
content: Quote from: Richard Crowley on July 27, 2014, 04:14:15 amI wonder about that,too,Richard.On many occasions,Ihaveused an Oscilloscope probe to look at Mains voltages,basically just to see if they are there.In this case,the probe earth clipis not connected to anything(& is probably best removed ) with the return path to Neutral made via the Earth connection included in the 'scope's power cord,&,in turn, the building Earth/Neutral connection.Obviously,this convoluted return path will affect accuracy of any high frequency transients on the Mains,but it works OK fot troubleshooting purposes.

author: Psi
author_rank: Super Contributor
timestamp: July 27, 2014, 12:09:22 pm
content: Quote from: vk6zgo on July 27, 2014, 11:32:53 amWhen i was probing the mains it was because i was working on an implementation of UPB (universal powerline bus).I needed to look at the data pulses i was injected into the mains waveform.

author: rob77
author_rank: Super Contributor
timestamp: July 27, 2014, 03:18:20 pm
content: i never felt like i need to probe the mains with a scope (why the hell would i ?). but checking the waveform of a inverter is a different story - that makes sense.but anyways:1. isolation transformer ! isolation transformer .... ah... and forgot to mention... isolation transformer !2. differential probe3. use a battery powered scopechecking the mains waveform with a wall-wart transformer or any step-down transformer is a very good idea ! (and actually it's the solution #1 - isolation transformer)not using isolation transformer makes you a very good candidate for the Darwin awardthe difference of potentials between neutral and ground might be several volts - enough to cook your scope's ground paths. if you connect your ground clip to the live by accident ... welll... you could make a much cheaper fireworks , rather than making one out of your scope

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2014, 04:17:40 pm
content: Quote from: tautech on July 27, 2014, 10:17:23 amWhat I always thought funny is the older Tektronix application notes recommending this very method. I think it is even used in some of their old service manuals.As far as cutting the earth ground to the device under test and using an isolation transformer with it, that does not preclude attaching earth ground to the point where the oscilloscope probe is grounded as long as it is neutral or the chassis.

author: saturation
author_rank: Super Contributor
timestamp: July 28, 2014, 02:06:14 pm
content: For working on such tasks you are best off with a portable scope or a differential probe; there are industrial grade active probes that are relatively low cost made for checking appliance grade electronics, these are made by testec or tiepie and resold under different brand names.http://www.testec.de/tastkoepfe_25mhz.0.html?&L=1There is also a similarly priced Taiwanese version sold also by many brands.http://www.pintek.com.twThe above come in various bandwidths for price, all are sold under many brand names.

author: othello
author_rank: Contributor
timestamp: July 29, 2014, 04:29:28 am
content: Although i never tried to measure mains AC with a scope, what would be the preferred way to do it ? My scope for example is rated at 300V RMS, so i assume 220V RMS should be no problem when using a 10x probe ? I read the probe specs and their rating says for 10X: CAT II 300V AC. But unlike the scope manual, the probe specs don't mention RMS. Is it still safe to use the probe in 10x mode to measure 220V RMS ? So lets say i'm looking at the wires coming out of the wall, 3 wires, 1 is earth. How would you make connection to measure using the 10x probe ? Scope itself is grounded, bench scope.PS: sorry for the newbie questions, appreciate your guidance

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 29, 2014, 05:17:19 am
content: If someone held a gun to my head, I would use a 10x probe. First I would leave the scope probe ground FLOATING!Then I would probe green-wire safety ground to see how much difference there is between the oscilloscope ground reference and the mains safety ground. That measurement by itself would be interesting. Then I would probe the "neutral" ("cold") side to see what is the differential between THAT and scope reference ground.THEN I would consider poking the probe at the hot side of the mains (with the scope set for the expected peak-to-peak voltage we expect based on the nominal voltage. i.e. 679 VAC P-P for a nominal "240V power mains. Even at nominal 120V mains voltage where I live, I would only probe the hot side if you pulled the hammer back on the pistol as an additional threatening gesture.I STILL don't get the morbid fascination with people here probing the power mains with their oscilloscopes! As far as I can see. it is an adult extension of what small infants do when they poke things into the wall outlets.At the risk of repeating myself: If I want to know the mains VOLTAGE, then I will use a proper DMM. If I want to look at the WAVEFORM of the power mains, I will grab a discarded wall-wart transformer, and safely probe the low-voltage secondary.

author: kripton2035
author_rank: Super Contributor
timestamp: July 29, 2014, 06:18:31 am
content: Quotethe problem with wall mart transformers is that they are made for 50-60Hz frequencyif you want to look at x10 signals around 100KHz, there is not a lot of anything to look at !

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 29, 2014, 07:03:33 am
content: Quote from: kripton2035 on July 29, 2014, 06:18:31 amPeople with such legitimate requirements....1) can characterize some transformers and screen them for HF response. It is not unusual for a small transformer to have response to 100KHz.2) can high-pass filter the tap to allow looking at the HF content while minimizing the hazardous energy from the baseband frequency.3) probably can afford and know how to safely use the proper equipment to do the job without risk.4) probably aren't asking such random questions with no apparent motivation in the beginner's forum.Advising people with dubious experience and unknown endowments common sense on messing safely with power mains has been a problem in electronics-related forums since the beginning of Usenet. And the special concentration on test equipment in this forum makes the problem even more prominent here. I am just very concerned about many newbies who come though here innocently asking about doing dangerous things. I guess that is why we have the Darwin Awards.

author: BravoV
author_rank: Super Contributor
timestamp: July 29, 2014, 08:13:34 pm
content: Quote from: Richard Crowley on July 29, 2014, 07:03:33 am+1Though not an expert, imo when it comes to mains line related voltage probing, its time to talk about isolated probing techniques.Each method has it's advantages and trade offs, click here ->HEREand download the Tektronix "Fundamental of Floating Measurements and Isolated Input Oscilloscopes".Yes, its not cheap when it comes to a "proper" high voltage and isolated probing using an oscilloscope.... sighFyi, a battery powered or handheld scope does NOT mean they're isolated scope, it must be designed from the 1st place, from the scope down to the probes, thats why usually they're quite pricey.A nice example illustration on how an isolated scope was designed, stolen from Tektronix.Simple example below on what you can do with an isolated scope, just watch carefully the different hook points at "both probes ground clips", its just impossible to do that with an ordinary 2 ch scope, even using an isolated transformer or floated scope/dut, or handheld/battery powered scope that is NOT designed as an isolated scope from the beginning.Just be careful before using the scope on mains line without a clear & through understanding on the details and dangerous involved, its not worth your life especially just for the sake of curiosity.

author: BillWojo
author_rank: Regular Contributor
timestamp: July 29, 2014, 10:59:50 pm
content: That TekScope print that you borrowed, does that pertain to older Tektronixs scopes as well such as my 465M? I'm troubleshooting a DC motor drive from the 50's that uses a pair of C16J thyratron tubes, lot's of nasty DC voltages involved. I picked up a 100X probe and an electrical tech that I know gave me a few very small value caps, said to put one of the caps between the ground clip and one end of the circuit. Ground lead to filiment and test lead to grid. Might not be 100% correct on that, my notes are not with me.BillWojo

author: Muanpuia
author_rank: Newbie
timestamp: July 30, 2014, 12:00:37 am
content: Please anybody know where to buy refurbished/used scope for a very cheap price? Too costly for me to buy in my country. Its kind of suffocating sometimes just reading about it and planning what id do with it. Im using my soundcard for almost three years now and when i try using it for digital realm its mostly impractical to multiplex/filter it around. The result i get hardly makes sense. It seems these days most are digital and atleast those old CRO would still be somewhere out of service...

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 04:48:06 am
content: Quote from: Richard Crowley on July 29, 2014, 05:17:19 amThank you for the advise ! It's better to at least know than to stay ignorant so i hope you don't mind the questions.1. I've measured the DSO ground ref with the mains ground (earth i assume), difference is about 1.3V, what does that tell me ?2. If you talk about "neutral" and "hot", you mean the earth of the mains and one of the hot wires ?3. I take it that a 100x probe is safer than a 10x probe ? Why if the 10x is rated 300V AC CAT II and DSO INPUT IS 300V AC RMS ?4. With "FLOATING" does it mean leave the ground wire unconnected of the probe itself ?5. Say i have a 100X probe, i assume i connect the ground of the probe to the earth wire and then probe one of the hot wires ?Thank you again, just want to understand more about measuring AC and do's and don'ts without doing it :-)

