I’m getting back into tube amp repair. Refreshing my knowledge that I’ve lost since the late 80’s/early 90’s. Thank you so much for your very informative video. You got a new sub! 🤟

Yeas Isolation Transformers are not so Isolated and some are shipped with an adapter which eliminates the earth ground connection. The USB Connection from your computer is indeed earthed and I am happy you pointed that.

This is the best video which covers most of scenarios including differential probe. Thank you! 👍

Great in depth vid. on isolation and safety. There is a work around to make the ground on the output of the transformer isolated and that is to use a 2 plug adapter (line, neutral) so that the ground is completely isolated from the primary. Hence, there is no connectivity between ground input and ground output.

Martin, very good and informative video. I use a variac/isolation transformer combination (actually from Sencore) and one trick I use is to only slowly ramp up the voltage to the minimum voltage needed to power up the circuit or test it. Many times you only need a few volts to get things to work enough to probe things. And I always do an ohms check across the input of the DUT to make sure there isn't a short or low resistance before applying power.

Excellent video Martin, right on the money also nice to see you publishing again

Wow, that answers all the questions in my mind! Thank you.

Truly appreciate the highly detailed explanation. Extremely helpful.

This was an invaluable lesson in measuring safety. Can not thank you more.

One of the better videos on this topic. Thanks!

Very important subject. Explained very well and with diagrams. Thank you.

Great video, clear explication, thanks a lot

Another very fine informative video Martin, now I question my equipment. *_Cheers_*

A isolated USB adapter is also a useful tool to have to help minimize the "what if" scenarios if you really must have that connectivity during testing. Thanks Martin!

Very helpful information. I like it

Excellent and informative video. Thank you.

Well presented video. I'm just building a variac/isolating transformer combination for my bench that I have been using for years as free-standing components, wired as required. The variac is an old Japanese brand, extremely well constructed and ranges from 20V to 260V (on our 240V system here in Australia). After cleaning up the windings where the wiper makes contact it is now silent, doesn't even hum. My iso transformer weighs about 7KG and is rated at 1KVA. So I'm building in a panel mounted voltmeter on the output of the variac and have included a primary switch, fuse and indicator lamp. I will add an earth lift switch on the secondary of the iso transformer. We used to have devices called ELCBs (Earth Leakage Circuit Breaker) that worked on voltage sensing. Now we use RCDs (Residual Current Devices) that obviously sense a trip current, usually 30 milliamps. You are so right about 'what if' situations. I have been an electronics hobbyist for more than 60 years and the only time I ever received an electric shock was when I was about six years old and thought it would be a good idea to remove a light globe from its socket and poke my finger inside. I think the globe had blown and I judged it prudent to put my finger in the socket to see if there was any power there. There was. That little episode taught me a respect for electricity that I have never forgotten.

Thank you for this video. It looks like an isolation transformer presents its own dangers if you don’t know what you’re doing.

Excellent explanation, thank you very much! Very helpful

Having watched a bunch of videos on this subject and having thought about it, I guess the real safe way to use an isolation transformer is probably: 1) don’t connect the earth pin of its output socket to ground of your household (ie floating), which makes it truly isolated; 2) use an RCD immediately after its output socket before the DUT (device under test); 3) never plug a (traditional) scope to an isolation transformer.

The demonstration of the GFCI was quite informative. I’ve been researching this topic quite intensely recently in preparation for building my own dim bulb tester. I think your recommendations are sound, because there doesn’t seem to be a way of making either the test equipment or the device under test completely safe. The best you can do is set up a situation in which you, as the technician, know where the danger lies, and what you must, and must not, do. My takeaways thus far are: (1) Don’t float the equipment. (You can get fired for that, even though that is what I learned in school.) (2) If you float the device under test (DUT), don’t touch ground. (3) The use of a Variac and/or dim bulb tester in conjunction with an isolation transformer is highly advantageous, as is the ‘one hand rule’, although the danger to the test equipment (without isolation) occurs before the ‘one hand’ goes to probing: it occurs when you clip the ground lead. As you point out, it really helps to know what you’re working on, and to proceed cautiously.

Best explanation so far thank you !

