Measuring Mains with Rigol DS1052E - Page 1
Forum section: electronics / beginners

author: logicnibble
author_rank: Contributor
timestamp: August 02, 2011, 01:40:08 pm
content: Hi!It's dangerous to measure mains voltage directly with an oscilloscope but since I've read many guys have done it, I'd like to know about the Rigol DS1052E and if there are any tips & tricks, like:-Not connecting Rigol earth ground.-Using diferential reading between channel 1 and 2 and not using the common aligator from both probes.Thanks

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 02, 2011, 02:16:17 pm
content: If you have to ask this you probably shouldn't be doing it at all.Lifting the scope ground is a very BAD idea. Diff probes often don't have sufficient range and/or common-mode rejection.The normal way to do it is power the device under test from an isolating transformer so you can then ground the circuit ground (seperate ground- don't rely just on the scope probe ground).Make sure your probes are suitably rated- you'd normally want a x100 probe for mains use. 100 probes are also handy to have for probing high impedance circuits like 32K watch crystals.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 02, 2011, 02:38:05 pm
content: I think that logicnibble had set one interesting question.And no it does not limited to the Rigol product range.As a electrician I would like to have on screen other than volts and the wave form too.So to be able to see and tell how perfect or clean is the AC wave at 220 or even at 380V.Or even, to be able to check DC/AC inverters and see how they respond against a specific load.Some people they do complains that the low quality DC/AC inverters cause noisy operation on small motors, usually small pumps.I like to be able to see of what is happening with the wave forms.If there is a tool capable to help me out there, I need to know which one is capable for the task.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 02, 2011, 03:33:03 pm
content: What are you trying to measure? You can measure phase-to-ground and neutral-to-ground with an ordinary high voltage probe. For phase-to-phase or phase-to-neutral measurements, hooking up two probes and doing math A-B on the scope has terrible common mode rejection, but it might be OK at 50 Hz. Simply doing separate measurements of phase-ground and neutral-ground may be the most illuminating -- the neutral-to-ground voltage is indicative of the current. Not enough for a true measurement, but if you turn on a reactive load you will probably see an out-of-phase voltage pop up on the neutral line.If you just want to see the AC waveform, see if you can find a small AC output wall transformer with <24VAC output and measure the secondary voltage. High harmonics will be filtered by the transformer, but if you purpose is to look for garbage on the mains caused by other devices you can still probably see plenty.

author: logicnibble
author_rank: Contributor
timestamp: August 02, 2011, 04:17:36 pm
content: Quote from: ejeffrey on August 02, 2011, 03:33:03 pmI'm trying to measure a uC circuit which is powered by a capacitive (transformerless) power supply.This means all circuit is LIVE.I also would like to measure a dimming circuit, which is LIVE too.Quote from: ejeffrey on August 02, 2011, 03:33:03 pmDoes Rigol DS1052E have one?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 02, 2011, 04:41:10 pm
content: Quote from: logicnibble on August 02, 2011, 04:17:36 pmIn which case use a mains isolating transformer, and connect the circuit's 0v rail to mains earth so you can ground the scope to it.

author: IanB
author_rank: Super Contributor
timestamp: August 02, 2011, 05:00:12 pm
content: Quote from: mikeselectricstuff on August 02, 2011, 04:41:10 pmPlease humour me here as an electronics noob...but if the transformerless device already had it's 0 V rail connected to the chassis and the chassis was connected to mains earth, what might go wrong with connecting a high voltage scope probe under those circumstances?I understand that if the low voltage supply was insulated from the chassis and completely floating then you would need to create a safe common ground for the circuit and the scope, and then I see why you would need the mains isolating transformer to do that. Do I have it right?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 02, 2011, 05:27:33 pm
content: Well I believe too, that with a mains isolating transformer, you do not need any connection with the ground.

