Measuring Mains Voltage with Oscilloscopes

Problems with measuring mains voltages with an oscilloscope, the correct and safe way to do it, and other dangerous methods that must never be used.
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[@ianharrison6597]

Thanks JW. Perhaps the clearest explanation of correct ‘scope/mains practice I’ve heard.

[@daveb5041]

*My six year old daughter loves this channel. She especially likes learning about electrical code and regulatory bodies. You should make a second channel called JWKids which would focus more on the laws governing electrical code*

[@m.9243]

Clear and precise explanation, going into detail that is easy to understand. Thanks for your effort to share your knowledge with the rest of us John.

[@Markus0021]

Very clear and thorough explanation. I 'sort of' understood this before, but now I 'really get it.' Thanks.

[@madbstard1]

My second lesson in two days on using a scope on mains - Play With Junk also talked about the dangers when he was probing a linear power supply. But your video gave far more detail on the different scenarios.

[@kenfoland]

If you are new to isolation transformers, research the subject thoroughly. Before you buy one, understand that not all isolation transformers are intended to be used in a test bench application. Some will pass the mains earth to the secondary coil by bonding the neutral lead to the primary earthing. This type is not suitable, and will require modification before use. Also, a cheap variac autotransformer alone will not work. It will not provide Galvanic isolation. Therefore, it still will require the use of a properly mains earth isolated isolation transformer.

[@amuthanshan]

You are right, you need to find an isolation transformer that has CTE (Centre Tap Earth). A centre tap is a connection made from the centre of the secondary winding of a transformer. It is labeled as Earth, Common or 0V. The other type of isolation transformer is grounding the secondary winding of a transformer by connecting the earth to the neutral lead, which you don't want. What you want is the earth is connected to the centre tap of the transformer instead of the neutral. This is known as Centre Tap Earth (CTE). CTE systems are very common in electrical distribution systems and site safety operations.

[@BjornV78]

A few sidenotes. 6:25 Those probes or "external box" you mentioned, are called differential probes, but differential probes are NOT isolated (There are fully isolated probes, but those are even more expensive then a differential probe). With a differential probe there is still a high resistance between both input + and - leads , and between each lead and earth, but no isolation like a real isolation transformer. 8:25 By disconnecting the earth to the scope has also 2 major side effects on scopes wich contain a EMI filter on the mains input inside the chassis. If you don't connect a earth to this EMI filter, then 50% from your main voltage (with a very low current) will leak through the safety caps from the phase (hot wire) to the chassis (and negative part of your BNC connectors). Even when there isn't any probe connected to the scope. In the UK with 110a120V AC, there will be 55a60V AC on the chassis and BNC connectors. In countries with 240V AC, it's 120V AC what is present on the chassis and negative part of the BNC connectors. When earth is connected to this EMI filter, this low current leakage is not a problem, and will be leaked to 0V. A second side effect by disconnecting the earth to a scope with EMI filter inside, is that the produced measurements on the screen are faulthy, because there is no 0V reference point, this 0V point is now 50% of the mains voltage.

[@hoggif]

Back when I made an electronic work bench wiring it was required that any open device you test/adjust must not be referenced to earth. And touching any single point in open device should be made safe. For measuring I used an (mains isolated) transformer to power AC load. However, when you connect a scope with earth referenced led connected to earthed case, device under test becomes mains referenced. So, scope and other measurement devices went behind a separate isolation transformer. At that point you can safely touch any single point of a powered circuit with scope connected (as required). That setup worked very well but it was bulky and heavy. I had 230V/16A rating for device and a variac added for testing at different voltages. That pack needed wheels under it to be able to move it by one person. Differential probes were out of hobbyist reach at that time for any reasonable bandwidth. I think back then it was recommended to make tables etc from non-conducting material. I think isolation requirement for professional setups was 10kVac, much more than 4kV isolation protecting the normal product user. I actually tested my isolation for 10 or 12kVac.

[@spiderjuice9874]

Saw your other video, was wondering myself about the details, so thanks for making this video!

[@Robonthemoor]

Find this man ace, wish I had thought of been an Electrition,, love your work immensely 😀

[@tarunarya1780]

Love your videos with clear reasoning and good solutions.

[@sumilidero]

I always used scope probes with removed ground clips (it just unhooks off the probe) thus leaving scope ground as mains earth, and put ground-clip-less probes to 3 phases . Never had problems with too high voltage or transients (scope and probe impedances are high enough) altough connected mostly on the input of AC/DC and AC/AC 4Q converters. Of course scopes were digital not the bulb ones. For single phase small power devices, always isolation transformers. Also, never tripped RCD refering probes to earth as their impedance is high enough and the current is very little. Unlucky that all newbie technicians have no idea about the problem you perfectly explained, they just wanna stow the probes to the outlet like normal multimeter and watch a fun show on the screen. In schools they always probe some low voltage isolated things and reading only 400V input they think is enough to not to worry about anything. Great vid!

[@Basement-Science]

yep, if you´re probing low frequency stuff and want to know the voltage to N/PE then that´s perfectly fine. There´s still some risk of voltage spikes of course, but that could happen in many DC circuits just the same as soon as there´s an inductor somewhere.