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 05:42:51 am
content: This topic was intertwining I must say.Only once was there any query to clarify the test aspect requirements. As a person that needed to scope up to 145kv regularly the principals are a all the same.For specific reasons different probing methods and probe types are used. Yes there is a difference safety gear when using x1 and x10,000 probes but the principles and safety are the same.All I read here is is a lot of people who have gotten complacent with their probe use skills because they only measure what they consider low voltage.There have been recorded deaths from as little as 10ma and 12v. I wouldn't be surprised that there are others I don't know about.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 30, 2014, 05:44:19 am
content: Quote from: othello on July 30, 2014, 04:48:06 amIt means that already you have 1.3V difference between what SHOULD be mains ground and what the scope thinks is "ground".There are a dozen unknowns here that prevent more detailed analysis of this finding. For example:1) What is the grounding scheme in your (unidentified) scope? Does it have a 3-pin, grounded mains plug?2) What is the connection between the power mains green-wire ground and the chassis/case ground of your scope?3) What is the connection between the scope chassis and the outside shell/ground/reference of the probe input(s)?4) How familiar are you with the power mains conventions in your country? WHERE are you?5) You don't sound very sure about exactly where you probed. Did you really probe the green-wire ground? Or the white (or blue?) wire neutral/low side?6) Did you probe the same outlet where the scope is plugged in?7) How good is the wiring to that power outlet? Faulty and/or degrading wiring will cause the ground and/or neutral nodes to rise above 0V ground.QuoteLow/Cold/Neutral is NOT the same as the green-wire safety ground. At least in most of the civilized world. But again, we don't know where you are?You might want to learn more about how power mains are wired before going anywhere near them with a scope probe.Now if you want to use a DMM of appropriate specs to investigate the voltages on various power mains nodes, that would be orders of magnitude safer for both you and for your equipment. If it rather scary to hear you wanting to probe something that dangerous, but which you seem to not understand yet.QuoteA 100x probe can likely take a higher voltage than a 10x probe. But that is relative, and you live in a Real World.Quote300V is not high enough to consider going anywhere near the hot side of a mains source.QuoteIt means that while the BNC connector on the scope grounds the shield around the probe wire, you leave the ground node out at the end of the probe NEVER touching anything.QuoteNO! That would assume that the earth wire is at zero volts relative to the scope ground. You have already shown that is NOT the case!That is where you start getting into trouble here. You CAN NOT assume that the "earth wire" is the same ground as the scope reference.Some of the reasons are those questions up at the top of this post. And those are just the most obvious questions. There are dozens more.This is NOT as simple as you might want it to be. This is a complex and DANGEROUS proposition and you would do well to take a much more cautious approach.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 30, 2014, 07:17:31 am
content: Quote from: othello on July 30, 2014, 04:48:06 amIF you live in most countries,your Electricity company supplies you with your house power at 240v,220v,or 120V,or whatever,in the form of Live (also called "Active' or "hot") & Neutral conductors.The Neutral may,or may not be physically grounded elsewhere,depending on the exact system,but it is almost always physically grounded at the entrance to your house.From the point this is done,a third ground,or "Earth" conductor is extended to your power point.At the power point (called a "GPO" in Australia,so for convenience I will use that terminology),there are normally two similar socket holes,& a third "different"one.For instance,an Australian GPO has two slanted flat pins above one vertical one.At first sight,grounding one side of the Mains seems mad,as there are many incidentally grounded objects in the average house,so you could get between them & Live & be zapped,but there is "method in their madness".Historically,most Electrical appliances had metal cases,for strength,as plastics were not sufficiently developed.Imagine with a system where neither Mains conductor was grounded,& two such devices developed short circuits between opposite conductors & the metal case.Both devices would continues to work OK individually,but there would be,say 220V between the cases.If you were happily working in the kitchen & touched both devices,you would be "toast",as the Mains fuses wouldn't notice the small increase in current draw needed to cook you!Enter the three wire "protective Earth' system.A short develops between the case & the Active conductor----- the Earth conductor is connected back to the Neutral side at the house entry point,so the Power supply sees a dead short,excessive current flows,the fuse in the fusebox blows,& you are protected.In an ideal world,"Earth" & Neutral" would always be at exactly the same potential.no matter what GPO you looked at,but in the real world,Neutral has to carry the same current as the Live conductor,whereas the Earth does not,so there will be some voltage drop,hence the difference in voltmeter reading. between the two at any given GPO.Let's look at (2)"If you talk about "neutral" and "hot", you mean the earth of the mains and one of the hot wires ? "From the above you can see that "NEUTRAL" & "Earth" arenotthe same thing."Neutral" & "hot" means exactly the same thing as "Neutral" & "Live".There are two special cases where referring to the two pins on a power outlet as both "hot" is legitimate.-(a)When you are looking at an old installation where you do not know which output of the GPO is the Live one----for instance,the now mandatory connection of Live as the left socket pin & Neutral as the right one was only a "recommendation" in Australia until the 1970s.(b) In the USA,some appliances require 240v as distinct from the normal 120v supply.This is done using a special transformer on the power pole which has a 240v secondary .The secondary is centre tapped to produce two Live 120v feeds,with the tap as Neutral.At a 240v socket,there are nowtwoLIve 120v socket holes & one Neutral.The device is connected between the two "hots" & hence across the 240v secondary of the transformer.Re (4):-Oscilloscopes normally have one side of the 'scope inputs returned to the metal case and/or internal metalwork of the instrument, hence, via the power cord, GPO & internal wiring to where the Earth & Neutral are connected.If you completely remove the grounding lead from a 'scope probe & probe the "Active" you will ne able to see the Mains waveform,as the return side of the probe's circuit is provided by the instrument Earth as described above.This is what Richard referred to as "Floating",as he has pointed out in his later posting.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2014, 07:22:43 am
content: Quote from: WarSim on July 30, 2014, 05:42:51 amWOW, I am in awe.Tell us more please.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 07:35:05 am
content: Quote from: tautech on July 30, 2014, 07:22:43 amHere is all the info I can offer. The RADAR systems I worked on needed needed to be maintained and tuned so it don't go boom.

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2014, 07:45:28 am
content: Quote from: WarSim on July 30, 2014, 07:35:05 amOK, by your profile , I guess military application.But 145 kV and 10,000:1 probes.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 07:52:22 am
content: actually nick is from an AI I wrote, in 1990. But yes some had militarized applications.Yeah the huge 10,000:1 probe with matching cuffs, helmet and boots. Very stylish

author: mzzj
author_rank: Super Contributor
timestamp: July 30, 2014, 07:55:07 am
content: Quote from: othello on July 30, 2014, 04:48:06 amMostly because 100x probes usually don't have switch like many 10x probes that you can accidentally switch to 1x position.10x is rated 300V AC CAT II ProbeFROM RELIABLE MANUFACTURERis good enough for probing mains at your workshop table.CAT II probe must be able to withstand up to 2500v surge peaks from the mains.Your average scope input will get fried with that 2500v surge.http://digital.ni.com/public.nsf/allkb/05D3F9C0F740BC2986256E9A00505421

author: tautech
author_rank: Super Contributor
timestamp: July 30, 2014, 09:53:26 am
content: QuoteWhat people fail to remember in posts, is mains voltages vary on this planet and IMO it would be unwise to use any 300 V rated probe or scope near 230 VAC mains, CAT rated or otherwise.Consider a simple 230 VAC SMPS, rectified mains for the primary switcher is ~325 V DC.You already exceed the probe voltage rating!IMO if you do not have at least 600 V 10:1 probes you are at risk.Even this will not give you much headroom when it comes to voltage de-rating with frequency.Better that you have at least one 100: 1 probe because safety demands it.

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 04:35:15 pm
content: Thank you all for the valueable input !After reading it seems a few more questions came up. For clearity, i'm not going to probe mains with my DSO, don't know enough about high voltage / dangerous currents vs safety. But if i would have to do it in the future, how would i do it, in practical terms ?Here is some info which applies to my situation and can be used as example:- Country: The Netherlands- Mains voltage: 220V AC (my DMM says 222V AC)- DSO Rigol 2202 (300V AC CAT I) ,- Probe 10X/1X (300V AC CAT II)- I have a surgeprotector from APC which shows a green led if the earth connection is good, it shows green.So i'm look at a normal socket in the house, 3 wires are coming out of it as follows;- 2 wires are hot- 1 wire is earthQ1: Now about the above, the wire i call earth should i call it ground or neutral ? Or what is the official word for it ?Q2: The 2 wires i call hot, should i call them live or what is the official word to call those 2 ?Measuring with my DMM 1 hot with earth wire shows 0V, same for the other hot wire with earth, also 0V.Measuring with my DMM the 2 hot wires gives about 222V AC.Situation with the DSO:- Bad idea to use a 10X from what i read- Good idea to use a 100X probeQ1: At the front of the DSO there is a ground hook (?), i measured with the DMM a difference between that ground hook and the earth wire from the wallsocket of about 1.3V. In practise, does this mean i'm dead if i use a 100x probe to try and display the waveform on the above DSO (see specs) ?Q2: Is it correct to connect the 100x probe ground to the earth wire of the wallsocket and then probe one of the hot wires ? If not, what is the best way to do it ?Situation ESD work (not directly related to above situations btw):- Anti-static wrist strapQ1: The proper way is to put it on then connect the wire to the earth clip of the wallsocket, is this correct ?Hope you guys have some patience and help me understand in practical words the answers you give to the above.Thank you !

author: Ribster
author_rank: Frequent Contributor
timestamp: July 30, 2014, 04:45:14 pm
content: Reading your post, i would discourage you to do anything with mains voltage.If you do want to power something from mains, for your own safety, use a separating transformer (Dutch: Scheidingstransformator).It's a simple transformer that converts 220V to 220V. So a transformation of 1:1.This is a safer situation. If you short out the secondary, the consequences will be less dangerous.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 30, 2014, 07:15:03 pm
content: Quote from: othello on July 30, 2014, 04:35:15 pmIt would be convenient if you could complete your user profile to at least identify your country so we have some clue what you are talking about.QuoteREMEMBER that is RMS, and peak-to-peak voltage is CONSIDERABLY higher than that!!!!!Quote300V is NOT adequate to probe the peak-to-peak voltage you would expect from ANY mains supply.QuoteIf truly "2 wires are hot" then your surge protector would have no frame of reference to tell "if the earth connection is good".I don't know anything about how the mains power is distributed in your country, but I question whether "2 wires are hot".You REALLY REALLY need to study exactly how mains power wiring is done in your country. THIS IS NO PLACE FOR GUESSWORK AND ASSUMPTION!!!!!QuoteClearly your measurements show that you do NOT have "2 wires are hot".If you are measuring 0V from a "hot" wire to earth, then it is either BROKEN, or else it is not "hot".PLEASE PLEASE study how mains wiring is done in your country before putting your equipment or yourself any more at risk!QuoteUntil you know more about mains wiring these are BOTH a bad idea.QuoteAs we already discussed, without knowing more about the situation, we cannot make any definitive diagnosis from this symptom.QuoteIt is probably safe if your remember to NOT connect the scope/probe ground to ANYTHING!!!!QuoteNO! You repeatedly report that you are seeing ~1.3V difference between the scope/probe ground and the mains earth/cold/neutral.What do you expect to happen when you connect that to the scope/probe ground? If that is caused by faulty wiring or an overloaded circuit, that could FRY your scope!QuoteFirst, learn how mains power wiring is supposed to be done in your country.Then, analyze your mains circuit to see why it is showing 1.3V difference where there should be 0.0 VThen, give us a better definition of exactly what you are trying to accomplish by probing the mains power?QuoteSuggest searching the archives. I must assume this has been discussed before.