author: jahonen
author_rank: Super Contributor
timestamp: August 02, 2011, 05:37:39 pm
content: For measuring normal "low-voltage" mains safely with an ordinary grounded scope:Regards,Janne

author: IanB
author_rank: Super Contributor
timestamp: August 02, 2011, 05:38:09 pm
content: Quote from: Kiriakos-GR on August 02, 2011, 05:27:33 pmWell ground has two purposes: one as a common voltage reference, and the other for safety to ensure a low impedance return path for fault currents.The question is whether the oscilloscope probe ground is connected to the mains earth or whether it is isolated and floating? I don't know much about oscilloscopes, but I suspect the scope ground must be connected to mains earth otherwise the probe would pick up all sorts of noise by inductive and capacitive coupling (much like a DMM generates random voltages on the display when the probes are not connected). If the probe ground is earthed, then connecting it to a point in a circuit that is not at earth potential is going to do bad things. The way to cure this problem is to ensure the circuit under test is completely isolated and floating, so that no current can flow between the circuit and the oscilloscope.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 02, 2011, 05:39:24 pm
content: Quote from: IanB on August 02, 2011, 05:00:12 pmThe transformerless device cannot have an external chassis connected to mains earth: it's ground point is connected to mains live or mains neutral (if the plug is reversible),so it will be classified as a "dangerous to touch terminal".A transformerless device must be built inside an insulating case (old TV sets were made this way).The headphone socket of my old TV set is driven by an insulation transformer.The antenna connector is insulated through low value, high voltage capacitors (that limit the current that can be sourced from the antenna socket).

author: IanB
author_rank: Super Contributor
timestamp: August 02, 2011, 05:50:52 pm
content: Quote from: ciccio on August 02, 2011, 05:39:24 pmThat makes sense. But in the UK (where I'm from) reversible two pin plugs are found only with electric shavers. Everything else has a three pin fused plug with an earth connection. Devices can either be "double insulated", which means there must be no potentially live parts that can be touched, or the case must be securely connected to the earth wire in the mains plug. In theory (I think) you can connect the mains neutral to the mains earth inside the device since the two wires should be at the same potential. If someone reversed live and neutral in the mains lead then a short circuit current would flow and blow the fuse in the plug or trip the circuit breaker if someone bypassed the fuse. This idea of connecting earth to neutral inside a device is the bit where I may be off base I think...

author: ciccio
author_rank: Frequent Contributor
timestamp: August 02, 2011, 06:13:32 pm
content: Are you sure that neutral on your socket is at ground potential?Measure it, please.Here in Italy neutral is grounded only at the electric company's medium voltage to low voltage transformer,The one serving my home is about 500 meter from me, and in this moment neutral is about 30 V respect to ground...An old electrician told me: live is hot, and neutral is hot..

author: Bloch
author_rank: Unknown
timestamp: August 02, 2011, 06:30:58 pm
content: Quote from: ciccio on August 02, 2011, 06:13:32 pmOne thing to note is that every contry have there own mains standardHere in Denmark it is "normal" at every house to have you own ground point (a 2 to 6 meter long plug in the ground)If i was you I would talk to the lokal electrician to check if you own ground point is ok. (My guess is not)I dont have a englich page but try to see herehttp://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&sl=da&tl=en&u=http%3A%2F%2Fcubus-adsl.dk%2Felteknik%2Finstallationer%2Fnulling.php

author: IanB
author_rank: Super Contributor
timestamp: August 02, 2011, 06:53:45 pm
content: Quote from: ciccio on August 02, 2011, 06:13:32 pmI'm not in the UK right now so I cannot measure, but I know there could be a potential difference between earth and neutral wires. In the UK the earth and neutral are either connected at the nearest substation as you describe, or are connected just before entering the home (in more modern systems). Either way, if there is 30 V between neutral and earth, it suggests there is some significant voltage drop where it ought not to be; for instance there might be an unexpected current flowing in the earth conductor?I suppose though that I am wrong to think you could connect earth and neutral inside a device; most likely you should not do so and I imagine I would find that out if I could locate the right regulations to read...

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 02, 2011, 09:43:51 pm
content: Quote from: ciccio on August 02, 2011, 06:13:32 pmYes and No.The neutral can show voltage , when and only if there is extreme loads on the phase.When the load drops, the neutral voltage gets lower too.The voltage on the neutral commonly called as " returning currents ".It is part of the energy that phase has, that it does not consumed by the electrical device,and so one part of it returns on the neutral wire.The major problem that we have in the industry is that the loads are so large ones, and the neutral gets a significant amount of voltage.You have no choice other than to ground it when ever is possible.But if you do so, there is no way to install protection relays afterward s.