[@ManchesterBlackSheep]

Fabulous video; beautifully, clearly explained. thank you very much. The "fairly fatal" amused me! :D

[@billinski42]

As always, thorough simple arguments and solutions.

[@ginbot86]

I built a circuit that used high value resistors for a voltage divider, then an isolation amplifier to keep the high voltage away from the oscilloscope. I find it especially useful for measuring non-sinusoidal waveforms like modified sine wave inverters.

[@julianhicks1666]

Hi John, Just watched this video. You can use the two channels of the scope and then there is a setting to display the difference between the two channels. Then you don't go anywhere near the external connection. No need to connect to the outside connection of the BNC connector. Just plug one channel to one side of the supply and the other channel to the other side.

[@UpcycleElectronics]

My favorite 'other' option I've seen used is to simply add a loop or two of loose insulated wire to any magnetics already present in the circuit. While the turns ratios and actual voltage levels of the circuit will still be unknown as far as the scope readings, this does create a small scaled, isolated voltage that will allow you to see the AC waveform.

[@blanchbacker]

That is very interesting I never would’ve thought to do that

[@DIMTips]

Thank you for the great explaination. Some great points. 👍

[@peckelhaze6934]

There are other videos on KZbin regarding this topic but this is the best I have seen. EEVblog is good too but not quite as clear in my opinion. I have seen people suggest disconnecting the earth which as JW has said is ridiculously dangerous.

[@____________________________.x]

I will do anything to avoid having to listen to that guy's annoying squeaky voice, Mr Ward is infinitely preferable.

[@jaycee1980]

Disconnecting the earth is also assuming that the earth and neutral are not readily commoned, which in PME/TNCS/MEN mains system they are. Floating your scope ground is NEVER safe to do. Either float the circuit via an isolating transformer, or float the scope with an isolator transformer - its safest to float the circuit under test

[@jaycee1980]

I should clarify on floating the 'scope - some 'scopes are not mains earth referenced but should still not be used to probe live appliances that are. For example Philips did a range of dual battery/mains 'scopes which when mains powered used a low voltage transformer, effectively making the 'scope isolated from the mains supply. You still need to be careful because there is a risk that wrong connection can make the chassis of the 'scope live. It is safest to power the device under test from an isolating transformer unless you are VERY sure of what you are doing

[@macman6421]

Unfortunately some of the information given in this video is not correct or misleading. For instance with his use of resistors he is not accounting for the input resistance of the scope so he doesn't need the last resistor. Also when he talks about isolated probes I think he is referring to differential probes that do not normally provide isolation but have 2 input connections that have a high resistance to ground. Using a differential probe is the normally the best method of making measurements on mains operated equipment. Isolated probes are available but they are extremely expensive and not necessary in most cases.

[@esuohdica]

@@____________________________.x I was just about to type the same thing! I can only take Dave Jones is small doses!!

[@pdrg]

Thank you JW first time I've understood the whole issue

[@NivagSwerdna]

Very good. I did not know about this at all! And now... I need to go shopping for an isolation transformer!

[@muzikman2008]

Get a big one 😉

[@jonpotato3366]

Very accurately explained! There are so many ways you can destroy an osciloscope or the equipment you are probing. This video should be watched by anyone thinking about buying and using an oscilloscope. It should be watched BEFORE using it. Unfortunately for my cheap China USB scope, I watched your video afterwards.

[@metalsnapper6728]

Hi John. Love watching your videos and you indeed one very clever bloke! 👍🏻. I’m looking for a handheld oscilloscope to be able to see the waveforms for 230v AC and also DC voltages up to 48V. This would be for testing the outputs on portable generators & DC to AC inverters. These will be installed on boats as some of these generators and inverters don’t produce and very nice sine-wave and cause issues for the boat owner. It would be really nice to be able connect a scope up to these and see if they are actually producing a nice waveform so I can 100% prove that it is their inverter or generator. Some of these inverters do actually say they are modified sine wave but some don’t. I’d like to know which handheld meter can do this safely and would I still require a voltage divider. Many thanks. 👍🏻

[@Agent24Electronics]

Excellent video!

[@drteeth7054]

Brill vid as always. Thanks.

[@regmigrant]

thanks John, would you consider an addendum covering USB based scopes connected to a laptop? - if its charging the usb ground pin can be connected to mains earth

[@MauroSedrani]

Great video! Thanks!

[@walker55able]

Thank you helpful advice

[@krnlg]

I understand the reason for the earth on the isolation transformer - or thought I did - but if fault current could flow from say the Line of the output of the isolating transformer through the earth in a fault situation, surely the same could happen through the oscilloscope's earth. So I presume this can't happen - in which case, I don't understand the isolation transformer's earth. Clearly I'm missing something here but I don't know what!

[@paulmorrey733]

Thanks John

[@stephanefernique9140]

thank's for your excellent video . I use a DMM for control the main DC voltage never use a scope ! But buy a differential probe is too much expansive better wishes from France

[@spelunkerd]

Well explained!