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 08:20:29 pm
content: Quote from: Ribster on July 30, 2014, 04:45:14 pmThanks, i read indeed about isolation transformers. Their secondary is still 220V AC but ungrounded ?

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 08:24:09 pm
content: Quote from: Richard Crowley on July 30, 2014, 07:15:03 pmThanks for the advise Richard ! Just to clarify, i'm not planning or touching anything AC. Only asking your views / advise.The reason why i call those 2 wires hot is that polarity doesn't matter with AC mains. So both wires are dangerous unless i'm wrong in assuming so.The 3rd wire is earth, but i don't understand why i would only have 1 of those 2 wires called hot and not both called hot ? I'm guessing this is where i'm confused about thinking of polarity for those AC wires and assuming both are dangerous ...Example:Lightbulb connected to those 2 wires, changing wires doesn't matter. That is why i call them both hot

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 08:31:47 pm
content: The author is referring to Plug F in the standards list, a CEE 7/4 and commonly called “Schuko plug”. Still looking up if they use whole house ground fault protection to explains the possible 1.3v offset.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 08:59:18 pm
content: Looks like the two prongs are hot and neutral which are treated as interchangeable the clips are PE Ground. So if there is a difference it is either ground fault protection, poor house ground, noise expressed as an offset on a poor DMM, disappointing protection method on the scope, or something else just as odd.Apparently your country is not concerned about the safety issues addressed by standardizing mains polarity. Apparently your country doesn't have a polarity convention. Either post could be the hot. I have not found a code book for your country to find out what your neutral reference is if it is. Believe it or not some countries depend on the home service losses to ensure line offsets. Some limits are 2400 unloaded. This is one of the reasons Fluke DMMs have 3000v protection and limits.Only safe way to test phase and offset on your mains is get a 100:1 or 1000:1 three channel differential probe. Be sure it has cat 3 protection. A bit expensive.If you are just concerned about wave formation only then use a 100:1 or 1000:1 two channel differential probe with AC coupling with cat3 protection. Less expensive. Ground clips are connected together not to the socketIf you don't mind the calculating the neutral offset error yourself you can use an AC coupled 100:1 or 1000:1 probe with Cat3 protection. This will only give you an estimation of wave form and treat both as hot until you figure out which is which by testing. Ground clip is not connected to the plug, which causes an error but keep your scope from trying to pull your house to any reference. Least expensive.I said 100:1 or 1000:1 cap because it is unlikely you will find a 100:1 with cat3 protection.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 09:08:30 pm
content: Oh yes a cheat for the CAT3 protection is use a battery operated scope but I personally do not trust the case isolation of specs of any of them. Too many spec the physical case but ignore the isolation of the controls and screen.Until someone can confirm how you house mains is referenced at lease cat3 protection is needed.Even using a transformer method the re-reference the mains is not a good idea if you can not confirm that the transformers isolation limits are greater than the reference offset limits of your mains. Not to mention you will only be able to measure a wave form estimation due to transformer action.

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 09:15:18 pm
content: @WarSimI'm going to double check what in the Netherlands is common for households. The sockets in the wall are grounded, 3 wires from which 1 is earth (aka ground ?) and the other 2 are hot because polarity doesn't matter. I use a Fluke DMM to make measurements btw. I'm now going to check see if i can find specification in english to reference to, brb

author: Ribster
author_rank: Frequent Contributor
timestamp: July 30, 2014, 09:16:20 pm
content: Quote from: WarSim on July 30, 2014, 08:59:18 pmI live in the country next to the netherlands and i can confirm that we do not give anything about polarity. The euro plug has a variant with earthwire and a variant without (class II as it is called). You could get fixed polarity with the connector that has an earth pin. Fixed color coding for cablling (blue & brown), it can be easily be reverse connected inside the wallsocket. In Belgium we measure the difference in current going in and out. If there is a leakage to ground, the differential picks it up and shuts off. It's TN/S (Earthing neutral separated).

author: Ribster
author_rank: Frequent Contributor
timestamp: July 30, 2014, 09:22:11 pm
content: Quote from: othello on July 30, 2014, 08:20:29 pmThe chassis of the transformer is connected to ground. The wires with the voltage difference of 220VAC willneverbe connected to ground. There is a neutral wire, that is connected to the starpoint of the distribution transformer in your street. The 'hot' wire will be connected to a phase of that distribution transformer. What you do with the isolation transformer, is disconnecting yourself from the distribution transformer in your street. The connection between the primary of the isolation transformer and the secondary is effectively air and isolation. There is no metallic connection (aka galvanically isolated). So when you short the secondary of that isolation transformer, things will not go as bad.

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 09:37:48 pm
content: Quote from: Ribster on July 30, 2014, 09:16:20 pmThank you, i believe your explaination will help to clarify the situation in the Netherlands for WarSim and others. So i hope now it is more clear why i mention 3 wires, of those 2 are hot (polarity doesn't matter) and 3rd wire is earth. So based on this information and using a 100X probe to display the waveform on the DSO it is a matter of connecting such probe to 1 of the hotwires and ground the probe to the earth wire ?PS: again, i'm not going to attempt it, just want to learn more about this like the how / why / what / where stuff@WarSim / OthersNot sure what you mean with standardizing polarity for mains AC ? I mean AC doesn't have polarity as far as i know. So how can it be standardized like if it was DC as in - and + ?

author: WarSim
author_rank: Frequent Contributor
timestamp: July 30, 2014, 10:48:10 pm
content: Quote from: Ribster on July 30, 2014, 09:22:11 pmOk so disembarkation referencing so if they limit tailing to 500m then offset should be below the lower safety limit of 1250v. If you are assured of a shorter tailing the isolation limit can be lower.Here we used distribution and disembarkation referencing and base our safety limits on the second failure point. In short we use 600v as our safety, because our tailing (drop + distribution) is limited to less that 250m. Also the reason why any main termination rated transformer in North America is rated for 600v isolation as a minimum.

author: othello
author_rank: Contributor
timestamp: July 30, 2014, 11:17:21 pm
content: Finally found something which clearly makes a difference between what might be common in the US or in Europe !http://www.eetimes.com/document.asp?doc_id=1272972For example:"Myth: A computer treats the neutral wire differently from the hot wire.International office product safety regulations (including IEC 950 and UL 1950) prohibit these wires from being treated differently. Each is considered a safety hazard under the regulations and is required to be disconnected from and have appropriate safety spacings from logic circuits. Furthermore, inspection of the wiring schematics of any computer equipment clearly shows that the input hot and neutral wires are connected to the same circuits in the same ways and are interchangeable. Belief in this myth is very uncommon in Europe since anyone can see that a European plug can be reversed."That is what i mean with the 2 hot wires, they're both hot / dangerous and only the 3rd wire is earth.Not sure how it is in the US but i see people talk about mains AC and mention HOT and NEUTRAL and GROUND (EARTH), so this makes it confusing since AC has no polarity.So the question is, what is this NEUTRAL wire ? Why call it NEUTRAL at all when it comes to mains AC ?PS: I'm reading / learning more about this but any info is always appreciated guys !Think i found it explained, i will paste the relevant part so other newbies can understand this thing better:"...The reason that one of the power wires is named "neutral" is because it is connected directly to the building ground connection at the circuit breaker panel. Therefore it is connected directly to the grounding (third) wire. In essence, then, two of the three wires at the wall receptacle are actually grounded wires, one being used for power flow, and the other connected only to exposed metal parts on the equipment.The power wire that is grounded is called the "neutral" wire because it is not dangerous with respect to exposed metal parts or plumbing.The "hot" wire gets its name because it is dangerous.The grounding of the neutral wire is not related to the operation of electrical equipment but is required for reasons of safety. To reduce the chance of electrical shock, it is important to provide a means to automatically shut down the electrical circuit if an exposed metal part becomes accidentally connected to a hot conductor or circuit. This is accomplished using the 3-wire system by an ingenious technique:Every electrical circuit is protected by a circuit breaker. The purpose of the circuit breaker is to prevent the building wiring from overheating as a result of excessive user loads being plugged in. However, in the 3-wire system, the circuit breaker provides another critical safety function. If a hot wire or circuit were to become accidentally connected to an exposed metal part on a piece of equipment, then a shock hazard would exist. However, if the exposed parts are connected to the grounding wire, then the hot wire becomes connected to the grounding wire. This would not cause anything unusual to happen except for the fact that the second power wire, the neutral wire, is also connected to the grounding wire at the circuit breaker panel. Therefore, for this safety hazard the grounding wire essentially becomes connected as a load. The low resistance of the grounding wire causes it to draw a very large amount of current when it becomes inadvertently connected as a load, which in turn causes the circuit breaker feeding the hot wire to trip. Therefore, the 3-wire system operates in a manner which transforms a safety hazard into an over current condition, causing the safety hazard to be automatically cleared by the circuit breaker. The circuit breaker is used as both an over current protector and a shock hazard protector...."

