Very good video. Im glad you told everyone at the end you can still get shocked. Someone new to electricity might think if they have an isolation transformer they are ok to do what they want to and wont get shocked. I like the one hand rule, where you only use one hand to probe equipment hooked to isolation transformer and keep other hand in your pocket, so you dont complete circuit and get shocked.
Too true. You don't even need to touch anything to lose your life to high voltage electricity. My father is a principal engineer for Honeywell Aerospace and an employee at one of his facilities was cleaning inside one of the test cells with basically a leaf blower. They were told to never use that kind of equipment because it can generate static electricity and their power supplies put out a TERRIFYING amount of power. Well. That was Christmas Eve. And while this guy never actually touched anything, he never saw Christmas. We were having a Christmas brunch when he got the call. He was crushed. Keep in mind, this facility is in another country, he got there that night to figure out what all exactly went down. That guy had a young family. It's just awful. Electricity is no joke. Never cut corners
I cannot be responsible for what others may say on the subject, but I can be clear about ONE thing this video lacks... any mention of the "isolation transformer which isn't!" An "isolation transformer" MAY come with a THREE WIRE CORD and have a THREE WIRE OUTPUT SOCKET. This allows the maker to loop the GROUND WIRE (so named because it is usually Green and attaches to a safety ground such as a ground rod outside the home) to the output socket. THIS IS POSSIBLY THE MOST DANGEROUS THING POSSIBLE! THE GROUND WIRE is usually attached to the electrical neutral wire (part of the power grid) at the entrance box. It makes the "isolation transformer" part of the electrical system you were TRYING to avoid! Older isolation transformers had a two wire outlet... most new ones have a THREE WIRE output plug WITH GROUND WIRE. If your device under test (DUT) has a THREE WIRE PLUG... Ouch! You can be back to having NO ISOLATION from the power system. This depends on how the device itself USES the "ground wire." This MAY appear to be "more information than is needed," but that is life. It is why "common sense is the most uncommon sense of all."
I've been "trained" in mains electric wiring while working in a telecom switch room. I had to test the building back-up batteries, wire up the BDFB bay, maintain the building rectifiers which was a very horrifying room to walk through, and I would need to test the back-up generator which was the same engine as a train/locomotive basically. I used to also have to work on live AC wall outlets. All the training I had was complete and utter BS, and none of our tools were insulated. I brought up my safety concerns with the higher levels of the company and they put an end to switch techs performing the high power electrical work.--(because qualification matters and training by a senior co-worker standing by with a broom stick to knock you off the 1" thick copper bus bars so that your family might be able to recognize you in an open casket funeral apparently wasn't acceptable for a large corporation.) They also equipped our tool chest with all insulated tools all within a week or two. They had been running things like this for years until I opened my mouth. Needless to say, the managers were pissed at me and retaliated which is why I don't work there anymore.
@@cat-lw6kq This was Voicestream (now T-Mobile) shortly after they acquired Aerial Communications. It was the legacy Aerial Communications management that was the problem. I still work for T-Mobile and they are extremely strict on safety standards. We're not allowed to climb ladders or even pick up boxes as office workers.
@@cat-lw6kq Also, the live AC wall outlets were with another company called IDL where I was a prototype engineer. I had no training of any kind to work on power. They just yelled at you to not touch both "negative and positive"--which is not how AC works as we all know. LOL.
AT&T was very strict about safety, especially in const. dept. I used to help maintain batteries, rectifiers and generators. Even a 24v plant is dangerous because of the amount of current those large lead acid batteries can produce.
Thank you for making this video! I’m in the beginning stages of this hobby-work, and I’ve been searching/researching the safest way to use these tools. Very informative video, honest, and eye-opening! Thanks
I was not trying to influence purchase decisions (variable transformers have many uses) just to point out that not all variable transformers include isolation.
The tenma unit shown still has mains and chassis ground with the secondary side neutral also tied to chassis ground. In other words, it's not isolated without some modifications.
Thanks for this video. It's very important. A lot of terrifying videos show "how easy" can you do something with just a couple of passive parts for e.g."capacitive dropper" - which could kill you at the end. There are a lot of videos including MOTs that shows how electricity is entertaining - could be someone a last show.
And I was going to purchase a Variac thinking that I needed it to properly protect myself... I now understand that while a Variac is a useful tool... So is a proper isolation transformer... Thank you for putting that out there..
The rule of thumb for me is...if is does not specifically state "isolation" then it is not isolated. If it does state it then it may or may not actually be isolated especially if it uses a 3-prong plug. It is just easier to assume it is not isolated until and unless I can see the circuitry myself to verify it.
I am glad that you made this video...folks really need to know these things before they get into this kind of hobby/work. The easiest way to know if a variable is a true isolated kind...if it is cheap...you can pretty much bet it is NOT isolated! Real VIT's are rather expensive...and I would not trust one from China even if it claims it is isolated.
You also have to worry about isolation from ground especially on new isolation transformers. You also have to remember that as soon as you hook up ground on your scope probe you are grounding your item under test, if you don’t know or don’t understand what you are doing don’t do it mains kills.
What becomes very clear when you look at the circuit diagram is that it depends on how you connect the plug to the mains: It is best to plug it in so that you always get the neutral conductor on the one output that is also grounded. So you only have dangerous voltages at the output at the higher voltages you set. Otherwise the hot side would always be present at an exit, which would be much more dangerous.
Very old tv,s that did not use a power transformer had a hot chassis depending which way the plug was inserted into the outlet. The same goes for most 5 tube radios up until transistor radios came along.
I wonder how common it was to work on hot-chassis equipment without an isolation transformer back in the 40s and 50s and even earlier, and if getting killed as a result was a regular occurrence. One of my early jobs was a summer helper at a power company. I remember being handed over to an old electrician who was a few months away from retirement and we were to replace all the old non-polarized and non-grounded 110-V outlets in a few old substation switch houses with the modern variety. There was no ground available at any of the outlets, and all the guy did was to take out the old outlet, tie the neutral lug to the ground lug of the new outlet and pop it in. He relied on the color of the wires to tell him which was hot and which wasn't, so for all he really knew, he could have made the neutral and ground sockets of those replacements hot. I guess he figured by the time anybody found out, he would be gone.