Thank you for the great video and for the comments below. I'm really earning a lot from your videos. I wouls feel safer using an isolated probe or a battery powered laptop or the battery-inverter system. At the moment I am using an old plugin transformer that steps down from 110Vac to 16.5Vac (probably served an old telephone system)

Thanks, very informative. Great job 👍👏

Hi and thanks for a great presentation. Can you advise re using an isolation transformer with built in gfci and and RCD protection on the secondary versus using an o pen (pen fault detection consumer unit for installation in a garage or outbuilding? In the uk even if the incoming mains is a TNS out in the street it could still be TNCS. Thanks a ton!

Great explanation. Thanks.

In Australia it is normal for the output socket on an isolation transformer to be NOT connected to earth, if you need an earth on what your working on, then that has to be provided separately. Anyway I would NOT use one to float a scope, or any other test gear. If you do, and connect the probes earth clip to any substantial voltage, then that will appear on the outer metalwork of your scope, as in the WHOLE chassis will be LIVE, very dangerous indeed. If you then go to connect a second lead to another input, you will be touching a live socket, and you will get a shock. A problem with floating equipment, is that the chassis will probably be sitting at anything other than earth, this is usually due to stray capacitance in the power transformer, and is usually exacerbated by mains filters that can easily put the chassis at half mains, whilst there is little to no current seen here, the static charge contained in a largish chassis, and the stray capacitance, can cause a current surge/voltage spike when you hook the earth clip, or an X1 probe doing damage to your gear. I once connected a floating video camera into a video input on a computer capture card, the spike destroyed a few hundred dollars of equipment - it does happen, floating gear is bad.

Martin, love your videos. I have a Micsig DP10013 differential probe. Would like to know if you could do a tutorial on using one. I'd like to test the output of my APC unit but don't want to blow up anything. I've watched one video where they put the probe into a power stip to show the sine wave. However the probe was powered by a wall wart, not the scopes usb port. TIA

A differential probe is the most elegant solution, but they often cost thousands of dollars while a good isolation transformer is only $100-200. I'd rather put those thousands towards other good equipment, and just be extra mindful when using the isolation transformer.

Great explanation. Thanks

Thanks Very Helpful Information

Great video, thanks!

Interesting & informative. Liked the whiteboard drawings - worth a thousand words. Thank you.

Very informative video dear

Can you use one of those adapters that is used like on an extension cord that just has 2 prongs, and connect isolation trany to the wall with this adapter in between? The adapter does not have a ground terminal on it going into the wall..

Very enlightening, thank you for the excellent video. So, it's okay to use this particular B&K isolation transformer with the caveat of disabling the ground lead coming into the transformer and putting the DUT onto the output of the transformer? Why do you think B&K added the ground?

@mjlorton .. Say that we are going to work on an old am receiver / radio from the early days - that doesn't have a transformer. AC line is connected directly into the radio circuitry. Most of the old ac cords only had two-prong plugs. So what if we remove the (green) ground wire from the BK that is present at the transformer's secondary? There isn't really any need for the bigger 3-prong (N__H__G) are they?

Hi Martin, love your channel, very informative. As an extra precaution i am thinking about adding a 12 volt li-ion pack to my r&s 1202 scope so it can run on battery power isolating it from mains and making it more usefull around the house and in my garage. Would there be any problem with that if i want to measure mains with the scope on battery power? I am interested in looking at high frequency noise (fft) as there are quite a lot solar panels in my surrounding area (i guess) interfering with short wave reception and i am suspecting some of it is coming over mains in my house. I want to measure on/through a 0-230 ac variable power supply i already have, it is basically a variac powering an isolation transformer, i need the variac as the input of the scope is max 200v p-p so it need to lower the 230v to 100 volt or so,, the isolation transformer is just an extra that is already there although i might disconnect it to check if it does not filter out the hf noise. I am aware that referenced to ground there still could be potentialy 1/2 ac mains on the chassis of the scope. I cannot think of anything that might hurt the scope or me but i am always for double checking... Another option i consider is differential measuring using 2 channels and the math function of the scope combined with 2 wideband high voltage probes, are there still any risks in that setup ?, also here i cannot see any problem. Maybe something to cover in a small follow up? Gr from Holland

Veri good video, thanks

Thanks very much!

A very good video.