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 31, 2014, 03:11:45 am
content: It is called "neutral" because it is the "low" side of the power. At least here in North America, back in the circuit-breaker distribution box, each branch circuit white-wire "neutral" is actually hard-wired to green-wire safety ground. And then typlcally connected locally to physical ground/earth. A copper water pipe, or an actual ground-rod driven into the crust of the planet.The typical mains service to a home or small office is 240V center-tapped and the center-tap is grounded. So you have two 120V "phases", each of which is referenced to ground. The grounded side of the branch circuit is called "neutral" because it is designed to be nominally at ground-potential. 120V branch circuits take their power from one or the other "phase". High-power loads will use BOTH "phases" for the full 240V experience. But that 240V is center tapped and grounded.It is interesting to see how mains power distribution evolved in different countries. I was the engineer for a musical group that performed on the Queen Mary just after it was opened as a "floating hotel". The entire steel hull of the ship was both "green-wire" safety-ground, AND "white-wire" branch circuit neutral because the power distribution was single wire only, and they just used the metal hull as the return path. The guitar players had an interesting time with those goofy 2-way power switches that would put a small capacitor to one side or the other of the mains plug (back before the plugs were polarized, even the un-grounded 2-prong ones.)

author: vk6zgo
author_rank: Super Contributor
timestamp: July 31, 2014, 03:40:35 am
content: Quote from: othello on July 30, 2014, 11:17:21 pmElectricity authorities are deeply distrustful folk,& assume that either,some sockets will be wired with Live & Neutral reversed,or that you may be in a country where the plug can be reversed.Accordingly,they require the same level of insulation for both conductors,so that if what would normally be the Neutral connection inside the appliance,with little or no potential to the Earth,& hence Earth/Neutral connection back at the house supply entry point,now becomes the Live connection,there will be no safety or operational problem.This can be very important,as the following anecdote shows:-Back in the early 1970s,I was involved with the installation of a Closed Circuit TV system.The pan & tilt heads for the cameras were unusual,in that they operated from the Mains,using appropriately rated cable to make the connections between the control room & each such device.A test box was made up which could be plugged into a nearby GPO,& switched the Live mains leg to each motor in turn.We had to stand on a ladder & adjust stops which operated microswitches ---these prevented you "panning" or "tilting" your camera lens into a wall or other obstacle.All went well,till I was adjusting one of these stops,received a nasty shock,fell off the ladder,& ended up on the floor.On investigation,we found that the manufacturer had insulated the "Live" wiring well,but hadn't worried about the "Neutral".I,of course,had "hit the jackpot"----a GPO with reversed connections!That said,as far as the power outlet is concerned,with acorrectly wirednormal 220v single phase supply,the only "hot" socket pin is the live or "Active" pin.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 03:59:12 am
content: The neutral wire is called neutral because the reference on that wire has a neutral reference. AKA the one attached to a distributed ground.You plug has two hots because either on could be connected to hot or neutral. One wire is hot one wire is neutral you just don't know which.Any myth that states the polarity of a single phase modern appliance is false. Current code everywhere I have looked prohibits neutral return trough user access able surfaces. One exception is light bulb sockets. The hot is suppose to be the deepest pad but many lamps ignore this.Lol how exactly did you achieve PE ground on a ship.. Now on a ship with hardened ring power that system actually is two hot wires.

author: othello
author_rank: Contributor
timestamp: July 31, 2014, 04:17:43 am
content: This stuff gets more interesting by the hour :-) Despite that it's not easy to understand for the untrained !So if some one touches by accident the neutral wire then nothing will happen ? I assume because the neutral is grounded to the earth (grounding) wire ?In the US the plugs can only fit in 1 specific way if this hot vs neutral is important for household appliances for example ?

author: IanB
author_rank: Super Contributor
timestamp: July 31, 2014, 05:01:48 am
content: Quote from: WarSim on July 31, 2014, 03:59:12 amWell if you are inside the ship then the ship's hull is the only neutral potential that exists, surely? So it doesn't really matter how much current you return through the hull, the potential difference between you and the ship is always going to be zero. There is no external ground to reference the voltage against. If you bond everything securely to the hull then I would assume nothing can float to any unreasonable voltages.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 05:05:35 am
content: Quote from: IanB on July 31, 2014, 05:01:48 amI wasn't disagreeing with the hull being the neutral on a ship with a non-hardened power distribution. What I though was amusing is you referred to it as a ground. Yes I would have found it amusing if you called it neutral and water also.

author: IanB
author_rank: Super Contributor
timestamp: July 31, 2014, 05:10:16 am
content: Quote from: WarSim on July 31, 2014, 05:05:35 amAh yes, "ground", I see. I wasn't the original poster, that was Richard Crowley, but apparently I didn't pick up on the joke. Oh well...

author: vk6zgo
author_rank: Super Contributor
timestamp: July 31, 2014, 06:08:55 am
content: Quote from: othello on July 31, 2014, 04:17:43 amIt shouldn't if all appliances were fitted with double pole switches.A lot of older stuff & also cheap newer appliances only have single pole switches.Another thing is that,if you open the Neutral line of a lamp circuit,motor,transformer primary or similar,you can still get a shock from the Neutral sideof the device's circuitry.This really only of importance to people working on the equipment,but is nevertheless, the reason Australian rules forbid fuses in the Neutral line of devices.Even with strictly regulated pinouts,you can get caught out.The dear old People's Republic sometimes make toasters with the "off" switch in the Neutral line..In these cases,even with no toast in the unit,there is always Live 240v available to zap you.A proper design uses a double pole switch & disconnects both sides.The term "polarity"does not really apply to the Live/Neutral relationship.An AC waveform reverses polarity on each half cycle,so that if you could sample the voltage of Live with respect to Neutral at the peak of each half cycle of a 220v RMS waveform ,it would give readings of +311v,-311v,+ 311v,-311v & so on (I started on the +ve half cycle for convenience).You can see that whatever half cycle you are on,a device connected between Live & Neutral will work.If it is connected between Earth & Neutral,it will not!

author: mzzj
author_rank: Super Contributor
timestamp: July 31, 2014, 07:11:47 am
content: Quote from: WarSim on July 30, 2014, 08:59:18 pmI have no idea about Netherlands but at least here in Finland it is common in older buildings that grounded outlets are wired with "pseudo-grounding" (in lack of better english terminology)With old "pseudo-grounding" the neutral and ground wire are connected already at the outlet and share same wire to the distribution/fuse box. This causes all sorts of trouble, including potential differences between outlet ground connections in different branches because of voltage drop on shared ground/neutral wire! (also a massive safety hazard if the house wiring becomes faulty with loose connection somewhere on the shared neutral/ground wire)Most of the Europe doesn't have polarity convention, or if there is such it is just based on "old habits". IE here in Finland we wouldusuallywire the "EUROPEAN "SCHUKO" CEE7" with neutral on the left side even that it really doesn't matter since the plug is non-polarized!

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 07:32:19 am
content: I think the point is being lost here. The point in single phase home wiring is not to know which wire is neutral. It is to know that one of the wires is referenced and where. If you need to measure it in someway in relation to PE Ground. Because most scopes are PE referenced in some way, this is important. Not to mention the operator is likely PE referenced some how. Unless of course they are wearing an isolation suit.As stated if referencing is from the feed you need at least 3000v isolation if referenced at disembarkation with a long tailing 1250v isolation. Referenced at disembarkation with short tailing 600v isolation. Referenced at distribution 600v isolation. A fully floating long mains feed is a dangerous thing. And this is the important thing about code. No mater what safety processes are used.The point was to make sure the author was safe not to judge another countries wiring conventions.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 07:38:06 am
content: HuvaI have Finish heritage and many relatives there, so it is good to see Finland represent.. Although my family has been out of country for three generations and now considered "outsiders" it's nice.

author: mzzj
author_rank: Super Contributor
timestamp: July 31, 2014, 08:05:45 am
content: Quote from: WarSim on July 31, 2014, 07:32:19 amI am not sure what are you referring to with long tailings and isolations?

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 08:14:38 am
content: Isolation is how much voltage difference will be prevented from arcing over. Usually given in Category classifications such as cat 1 - 4 protection.Tailing is the service wire that connects to the mains receptical. Specifically the length of wire used from the reference point to the load. Reference point being where the AC cct is referenced to a global potential. Which is PE ground in most cases. Sea basic for ships. Derived neutral for delta etc.

author: mzzj
author_rank: Super Contributor
timestamp: July 31, 2014, 08:54:09 am
content: Quote from: WarSim on July 31, 2014, 08:14:38 amumm, sounds like a'murican or boat standards.Any sort of appliance transformer here(europe) is rated for 2.5kV and Safety isolation transformers(including "wall-wart" type transformers) have to be rated for 4kV insulation.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 02:53:38 pm
content: QuoteAny sort of appliance transformer here(europe) is rated for 2.5kV and Safety isolation transformers(including "wall-wart" type transformers) have to be rated for 4kV insulation./quoteYes that would be about what is required with your power delivery system. Here the minimum is 600v because of how this power system is setup here.Also the key why people here are saying you need a 10:1 probe assuming our system. Also the reason I started with my replies pushing for 100:1 and the 2.5kv isolation.I hope this puts the x10 and x100 discussion to rest.

author: Ribster
author_rank: Frequent Contributor
timestamp: July 31, 2014, 03:05:19 pm
content: Just use a differential probe and be done with it. Damn.This is how it's done. TN/S. The earthing and neutral are separated at the transformer side and being brought out.This is wat everybody means and refers to neutral / hot. Hot being Lx and neutral being N.Use a differential probe so you aren't referenced to anything, no problems. Dave has some video's using differential probes.

author: IanB
author_rank: Super Contributor
timestamp: July 31, 2014, 04:44:33 pm
content: Quote from: Ribster on July 31, 2014, 03:05:19 pmWell surely you are referenced to the chassis ground of the oscilloscope? So if there were a 4 kV spike on one of the probes it might find a path to ground through insufficient insulation or isolation?

author: Ribster
author_rank: Frequent Contributor
timestamp: July 31, 2014, 05:03:51 pm
content: Quote from: IanB on July 31, 2014, 04:44:33 pmUse a proper isolated one then offcourse. Your CAT rating will tell you how high you can go. The right tool for the job. If your circuit can go up to 4kV potentially, then well, take the differential probe that's rated for 4kV. No ?