Thank you for this informative video. I had always thought that using a Variac-style transformer protected you from the hot chassis. I’m just starting the process of restoring an older radio and had planned to use a variable auto transformer in the process. Thanks!!
I'm glad you liked the video. As I am sure you realize, they do make variable isolation transformers-in fact one is mentioned in the video. Or you can buy a variable autotransformer and an isolation transformer to use in combination. Usually you want to put the isolation transformer after the variable one so that you can make up for losses by raising the voltage on the variable transformer. But if you get both in one unit I think it is better (though sometimes more expensive.) Good luck with your radio restorations and I hope you will post some of them on UA-cam sometime. By the way, VARIAC is a trademark the is, unfortunately sometimes used generically to refer to all variable transformers, not just those made by the company holding the trademark rights to VARIAC. I appreciate those, like yourself who understand this and do not refer to all variable transformers with that name. Finally, think that there are some VARIAC brand variable isolation transformers, but I am not sure.
The worst shock I ever got was when I was testing a car alternator on a bench tester. The alternator had a short in it and it sent all of it's energy through my chest. It felt as though someone hit me in the back with a bat. It was so bad, I was lucky it threw me back. If it hadn't I probably wouldn't be typing this. It was that bad. I have been hit by all kinds of ac, but nothing like that.
I think it's worth noting that the RCA/ VIZ Isotap WP-27 is an affordable solution which solves both the problem of varying voltage, and the problem of isolating any device which is plugged into it from earth reference. Not only that, but it is specifically designed for *exactly* the purpose of increasing the safety margin for those who are servicing/ repairing/ restoring mains-powered electronics. It won't bring up the power absolutely continuously, or as smoothly as a variable autotransformer (aka "Variac", "Adjust-A-Volt", or "Powerstat"), but it does solve both problems in one compact design which can generally be found at affordable prices on the 'net. And most importantly, it is *truly and verily isolated*, unlike almost all variable autotransformers (aka "variacs"). At a rating of 3.35 amps @ 120V, it can handle the current draw of just about any decent-sized radio, TV, stereo, guitar amp or piece of test equipment one would care to work on, and it is a genuine self-contained isolation transformer. These are fairly common on eBay and on other electronics surplus and auction sites. Here's a video demonstrating it. The guy doesn't quite understand everything about the unit, but he does seem to understand the importance of using an isolation transformer, and one can get a sense of how it works and why it's a good solution: ua-cam.com/video/G6CZVKD-Hqc/v-deo.html
Yes, I wish I had known this when I started out. Until I got a mentor I was flying blind. Of course, I was also building rockets with fuel made in my backyard over an open fire - I could have been the first astronaut (although a little charred) if things went wrong. Lucky to be alive despite my ignorance.
Hello, nice video. I just did some AC work, and ended up safe...I work by the old Austrian scheme of "having the plug in your pocket". Either way, I am always happy to see a safety related video in my timeline! Tam
It is true. Accidents are terrible...people usually find them funny until it hits them or someone close to them. To be honest: I (as one can see on my channel) am paranoid of high voltages. Almost got fried in an accident some years ago...
One thing that a lot of isolation transformer videos do not do is to teach an operator how to test if a piece (even if it claims to be) is an isolation transformer. If we look at the right hand side diagram. Would a continuity test do this. So, one probe on the AC side (first on on line the on the other) and the other probe on the transformer outputs (again first on one and then the other). A total of 4 combinations. If any one of these is continuous then there is definitely potential for trouble and you should stay away from it. The question is, if you don't find continuity is it still safe, and what are possible failure modes.
No matter if on primary side or isolated on secondary side: higher voltages are ALWAYS dangerous. It is not save to work with isolated transformers at higher voltages. It can kill you, too.
should an isolation transformer be grounded? Also can you run a variable auto transformer off a isolation transformer, if so should it be grounded? Could i ground both devices to a outside ground rod? Thanks
This is only half of the story on isolation transformers. Most so called isolation transformers are only isolated on the line and not the ground. This can be bad for pieces of test equipment such as oscilloscopes. Even that Tenama you have there, shares the same ground for both input and output. I disconnected the ground on the output socket on the inside of mine. If you have ever clipped your o-scope probe up to a hot chassis and didn't have ground isolation between your device under test and scope, then you know what I am talking about.
@@Route66Wanderer You would be surprised what you run into. Ran across an old Silverstone guitar amp that someone had installed a three prong cord improperly without bypassing the ground switch. Luckily I had done a thorough inspection on it before powering it up. It just so happened the switch would have had the hot dead-shorted to ground had someone powered it up.
@@srtamplification Doing electronic repair for 43 years, I’m not surprised by anything. The good thing in the situation you found is it would have blown a fuse or circuit breaker instantly. The only thing an isolation transformer without a U ground can protect you against is AC mains. Not having a U ground will not protect someone from HT/B+. While I use a Sencore PR57 and have more many years, I use it mainly for the variac and meters but the Isolation transformer is definitely required working on a transformerless amp or old AA5 radio.
Your isolation transformer is not fully isolated as ground remains connected. You need a 'cheater plug' connected to that output to get full isolation. Your drawing shows full isolation as ground is not connected to the primary, but the transformer is not. If you open up your transformer you will see a ground wire connected to the chassis or connected to the primary.
A short extension cord with a single gang box install a GFCI receptive this provides some protection because it watches the difference between hot and neutral. But always use protection in all you do around high voltage
You forgot to talk about the ground lead? You are still exposed to mains ground if the DUT has a ground plug. Most isolation xformers I've come across still give the mains ground on the secondary outlet. Sure if you're working on old radios or equipment without the ground plug - then you can assume you're isolated from mains ground. If your DUT has a ground plug then you're kidding yourself.
If you make any follow up videos can you address the ground wire which I believe is connected between both sides (not isolated). It's somewhat confusing since the ground and neutral, at least in the US, are connected together in the breaker panel and I don't quite get if you are isolated with a 3 prong plug on the isolated side. Thanks.