author: Unknown
author_rank: Unknown
timestamp: August 02, 2011, 11:01:33 pm
content: Quote from: IanB on August 02, 2011, 06:53:45 pmHow about current flowing through the neutral conductor? V=IR, so if there is a lot of current and some resistance, there will be a voltage drop. 30V sounds like a lot though.Quote from: IanB on August 02, 2011, 06:53:45 pmThe only point were theymaybe connected is at the distribution level (may be in your own home, may be somewhere far away depending on the topology). Connecting them somewhere else may result in fireworks and will likely trigger the GFCI. They may also not be connected at all, the ground may also be in the middle between neutral and ground. A break in the neutral wire somewhere may also change the voltage on the neutral wire, since the equipment connected to the phases now forms a voltage divider, so a large unbalance between the phases will cause neutral to be far away from ground.Equipment with the chassis connected to neutral has not been allowed for a long time, for good reasons. I'm not sure about the UK, but in many countries live and neutral are often wired incorrectly. Reversing ground and neutral is also somewhat common, but that gets noticed fairly quickly because it will trip the GFCI. Reversing live and neutral can remain undetected for a long time since most equipment developed for an international market doesn't care. Most safety regulations I'm aware of treat live and neutral exactly the same, both require the same clearance, creapage and isolation.If you're not familiar with this, I think building something connecting directly to mains is a dangerous plan.

author: Unknown
author_rank: Unknown
timestamp: August 02, 2011, 11:18:34 pm
content: Quote from: Kiriakos-GR on August 02, 2011, 02:38:05 pmCrest factor (http://en.wikipedia.org/wiki/Crest_factor) might be a useful indicator for what you are describing. Measurement of crest factor usually comes as a measurement in DMMs, oscilloscope and scopemeters. Of course, having a proper display of the waveform gives more insight, but a DMM with crest factor measurment should be able to tell you if you need to get the scope out. Just a thought.

author: IanB
author_rank: Super Contributor
timestamp: August 02, 2011, 11:19:48 pm
content: Quote from: alm on August 02, 2011, 11:01:33 pmI thought it seemed like a lot. A 30 V drop on a main conductor would seem to me something like a brown out.Quote from: alm on August 02, 2011, 11:01:33 pmFair enough. I don't do electricity for my day job and I also don't like to just follow rules by rote without understanding them. But once I followed through the reasoning as you explained above I had no problem seeing the picture. My incorrect thinking was not something that was going to follow through into a real project without me conducting a proper investigation. Any exposed metal parts should be earthed and current carrying conductors should be properly insulated and should be connected only to the parts of the circuit that are meant to be energized.

author: PetrosA
author_rank: Frequent Contributor
timestamp: August 03, 2011, 02:46:38 am
content: There have been some good (and some not-so-good...) opinions on the 30V to ground measurement. In most modern systems where the neutral is grounded at or near the main disconnect or last transformer feeding a service, 30V could likely indicate one of a number of possible serious problems.1 - there is a bad (high impedance) neutral connection between your outlet and the neutral-earth bond2 - you are connected to one of three phases (typical in EU) and there is a large imbalance between the phases3 - you are connected to one of three phases and there are a lot of non-linear loads on the transformer causing an overload on the neutral (this is actually more likely now that the EU is forcing the use of compact fluorescent bulbs).4 - the neutral from the transformer is undersized and overheating5 - the ground is actually disconnected and you are measuring induced voltage on the ground wire to the properly grounded neutralThere are other possibilities, but these would be the main culprits.If your service is protected by an RCD then you should be relatively safe. I would still have it checked by an electrician though since any of the neutral problems could potentially create a fire or shock hazard.If there are large voltages like this between ground and neutral, please be VERY careful testing it because there's the possibility that connecting the two will shunt a large load (10s of amps) through whatever is jumpering them.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 03, 2011, 05:48:58 am
content: Thanks PetrosA for your response:I've verified this years ago with the Electric Power Company's technicians, after one of my many calls for service.-The neutral to hearth bond is at more than 400 meters from my house (in the company's big MV to LV transformer). Lots of new building between.-I use a single phase of a 3-phase network (the other 2 are used only occasionally by the condo's common system (stair and external lights, etc) and most of the time they are unloaded. The other buildings may be connected to the same phase than mine, or to one of the others. Nobody knows...-The ground is connected to hearth at my house, with 3 earthed poles (verified with a Megger type instrument) so my RCDs work reliably.-The neutral is connected to ground at the transformer.-The neutral (and also the live) ables that supply my house are actually undersized, but the power dropped is acceptable to the company's standards.The voltage between neutral and ground varies with the day of the week (different loads), with the hour of the day and with the season (summer: more air conditioners: more current draw).Early this morning I has less than 5 volts, but in no case, even at it's maximum, it could light a neon voltage tester..... If this happened, the Power Company would surely intervene (or at least I was told so by one of the technicians)This situation is "normal": when I was younger and was a touring "sound engineer's helper", we had similar problems with power supplied to our stage's systems, and we never trusted both live and neutral.