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2014, 05:08:53 pm
content: Quote from: IanB on July 31, 2014, 04:44:33 pmBoth probes are referenced to chassis ground through the oscilloscope so an overload will be shunted to chassis ground either through the probes or the oscilloscope input protection circuits. Note that this means many differential probes will *not* work on a galvanically isolated vertical input depending on how they are configured because their common mode voltage may drift out of range.If the oscilloscope itself is floated, then breakdown through the insulation of its power supply transformer or EMI filtering networks is possible or even likely. Old Tektronix application notes which discuss making measurements with the oscilloscope chassis floated mention this danger.Personally I do not worry about making an occasional single ended measurements using a 300 VAC catagory 2 x10 or x100 passive probe of a power outlet up to 240 VAC while the oscilloscope is properly grounded and the probe ground lead is not used but all of the common mode noise between the oscilloscope ground and the measurement point will be included. x100 passive probes rated to 2000 volts peak or higher are available and inexpensive if higher levels of protection are warranted.Good oscilloscope high impedance inputs have no trouble handing an direct connection to 240 VAC at any sensitivity without damage.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 31, 2014, 05:20:58 pm
content: When do all these magic 4kV spikes occur?If they were at all common,we would have equipment being zapped everyday,all around the world.In fact we do not,& the Power supply Companies take good care that we don't,with protective devices of various types.Many of these very high voltage ratings are to provide a very large amount of "headroom" over & above the operating voltages of the system.In thereal world,a decent Tektronix X10 probe can be used to look at the 240v Live conductor,provided you don't connect the Grounding clip toanything& rely upon the 'scope Earthing via its own power cord to provide the return path.And guess what? You can pre-attach the probe & then turn on the power---then,if I & the tens of thousands of other people who have used this method are wrong,all you will do is blow up your probe,or possibly your 'scope.You will not be touching anything,so will be safe.

author: mzzj
author_rank: Super Contributor
timestamp: July 31, 2014, 05:34:29 pm
content: Quote from: vk6zgo on July 31, 2014, 05:20:58 pmMany of the X10 probes are rated CAT II 300V, so they should be good for up to 2.5kV anyhows....Ground clip is the biggest safety risk for beginners and even intermediate hobbyist.I have seen scope probes with fuse holder on ground clip in one particular company...guess they had some issues

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2014, 05:43:13 pm
content: Quote from: mzzj on July 31, 2014, 05:34:29 pmI had a ground clip turn white hot and melt once. Luckily that was the only damage that occurred.The safest way to handle the ground clip when making this kind of measurement is to remove it.

author: othello
author_rank: Contributor
timestamp: July 31, 2014, 09:03:50 pm
content: So as a newbie i think i understand it as follows for a household mains:- HOT wire carries the current and polarity keeps changing 50 or 60 times a second only on this wire and shows voltage when check against GROUND ?- NEUTRAL wire is the returnpath and does not carry current and has 0V when checked against GROUND ?I just tested the main with my fluke dmm, hot wire against ground has about 223V and neutral against ground has about 0V.

author: WarSim
author_rank: Frequent Contributor
timestamp: July 31, 2014, 09:47:43 pm
content: Quote from: vk6zgo on July 31, 2014, 05:20:58 pmNo you would not the high voltage transients are either filtered or suppressed by the inputs of your appliances. Scope probes do not have such suppression because it would corrupt the test signal. They do have input protection but that just tries to clamp the signal at their theoretical maximums.+1. For Darwinism.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 31, 2014, 10:23:47 pm
content: Quote from: othello on July 31, 2014, 09:03:50 pmCorrectQuoteProbably, not not necessarily. The actual mains voltage occurs between the "HOT" and the "NEUTRAL" (or "other hot" in places where they don't care!)QuoteCorrectQuoteNO. If the neutral is the "return path", then, by definition, it carries the return current!QuoteAgain, that depends on the mains wiring convention in your country (or whatever). Apparently there are several competing standards for this.QuoteSo that shows that in your area, you have a neutral that is connected to ground somewhere.REMEMBER, the green-wire safety GROUND/EARTH wire DOES NOT carry current for that circuit! It is only there as a safety feature to take any fault current if something goes wrong. Else it might be going to ground through YOU!

author: othello
author_rank: Contributor
timestamp: August 01, 2014, 12:07:19 am
content: Quote from: Richard Crowley on July 31, 2014, 10:23:47 pmThank you, sounds like it's clearing up for meSo for example, say some one uses a 1000X probe rated to 10kV, he first would identify the NEUTRAL wire, put the ground of the probe on it and then probe the HOT wire to show the wave form on his DSO ?

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 01, 2014, 12:33:15 am
content: Quote from: othello on August 01, 2014, 12:07:19 amNO! Just because the wire is called "neutral" (or even "ground") does NOT mean that it is safe to connect the probe/scope ground to it!If they have ANY voltage on them (more than a few millivolts of noise), relative to scope/probe ground, then it is NOT SAFE to connect the probe/scope ground! NEVER EVER connect the scope/probe ground to anything that isn't really. truly GROUND! (or is floating and it doesn't matter) The power mains neutral and even power mains safety ground are particularly hazardous because of how much current they may have behind them even if you are only measuring a volt or two. And because people just assume they are "ground" when they frequently ARE NOT (as you have shown in your own case!)Did you not see others saying to NEVER connect the probe ground to any mains terminal? Did you not read the story of the ground clip turning white from heat? Did you not read the description of the scope probe that had a fuse in the ground wire? There are many things here that will trip you up, or blow up your scope or even electrocute you and burn down your house. This is NOT something to mess with unless you really understand what is happening here.

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 12:37:58 am
content: Quote from: othello on July 31, 2014, 09:03:50 pmLike othello says, the NEUTRAL wire carries the return current from the HOT wire.When no current is flowing it will be at the same potential as the GROUND wire so it may appear safe however when high current is flowing, its potential will rise above GROUND and this presents a problem for grounded measurements because if the oscilloscope ground lead is attached to NEUTRAL, part of that current will flow through the oscilloscope probe, the oscilloscope, and into the oscilloscope chassis GROUND connection.Even worse, NEUTRAL may be swapped with HOT. Almost as bad is when NEUTRAL fails for some reason like if the plug gets pulled out part way. Then the entire NEUTRAL current may flow through the ground connection of the oscilloscope probe.In general as far as safety, it is best to treat NEUTRAL and HOT the same and off-line power supplies are not suppose to distinguish them because mistakes sometimes happen.

author: othello
author_rank: Contributor
timestamp: August 01, 2014, 01:24:20 am
content: Quote from: Richard Crowley on August 01, 2014, 12:33:15 amThanks for clarifying that. Also don't worry, i'm not going to try it, just trying to understand where things go wrong when trying something with not enough knowledge / safety precautions.Quote from: David Hess on August 01, 2014, 12:37:58 amThank you for the extra info on this !What would be the official way to display highvoltage A/C waveforms on a DSO ? Not looking to cheat on costs, so just how would one go about it in a professional way ?

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 01:39:56 am
content: Quote from: othello on August 01, 2014, 01:24:20 amThe best is to use a high voltage differential probe because the ground connection may go to actual ground or not be used at all in some cases. Two vertical channels may be operated differentially to achieve the same thing although the performance will not be as good.Second best is to use an oscilloscope which has galvanically isolated vertical inputs. Some handheld oscilloscopes have this feature as well as the Tektronix TPS2000 series.Third best is to use an isolation transformer so hot and neutral are isolated and then ground may be attached at the same point that the probe grounds connect to. In extreme cases I do this while also using a differential probe.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 01, 2014, 01:54:50 am
content: Quote from: othello on August 01, 2014, 01:24:20 amAs several people have mentioned in this (and similar) discussions, using TWO high-voltage probes into a DIFFERENTIAL input on an oscilloscope would be the "professional" way of looking at the waveform of a signal that isn't really "referenced to ground". You would connect one probe to the "hot" side, and the other to the "neutral" node, and then the oscilloscope "subtracts" the neutral voltage from the "hot" voltage, yielding exactly what the voltage is between them, but without having to connect EITHER side to ground (because "neutral"/cold is not necessarily "ground" as you have proved with your own tests.

author: othello
author_rank: Contributor
timestamp: August 01, 2014, 02:00:43 am
content: Thanks, i've looked up for my Rigol scope the available differential probes and looks like there are a few options. Pricewise they're expensive but better safe than sorry if i ever would need to measure dangerous AC !One of them seems to be good enough with these specs:High Voltage Differential Probe, DC-25 MHz, 1400 Vpp1400Vpp is peak to peak, so half wave would be 700V ? And the 25Mhz is plenty i assume because here the household AC is 50Hz ?PS: I just saw the reply from Richard, so if my scope would have a differential input then that would be also the good way to go.

author: Ribster
author_rank: Frequent Contributor
timestamp: August 01, 2014, 03:48:21 am
content: Quote from: othello on August 01, 2014, 02:00:43 amYou should really consider an electricity course. Relation between 220VAC RMS and Vpp is "square root of 2". So in your case if you need to need to go backwards its 1400 / 1,41 -> 992,9VAC RMS.25Mhz will suffice to check a plain 220V sinewave. Once the waveform starts to deform (eg. motor voltages, triac voltages), the high frequency harmonics start to come into play. Look up harmonics and bandwidth, read up onto this..Knowledge is power!