I would like you to view the Tech Tips Tuesday video published by Mr. Carlson's Lab on December 22, 2015, if you have not already done so, to see if this covers the issue to which you are referring.
tomtektest - I forgot to originally say thank you for all of the videos that you are uploading. I find them all very interesting and hope to watch them all. I have seen that video and have most connected the dots on his recommendation. Personally, I won't be checking my wall voltage with an oscilloscope but would like to work on an AA5 radio. In that case I'll probably just stick to my voltmeter for measurements. I get the concerns about liability of posting such videos but it would be helpful to see an actual do's and dont's video on this topic. I'm an EE with several decades of experience but have not spent much time working with HV circuits and I find it confusing so I would imagine a lot of folks out there do also. Thanks again. Joe
There is one aspect of this that confuses me. In Europe we have RCDs (probably called something different in the US). Touch a 240v live connection and the power trips out in about 30ms. It will give you a zap but generally no harm done. There is a leakage rating of about 30ma total for the installation. So the live side can be fairly safe between live and ground. I have purchased an isolation transformer because I want to float an oscilloscope. However if the device under test has any path to ground them my isolated circuit becomes lethal as the isolation transformer prevents the RCD tripping. This is almost the opposite of what you are saying above.
In the US they are called GFI or GFCI for Ground Fault Interrupter or Ground Fault Circuit Interrupter. They are usually used in locations near grounded water such as outdoors, bathrooms, etc. A more advanced form is the Arc Fault Interrupter that not only trips when a path to ground exists, but also opens when a spike is detected on the hot side, designed to prevent an arc (such as due to a broken wire touching and breaking) from causing a fire. All are good things to have but many homes and labs do not have them on all outlets. My home lab is equipped with Arc Fault Interrupters but I still use an isolation transformer on everything just to be extra safe.
mbaker335 - First, not all of Europe uses RCDs, 230 VAC, 50 Hz, or even the Euro. German's, for example, seem to think Europe is all the EU and everyone is the same. Tell that to a Scotchman. Second, RCDs don't trip any faster than GFCI or AFCI units in North America. They are all too slow. They are designed to protect the wiring system and the house in the case of AFCIs (and in the case of whole house RCDs, the power company transformers.) You will be just as dead either way. They are too slow. Last, open frame gear (old stuff) does not have a ground (protective earth is the correct term in the US,) and so none of these fault detectors even see the stuff. In any country in the world, if you plug an open frame device into mains, you have a 50:50 chance that the frame will be live verses PE or neutral. Use isolation transformers, or the like, when you work on anything. No exceptions.
I wish everyone that speaks of how dangerous the mains is would take the time to teach about it, and how it all works, because in the real world UA-cam is the mentor of most learners these days, like it or not its up to us to do the mentoring......if there were mentors everywhere for those wanting to learn, there wouldn't be much of a UA-cam audience.......most come on to learn. And def not trying to be a Pain in the butt, but these are the facts in 2017.
Harb's Elecronics Lab Wow what a comment . That is what I myself think. Most mentors took there whole life to learn something and don't let off good teachings. Plus like you say you tube is where we all look , young and old. Share the knowledge. I just bought Variac because it looked and sound cool. Sure glad God sent me this fellows video.
Tenma makes sub standard test equipment. I know because I purchased a few of them for my company. I took one apart for evaluation and was horrified by the workmanship. They were purchased at the lowest price of all auto transformers I evaluated, however all the Tenma products were sent back to the vendor.
I'm trying to determine exactly how the isolation transformer will help me in the following situation??? How dangerous it would be if I have isolated the equipment I am working on as you said, but the chassis of the isolated equipment is electrically hot with 120v AC, and while touching the hot chassis with my left hand, I accidentally touch something that is earth grounded, on my bench, with my right hand??? Will electrically isolating the equipment I am working on save me in this scenario???
That is the point of having isolation - the only connection to the mains is inductive, not conductive, so no current flows to installation ground. It's essentially just a wire in space, but, touch both ends even of that isolated supply at the same time and you will still get a lethal shock. When testing, always probe with right hand only and keep your left hand in your pocket. The left hand is directly next to your heart, so keep that hand out of the way. Better yet, switch off before connecting meter probes if at all possible, then switch back on to take the measurement. Oh, and if you're thinking of using an oscilloscope with your isolated supply - don't. Check out the safety videos on here first.
Yes, in fact I used just such a setup for some time with a Tenma variable transformer feeding a Tenma isolation transformer. I gave the isolation transformer to my brother some time ago (he decided to take up vintage radio restoration) and I purchased a Tenma variable isolation transformer. Later I purchased a Sencore PR57 which is what i use now.
AC 120 or 240v isnt the problem i have being zapped with 240v Ac more times than i can remember ,like terry says never put two hands in a item you are working on and also beware of transformers and capacitors inside audio amps ect carry 450v DC volts and upwards always use earth leakage trip switches aswell as isolation transformers never put yourself in a earth loop .learn how to check for earths .chassis shorts and earth loops .
the only reason a variac is not an isolation transformer is that the neutral wire is common to both in and out and that is tied to earth at the power station in europe or at your supply box on a pole in the us but a variac with a seperate isolation 1:1 transformer that connects only by magnetic field and has no chassis earth to the output socket is fully isolated!
Can you elaborate please. I see in the schematic that the isolated output outlet does have a ground connection and the neutral carries through from the mains to circuitry on the secondary side of the transformer. I'm not yet well versed in all this but it doesn't look truly isolated to me. Wouldn't this type circuit provide EMI isolation but not necessarily electrical isolation for the device being tested (hot chassis)? I'd like to understand more about this. Thanks
There are other videos on UA-cam that can show you better than I can write it out in the comment section, but yes, you need to understand that this device is not "truly isolated". Follow me: In US households your electrical panel has a bond between ground and neutral. This means at each outlet you have continuity between ground and neutral. The Sencore PR57, like many other isolation transformers, does in fact have the hot and neutral wires separate between input and output. So great, those are isolated. BUT - the ground wire carries from the input to the output. So now you have continuity between the OUTPUT GROUND all the way back to the electrical panel NEUTRAL! One of the problems with this is if you have, say your oscilloscope plugged into a regular outlet and you accidentally put the probe ground on a live voltage spot on your test device (plugged into the isolation transformer), then BOOM - you just blew your scope probe or your scope or maybe your finger. The most common way around this is to disconnect the ground on the isolation output outlet, or "float the ground". Of course, that has it's own set of dangers, so you do so at your own risk. Hope that helps. Be safe!