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2011, 06:49:48 am
content: It reminds me of a time many years ago when I was delivering and setting up some equipment on an industrial site on the other side of the world. We had some low voltage power supplies with earthed metal cages and when we happened to rest them on metal strips under the floor there were sparks flying. We measured voltages and the "earth" wire was at a potential of about 100 V. As I recall the situation, someone had wired up the supply wrongly at the distribution panel and at least two of the live, neutral and earth wires were swapped.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 03, 2011, 08:30:09 am
content: Another way of looking at this is to say:-"One side of my Oscilloscope goes to the mains earth,& the neutral line is ultimately connected to the earth,so I can geta fair ideaof the 50/60 Hz signal bynot connecting the probe earth to anything,or by connecting the probe earth to the earthed case of the device,& just probing the mains active lead."Your Oscilloscope safety earth still protects you in this case,plus you can determine if your neutral & active connections are transposed.The best way is, of course to use an isolation transformer so you can get a better idea of the mains waveform,as some HF stuff may be lost using the method referred to above.In many cases,all you need is a rough idea,though.On another point,I'm not sure why everybody freaks out about measuring mains voltages.OK,the mains has a very low internal resistance/impedance,& can source huge amounts of current,but thepeak voltage of say,a 240v supply is less than 400v.You can definitely look at voltages of this level with a Tektronix, HP,Siemens,Philips or BWD x10 probe!Maybe the "El Cheapo" probes aren't as well rated!VK6ZGO

author: logicnibble
author_rank: Contributor
timestamp: August 03, 2011, 09:44:31 am
content: So, the best way to make those measurements is to use an isolation transformer. Got it.But if I had to measure it directly, can I use the Rigol DS1052E not connecting its mains plug earth connection?If the oscilloscope is battery powered, I should no problem, right?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 03, 2011, 09:58:40 am
content: Quote from: vk6zgo on August 03, 2011, 08:30:09 amI will agree that fear its not a good adviser, you are less concentrated and you can cause a mess due your own panic.Please exclude the electricians from your list, we love Mains ... LOL

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 03, 2011, 10:06:53 am
content: Quote from: logicnibble on August 03, 2011, 09:44:31 amEvery device on this planet is safe to use if : The live phase is not connected with the exterior casing.But hey wait a minute, those Rigol looks to have plastic casing !!

author: vk6zgo
author_rank: Super Contributor
timestamp: August 03, 2011, 11:07:09 am
content: Quote from: logicnibble on August 03, 2011, 09:44:31 amIf you read my posting above,you will see there is a work-around of sorts.The point I was making was that if you simply want to have a look at the mains,& are not chasing very small high frequency signals riding on the 50/60 Hz mains frequency,you do not have to connect the ground clip to either one of the Active or Neutral lines.As the Neutral line is,if everything is wired correctly,very close in potential to Earth,connecting the ground clip to a mains Earth point & probing the Active line will give a result which will be satisfactory in most cases.Removing the Earth lead from the 'scope & connecting the probe between the Active & Neutral is an enormousNO NO!!,as,if the Neutral & Active are transposed,any point on the Oscilloscope which is meant to be grounded will be at,in Australia at least,240v above Earth,& can KILL YOU!I REPEAT! Do NOT connect the probe across Neutral & Active.UNLESS you use an isolation transformer which will 'float" the two sides of the supply with respect to Earth.If youdon'thave an isolation transformer measure between EARTH & ACTIVE.(ground lead on Earth).It is almost as good,& you will not put yourself in danger.VK6ZGO

author: PetrosA
author_rank: Frequent Contributor
timestamp: August 04, 2011, 12:18:50 am
content: Ciccio, from reading some of your other posts, I'm guessing you have a pretty good idea of the dangers your situation presents, so I'll direct this warning to the general publicWhat's happening in Ciccio's main's (high voltage between neutral and ground) is potentially dangerous to a curious experimenter because if someone were to shunt the two at an outlet with a DMM set to measure current ( or a piece of wire) or somehow connect the two in a device, that connection would carry quite a large amount of current. There is a high impedance in the neutral serving his home due to an undersized utility neutral wire being overloaded by a poorly balanced load. If the buildings are loaded that far from the transformer and there's a large load on the neutral from imbalancing, any connection made between the ground and neutral in any of the buildings will split that load between the undersized neutral and the grounding conductors back to the transformer through the house wiring. The RCD device would trip if it's working properly. If it's not or if there weren't an RCD main breaker, the neutral and ground in the house wiring could carry a load many times greater than what it's rated for.Ultimately it's the utility's problem, as it's their wires that will burn up at some point and when that happens, customers will probably suffer some equipment losses due to a floating neutral and the high voltage that will ensue. Hopefully the utility is insured