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2014, 03:55:03 am
content: Quote from: WarSim on July 31, 2014, 09:47:43 pmMains supplies have been around for many years.Only comparatively recently have devices had such protection,& even now,not every piece of equipment has protection---legacy stuff,kitchen appliances,etc.Huge voltage spikes on the Mains supplies are incredibly rare,& even in the unlikely event of them occurring,most are of such short duration,as to (1) have very little energy, (2)be substantially attenuated by the inline inductance & resistance of the distribution system.We have to synchronise you looking at the Mains with your 'scope,& the very rare occurrence of a spike capable of damaging your probe.It's like winning the Lotto every day for 20 years!For some reason,some people like to think our occupation is incredibly dangerous.Therearedangers,but a little forethought can allow you to work safely for a whole career.Running around squawking like Chicken Little,on the other hand,is pretty much counter-productive.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2014, 04:14:57 am
content: Quote from: Ribster on August 01, 2014, 03:48:21 amOf course,you do realise,that when looking at an AC waveform,"peak to peak" is only a"pretend"figure,to make it easier to measure voltages on a 'scope.(OK,it is also useful for calculating the operating point of amplifiers,ADCs,etc.)The +ve & -ve half cycles are not present at the same time,so the realpeakvoltage,Vpof a 1400v p-p waveform is 700v.The relationship between Vpp & RMS is :Vrms=Vp /root2From the values given,RMS is 494.97 v,or,near as dammit,500v."A little learning is a dangerous thing!"

author: WarSim
author_rank: Frequent Contributor
timestamp: August 01, 2014, 04:18:15 am
content: Quote from: vk6zgo on August 01, 2014, 03:55:03 amThat is called a risk assessment, not a guarantee of safety. If you can't understand the difference between safe and safe enough, say high to Darwin when you meet him.Are you using the magnanimous "our", or are you trying to imply we are in the same field? By you statement I can assure you that we do not have similar work experience.We obviously have different opinions. I state what is possible and you attack the statement with probability. The question was what could happen not what will happen. Im not like you, I would personally feel very bad about advising someone to do something that kills them.I would say stop but this is hilarious. You are actually trying to justify the risks you have decided to accept by convincing someone else to take them. I have read about this in school, it was called lemming theory. It was a study of the Tobacco industry marketing process but I think it applies here too.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2014, 04:58:38 am
content: "That is called a risk assessment, not a guarantee of safety. If you can't understand the difference between safe and safe enough, say high to Darwin when you meet him."Every day,we make risk assessments-----"safe enough" is the best we can ever get.I guess we could hide under our beds,but we might get bitten by a spider,or choke on dust bunnies.If Charles Darwin had needed absolute safety,he wouldn't have ventured down into the "very dangerous" South Atlantic in search of Iguanas to study.Are you using the magnanimous "our", or are you trying to imply we are in the same field? By you statement I can assure you that we do not have similar work experience.40years "hands on" in Electronics,mainly in the field of high power Transmitting equipment.--------Your turn!I would say stop but this is hilarious. You are actually trying to justify the risks you have decided to accept by convincing someone else to take them. I have read about this in school, it was called lemming theory. It was a study of the Tobacco industry marketing process but I think it applies here too.I think "school" gives you away,sonny!Cigarette smoking is not necessary,it is a choice ---unfortunately,walking down the street,catching a train ,working,in any capacity,are necessities & all have an element of "risk assessment".None of my suggestions put anyone at risk,providing they use the basic intelligence that God/Evolution gave them.

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2014, 07:32:24 am
content: Quote from: vk6zgo on August 01, 2014, 04:58:38 amI guess you missed this:https://www.eevblog.com/forum/beginners/how-to-mesure-240v-ac-with-an-oscilloscope/msg487684/#msg487684QuoteAnd this:https://www.eevblog.com/forum/beginners/how-to-mesure-240v-ac-with-an-oscilloscope/msg487729/#msg487729

author: nitro2k01
author_rank: Frequent Contributor
timestamp: August 01, 2014, 08:20:35 am
content: Quote from: Ribster on August 01, 2014, 03:48:21 amNitpick time. Ever since the line voltage harmonization, all of EU is required to provide a nominal 230V instead of the 220V that was previously commonplace in most of Europe. I can almost guarantee you that your socket will provide you with 230 V +/-1% in practice, even if the standard allows for +10%/-6%.

author: Ribster
author_rank: Frequent Contributor
timestamp: August 01, 2014, 08:31:34 am
content: Quote from: nitro2k01 on August 01, 2014, 08:20:35 amI'm glad that you don't dispute the technical baggage of what i said. And yes, you are right. It was changed to 230V, a habit of speech i guess.. Allows for more power throughput though, was a nice move from the EU.

author: Niels
author_rank: Newbie
timestamp: August 01, 2014, 08:43:25 am
content: Quote from: Ribster on August 01, 2014, 08:31:34 amNo, as far as I remember/know it was to harmonize the specification around Europe.UK had 240Vac +-10%, this was changed to 230Vac +10% -6% in a transition periodThe rest was 220Vac +-10%, this was changed to 230Vac +6% -10% in a transition period.And now it is 230Vac +-10% in Europe.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2014, 09:53:00 am
content: Quote from: tautech on August 01, 2014, 07:32:24 amShould I genuflect?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 01, 2014, 11:13:45 am
content: WarSim,I withdraw the "sonny" crack,having looked back at your postings over the past few months.---sorry!No,I'm not stalking you!It is rather unusual for someone to refer back some 30 years to their schooldays to make a point,though!

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 03:02:55 pm
content: Quote from: vk6zgo on August 01, 2014, 03:55:03 amAt least where I am in the central US, we have a lot of problems with power. We have like 2 or 3 significant blackouts per year mostly associated with thunderstorms. The half life for compact lamps which include electronic ballasts is about 6 months so incandescent lamps are a lot more economical simply because they are less expensive to replace. All of my computer and test equipment is protected by an online UPS and I have considered doing something similar for the lighting circuits but it would be a lot of work and probably not possible to do to code.

author: WarSim
author_rank: Frequent Contributor
timestamp: August 01, 2014, 03:31:02 pm
content: Quote from: vk6zgo on August 01, 2014, 11:13:45 amI can see how it could have suggested that. I also regret my defensive response. Mostly because I still can't directly say what I did or know even 25 years latter.But I can say my start in electronics was in 1970 back when child labor laws didn't exist.. Even as a child I repaired cct involving 53kv. In school I was a student instructor. After school that is when I can only say I had a very unique careers, that I would not wish on anyone. This is why I mentioned school because the last 25 years is basically a non subject.In the end I just wanted to make sure the possibly dangers where known. There where plenty of others giving empirical examples of safety. I agree it is unlikely the tailing will surge or float to 1250v PE referenced. Even less likely it will contain damaging transients up to 4000v mentioned. What I am saying is it can and it has, so the author has the right to know.

author: WarSim
author_rank: Frequent Contributor
timestamp: August 01, 2014, 03:42:03 pm
content: Quote from: David Hess on August 01, 2014, 03:02:55 pmThere are whole house UPS systems available in the USA. Some are even made in the USA. But the ones that are constant conversion are very expensive and large. In some ways a bit dangerous, just because you are storing power in huge sealed lead acid batteries. My uncle uses 3 heavy duty deep cycle fork lift batteries. So even his cheeper solutions involves $24,000 in storage alone.Just saying the option is there, it is up to you to decide if it is obtainable.

author: mzzj
author_rank: Super Contributor
timestamp: August 01, 2014, 04:17:23 pm
content: Quote from: David Hess on August 01, 2014, 03:02:55 pmSame in my parents home in countryside Finland, countless electronic bulbs and 1-2 adsl-boxes per year even if you run around your house like maniac to unplug everything at the first hint of approaching thunderstorm. In a really bad weather the electric sockets are flashing over and you can see blue flashes and hear pops inside the sockets around the houseDuring summer months I wouldn't poke/probe outlets here with anything "probably good enough"

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2014, 05:07:17 pm
content: Quote from: WarSim on August 01, 2014, 03:42:03 pmIt does not make much sense to condition the power for things like the stove, oven, electric dryer, welder, furnace, etc. but expensive light bulbs in fixed circuits which do not draw much power and are expensive are another matter. Double conversion power conditioning without backup would be sufficient so expensive perishable batteries would not be needed. I have a couple of smaller online UPSes which are currently operating without batteries but they are not quite large enough.Even then there is no payoff as long as I have access to inexpensive light bulbs.It might be fun to leave a DSO running to capture power line transients but all of mine are CRT based and I am not inclined to add that many hours to them.

author: othello
author_rank: Contributor
timestamp: August 01, 2014, 06:15:21 pm
content: Wow, despite various standards it looks like in the real world things don't always match the theory :-) But i appreciate this topic greatly, i bookmarked it and hope newbies will find it also fruitful.PS: I found out that the metal sticking out consists of 2 separate hooks, 1 is ground and 1 is according to scope manual the "Compensation Signal Output Terminal". So never mind my previous statement about measuring 1.3V between ground and earth, it was the CSOT and ground i was measuring. So now when i measured the ground of the scope and earth it shows about 4mV average.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 02, 2014, 07:42:22 am
content: Quote from: WarSim on August 01, 2014, 03:31:02 pmSorry about the "Chicken Little",too mate!it was uncalled for.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2014, 08:24:39 am
content: QuoteAll this kiss and make up is getting sickly.Nice to see though.We often omit a quick search of a members posts to gauge their expertise.