[@michaelcostello6991]

Just connected my Rigol DS1054Z to mains outlet and no problem. I did not bother connecting the probe ground/earth clip. Wave looked clean with perhaps 650 volts peak to peak that give 240 V rms 50 Hz her in Sydney. Probes that came with it are Class 2 rated for 300VAC when on 10X which i assume is rms value as the 650V peak to peak displayed on the screen was fine. I assume as the mains voltage is referenced to earth ground this explained why i did not need to apply the probe earth clip to the mains neutral. The question is do you ever need to apply the probe earth clip when measuring main voltage that is earth referenced. Please clarify. The next question is if i measure mains on an appliance with motor capacitors then i may have a problem as the capacitors could increase the voltage such as on a fridge etc ?? Any advice appreciated. Michael

[@ziadfawzi]

Thank you so much.

[@markyoung5608]

Did the voltage divider have an interaction with the scope input impedance and affect the reading? My cheap scope has a 1M input impedance, which would match the 10th resistor making the final resistance 0.5M. Appreciate that the original setup was more about the wave forms rather than the value though.

[@jwflame]

Probably, although actual voltage wasn't important for that demonstration.

[@johnarmstrong3782]

Does current flow though the probes to the scope if there is no earth connection? I have a 10x probe and a battery powered hand held scope but have never dared try mains with it. Thanks in anticipation for any reply.

[@p23q]

@John Ward, in the diagram at 17:00, would it not be better to add another 1M resistor between Neutral and the BNC Ground? That should not have any significant result on the measured voltage (besides a higher input impedance), but would secure against N-to-PE loops with other devices connected to the same line as the DUT, and also in the case that the DUT is connected via a isolation transformer, would it not?

[@peto22]

As you have a oscilloscope, could you show us the current wave form for those led lamps you tested earlier? Or even better if your oscilloscope can draw multiple waves simultaneously, compare voltage and current wave forms

[@bootsowen]

Hi John, will a Variac, specifically the Zenith Variac that you have previously restored (because I have one of those), work as an isolating transformer similar to the blue one that you have demonstrated in this video? Of if not, is it possible to rewire a "yellow 110v site" transformer to fit this purpose? Many thanks for the excellent videos, Owen

[@jwflame]

No, variacs are autotransformers with a single winding and do NOT isolate anything. A site transformer might be possible to reconfigure, although the voltage output will still be 110V rather than the 240V input. Also note that many of them have the actual transformer potted in resin which makes any changes internally next to impossible.

[@bootsowen]

@@jwflame Thanks John, all the best

[@rarbiart]

i like the luxury of having the DUT and the oscilloscope on an isolation transformer. So i do not have to worry where put the scope's ground, even if the DUT is connected e.g. to the house antenna system (which would be otherwise on the same ground reference as the scope throuth it's N)

[@ethelbertfogg2575]

What is the ordering information for the isolation transformer?

[@tarunarya1780]

@jw - sorry if i have missed something, but the 10:1 divider chain is only good if you know what you are doing, as if you attach the neutral end of the resistor chain to live by mistake you still end up with 240v live to earth if no isolator, rather than 24v, would it be better to have resistors on both sides of the taps to the leads for the oscilloscope..

[@johncrunk8038]

One more alternative: 2 channel scope and use a differential setting. Not ideal, but sometimes can be used since no ground (earth) is used. Noise can be a problem.

[@rickhunt3183]

Use the fluke to measure voltage and use a 6 volt step-down transformer to lower the voltage and view that waveform.

[@reddragon27284]

What function does the earth being passed through on the isolation transformer serve? If the output is isolated but one side accidentally contacts earth in the item connected to the transformer doesn’t that make it redundant? I have the same/similar transformer and always wondered that. It hasn’t been a problem for me as most of the kit I work on (audio/video gear) doesn’t require an earth.

[@Basement-Science]

If any terminal on the transformer´s output is connected to earth (intentional or not), then the other terminal always becomes dangerous. Actually it potentially becomes more dangerous than the mains itself since the RCD cannnot protect you in that case. If you connect the PE of the output socket, but do not connect either output of the transformer to the PE, then in case there is no earth fault in the device under test (DUT), both terminals of the transformer are still safe. In case there is an earth fault, now ONE (or in rare cases both) of the terminals is dangerous. However the Case of the DUT is still safe. In my own isolation transformer unit, I did not connect the PE however. To me it is more important to know for sure that there is no way either terminal can become earth-referenced. Basically it´s a tradeoff.

[@michaelcostello6991]

Interesting how the isolation transformer lets the supply voltage/current from the supply pass but not the current from the probe earth via the scope. I would like this explained a bit more. ie the theory behind it. Thanks for great video.

[@Shaggy_Campbell]

All I did was connect my 10/1 probe to the power and set the scope and measured it , set it on ac and hz and 10/1 and away we go ;)

[@abooli2]

Nic vid as always Can you suggest a beginners oscilloscope for those interested to get into electronics

[@MalagasOnFire]

Very good lesson om isolation and protection . The resistor array ressemble an HV probe , what about using a small 230 to 12V AC transformer if resistors not handy ?