Thanks, that is helpful. I'm using a viz wp-26a which should "float the device under test" since it doesn't have a ground on the isolated output outlet and the secondary is not tied to the neutral of the mains. I appreciate you taking time to discuss.
Yes, that model device is truly isolated on the Isolated outputs only. Instead of floating the ground, you could of course just use one of those little 3-prong to 2-prong adapters, again at your own risk.
What if you use diodes on the ground wires? I'm thinking that it should stop voltages leaving one piece of equipment and circling around to another... Otherwise you will have to get a second ground installed away from the original ground to have it isolated and also that is wired through a separate ground leakage interrupt to properly protect both yourself and your equipment... I'm not entirely sure about what is the best possible setup on this topic but I know that I don't like having to make sure that that I have correctly setup my equipment to test something because at the end of the day we are only human and if I should be distracted by anything I could possibly kill myself or my equipment or both... I guess that I'm going to have to dig a little deeper into it and try to understand it as much as possible...
Variac transformers are based on the "Auto transformer", one winding with multiple taps selected by a brush set up. Bottom of the windings connected to neutral, and a tapped point several turns from the top connected to line, several "overturns" to get the higher voltage than mains voltage. There is no secondary winding on an auto transformer!!!!
You don't need an isolation transformer when working on tube amps. The amp's power transformer acts as an isolation transformer (widowmaker amps are the exception). An isolation transformer's purpose is to protect expensive test gear.
I work on a farm I often do repairs on live 120v or 240v circuits. I don't always have time to find the breaker that services a certain outlet so I just do it live I was shockId 3 times this year by 120v. Ive been shocked about maybe 10-12 times in my life starting as a baby putting a key in a 120v plug. my fascination with electricity continues Ill get shocked again soon Im used to it 120v has never hurt me but I'm as careful as I can be.
Sorry but I have to say, I found this video a bit disappointing, especially from someone so knowledgeable. In my opinion, you didn't clearly explain WHY an isolation transformer is important (critical for some equipment), and what the downsides may be. You neglected to explain the rather critical difference between working on antique equipment with hot chassis (e.g. televisions), were isolation is a absolute must have, and newer types of units, where it is normally quite difficult to get shocked. As it was mentioned in another comment, the issue of modern Ground Fault Interrupt (GFI) devices plays a role here. In the video you give the impression, that it is always better and safer to use an isolation transformer. But is that really correct? As I understand it, if you are working on a unit powered through an isolation transformer, and you happen to make contact with both the live and neutral AC simultaneously, the GFI in your house will NOT trip, and cut the power. In other words, an isolation transformer lowers the risk of an electric shock occuring, but if it does occur, you then have a much greater risk of actually being electrocuted. A two edged sword in this case. There is also the topic of grounding of the DUT, and the risks in that. I would be very interested to hear your views on this.. Thanks for your many good (other) videos though! Regards.
When I was a kid we used to dare each other to stick our fingers in a light bulb socket and have someone else switch on the electricity, I guess we got used to the shock, this was back when we had 220Vac in Europe (+5/-10%). We were only 10-12 years old at the time, but it was very exhilarating. Funniest mains shocking I've ever experienced was my friend modifying an old TV to accept a headphone socket because he was still living at home and his parents demanded he be quiet after 9PM, so he added a headphone socket in parallel with speaker, knowing the headphone would need much less power and the speaker should be inaudible at the same time, unfortunately what he didn't know is that the TV set was wired with a hot chasis, so when he put on his headset and plugged it in, it looked like he was hit by some explosion and his body was thrown back into the wall and he was a blur for several minutes. I was in the room at the time and didn't immediately realize what had happened. We both still laugh at it though, he probably suffered permanent brain damage, but who cares.
Growing up, I worked with my brother-in-law, in the construction trade, specifically framing houses. In Oregon, you work whether it rains, or not. Most of the places we framed had crappy power supplies. The older worm drive Skil saws, (this was the late '60's- early '70's), were all metal. When it got real wet, you'd pick up the saw, soaking wet, (you and the saw), I there would be a steady 'buzz' in the saw from an insufficient ground, just at the edge of painful. My bro-in-law would get pissed off at me because I absolutely refused to use the damn thing when it was like that. He'd piss and moan, then grab the saw, in spite of the voltage leak. I watched him saw for hours, ignoring the damn thing. I just couldn't do it.
Very good video. Im glad you told everyone at the end you can still get shocked. Someone new to electricity might think if they have an isolation transformer they are ok to do what they want to and wont get shocked. I like the one hand rule, where you only use one hand to probe equipment hooked to isolation transformer and keep other hand in your pocket, so you dont complete circuit and get shocked.
Too true. You don't even need to touch anything to lose your life to high voltage electricity. My father is a principal engineer for Honeywell Aerospace and an employee at one of his facilities was cleaning inside one of the test cells with basically a leaf blower. They were told to never use that kind of equipment because it can generate static electricity and their power supplies put out a TERRIFYING amount of power. Well. That was Christmas Eve. And while this guy never actually touched anything, he never saw Christmas. We were having a Christmas brunch when he got the call. He was crushed. Keep in mind, this facility is in another country, he got there that night to figure out what all exactly went down. That guy had a young family. It's just awful. Electricity is no joke. Never cut corners
Yes. Electricity is a perfect servant but a demonic master.
I cannot be responsible for what others may say on the subject, but I can be clear about ONE thing this video lacks... any mention of the "isolation transformer which isn't!" An "isolation transformer" MAY come with a THREE WIRE CORD and have a THREE WIRE OUTPUT SOCKET. This allows the maker to loop the GROUND WIRE (so named because it is usually Green and attaches to a safety ground such as a ground rod outside the home) to the output socket. THIS IS POSSIBLY THE MOST DANGEROUS THING POSSIBLE! THE GROUND WIRE is usually attached to the electrical neutral wire (part of the power grid) at the entrance box. It makes the "isolation transformer" part of the electrical system you were TRYING to avoid! Older isolation transformers had a two wire outlet... most new ones have a THREE WIRE output plug WITH GROUND WIRE. If your device under test (DUT) has a THREE WIRE PLUG... Ouch! You can be back to having NO ISOLATION from the power system. This depends on how the device itself USES the "ground wire." This MAY appear to be "more information than is needed," but that is life. It is why "common sense is the most uncommon sense of all."