author: SeanB
author_rank: Super Contributor
timestamp: August 02, 2014, 05:51:09 pm
content: Gettin back to the topic of probing mains, and the nasties on it I have always had a few dozen plugtops with a VDR in them across live and neutral, simply as a plug to occupy a spare outlet, preferably the first in a strip. This has worked well to attenuate spikes for me, and I also have collected a lot of surge supressor plugtops over the years ( thanks to our Telecom supplier who will often simply change them out if they cause tripping of EL breakers due to surges, often replacing with a regular plugtop as well) and as they also have 3 VDR's in them between pins they also work well. Phone lines also have transient suppression as well, and though this may not be the best for ADSL ( adds to loss so I modify them to leave only the spark gap and remove the silicon switch inside, but have a unmodified one after the ADSL filters) It did save all my electronics when lightning hit a building 50m away a few years ago. Neighbours lost TV sets and electronics, I had no damage at all aside from tripped power. I did have some VDR units go short circuit a while later ( they did the job though) and it was a few minutes work to find the faulty one and simply replace the VDR inside.I did recently buy a Thermaltake PC power supply, and see that they did a very interesting thing to clip spikes, by using instead of a VDR a spark gap. Hope the fusing is good, though I think those will need the warranty voided if it does operate to replace the fuse. I do not think the NTC thermistor will cope well with being used to current limit the mains, they tend to catch fire and burn in that application. I have done that before......., but it survived slightly crispy and still works even though it is now pacman shaped.

author: Xieos
author_rank: Newbie
timestamp: July 07, 2017, 02:33:41 pm
content: Hi Every One ....I wonder why there is no circuit diagrams / grapical explnation in all of this long thread ??... ..mmmso I live where there is a 220VAC 50 Hz, and normally the power plugs doesn'y have an Earth connectionmy question here is .... Will the below Circuit is going to work or not and why ? (Check the Photo )

author: alm
author_rank: Super Contributor
timestamp: July 07, 2017, 04:20:23 pm
content: No, it could easily shock you when you touch your scope. What does the user's guide of your scope say about operating it without an earth connection? I would not use a non-isolated scope at all near dangerous voltages without an earth connection.I suggest you go back to the beginning of this thread ask yourself why you desperately need to measure mains and what the proper tools and alternatives might be (e.g. a small transformer).

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2017, 04:44:40 pm
content: @Xieos:Just to point out another hazard. If you only have 2-pin, non earthed sockets then you can't be sure which way round Live and Neutral are wired. They might be different on every socket in your house too!Please don't try to scope the mains like that - there really isn't anything interesting to see anyway. Don't try it because you can (or think you can!).

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2017, 08:42:28 pm
content: Please tell us WHY you think you want to put a scope probe directly on the power mains outlet?Yes, if all the planets align properly and your bio-rhythms have a triple-peak and you hold you tongue just right, you might be able to safely do this. But there are so many potential problems here that you could just as likely kill yourself AND burn down your house and blow up your oscilloscope (not necessarily in that sequence).It continues to be a mystery to me WHY people think they want to put their oscilloscope on the power mains! Nobody has ever offered a cogent justification. "Just curious." doesn't really cut it. Now when weighed against the considerable potential consequences.

author: djnz
author_rank: Regular Contributor
timestamp: July 07, 2017, 08:46:20 pm
content: Quote from: Richard Crowley on July 07, 2017, 08:42:28 pmExactly my thought. Why would you want to do this? Just to see some sine waves?

author: Xieos
author_rank: Newbie
timestamp: July 08, 2017, 07:46:03 am
content: ,,,Thank You Guys for the reply ,the photo I put there was Just for the sake of this discussion and long thread....but ....lets just say we have a 18 pulse VFD and I want to measure how much noise/harmonics is introduced back to the mains line (not only current harmonic..but voltage too) ...(I used to do this with a handheld Fluke Scope)...(yes I know it's battery powered)(and for Netural and Line...can be easly detected with multimeter referenced to Earth)

author: Electro Detective
author_rank: Super Contributor
timestamp: July 08, 2017, 09:42:21 am
content: There are a LOT of reasons why one would want to see the mains waveform on an oscilloscope,unfortunately none are acceptable to a lot of nay sayers who have no need for such 'eye candy' when diagnosing MULTITUDES of power related issues in the real world.Either way, do it SAFE guys, or not at all,the safety earth/ground system in your house is your family's best friend,but worst enemy for your earth/grounded test gear and DUTs.LEARN about it and do it right, if you must.If you don't do it right, your probe ground leads will go POP and or will take out your oscilloscope,or it just won't ever work as it should.. if you survive the INSTANTANEOUS fireworks/BBQIf safety doesn't interest you, how much will a replacement scope cost ?

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 08, 2017, 02:33:08 pm
content: The premise here is that if someone has a legitimate reason to view the mains power waveform, they are more likely to understand the issues with safety. It just seems to me that virtually ALL of the "how do I view the mains waveform" questions are from newbies who just got their first 'scope and are itching to look at something with little or no understanding of the safety issues.

author: Xieos
author_rank: Newbie
timestamp: July 08, 2017, 02:41:33 pm
content: .........what problems will the circuit (I posted above) will have ?can some explain with a drawing ....please ...

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 08, 2017, 03:06:22 pm
content: A significant problem is that innocent-looking ground connection between the probe and the "neutral" socket pin.How do you KNOW it is "neutral"? Especially if you have no Earth reference? In many parts of the planet you have a 50/50 chance of it being "Line" 240V.Anything you clip the ground lead to will essentially connect it to the entire outer enclosure of the oscilloscope. If you clipped it to Hot instead of Neutral, you have the equivalent of poking your finger into the mains socket.And if your scope power plug HAS an Earth pin (and assuming the mains outlet is wired properly), then if you clip your probe ground lead to the Line pin, you can hope that your scope probe blows first as an expensive fuse to protect the even more expensive part of your fault circuit (i.e. the scope itself).The problem with that drawing is that it ASSUMES a 100% "Happy Path" which rarely exists in the Real World. It is very misleading especially to newbies who don't understand the risks. If someone is holding a gun to your head and forcing you to do this, at least do NOT connect the ground clip from the probe. Leave it floating for safety sake. By probing EACH pin, you can at least get some indication of which side is "hotter" than the other.

author: Xieos
author_rank: Newbie
timestamp: July 08, 2017, 03:55:08 pm
content: I pointed out up there that the Neural and Line can be determined by a Multimeter referenced to Earth...Before doing the measurement( a friend of mine actually took a reading for a VFD mains input with a bench scope before with just leaving the earth return path open).... let's just say in an Ideal world .... will the above circuit work ... ??Yes .... or ... No Question

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2017, 04:23:53 pm
content: Quote from: Xieos on July 08, 2017, 03:55:08 pmNo one here should answer this as we could have a persons demise on our conscience.It MUST beyourand ONLYyourchoice to undertake such unsafe practices.Read this and please do not ask again.https://www.eevblog.com/forum/testgear/floating-scopes/

author: tronde
author_rank: Frequent Contributor
timestamp: July 08, 2017, 05:09:00 pm
content: Xieos has made it perfectly clear that he has NO earth connection. Therefore it does not make any sense to involve the earth/neutral problem into the discussion. It does in fact exist legally installed non-earthed outlets in some countries. My house is full of them for instance.It is impossible to consider what is normal in one country as normal in another country. Please do remember that.The danger by measuring as Xieos suggest is that one of the phases will be connected to the chassis of the scope, and therefore you will need to treat the entire scope as "live"; that means you will in fact be connected to mains when you touch it. Even if you can measure which phase is "live" and which is "neutral" you should consider both to be "live".The circuit you indicate in your drawing will work,given the scope is made for the voltage difference between its chassis and both supply wires. Many scopes are not made for that, and you can weaken the insulation so it will fail later on.I do not recommend you to measure as you suggest, but it will work in theory.Before you connect the scope as you suggest, you should consider if you can use two probeswithoutground clips and invert one channel on the scope and add the channels mathematically. Many scopes are prepared for this, so consult the user manual.Regardless of how you measure live circuits you need to switch on your brain. No equipment will make it "safe". You can only achieve some level of "unsafe".