[@Agent24Electronics]

The transformer is not likely to have a linear frequency response and will affect the measurements.

[@sdgelectronics]

@@Agent24Electronics in the dimmer example, it'd also kill the dimmer switch from the inductive spike when you chop the waveform

[@diydsolar]

Hi, thanks for the video, could you use your solution about 10x and then a 10x probe??? FInally you will get a 100x from the original signal, isnt it?

[@EyesOnReality]

Beautifully Explained Mr. West! Two Questions: If the scope was isolated with a DC to AC inverter, would it have to be a ‘Pure Sinewave’ inverter or could a cheaper inverter be used and still get accurate measuring results? Also, it seems the Voltage-Divider resistors would or could be 1/4 watt since the scope's input is of low loading high impedance, is this correct?

[@jwflame]

Electronic equipment should normally be supplied from a pure sinewave type - cheaper non-sinewave inverters may work but that depends on the equipment. Some might malfunction, or even be damaged. 1/4 watt is more than plenty - with 10Mohms across 240V, the current is only 24 microamps.

[@andypdq]

When I was doing A level Physics in 1980/81, the scopes were labelled as not earthed and that was deemed to be safe for teenagers to operate .I grew up on a farm and we had chainsaws without chain brakes, considered to be an unacceptable safety risk now but was perfectly acceptable back in the day. No doubt in fifty years time, our safe practices today, will be deemed recklessly unsafe. It's all a matter of considered relative risk.

[@davidbayliss3789]

I agree. :) I assume some considerations are made based on data analysis and availability of mitigating solutions. If there are x-number of fatalities or non-fatal but bothersome injuries from certain practices and the mitigation to statistically reduce that is affordable, then that's when standards should change I think. As humans - on an individual and empirical level we struggle to appreciate the wider-statistics. On the other hand of course (I think), and as we sometimes assume, or as so I interpret others, there can be over-anxious liability avoidance impinging on pragmatic acceptance of risk. I presume what we mean by balance is something like this ... it would be silly to die of heart failure because of lack of exercise when the lack of exercise is caused by a fear of the real risk of being run over or attacked and so on by venturing outside, and when the cost of mitigation - large indoor space with exercise equipment is not accessible to most, or armed guards for outside ventures or heavily policed traffic and so on, is out of reach of most individuals. Within the status quo in which we find our selves we balance these risks. We don't just give-up on life because it's too hard. (Usually. Of course sadly, some do, when their personal circumstances overwhelm them). I think in 50-years time it might be right for some practices considered safe today, to no longer be acceptable when "safer" practices are accessible and available.

[@dexterroy]

How about this- Use an insulating tape to cover the ground of the scope power supply and then use a step down transformer to reduce the voltage from 240V to say something around 12V AC. Then you can use X1 probe.

[@muzikman2008]

Great video again JW very well explained. I must admit, I have used a scope to measure mains voltage using the 10X probe (oops naughty step) , but your idea of 10 1M resistors is a much better idea. I really should get an isolation transformer toofor my work bench. Cheers JW.

[@sdgelectronics]

So long as you understand the limitations, you could rely on the mains earth as the return path and just probe with the signal line of the probe tip. It's the probe ground lead that causes all the issues

[@sparkyprojects]

Using a battery powered scope is just as dangerous as disconnecting the earth, the scope can become live in the same way Always isolate the device under test If the device is mains powered, but the internal circuit is low voltage, consider powering the device under test with batteries or a psu

[@uK8cvPAq]

I think the "legit" solution is to use one of those handheld style scopes which look like multi-meters.

[@sdgelectronics]

@@uK8cvPAq just use a differential probe, they aren't expensive. The Micsig I did some videos on works well if you're on a budget

[@PhilipKloppers]

Would it be safe to run the 'scope off a UPS/inverter that is NOT connected to mains, and then the test item on mains supply? Then the scope is running off battery and not earthed?

[@jwflame]

Probably not, would depend on how the inverter is configured. The output might be totally isolated with the earth terminal not connected, or it could be referenced to the output supply in various ways.

[@mikeZL3XD7029]

Hi JW, what sort of wattage resistors would you use with respect to a voltage divider like that? Maybe 1/2 or 1 Watt resistors?

[@jwflame]

Pretty much any power rating will do - the current is so low as to be almost irrelevant. Even 500V across 1Mohm is only 0.25 watts, lower voltages across 10M+ is insignificant.

[@mikeZL3XD7029]

@@jwflame Thank you JW, that makes perfect sense. Hope you had a good Christmas.

[@johncook5872]

I would personally use a 240:12 isolating transformer instead of a resistor divider (after some calibration), with the other measures in place.

[@simontay4851]

But a transformer will only work with AC signals and only a limited frequency range.

[@sparkyprojects]

An isolating transformer on the probes will not work with DC.

[@johncook5872]

@@simontay4851 I have used this technique interfacing between an 11Kv motor (gas circulator in an AGR - Heysham I and Hartlepool) to a two phase power meter design utilizing an AD633 to measure rotor bounce at 44.5 Hz and correlating this to vibration limits.