I've been "trained" in mains electric wiring while working in a telecom switch room. I had to test the building back-up batteries, wire up the BDFB bay, maintain the building rectifiers which was a very horrifying room to walk through, and I would need to test the back-up generator which was the same engine as a train/locomotive basically. I used to also have to work on live AC wall outlets. All the training I had was complete and utter BS, and none of our tools were insulated.
I brought up my safety concerns with the higher levels of the company and they put an end to switch techs performing the high power electrical work.--(because qualification matters and training by a senior co-worker standing by with a broom stick to knock you off the 1" thick copper bus bars so that your family might be able to recognize you in an open casket funeral apparently wasn't acceptable for a large corporation.) They also equipped our tool chest with all insulated tools all within a week or two. They had been running things like this for years until I opened my mouth. Needless to say, the managers were pissed at me and retaliated which is why I don't work there anymore.
Where did you work ? I used to do power routines at AT&T and they are pretty strict.
@@cat-lw6kq This was Voicestream (now T-Mobile) shortly after they acquired Aerial Communications. It was the legacy Aerial Communications management that was the problem. I still work for T-Mobile and they are extremely strict on safety standards. We're not allowed to climb ladders or even pick up boxes as office workers.
@@cat-lw6kq Also, the live AC wall outlets were with another company called IDL where I was a prototype engineer. I had no training of any kind to work on power. They just yelled at you to not touch both "negative and positive"--which is not how AC works as we all know. LOL.
AT&T was very strict about safety, especially in const. dept. I used to help maintain batteries, rectifiers and generators. Even a 24v plant is dangerous because of the amount of current those large lead acid batteries can produce.
Yeah know how you feel about being in a room like that. Large panels all vibrating to a very scary 60 cycle hum...
Thank you for making this video! I’m in the beginning stages of this hobby-work, and I’ve been searching/researching the safest way to use these tools. Very informative video, honest, and eye-opening! Thanks
This is a very important video. I almost bought a variac, but now I will hold out for one that is isolated. Thanks. Subscribed.
I was not trying to influence purchase decisions (variable transformers have many uses) just to point out that not all variable transformers include isolation.
This the most important video you have ever done for young people.
The tenma unit shown still has mains and chassis ground with the secondary side neutral also tied to chassis ground. In other words, it's not isolated without some modifications.
Thanks for this video. It's very important. A lot of terrifying videos show "how easy" can you do something with just a couple of passive parts for e.g."capacitive dropper" - which could kill you at the end. There are a lot of videos including MOTs that shows how electricity is entertaining - could be someone a last show.
I'm a trained electrician (and electronics engineer) and I can only wholeheartedly agree with what's been said this video. 🖖😎👍 ❗
Thanks for the great advice.
And I was going to purchase a Variac thinking that I needed it to properly protect myself... I now understand that while a Variac is a useful tool... So is a proper isolation transformer... Thank you for putting that out there..
The rule of thumb for me is...if is does not specifically state "isolation" then it is not isolated.
If it does state it then it may or may not actually be isolated especially if it uses a 3-prong plug.
It is just easier to assume it is not isolated until and unless I can see the circuitry myself to verify it.
Thanks for this, lots of misunderstanding around this subject which you have dealt with in a clear and plain manner.
I am glad that you made this video...folks really need to know these things before they get into this kind of hobby/work.
The easiest way to know if a variable is a true isolated kind...if it is cheap...you can pretty much bet it is NOT isolated!
Real VIT's are rather expensive...and I would not trust one from China even if it claims it is isolated.
You also have to worry about isolation from ground especially on new isolation transformers. You also have to remember that as soon as you hook up ground on your scope probe you are grounding your item under test, if you don’t know or don’t understand what you are doing don’t do it mains kills.
Got shocked as a kid by touching the base of an old lamp that was at my grandma's house.... ~40 yrs ago but will never forget....
thanks for the video, I've been thinking about this all week.
Every live save is by this video is a succes.
This is why I run a non grounded plug pn my variac after powering it FROM an isolation transformer.... Total isolation from mains power and ground.
Yes but the neutral is connected to ground in most breaker panels
I always plug vintage equipment into an isolation transformer then plug the transformer into the variac,
Rich Kudlick yup. Me too. A good precaution..
Thank you for the video. I am getting a scope and did not know about this. I will order one when I order my scope.
Thanks for this video this is a subject we all need to take seriously.
What becomes very clear when you look at the circuit diagram is that it depends on how you connect the plug to the mains: It is best to plug it in so that you always get the neutral conductor on the one output that is also grounded. So you only have dangerous voltages at the output at the higher voltages you set. Otherwise the hot side would always be present at an exit, which would be much more dangerous.
Very old tv,s that did not use a power transformer had a hot chassis depending which way the plug was inserted into the outlet. The same goes for most 5 tube radios up until transistor radios came along.
Yes, and when you started to fix it, you up your legs up checking what is swinging on a chassis by touching it with a nails instead of your fingers.
I wonder how common it was to work on hot-chassis equipment without an isolation transformer back in the 40s and 50s and even earlier, and if getting killed as a result was a regular occurrence. One of my early jobs was a summer helper at a power company. I remember being handed over to an old electrician who was a few months away from retirement and we were to replace all the old non-polarized and non-grounded 110-V outlets in a few old substation switch houses with the modern variety. There was no ground available at any of the outlets, and all the guy did was to take out the old outlet, tie the neutral lug to the ground lug of the new outlet and pop it in. He relied on the color of the wires to tell him which was hot and which wasn't, so for all he really knew, he could have made the neutral and ground sockets of those replacements hot. I guess he figured by the time anybody found out, he would be gone.
Excellent information from an experienced person, well done.
Thank you for safety advice. Isolation transformer to be used..
Thank you for this informative video. I had always thought that using a Variac-style transformer protected you from the hot chassis. I’m just starting the process of restoring an older radio and had planned to use a variable auto transformer in the process. Thanks!!