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 04, 2011, 01:10:07 am
content: I do not know what the Italians do, the Greeks haves 0.09V at 3V AC manual range ( U1272A) between neutral and earth,and they snoring peacefully.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 27, 2015, 04:32:27 pm
content: Sorry for sparking old topic, but it seems a hot question among beginners about the earth, neutral and oscilloscope ground clip.Actually, just after watching Dave's video - How not to blow up your oscilloscope - I was sure I knew what I was doing ....Yet, I managed to short a bridge rectifier circuit of Switching Power supply, which resulted in:* Tripped fuse* Tripped Residual current device in the fuse box* blown fuse of the power supply under test* blown (shorted) one of the rectifiers diode.I promised myself never to connect a CRO to mains or the primary side of power supply ..... again ...Thankfully, the CRO and the probe survived - probably due to the fact that all the safety/fuses worked perfectly.Now, on the topic and this is just in theory, I really don't intend doing it.I just want to learn more about safety and how things work.I already read a ton of stuff of grounding and earthing systems in a household, after the accident.Now, turns out the in most cases the Neutral wire is connected to the Earth wire in the basement of the building.Actually, there is just one Neutral wire coming into the building as Neutral, which is then split into Neutral and Protective Earth wire.This is the most popular system TN-C-S or PME.At least in Germany, where I live.If that is true, then Neutral and Earth in such a household should be at the same potential.Actually, I measured the voltage between Neutral and Ground (with a decent multimeter, taking many precautions) and I got 0.2 V difference between them.In such case - what will happen if you connect the ground clip of the CRO to the Neutral line and probe the Live wire with the tip of the probe?I see you say it is a big NO, but actually, if your household electric system is OK and there is no currently Lightning storm going on - it should be pretty harmless. Maybe if at the same moment some electric fault occurs - i.e. an appliance goes bad, there is a potential for things to go bad.I understand that if something goes wrong with the household electric system - it will always be a hazard, not just because you are connecting your CRO to something but in general.I am interested in theory and if all systems are working well up to standards.Opposed, to a case where you accidentally connect your CRO grounding clip to the Live wire - obviously not matter what - there will be a big bang.What are your thoughts on this topic?

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2015, 06:30:39 pm
content: Connecting the neutral and earth will certainly cause the RCD/GFCI to trip.If there's no earth leakage breaker, then a current will flow from the neutral to earth. How big the current is and whether it will cause any problems depends on the potential difference between neutral and earth and the resistance. If may not cause any problems or it could result in smoke.I don't see why anyone would need to measure the mains with an oscilloscope. It should be safe to just connect a suitable CAT rating oscilloscope probe to the phase conductor and not connect the chassis clip to anything. That will give you the waveform on the phase conductor which won't be much help.If I were using an oscilloscope to look at the waveform on the primary of an SMPs, I'd simply power it from an isolating transformer and be aware of the fact that the RCD won't be any use.

author: IanB
author_rank: Super Contributor
timestamp: September 27, 2015, 09:47:41 pm
content: Quote from: Hero999 on September 27, 2015, 06:30:39 pmOne case might be to examine the current in a mains conductor, as done by AvE here:https://youtu.be/IAG61bSTT7k