[@johncook5872]

By the way I also manged to synchronize the short break supply to the no break supply by connecting the Blue phase of one to the Red phase of the other - it was along day and the lights in the basement finally got into phase.

[@proyectosledar]

Hi John. Keysight gave me an oscilloscope for my youtube channel. I am shure they can give one to you too. Let me know

[@jwflame]

I rarely use an oscilloscope, so it would just be a shelf ornament.

[@dangoldbach6570]

Would it be useful to install a dimbulb in the oscope ground to indicate a fault? I’m envisioning a scenario where you run the scope ground through a 15 watt pilot lamp (or smaller, Christmas light maybe) to limit the parallel ground current if present AND give you a visual warning the scope chassis is hot? Say if you had a brain fart and didn’t use an isolation transformer and connected your probe ground to neutral on the DUT, you would immediately see the bulb illuminated if there was a fault and it would limit the fault current through the scope… but still bleed off charge safely once you disconnect the probe…and limit arcs… they sometimes put a monster 2 ohm resistor on grounds in big switchgear to greatly reduce fault currents in buildings, it still allows breakers to pop but limits dead short current so you get a pop but not a boom in those applications… thoughts anyone?

[@ricobass0253]

"fairly fatal" I love it. LOL Fatal is fatal in my book!

[@Aint1S]

What about a secondary ground to the oscilloscope that's only wired to the scope itself. The oscilloscope doesn't use the ground supplied from the mains and uses it's own ground for reference. It's how I usually run all of my electronics. I'm about to do this myself and I can't see a problem with it. Of course, I'll be stepping down the active reading from the 120v through a 30v/30w appliance transformer to lower the loading and the potential of the measurement... Probably from the inside of a stereo whole it's playing. We've got some unusual power and I need to see if it's a brown out scenario or our service transformer. It's bad. So please punch a hope in my idea!

[@willandrews3924]

Does your Isolating Transformer have a tapped Earth at Neutral or Centre, or do L&N lines float? He asks, as if they were floating, attaching the Probe GND to Isolated Neutral would effectively tap the Earth at Neutral, doesn’t touching Live becomes deadly? Hence spoil the isolation a bit.. Also couldn’t you just rely on the Scopes Internal Earth, to act as reference, and only Probe if the POS lead?

[@Mark1024MAK]

Most isolating transformers don’t have any secondary tappings connected to earth or to mains neutral. The whole point is that the output (both lines) are isolated / insulated from the mains (which is referenced to earth). Hence the output is floating with respect to earth.

[@Basement-Science]

@@Mark1024MAK Actually not all isolation transformers are built that way. For example for medical equipment, there are isolation transformer units where the output _is_ connected to earth because its purpose is just to act as a sort of mains filter. So if you´re buying one used, always check how it is set up.

[@Shaggy_Campbell]

If you're using 2 probes connect them to the grounds to the same point of reference ;)

[@Jnglfvr]

Don't see why disconnecting oscilloscope earth ground at 13:15 and probing either wire from secondary with ground clip would place 230 V on chassis of scope. If you were to touch scope it would be equivalent to touching a single wire on the secondary. No shock. That 230 V is not referenced to ground (where you are ) but to the other wire on the secondary. Now touching both wires on the secondary would shock you by completing the circuit. Let me add that the real reason to not bypass the oscilloscope ground, is not as you mentioned, but because should there be a ground fault within the scope itself that line voltage IS referenced to ground via the neutral wire connection 1) at the distribution panel (TN-CS) and/or 2) at the step down transformer (TN-S) from the utility company.

[@colibrimecatronic9922]

Is it possible to use a small petrol generator instead of a isolation transformer?

[@jwflame]

Yes, although the output from a generator may not be suitable for powering electronic equipment.

[@sdgelectronics]

Fairly fatal 😂

[@changeagent228]

I hear frequently mains power is increasingly poorly regulated in uk. Can you confirm this?

[@uK8cvPAq]

How often do these spikes appear on the mains?

[@Basement-Science]

These spikes are coming from somewhere, so it depends what is connected to the mains, and what the devices are doing.

[@t_Gecko]

What if i have a really old oscilloscope where the connector is not connected to earth at all? Should be all fine to measure mains, right?

[@t_Gecko]

Nvm, watched the rest of a video, where it is explained what happens if you disconnect the earth connection which is basically the same

[@t_Gecko]

i.imgur.com/nPRav37.png This is the power supply circuit of my scope, there is no connection between earth and secondary ground side

[@bobroberts374]

Ignore videos like these,. Just connect the single red oscilloscope lead to mains power, set lead to times 10 and multiply that value. The black lead is not needed to get a reading. I do it all the time, and no,. Your oscilloscope won't blow up,. As long as you don't connect the black lead to power you are fine.

[@ZENERVOLTAGE]

If I were to make this probe modification, what size (wattage) 1M resistors should I use?

[@jwflame]

1/4 watt will be fine - the current through them is almost nothing.

[@ZENERVOLTAGE]

@@jwflame That's brilliant of you to reply so fast John. Thanks very much and thanks for all the videos!