The first part of the video, I understood, where the RCD tripped. My grid supply in Australia is 240 vac. I use a RCD $15. I can bare handed put a pin into the active on my outlet and the RCD trips at 15 mA. Tested all my 22 of them many times. I recently bought an isolation transformer for my 14 year old future electrical engineer. It is truly isolating, no reference to ground on the outlet,,, but he can still get a shock of 240 vac if he by mistake touches both N and A. Without the isolation transformer the RCD would trip.....You demonstrated that,,, I think you showed voltage and not Amps. My RCD's trip at 15 mA,,, what more do you need? Why a differential probe? why not and RCD which does the same thing,,, except the scope also temporarily dies until a reset of the RCD ???? I am curious. Thanks for your videos, subscribed, and watched you for years.

For the isolated transformer output GFCI (orange colored isolated ground type, ground isolated from the receptacles frame and mounting screws) for the device under test (DUT) would have a separate isolated ground return to the output of the isolation transformer at the neutral terminal, much like from the transformer at the power pole but without an earthing connection at the pole or at the power entrance panel, the ground and neutral are connected together at the service breaker panel. This way there is a dedicated separate safety ground which would carry dangerous chassis AC leakage directly back to the supply (the output of the isolation transformer), and having a separate path would create an imbalance in AC current which would trip the GFCI. For the DUT GFCI we want to protect the operator from dangerous AC chassis leakage and not to guard against someone (a qualified electronic technician) from purposely electrocuting themselves. Since we are using an isolation transformer the output is floating so essentially we can assign the output as we like. So, one terminal is hot (feed) and the other one is neutral (return) with a separate safety ground connected directly at the transformer at the neutral connection, which is obviously not earthed. Most of the commercial variable isolated AC power supplies for bench troubleshooting do not have the output receptacle ground isolated from earth or the chassis, this is due to having to have the equipment UL listed. Most duplex receptacles have the frame and screw attach connected to the receptacles ground screw connection, so you can get a ground return from the metal chassis or the ground screw on the receptacle, so many times by just attaching the receptacle to a metal chassis you have grounded it, obviously not very effectively or as per the NEC. What you will find is you have to manually change the design of the off the shelf AC supply to completely float the ground. There are several ways to do this, but you will need an isolated ground receptacle to float the ground when the receptacle is metal chassis mounted. To completely isolate a DUT from both mains and earth ground, which is really what you want to do to not have to float the scope or the bench, you would need to use an isolated ground receptacle (orange in color). With an isolated ground receptacle you have the option of where to establish the receptacle ground connection and it is internally isolated from the receptacle frame and mounting screw connections. To test a USB device input power supply many times you can cross connect the output sense to activate the internal power supply without a computer attached. If you want to test it using a PC application after repair the best solution would be a battery operated notebook computer running on only battery power. This way the computer is also isolated from mains and ground. Usually, however, the fault is in the power supply portion of the device, so you just need the device to power up. In most well funded large corporation repair shop depots you will see a custom isolated variable AC power supply for troubleshooting at each repair bench, with many very nice features, like output current limit, selectable output breakers, analog meters and both regular and GFCI isolated ground receptacles. So, now the bench test equipment is essentially protected. However, the procedure to do the basic troubleshooting is to use isolated battery powered test equipment (in addition to the isolated bench) like a Fluke 289 DMM (or similar) and Fluke Scopemeter (or similar, the Scopemeter is isolated from ground and the channels from one another). You would only use a sophisticated bench oscilloscope (or other expensive piece of test gear) if absolutely necessary. Once the unit is repaired, it will go to a different outgoing test bench which has a sophisticated AC isolated power supply (mains isolated only) of the type that actually generates the sinewave output (very low harmonic distortion of both voltage and current into a purely resistive load). The DUT would then be calibrated and tested, after burn-in it may have a limited test, and then if passed, returned to the customer.

If the DUT only has a two prong power cord (live and neutral) and is plugged into the isolation transformer, does this still apply?

You can also use a 230v inverter powered by a battery and hook the oscilloscope on it. I did this until I had a proper isolation transformer. Now I use the proper isolation transformer for the DUT with removed earth connection.

I have some scope probe ground leads modified with a 100mA fuse inline. Not going to protect you but saves putting 100s of amps through your scope PCB

Martin, I've seen teardowns of differential probes (such as on Dave Jones' EEVBlog), and they seem to just contain passive components. So why do they cost several hundreds of dollars? I wouldn't mind buying a couple of differential probes, but not if you have to pay hundreds of dollars for a couple of resistors...