I'm glad you liked the video. As I am sure you realize, they do make variable isolation transformers-in fact one is mentioned in the video. Or you can buy a variable autotransformer and an isolation transformer to use in combination. Usually you want to put the isolation transformer after the variable one so that you can make up for losses by raising the voltage on the variable transformer. But if you get both in one unit I think it is better (though sometimes more expensive.) Good luck with your radio restorations and I hope you will post some of them on UA-cam sometime. By the way, VARIAC is a trademark the is, unfortunately sometimes used generically to refer to all variable transformers, not just those made by the company holding the trademark rights to VARIAC. I appreciate those, like yourself who understand this and do not refer to all variable transformers with that name. Finally, think that there are some VARIAC brand variable isolation transformers, but I am not sure.
Thanks for sharing your knowledge Boss
Thanks for sharing this. It is good solid and important knowledge.
The worst shock I ever got was when I was testing a car alternator on a bench tester. The alternator had a short in it and it sent all of it's energy through my chest. It felt as though someone hit me in the back with a bat. It was so bad, I was lucky it threw me back. If it hadn't I probably wouldn't be typing this. It was that bad. I have been hit by all kinds of ac, but nothing like that.
I think it's worth noting that the RCA/ VIZ Isotap WP-27 is an affordable solution which solves both the problem of varying voltage, and the problem of isolating any device which is plugged into it from earth reference. Not only that, but it is specifically designed for *exactly* the purpose of increasing the safety margin for those who are servicing/ repairing/ restoring mains-powered electronics. It won't bring up the power absolutely continuously, or as smoothly as a variable autotransformer (aka "Variac", "Adjust-A-Volt", or "Powerstat"), but it does solve both problems in one compact design which can generally be found at affordable prices on the 'net. And most importantly, it is *truly and verily isolated*, unlike almost all variable autotransformers (aka "variacs"). At a rating of 3.35 amps @ 120V, it can handle the current draw of just about any decent-sized radio, TV, stereo, guitar amp or piece of test equipment one would care to work on, and it is a genuine self-contained isolation transformer. These are fairly common on eBay and on other electronics surplus and auction sites.
Here's a video demonstrating it. The guy doesn't quite understand everything about the unit, but he does seem to understand the importance of using an isolation transformer, and one can get a sense of how it works and why it's a good solution:
ua-cam.com/video/G6CZVKD-Hqc/v-deo.html
Good info. This can save lives.
Yes, I wish I had known this when I started out. Until I got a mentor I was flying blind. Of course, I was also building rockets with fuel made in my backyard over an open fire - I could have been the first astronaut (although a little charred) if things went wrong. Lucky to be alive despite my ignorance.
I realy likes your videos . Thanks from Norway
Hello,
nice video. I just did some AC work, and ended up safe...I work by the old Austrian scheme of "having the plug in your pocket".
Either way, I am always happy to see a safety related video in my timeline!
Tam
Thank you for your comment - I was pleased to see so many viewers appreciate safety.
It is true. Accidents are terrible...people usually find them funny until it hits them or someone close to them.
To be honest: I (as one can see on my channel) am paranoid of high voltages. Almost got fried in an accident some years ago...
thank you
Good information. Thanks for making this video.
One thing that a lot of isolation transformer videos do not do is to teach an operator how to test if a piece (even if it claims to be) is an isolation transformer. If we look at the right hand side diagram. Would a continuity test do this. So, one probe on the AC side (first on on line the on the other) and the other probe on the transformer outputs (again first on one and then the other). A total of 4 combinations. If any one of these is continuous then there is definitely potential for trouble and you should stay away from it. The question is, if you don't find continuity is it still safe, and what are possible failure modes.
No matter if on primary side or isolated on secondary side: higher voltages are ALWAYS dangerous. It is not save to work with isolated transformers at higher voltages. It can kill you, too.
Very educational video
should an isolation transformer be grounded? Also can you run a variable auto transformer off a isolation transformer, if so should it be grounded? Could i ground both devices to a outside ground rod? Thanks
This is only half of the story on isolation transformers. Most so called isolation transformers are only isolated on the line and not the ground. This can be bad for pieces of test equipment such as oscilloscopes. Even that Tenama you have there, shares the same ground for both input and output. I disconnected the ground on the output socket on the inside of mine. If you have ever clipped your o-scope probe up to a hot chassis and didn't have ground isolation between your device under test and scope, then you know what I am talking about.
I’ve never seen a hot chassis with a 3 wire AC connection, they are always 2 wire. You have no worries with the U ground connected.
@@Route66Wanderer You would be surprised what you run into. Ran across an old Silverstone guitar amp that someone had installed a three prong cord improperly without bypassing the ground switch. Luckily I had done a thorough inspection on it before powering it up. It just so happened the switch would have had the hot dead-shorted to ground had someone powered it up.
@@srtamplification Doing electronic repair for 43 years, I’m not surprised by anything. The good thing in the situation you found is it would have blown a fuse or circuit breaker instantly. The only thing an isolation transformer without a U ground can protect you against is AC mains. Not having a U ground will not protect someone from HT/B+. While I use a Sencore PR57 and have more many years, I use it mainly for the variac and meters but the Isolation transformer is definitely required working on a transformerless amp or old AA5 radio.
Your isolation transformer is not fully isolated as ground remains connected. You need a 'cheater plug' connected to that output to get full isolation. Your drawing shows full isolation as ground is not connected to the primary, but the transformer is not. If you open up your transformer you will see a ground wire connected to the chassis or connected to the primary.
A short extension cord with a single gang box install a GFCI receptive this provides some protection because it watches the difference between hot and neutral. But always use protection in all you do around high voltage
BTW do not use a GFCI on the output of an isolated unit and expect extra protection against shocks. It will not work without a reference to Ground.
So the output from an isolation transformer isn't potentially lethal?
You forgot to talk about the ground lead? You are still exposed to mains ground if the DUT has a ground plug. Most isolation xformers I've come across still give the mains ground on the secondary outlet. Sure if you're working on old radios or equipment without the ground plug - then you can assume you're isolated from mains ground. If your DUT has a ground plug then you're kidding yourself.
If you make any follow up videos can you address the ground wire which I believe is connected between both sides (not isolated). It's somewhat confusing since the ground and neutral, at least in the US, are connected together in the breaker panel and I don't quite get if you are isolated with a 3 prong plug on the isolated side. Thanks.