[@cookieboi4449]

I bought a Hantek scope and an I/T for my son. I don't know how to use it. Haven't used a scope for more than 30 years. The Hantek came with non earthing pin. So I decided to use a cable with local earth pin. I have 22 circuits in my house, all on mcb/rcd's. If I probe on a board 240 v via an I/T and I get a short on my probe, then my RCD would trip. The scope as well as the work are both connected to the mains via a RCD. How do you stop that for tripping? Why am I using an I/T when it only protects the scope and not my life? The RCD protects my life but will probing not trip the RCD? Cheers, and thanks.

[@jwflame]

The item under test is connected via an isolating transformer for 2 reasons . First, touching either of the conductors on the output of an isolation transformer won't result in an electric shock, that's only possible if in contact with both of the conductors simultaneously which is unlikely. The second reason is to avoid shorting out the mains supply by connecting the scope earth/ground lead to a part of the device which isn't at earth potential. Doing that is very likely to damage the scope, regardless of whether an RCD is installed or not. RCDs do not limit the fault current - they just limit the duration of it, primarily to reduce the effect of that current on the human body and avoid permanent damage, or in some cases to prevent fire.

[@cookieboi4449]

@@jwflame Many thanks Mr. Ward, I also bought an Aus made $250 I/T for my son at the same time,,, just couldn't understand why I should use it. Thanks for your explanation. So, are you saying the the scope will not survive 15mA for a duration of 10 ms? I have tested my MCB/RCD's with bare feet and metal hand held rod, and I am still writing this... however on you advice, my 14 year old future electrical engineer will be using the I/T with one hand in his pocket or sitting on it when probing 240 v. again, thanks... but please tell me that the scope will not survive 15 mA before the RCD trips.... I cannot afford a new scope by testing it myself,,, although I am tempted. Cheers, stay save from this wicked virus.

[@jwflame]

@@cookieboi4449 The RCD won't limit the current, 15mA is just what it trips at. If you short out between L&E, it's a very low resistance circuit, the current can easily be 100s of amps, which even for a short time will totally destroy components in the scope.

[@cookieboi4449]

@@jwflame Many Thanks Mr. Ward,,, my son will only be using his gadget using his new I/T, you have explained it well for me, and I will ensure that this young 14 year old future electrical engineer follows you advice and be grateful for it, Thanks and Cheers,,, and stay safe from this virus,, KZbin needs you for many many years to come.

[@Equiluxe1]

You can use two probes on a dual channel scope (preferably 100/1 probes ) and set the channels too differential mode/addative mode.That way you are not referenced to earth as you are not using the outer connection of the leads.

[@sdgelectronics]

Usually poor CMRR though

[@emteepeeess]

@@sdgelectronics I'd be interested in an explanation of why there'd be poor CMRR (at least if the two probes are properly balanced)

[@SureshKumar-nk2ok]

sir ,instead can i use step down transformer for measuring frequency

[@jwflame]

Yes, but only if the transformer is suitable for the frequency you are measuring.

[@jamestaylor1934]

I'm a manufacturing engineer. I don't let anyone anywhere near an oscilloscope that can't explain the difference between floating and mains reference. 'Elf 'n safety...

[@German_byte]

I’m confused now as the video clearly shows continuity between the earth pin on the socket on the isolation transformer and the earth pin on the isolation transformer’s power plug

[@tablatronix]

Instead of resistors, why not just a 24v or 12v transformer, for a 10:1 reduction, do transformers cause a skew in the frequency or something ?

[@jwflame]

A 240-24 transformer would only be suitable at very low frequencies, and even then the results would be distorted. High frequency signal transformers exist, but are expensive.

[@tablatronix]

John Ward thanks, I figured as much, thanks for the video, very informative!

[@RwP223]

How about a battery powered o-scope? Or use a power pack or generator

[@jwflame]

Battery scope is by far the best option, no transformer required.

[@Wilksey37]

Does it matter on the power rating of the 1MR resistors?

[@jwflame]

No, with 10M across 240V, the current is so tiny, that even the smallest resistors are still massively overrated for power.

[@sdgelectronics]

No, you only need to care about the voltage rating, but with 10x resistors, bog standard 1/4W resistors are fine

[@donkrapf]

Plug the Device Under Test into a UPS with the UPS disconnected from mains.

[@TheDigitalAura]

I use a high voltage differential probe for such measurements, both methods are perfectly valid however. The biggest threat to my scopes are kitties jumping all over them.

[@greywolf271]

Or spilling a glass of beer on them. Waste of good beer

[@jensonmarugan6000]

i don't understand why the option not good if isolation transformer for the oscilloscope .......... also you talking isolated probes isn't same as differential probes

[@ronbrideau8902]

ELECTRO LAB Oscilloscope Model 34, no ground to mains, manual says 1000 volts, caps say 1600v, 10M ohm resistors in its circuits . 😜

[@johnflower8648]

The earth connection on the socket on the isolating transformer not required as it is referenced to nothing.

[@Mark1024MAK]

So where does any interference get grounded to? Good quality equipment/ appliances have interference/ suppression capacitors that connect to the earth of the supply lead. Also the casing or sometimes the low voltage ground / 0V connects to earth of the supply lead.