I agrre to what you said .but there is small thing..even if that transformer isn't 100% isolated..it is isolated enough i should say. You could always do as you said just snip there wire..but then you should use some grci protection ..because even if there isn't problem with that as such..(i mean you will not die even if volate will be on ground of your equipment you are testing) you should not have that there...soo make small adjustment..add it there..and that light is very very good idea...also small switch and ofc suppression capacitor..that is a must .. Maybe it isn't problem with 100W isolation transformer..but they to connect 500W or 1kW transformer and you see that you blow fuses in no time...even if there isn't reason for it ..as you are not connecting to it ...

I own one. I added a power switch and a pilot light. I considered adding a ground lift but changed my mind. My issue with this transformer is that it bumps my 124vac house voltage to 134 vac. I need it to keep it the same or reduce it in most cases.

Great video!! I’m always afraid of the “what-it’s”. In your scenario with the USB device, would that risk just be mitigated if you plugged anything associated with the DUT into the iso transformer? If you just make sure that anything needing power that touches your DUT is connected to the iso transformer, you should be safe. Right?

I found an industrial isolation transformer. It's just a 1:1 transformer in a huge steel box. I can barely lift it. It states 2000VA but i would imagine it to be 20,000Watts judging by the size of it. Just wondering if i should bother installing it or not. If the earth is floating does it then not become even more dangerous since the RCD won;t trip? You could use 2x cheap separate battery powered oscilloscopes. I have a battery powered 2ch i will have to check to see if the shields between the two channels show any continuity.

I found an old isolation transformer, very heavy 115v .500va. I replaced the incoming cord because the insulation was cracking. The transformer looks unused. I checked for continuity and short to ground on both sides and it was good, it has a single receptacle. After plugging it in to check output voltage at 127v, but there was also 17v on the ground side to chassis ans 96v from hot side to chassis. I am no electrician, is this transformer trash ? Thanks.

I added a switch for power and a switch for ground and marked them. I can then decided how to use the iso transformer.

Thanks Martin! You don't have to open up the isolation transformer to check if it's really isolated. Just Ohm the ground pin of your input plug to the ground of the output plug socket.

Would a DC to AC power inverter work as an isolation transformer? If not explain why not. I have never seen a power inverter with an earth ground. Another video?

In minute 17:00 you say that it would trip the protection switch. Why Is that? The primary phase is not leaking into ground, is the secondary only

I have looked at your sketch again,,, and I see what we in Australia call a RCD is in the USA called a GFCI,,, and you have one such prior to the I/T if I understand it correct... but what I have is such a gadget GFCI on the circuit supplying the scope as well as the the board for testing. Can you please place such a GFCI on the circuit supplying all your power to the lab and then deliberately short something out, and then tell me that you are safe as well as the scope ??? I have tested my devices and I can with bare feet, bare hands, poke a metal rod into my power supply and the RDC (GFCI) trips within 30m/s and at 15 mA imbalance.

I like the safety of an isolation transformer (or a generator) and it has to have earth connection. But the first thing you have to learn when working with voltage above the safe limit is to keep one hand in a pocket (or at least away from the area). When doing that you are basically safe. Differential probes are nice for checking mains voltage, but they have a problem with checking a circuit on mains voltage, because they are not made to measure mV or a few volt.

Stupid question, but why can't you just add another transformer for the ground? If the GFI thing requires a connection, just use the bare minimum resistor (or fuse) and put it in a spot where it's easy to replace. Would that be feasible?

Connect either oscilloscope or device under test or both through an isolation transformer from the moment that the probes negative inputs are connected to the ground of the oscilloscope and to the electric earth grounding of the mains then every time that the scope is connected to the DUT one output of the isolation transformer will be grounded through alligator clip and connected to the neutral of the electricity.So every time the scope is connected to the circuit there are also 50% possibilities for electrocution even with the isolation transformer.If remove the earth from the connector cable of the scope there is also 50% danger for the user if touches any metallic of the scope for electrocution but there is no possibility for short circuit.IF isolation transformer will be used and scope earth connection is removed there is no danger neither for short circuit nor for electrocution to the user.Battery operated scopes if connected without isolation transformer there is no danger with the connected polarity for short circuit but there is 50% danger for electrocution if touched anything metallic of the scope like another BNC input.

Good video. I never float my 'scope, only DUT.

Thanks. Great video. I think you are being silly with the light and switch for the isolation transformer. Its essentially a wall outlet. You just always treat it like its powered unless you confirm that it isn't. Most wall outlets and extension cords and some outlet strips don't have any indication they are on. You have to treat it like a gun and always treat it like its loaded.