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2017, 05:14:30 pm
content: Quote from: Richard Crowley on July 08, 2017, 03:06:22 pmThere is actually a way to do this without a wired connection to a known good Earth reference. Hold the common lead of your good handheld AC voltmeter in your hand and probe with the volts lead. Capacitive coupling between you and true earth will provide a reference allowing you to determine if the wired Earth ground, neutral, and hot are wired incorrectly.This works best with a voltmeter which has a 10 megohm input resistance for AC volts but a 1 megohm meter will work also.

author: Electro Detective
author_rank: Super Contributor
timestamp: July 08, 2017, 11:17:47 pm
content: There are other fun things to aid in the death of the probe ground clips, oscilloscope, operator's eyes, heart rate, and life:Mains mis-wires including faulty wall cabling and termination, REVERSALS (everyone's favorite..) DIY extension cords, reversed or cheap suspect IEC leads..all waiting to bite you and your pocketEven if you are sure of your abilities and everything checks out, there is still a 50 / 50 chance of it going wrongXieos, even if you know what you are doing, the guys here do not want to flip the coin for Heads or Tails at your expenseand besides x10 may not be enough to see all the waveform at 220v, you may have to back off the vertical gain trim all the way aka vintage 'uncal'and I don't favor your method above, I would rather play bite me with a poisonous snakeIf you have 2 scope channels and 2 probes, at least go the differential mode way, remove those croc ground death clipsand find out how it's done properlyPLEASE

author: onlooker
author_rank: Frequent Contributor
timestamp: July 09, 2017, 01:05:32 am
content: I think one of the risks is that the beginner scope users tend to think and treat a scope as just an extended version of multi-meter without fully understanding that, when dealing with lethal voltages, the operator may be exposed to much more danger with a scope than with a multi-meter.Another point to also realize is that theoretically valid does not equal to practically proper. For using scopes dealing with lethal voltages, forming a good habit with rules that are easy to remember and follow is as important as theoretically understand where the dangers are. This is because people make mistakes.

author: Xieos
author_rank: Newbie
timestamp: July 09, 2017, 04:04:57 am
content: @onlooker"theoretically valid does not equal to practically proper"Totally Agree ... with you ... but I have seen a lot of fuzz about this here in this thread .... without a single drawing or graphical representation that clearly showing that if you have that earth path open the scope will not blow up .... but you may get electrocuted ....I used to measure the input/output of Medium and Low Voltage VFDs... but I do it safely with active differential probes and battery powered scope .....and I understand the risk of actually plugging the probe into a wall outlet... which of course I'll not be doing ..... but the way this discussion was going it is a No No Answer without a single diagram or a circuit that shows a problem ... "A picture is worth a thousand words"-------------------------@trondeInteresting .... how would we know if the connection in the circuit I have posted will affect the insulation ...?....New Challenge (how to Check Scope Insulation to determine if's going to weaken after the test)-------------------------------------------------------------------------------------------------------------------------------------------------------------------@Xieos....I think I understand why people say it is NO NO taboo thing ...maybe if someone got hurt because we took the risk of saying <<yes it's doable >> we will feel guilty

author: onlooker
author_rank: Frequent Contributor
timestamp: July 09, 2017, 06:03:10 am
content: As others have pointed out, one obvious danger is to have live voltage on the exposed metal parts of the scope. This fact may easily get forgotten since one is so used to touching these metal parts freely.Picture is not needed, if you think an anology of sticking a screw driver into the live hole of a mains' socket. It may not always harm someone, but it can be considered a bad habit that adds the chance of harming someone, espetially when there are better alternatives to do it in a safer way.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 09, 2017, 06:57:11 am
content: @Xieos, you don't seem to be getting the point here. Anybody who says "sure, that circuit will work" is a fool and you shouldn't be taking advice from them. At least not if you value your gear, your house, and your life.If you make many assumptions not in evidence in your Happy Path Diagram, and assume that the diagram floated down to us from Planet Ideal, sure, it would work. But this is NOT "Planet Ideal"Just the absence of an Earth connection, (either for the 'scope mains power plug, OR for the measured outlet), makes it a Show-Stopping Risk, IMHO. You are literally betting YOUR LIFE that you know positively 100% which is Line and which is Neutral. You are further betting that the "Neutral" is some voltage reasonably close to zero. We have seen and heard too many reports of "Neutral" and even "Earth" not being what they are supposed to be. And people have literally died from those kinds of errors.The very fact that you are asking that question indicates to us that you are not cautious enough for anyone to tell you "Yes, that will work." without fearing that they are encouraging you to do something that will kill you.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 09, 2017, 07:04:06 am
content: Quote from: David Hess on July 08, 2017, 05:14:30 pmYes, that is what I suggested in my "gun to your head" scenario in a previous response.But even that technique (with a meter or with a scope) will NOT tell you if the lower-voltage pin ("Neutral"?) is properly within a few volts of Earth Zero.

author: newbrain
author_rank: Super Contributor
timestamp: July 09, 2017, 07:56:53 am
content: Quote from: Richard Crowley on July 09, 2017, 06:57:11 amQFT.And I'll repeat myself, you know it comes with age...There are places where mains distribution does not use a "neutral" at all: every house I've lived in in Rome (including the one where I am now!) has two live wires, with about 120V to ground from either side. This stems from the conversion from 110V to 220V that happened some decades ago.Using the connections in the picture will make the scope chassis live, leaving your life in the hands of the GFCI/RCD (called a"differential" or "life-saver"in Italy).

author: Electro Detective
author_rank: Super Contributor
timestamp: July 09, 2017, 10:36:45 am
content: Quote from: newbrain on July 09, 2017, 07:56:53 amSo your local street transformer has a centre tap that is also tied to earth/ground ?Otherwise the life-saver may disappoint when required

author: newbrain
author_rank: Super Contributor
timestamp: July 09, 2017, 11:33:07 am
content: Quote from: Electro Detective on July 09, 2017, 10:36:45 amPresumably yes, as the GFCIs work as expected (I might not be writing this otherwise).The old distribution was 125V (phase-neutral) for regular sockets, and 220V (phase-phase) for heavy uses (washing machines, boilers etc.) called "industriale".Everyone called the 125V "110V"With the switch to 220V, what was the neutral was "simply" connected to another phase.I can still measure ~127V phase (any) to ground, and remember, as a boy, to have lit 110V bulbs from phase to "ground" (=heating pipes): no real PE in sockets or GFCI, at the time.Newer areas have a more normal (and safer?) phase-neutral distribution (derived from 380-400V triphase).But the main, in topic, take away is: do not take risks with improvised "solutions", the assumption they rely on might not be valid!

author: tronde
author_rank: Frequent Contributor
timestamp: July 09, 2017, 03:14:43 pm
content: Quote from: newbrain on July 09, 2017, 07:56:53 amIt is typical for people living in certain countries to believe that the entire world is exactly the same as what they have at home... It is just impossible for them to understand that the world is a very diverse place.It can be that your distribution system is IT (isolated terra). An example of IT can be what is common in Norway. The source transformer is connected as star (wye) with no earth connection from the center. It does have a spark gap in case of lightning, but no permanent connection. The house is connected as delta with only local ground/PE. We have 230V available and if the load is symmetrical on all three phases from the transformer you will measure 230/1.73=133V between one phase in your house and earth. Because of this we do not have "live" or "neutral". We only have "live". If the load is not symmetrical (or if there is a short to ground somewhere) you can measure anything between 0V an 230V between a given phase and PE.Until the late 1990's it was common to use wall sockets without earth if the room was defined as "dry". Bedrooms and livingrooms will most likely not have any PE in houses built before the late 1990's. I know that this was common practice in some European countries.Only very few European countries have a defined "live" and "neutral" pin in the wall socket. The schucko socket is not polarised, and the "French" socket that is polarised by design is implemented in different ways in the countries using it.Because of this, equipment with mains plugs sold on the European market must be made so it will be safe regardless of which supply wire is "live" or "neutral".

author: tronde
author_rank: Frequent Contributor
timestamp: July 09, 2017, 03:41:08 pm
content: Quote from: Xieos on July 09, 2017, 04:04:57 amYour scope is fed from say 230V mains. Inside your scope you have some kind of a transformer that will insulate the electronics in the scope from the 230V mains so you can use it safely.The problem I mention is caused by the fact that the 0V input socket (the screen of the probe) is connected to the scopes chassis and therefore also the PE wire. This wire is not meant to be at an elevated voltage during normal operation. If you connect an elevated voltage to the chassis, you will have a voltage difference between the chassis and the transformer providing the insulation (and safety) from the mains voltage. Some manufacturers allow only 50V, while others allow something else.Because of this you can end up by weakening the insulation when you measure as you indicate in your drawing.I do really encourage you to find another way to do the measurement. You said your intention was to measure noise feed back to the mains. You also said"and I understand the risk of actually plugging the probe into a wall outlet... which of course I'll not be doing ...".I don't actally understand why you asked about connecting as shown in your drawing if you dont need to connect to mains.You must remember that if you measure anything that is fed from mains without an isolating barrier, you are actually connecting directely to mains safety wise.Many people seems to forget this. It is not only direct connection to the wall socket that is dangerous.

author: Xieos
author_rank: Newbie
timestamp: July 10, 2017, 02:40:59 pm
content: I'm not actually against safe practices ... but I mean this type of measurement is doable but it's not safe and it's not recommended and whoever will do this has to know what he is doing and proceed at his own risk ...@tronde .... as long as I have a safer alternative I will not take such a measurement ..... I'm not trying to measure anything like this now but my example was for the sake of this long thread.

author: tronde
author_rank: Frequent Contributor
timestamp: July 10, 2017, 04:08:40 pm
content: Quote from: Xieos on July 10, 2017, 02:40:59 pmOK.When somebody ask about a safety related issue on an open forum we have no idea about theiractualknowledge or intention. I have met a lot of people that ask a question, and use the answer for something completely different. When this happen in a face-to-face setting, you expect worse on the internet.People can for instance say they are a certified electrician so they know about these things. The problem is that a certified electrician is not the same in all countries. In some countries they will have a lot of theory in school, while in other countries the education is more about how to follow the rule-book and do the wiring.Even if you know what level of knowledge is expected in a certain country, this is not the same as the person in question actually has this knowledge.The education can also change over time. 20 years ago a Norwegian electrician would do basic calculations on three-phase systems and phase compensation in the first year at school. Today you can find pupils leaving school after two years that believe plus and minus is only about which way you connect your voltmeter. This is not a joke.You will also find certified electricians that don't know how many volts you are expected to measure between the different wires in a 400V TN-system, or why you can get more than 230V in the wall socket if the neutral conductor from outside of the house works loose in the distribution panel.