[@allthegearnoidea6752]

Although the method of using an isolation transformer has its advantages I would definitely recommend the use of the differential probe and they cost as little a couple of hundred pounds now so they are actually relatively low cost. I have recently bought diff probes by picoscope and Micsig, both are very good. I would not recommend the use of battery scopes as most of these still have unshrouded BNC and require the correct shrouded probe. Interesting I have a scope (powerscope) made in Australia that is specifically designed for use on three phase power systems. Great Video John but I would just recommend you check out those differential probes. Your very unlikely to damage a scope input by high voltage transients however you should not use them on high power system for the same reason as low CAT DVMs. You may find this interesting but skip the first 16 minutes kzbin.info/www/bejne/rYiqeJV-o7ekpZI this is the second part kzbin.info/www/bejne/h3u7oa2Bnp6nb5o both these video are early ones for me and hence a bit crap but the powerscope is very cool and worth a look. Thanks Regards Chris

[@Basement-Science]

I´m sorry, but a couple hundred pounds is not "low cost" for most people. Plus you can easily get a big isolation transformer used for way cheaper. On top of that, it reduces the risks in case you come into contact with the DUT while it´s powered.

[@allthegearnoidea6752]

Matt Quinn I would suggest if you have a requirement to measure main voltage on a scope you have a perfect usage case. The use of an isolation transformer does not make you safe and takes some skill to appreciate the risk and use one safely I know I own several isolation transformer and use them on a daily basis. I wouldn’t have invested in a diff probe if not for the utility they offer. I’m slightly surprised that anyone would make comment on it being expensive. You know a good fire extinguisher is very expensive but not so costly when your on fire.

[@allthegearnoidea6752]

Matt Quinn I assume you lectured in art. LOL

[@banananaa]

If you got an RCD in the circuit it probably won't kill you.. It just hurts like hell. Still would suggest using diff probes

[@sdgelectronics]

Diff probes are the only safe way really

[@hannahgleeson1271]

what would happen if i used an 24v mains powered transformer, take everything out like caps and diodes etc, and just run the transformer on its own, would that also work?

[@lelandclayton5462]

You're thinking of a Stepdown Transformer. That would work but you would only have 16 to 24 volts output and a light bulb in the Video example uses full mains voltage. Now you can take two of the same Transformers and have a little full mains isolation Transformer by taking the set of Secondaries from the first Transformer (24 volts out) and connected it to the Secondaries of the second Transformer you'll get full mains output (plus or minus a couple of volts) on the primary of the Second Transformer.

[@dom1310df]

The type of person to remove the earth connection is probably also going to replace the fuse wire in their consumer unit with a nail "because it keeps blowing".

[@sikkepossu]

You don't need to connect probe's ground lead to anything. Just use the center lead to measure mains connected circuits and there is no danger or problem.

[@Mark1024MAK]

sikkepossu - I hope you enjoy all the induced mains frequency hum then... In practice, to get a nice stable signal, you do need to follow John’s advice.

[@GripElectric]

thats correct because that oscilloscope already have reference point from mains supply

[@maxerhard]

sikkepossu - I think you're right, he didn't really cover this. The danger seems to be the ground lead, rather than anything else. Also, the isolating transformer doesn't seem to provide any increase in safety as you've added the ground reference back again via the oscilloscope. I suppose you need to be aware of the fact the lead is ground, although if you're not aware of this you're unlikely to know or understand what an isolating transformer is...

[@bask2140]

Why is the earth of the isolating transformer necessary? If you disconnect that earth, is the security problem then not solved?

[@ElliottHurst]

Maybe because it's safer in the event of a fault with the appliance

[@sdgelectronics]

It prevents the chassis of any Class I equipment plugged into it rising to a dangerous potential with respect to mains earth and also, more importantly, you.

[@ElliottHurst]

@@sdgelectronics surely that's not a problem though, because the transformer isolates you so the potential it rises to would not be referenced to mains earth or yourself?

[@sdgelectronics]

@@ElliottHurst the issue would be related to equipment connected to the scope or in the vicinity. If the device was used standalone, the only reason would be for correct shielding for EMC purposes

[@rsattahip]

What's to test? If you plug in the oscilloscope and it comes on, there is nothing wrong with mains power. No further tests needed.

[@krnlg]

Lol :) Of course, the answer is that you may be probing a mains powered device somewhere on the high voltage side of its circuitry.

[@williamsquires3070]

You need one more 1 Megohm resistor; you’ve got a ratio of 9 Megohm : 1 Megohm as it stands right now. But don’t feel bad; you’re not the only one I’ve seen fall into that trap. 🤭

[@darkstar_448]

Nah mate, John's got it correct. The divider is Rtotal, meaning Vout = Vin x 1 / (9+1). Best way I like to think of it is with a potentiometer-he's one tenth of the scale along.

[@Brian_Of_Melbourne]

Sorry William, but YOU are the one who is wrong here. The voltage division ratio is equal to 1 MOhm / 10 MOhm i.e. the resistance of the part of the divider from which the output is derived divided by the total resistance of the entire divider. Edited (July 2019) to add: There is also an issue with the input impedance of the 'scope being in parallel with the 1 MOhm resistor. See my later post (way down) for a full explanation.