I've been looking at some medical isolation transformers, which I believe do give full isolation, but I'll definitely not be taking anything for granted. I think I want a differential probe, too, which is probably a better bet as far as scoping goes.

Great explanations! I might add to your 2B scinerio (use iso trans on scope and disconnect the earth). When the ground lead of the scope is connected to a hot, the chassis of the scope becomes hot, and if you were to touch the scope to make an adjustment, or just touch it, you could get a shock if you were also grounded. I have done both isolation methods due to the small size of my iso trans and the large load of the DUT, but I make SURE that I am NOT grounded when I have to touch my scope. I also use a portable scope which eliminates all the ground loop problems.

Watch out for those battery operated charging transformers when using Chinese equip. I'll bet you can't find and isolation rating even after disassembly / reverse engineering of the charger. I wonder what the input / output capacitance is... That only really matters if the ground bonding remains. My unit's OP is not connected to IP ground.

At 18:00, am I wrong or current wouldn't want to go back to earth so won't go back to scope?

An even simpler method but costs 2 channels of your scope is to disconnect the earth clamp from both probes and set up a math function on the scope to take ch2 - ch1 and use it as a differential probe setup. That functionality is already built in and only costs an extra channel

I have two isolation transformers. Neither passes the earth through. Is it ok to use one on DUT and the other on oscilloscope? Is that configuration safe and does it make any sense?

What if you connect the secondary neutral to ground via a resistor so that a GFCI on the secondary side will still work?

Old but good.

Isolation transformer seems a meme from the pre-gfci past, which amazingly persists.

For a true isolation transformer, you must also isolate the (thru) center ground connection.

Why not power your oscilloscope with a power inverter? - makes perfect sense to me, but out of all the videos I have watched on this subject - not one person has even mentioned it, but a power inverter would give you the isolated power source you need for the oscilloscope wouldn't it?.

Use your laptop instead of desktop PC. The laptop power supply might be grounded so just use battery power.

I have thought about putting a diode in the ground circuit. My theory was that this allows current to only flow in one direction. Although, maybe this doesn't make any sense. Also, your Amazon store links never seem to work for me. Maybe your Amazon store is 'isolated' from your KZfaq account.

18:25 ISOLATED --- Not ISOLLATED from the Latin SOLO meaning Single / One / by itself / sola(r) One Sun

I think it remarkable that folk with the faculty to test and play with such equipment readily fail to see the primary concern of not electrocuting themselves. All the talk of removing the protective conductor, floating mains chassis and general nonsense misses the point. You are dealing with a high energy supply. It is enough to kill you, destroy any dut and or equipment and at the least be the cause of a need for new underwear. Are the same considerations given for battery powered devices. High current supplies. Increased voltage D.C... probably not. Certainly not. But the issues are still the same. Short it out and you are toast. Isolation only provides one extra level for mistakes and then the problems are back again. One probe clip one circuit.. 2nd probe clip.. bang. Touch tracks of different potentials and you won't be pointing out what went wrong. Earth. That's what it is, as was demonstrated here. You don't need to hard wire it for you to become a conductor. So you are better using the installed earth from the mains and ensuring it is in good order. Have your sockets tested by competent persons. Then maintain protective earth where it is intended to be. And if you think you are in an environment where are are not earthed you should consider that pvc electrical tape is NOT an insulator. Vynil flooring is not an insulator. A closed pin hole in classified insulating gloves is enough to prove fatal. The only thing an isolation transformer can offer is electric potential separation for the test equipment until the first connection is made. Stay safe.

Are you south african?

Only care that I don't blow my equipment, not for safety at all. Safety is "Me", based.

How can an isolation transformer have an earth/ground wired through to the output side? That's definitionally idiotic.

Just change SMPS in scope to transformer, problem solved. Remove grounding from scope, it will be like those cheap battery operated ones, that you plug any way you like, or like your DMM, plug any a way you like, NO BANG. Isolation transformers are expensive, so isolating cheaply is like powering from batteries just change SMPS in scope to transformer, problem solved. Recommending to buy battery scope is great if you get free cash flow, like you do.

diagrams with explanation are confusing!

Martin, move to bitchute.com ... KZfaq is pox!

What planet are you living on ? The average hobbyist and even some techs can't afford your recommendations. You get the stuff for nothing .

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Great vídeo, thanks!!!