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 28, 2015, 06:54:11 am
content: Hi,thanks for the replies, I was thinking the same - that there will be current flowing from N to PE - just wanted to confirm with you guys.No, after I blew up the fuses in my previous experiment - I do not intend to measure the mains ... at all.However, so many people are asking and trying - that I think this topic should be periodically re-answered.You know, it seems that the beginners are drawn to the mains the same way as moths to the flameThere is something magical in trying to fiddle with the mains power, especially with doubtful Chinese instrumentsOne more question, which is more like clarification:What is causing the potential difference between N and PE? In my building they are connected to each other in the basement. (By the way this is the most common case in Europe.)Is this potential difference caused by the mere length of the cable and the fact that parts of the Neutral wire are carrying current (people are using things in their homes.) - so there is voltage drop - meaning the Neutral is with lower potential compared to PE, even though they are connected in the basement?Also the resistance between N and PE is minimal - I think it is about 0.3 ohms - required by law.If there was bigger resistance I couldn't have been able to blow my power supply - because it is not Earth referenced - it has just 2 prongs plug.So, when I connected the CRO, the current flowed from N, via the PE, to the Live wire - therefore shorting N and L via the PE and the CRO casing.If there was big resistance between N and PE - this shouldn't be possible.I just imagined myself connecting an amp meter between N and PE and turning the power on from the other side of the room.Just to see what happens...... as I said moths to the flame/No, I am not doing, but if someone had already done it - and I am sure people have done it - please let us know the results./

author: mzzj
author_rank: Super Contributor
timestamp: September 28, 2015, 09:05:42 am
content: Quote from: pplaninsky on September 28, 2015, 06:54:11 amYes, the potential difference is caused by resistance (or impedance more exactly, but wire inductance is tiny compared to resistance at 50Hz so we can ignore it)If you connect amp meter between N and PE you should see about half of the load current.If the branch of wiring doesn't have load there is no current and no potential difference. But the load can be also somewhere else than the outlet in front of your nose, room lightning and outlets may share same branch of wiring for example.

author: pplaninsky
author_rank: Regular Contributor
timestamp: September 28, 2015, 12:27:16 pm
content: Hey guys,thanks for all the replies.I think this is a hugely useful topic and I have a better understanding now about CRO's ground and AC mains ground.I just measured the voltage between my N and PE and it was 0.025 V AC.I doubt this measurement has any meaning at all.I am just putting this info here.Thanks once again!

author: djacobow
author_rank: Super Contributor
timestamp: September 28, 2015, 03:59:45 pm
content: I just wanted to chime in to say that knowing what you are doing is NOT sufficient for safety with live mains equipment. You also have to not make mistakes.That is harder than it sounds, since we all make mistakes all the time. Discipline and the use of checklists can be assets. Sure, you can look over the isolation xformer and rats nest of power cords on your bench, but did you actually make sure that the DUT you are working on right now is the one plugged into the xformer? Or you could have a policy of always powering the "DUT side" of your bench from isolation, but then you have to remember that when you're working in the field, etc.Mistakes happen all the time. We've all let the smoke out many times. And yet, with mains AC, the cost of a mistake can be much more exciting.Just sayin'.Personally, I prefer to use a HV differential probe. They're not that much more money than an isolation transformer, particularly if your application requires a large one, like 2kVA, and obviously much more convenient if you have to work in the field.

author: newbrain
author_rank: Super Contributor
timestamp: September 28, 2015, 04:07:07 pm
content: Just to add one data point:There are places in Italy where two phases (out of three) are distributed, with no meaningful distinction between Neutral and Phase.Protective Earth is, electrically, 127V from each phase, and the difference between phases is 220V.127 x sin(60) x 2 = 220.I have personally verified this in all the apartments I've been living in (Rome area);I just checked here in Sweden and I have real Phase and Neutral.Is this type of distribution common elsewhere? I think it has something to do with the conversion from 110 to 220 that happened some 40 years ago, but I'm not sure.

author: Nivr
author_rank: Newbie
timestamp: October 30, 2019, 08:54:50 am
content: Quote from: mikeselectricstuff on August 02, 2011, 04:41:10 pmIsn't this creating a very dangerous situation, something powered by an isolation transformer, but then earthed nonetheless?If you touch anything connected to the non-earthed secondary side of the transformer, nothing (no rcd/gfci) would be able to interrupt the current and save your life.

author: Zero999
author_rank: Super Contributor
timestamp: October 30, 2019, 07:27:57 pm
content: Quote from: Nivr on October 30, 2019, 08:54:50 amYes, it does increase the risk of getting shocked somewhat, but I wouldn't describe it as a very dangerous situation.QuoteWell no, the secondary of the transformer would be be earthed.Expanding on my previous comment. It's true that using an isolating transformer, then earthing the secondary, would defeat and RCD/GFCI, thus increase the risk of shock, but that isn't necessarily a very dangerous situation. It's only dangerous if you touch a live part of the circuit, which can be avoided by adhering to safe working practices. People worked on live circuit fine before RCD/GFCIs were invented and didn't get electrocuted.