[@Brian_Of_Melbourne]

@@r.h.8754 Hi. You're right of course. And I did make exactly that point in a slightly later (and longer) reply post to a user called 'Elektronik mal einfach'. That part of the comments thread seems to be a long way down now. My post here was just intended to to correct the 9:1 vs 10:1 misunderstanding by Mr Squires.

[@Brian_Of_Melbourne]

@@r.h.8754 And sadly there's been no follow up video from John Ward to deal with the issues. He seems like the sort of chap who likes to get things right, especially 'on the record' so to speak. I've edited my earlier post in this thread now.

[@bluelightningnz]

Great video, thanks. Presumably an alternative to the "battery powered scope" option would be to connect your normally mains powered scope to a UPS, if those are two things you have handy of course (and the UPS's modified sine wave doesn't cause problems with the scope).

[@jwflame]

Many UPS outputs are not isolated either, so the UPS would need to be totally disconnected from any power input.

[@bluelightningnz]

@@jwflame Indeed, that's what I would have assumed.

[@kaybhee6]

where to get volt divider cable,,,, u dont mean x10 of probe huh

[@UberAlphaSirus]

If you where not fussed about measuring precise voltages and only wanted to view a waveform and timings. Could you not make a probe adapter, have a string of 11 or 21 1M resistors. the ends of which connect to the probe and the scope being across the central resistor. I blew a preamp out circuit on my audio amp the other day, as I was checking phase between the speaker outputs of that amp and slave amps. usb scope on a tablet. all floating from a battery pack. But stupidly forgot to put 1M resistors before ground clips, so stuck a ground clip on a phase of one amp which shoved that grounded thru the rca subout frying the opamps ground somewhere. Fixed it now though, but was I pissed. So glad old sony equipment stick fusible resistors everywhere.

[@woody412]

Just use a battery powered scope and save all the hard work

[@sdgelectronics]

This still requires extreme caution

[@Jaycsee66]

9:10 Disconnecting the earth is never a sensible option on anything, just not the oscilloscope.

[@ivok9846]

probably more than half electrical installations over the globe have no earth (ground) connection. now what? one more interesting thing: grounding was put into practice to prevent overvoltage killing the gear in power station, but it caused many human deaths. if the ground was floating, it would kill the equipment (discharge via transformer/generator windings), but it wouldn't expose humans to easy return path for electricity under their feet. simillar could be said for the circuit you're discussing above ( www.globalspec.com/ImageRepository/LearnMore/20144/FARR_LAMBDA01_OCT20083cd0450d7a5a494fa6f3f0f364c7e402.gif that's inside most pc psus and scopes, it's EMI/RFI filter), to prevent producing more noise on the mains, some people will be jolted by this 120V voltage, if they have connection to earth via their feet, some might be jolted at wrong moment, cause them to fall from ladder etc. (yeap., most of those homes not having earth won't have RCD either) overall and paradoxically, i would say grounding killed more people than it saved....

[@ivok9846]

@@mernok2001 lightning hitting nearby would probably yield simillar (unpredictable) scenarios in both (grounded and ungrounded house) cases, house wiring practices would mean the most in that case...ie are there any wires near humans. more dangerous is the metal plumbing, which, just like ungrounded mains at home, you still have all over the world. ungrounded electric substation would take most of the lightning, destroyed gear etc. back at home, having devices with plastic cases also helps, much more than grounding metal case, i would say. as for few kvs over the mains, it would mean el. equipment hooked to it would be damaged, so that couldn't continue anyway. also, there are cases with grounded design where you get 380v on monophase systems. that also causes damage to equipment and is usually fixed promptly. to conclude, humans rarely touch live wires or any current conducting devices, but when they do, having a grounded system (grounded at local hivoltage/mains transformer substation) will usually be the thing killing them. hand->leg current thru the heart. not lightning strikes at home or overvoltage on mains.

[@felixcat4346]

@@ivok9846 The example your using to describe the lack of grounding in at least half the globe is a bit misleading since most of these installations have the neutral grounded at the utility pole just outside the house.

[@ivok9846]

@@felixcat4346 no, they don't, you're probably describing your surroundings (in north america), but you would need to visit poorer countries to figure it out. still, i doubt these places have higher electrocution rates, i think americans are less cautious with their mains than just about anyone else...and yes, in most of these deaths current will flow thru the earth and someones heart...

[@mikewardUK]

BigClive is shooting mains across bananas today!

"You touch the case, you die..." No you dont. Are you teaching or trying to scare a 10 year old? 240v gives you a shock if you touch a lead by accident. Everyone does that mistake at some point and we are all here arent we? So wtf?...... Besides all where is the MEASUREMENT? Whopping 20 minutes of video and no actual measurement?? Again wtf......

[@jwflame]

240V can kill people. So can 120V. Many people can get a shock and survive. Others are not so lucky.

@@jwflame Just stop advertising power from a wall outlet as it is a death ray because it is totally bullshit. Yes it CAN kill you. It is possible. Won't turn you into dust in an instant.