I would like you to view the Tech Tips Tuesday video published by Mr. Carlson's Lab on December 22, 2015, if you have not already done so, to see if this covers the issue to which you are referring.
tomtektest - I forgot to originally say thank you for all of the videos that you are uploading. I find them all very interesting and hope to watch them all.
I have seen that video and have most connected the dots on his recommendation. Personally, I won't be checking my wall voltage with an oscilloscope but would like to work on an AA5 radio. In that case I'll probably just stick to my voltmeter for measurements.
I get the concerns about liability of posting such videos but it would be helpful to see an actual do's and dont's video on this topic. I'm an EE with several decades of experience but have not spent much time working with HV circuits and I find it confusing so I would imagine a lot of folks out there do also. Thanks again. Joe
There is one aspect of this that confuses me. In Europe we have RCDs (probably called something different in the US). Touch a 240v live connection and the power trips out in about 30ms. It will give you a zap but generally no harm done. There is a leakage rating of about 30ma total for the installation. So the live side can be fairly safe between live and ground.
I have purchased an isolation transformer because I want to float an oscilloscope. However if the device under test has any path to ground them my isolated circuit becomes lethal as the isolation transformer prevents the RCD tripping. This is almost the opposite of what you are saying above.
In the US they are called GFI or GFCI for Ground Fault Interrupter or Ground Fault Circuit Interrupter. They are usually used in locations near grounded water such as outdoors, bathrooms, etc. A more advanced form is the Arc Fault Interrupter that not only trips when a path to ground exists, but also opens when a spike is detected on the hot side, designed to prevent an arc (such as due to a broken wire touching and breaking) from causing a fire. All are good things to have but many homes and labs do not have them on all outlets. My home lab is equipped with Arc Fault Interrupters but I still use an isolation transformer on everything just to be extra safe.
mbaker335 - First, not all of Europe uses RCDs, 230 VAC, 50 Hz, or even the Euro. German's, for example, seem to think Europe is all the EU and everyone is the same. Tell that to a Scotchman.
Second, RCDs don't trip any faster than GFCI or AFCI units in North America.
They are all too slow. They are designed to protect the wiring system and the house in the case of AFCIs (and in the case of whole house RCDs, the power company transformers.)
You will be just as dead either way. They are too slow.
Last, open frame gear (old stuff) does not have a ground (protective earth is the correct term in the US,) and so none of these fault detectors even see the stuff.
In any country in the world, if you plug an open frame device into mains, you have a 50:50 chance that the frame will be live verses PE or neutral.
Use isolation transformers, or the like, when you work on anything. No exceptions.
I wish everyone that speaks of how dangerous the mains is would take the time to teach about it, and how it all works, because in the real world UA-cam is the mentor of most learners these days, like it or not its up to us to do the mentoring......if there were mentors everywhere for those wanting to learn, there wouldn't be much of a UA-cam audience.......most come on to learn.
And def not trying to be a Pain in the butt, but these are the facts in 2017.
Harb's Elecronics Lab Wow what a comment . That is what I myself think. Most mentors took there whole life to learn something and don't let off good teachings. Plus like you say you tube is where we all look , young and old. Share the knowledge. I just bought Variac because it looked and sound cool. Sure glad God sent me this fellows video.
Tenma makes sub standard test equipment. I know because I purchased a few of them for my company. I took one apart for evaluation and was horrified by the workmanship. They were purchased at the lowest price of all auto transformers I evaluated, however all the Tenma products were sent back to the vendor.
So what brand did you end up purchasing?
I would also suggest getting a thick rubber mat to stand on
Black rubber can contain carbon and not be an insulator...
Thankyou
I'm trying to determine exactly how the isolation transformer will help me in the following situation??? How dangerous it would be if I have isolated the equipment I am working on as you said, but the chassis of the isolated equipment is electrically hot with 120v AC, and while touching the hot chassis with my left hand, I accidentally touch something that is earth grounded, on my bench, with my right hand??? Will electrically isolating the equipment I am working on save me in this scenario???
That is the point of having isolation - the only connection to the mains is inductive, not conductive, so no current flows to installation ground.
It's essentially just a wire in space, but, touch both ends even of that isolated supply at the same time and you will still get a lethal shock.
When testing, always probe with right hand only and keep your left hand in your pocket. The left hand is directly next to your heart, so keep that hand out of the way. Better yet, switch off before connecting meter probes if at all possible, then switch back on to take the measurement.
Oh, and if you're thinking of using an oscilloscope with your isolated supply - don't. Check out the safety videos on here first.
Yes! In short, the answer is NO!!! The result is not good.
If you have an UN-ISOLATED variac, can you plug it into an adequate ISOLATED transformer, and be OK?
Yes, in fact I used just such a setup for some time with a Tenma variable transformer feeding a Tenma isolation transformer. I gave the isolation transformer to my brother some time ago (he decided to take up vintage radio restoration) and I purchased a Tenma variable isolation transformer. Later I purchased a Sencore PR57 which is what i use now.
@@tomtektest8042 I can't find either unit for sale that isn't one that is non-functional and/or listed for parts only or is way overpriced.
AC 120 or 240v isnt the problem i have being zapped with 240v Ac more times than i can remember ,like terry says never put two hands in a item you are working on and also beware of transformers and capacitors inside audio amps ect carry 450v DC volts and upwards always use earth leakage trip switches aswell as isolation transformers never put yourself in a earth loop .learn how to check for earths .chassis shorts and earth loops .
the only reason a variac is not an isolation transformer is that the neutral wire is common to both in and out and that is tied to earth at the power station in europe or at your supply box on a pole in the us but a variac with a seperate isolation 1:1 transformer that connects only by magnetic field and has no chassis earth to the output socket is fully isolated!
The Sencore PR57 is not truly isolated - the ground (and chassis) are coupled between the input and output.
Can you elaborate please. I see in the schematic that the isolated output outlet does have a ground connection and the neutral carries through from the mains to circuitry on the secondary side of the transformer. I'm not yet well versed in all this but it doesn't look truly isolated to me. Wouldn't this type circuit provide EMI isolation but not necessarily electrical isolation for the device being tested (hot chassis)? I'd like to understand more about this. Thanks
There are other videos on UA-cam that can show you better than I can write it out in the comment section, but yes, you need to understand that this device is not "truly isolated". Follow me: In US households your electrical panel has a bond between ground and neutral. This means at each outlet you have continuity between ground and neutral. The Sencore PR57, like many other isolation transformers, does in fact have the hot and neutral wires separate between input and output. So great, those are isolated. BUT - the ground wire carries from the input to the output. So now you have continuity between the OUTPUT GROUND all the way back to the electrical panel NEUTRAL! One of the problems with this is if you have, say your oscilloscope plugged into a regular outlet and you accidentally put the probe ground on a live voltage spot on your test device (plugged into the isolation transformer), then BOOM - you just blew your scope probe or your scope or maybe your finger. The most common way around this is to disconnect the ground on the isolation output outlet, or "float the ground". Of course, that has it's own set of dangers, so you do so at your own risk. Hope that helps. Be safe!
Thanks, that is helpful. I'm using a viz wp-26a which should "float the device under test" since it doesn't have a ground on the isolated output outlet and the secondary is not tied to the neutral of the mains. I appreciate you taking time to discuss.
Yes, that model device is truly isolated on the Isolated outputs only. Instead of floating the ground, you could of course just use one of those little 3-prong to 2-prong adapters, again at your own risk.
What if you use diodes on the ground wires? I'm thinking that it should stop voltages leaving one piece of equipment and circling around to another... Otherwise you will have to get a second ground installed away from the original ground to have it isolated and also that is wired through a separate ground leakage interrupt to properly protect both yourself and your equipment... I'm not entirely sure about what is the best possible setup on this topic but I know that I don't like having to make sure that that I have correctly setup my equipment to test something because at the end of the day we are only human and if I should be distracted by anything I could possibly kill myself or my equipment or both... I guess that I'm going to have to dig a little deeper into it and try to understand it as much as possible...
Variac transformers are based on the "Auto transformer", one winding with multiple taps selected by a brush set up. Bottom of the windings connected to neutral, and a tapped point several turns from the top connected to line, several "overturns" to get the higher voltage than mains voltage. There is no secondary winding on an auto transformer!!!!
You want true isolation on that isolation transformer? Tear out the 3 prong output socket and replace it with a 2 prong vintage socket!
An isolation transformer can also save your testing equipment.
You don't need an isolation transformer when working on tube amps. The amp's power transformer acts as an isolation transformer (widowmaker amps are the exception). An isolation transformer's purpose is to protect expensive test gear.
Galvanic connection vs electrical connection. Electrical connection can still kill you. Treat everything as it can kill you.
Many shotguns are not sniper rifles... And these days many people are not capable of having a critical thought.
Variac has become a generic term.
I work on a farm I often do repairs on live 120v or 240v circuits. I don't always have time to find the breaker that services a certain outlet so I just do it live I was shockId 3 times this year by 120v. Ive been shocked about maybe 10-12 times in my life starting as a baby putting a key in a 120v plug. my fascination with electricity continues Ill get shocked again soon Im used to it 120v has never hurt me but I'm as careful as I can be.
That 240v.....that one's a shocker......be careful my friend.
@@jimijamesnoob685 I touched the b+ on a tube amp I was working on 630 volts DC
Dang dude. You can probably charge your phone without the charger!
Best stay away from 3-phase then if you're working in UK!
Takes no prisoners...
Life is to valuable to put a value on........ just buy a stand alone Isolation transformer you can trust.....they are cheap.
Sorry but I have to say, I found this video a bit disappointing, especially from someone so knowledgeable. In my opinion, you didn't clearly explain WHY an isolation transformer is important (critical for some equipment), and what the downsides may be. You neglected to explain the rather critical difference between working on antique equipment with hot chassis (e.g. televisions), were isolation is a absolute must have, and newer types of units, where it is normally quite difficult to get shocked. As it was mentioned in another comment, the issue of modern Ground Fault Interrupt (GFI) devices plays a role here. In the video you give the impression, that it is always better and safer to use an isolation transformer. But is that really correct? As I understand it, if you are working on a unit powered through an isolation transformer, and you happen to make contact with both the live and neutral AC simultaneously, the GFI in your house will NOT trip, and cut the power. In other words, an isolation transformer lowers the risk of an electric shock occuring, but if it does occur, you then have a much greater risk of actually being electrocuted. A two edged sword in this case. There is also the topic of grounding of the DUT, and the risks in that. I would be very interested to hear your views on this.. Thanks for your many good (other) videos though! Regards.
But you can clearly see WHY it is important by the schematic.
Jonas vH. Berg 1:55 “if you get between that equipment and ground you could be killed”..
When I was a kid we used to dare each other to stick our fingers in a light bulb socket and have someone else switch on the electricity, I guess we got used to the shock, this was back when we had 220Vac in Europe (+5/-10%). We were only 10-12 years old at the time, but it was very exhilarating.
Funniest mains shocking I've ever experienced was my friend modifying an old TV to accept a headphone socket because he was still living at home and his parents demanded he be quiet after 9PM, so he added a headphone socket in parallel with speaker, knowing the headphone would need much less power and the speaker should be inaudible at the same time, unfortunately what he didn't know is that the TV set was wired with a hot chasis, so when he put on his headset and plugged it in, it looked like he was hit by some explosion and his body was thrown back into the wall and he was a blur for several minutes. I was in the room at the time and didn't immediately realize what had happened. We both still laugh at it though, he probably suffered permanent brain damage, but who cares.
Did he wire the socket ground to the chassis? Or was the metal part of the socket touching the chassis?
Growing up, I worked with my brother-in-law, in the construction trade, specifically framing houses.
In Oregon, you work whether it rains, or not. Most of the places we framed had crappy power supplies. The older worm drive Skil saws, (this was the late '60's- early '70's), were all metal.
When it got real wet, you'd pick up the saw, soaking wet, (you and the saw), I there would be a steady 'buzz' in the saw from an insufficient ground, just at the edge of painful.
My bro-in-law would get pissed off at me because I absolutely refused to use the damn thing when it was like that. He'd piss and moan, then grab the saw, in spite of the voltage leak.
I watched him saw for hours, ignoring the damn thing. I just couldn't do it.
I dont know if this is a good thing to write publically........some kid may read this, try it and die.
⚠️👍🏻⚠️ Time to buy one and send it to 1600 Pennsylvania Ave. cures Dementia!?