Also, your closing summary. I agree, do NOT get complacent around electricity. I live in a 240V country and I've lost count of the number of times I've come in to contact with both active and neutral at the same time and got a shock. Across uninsulated fuse holders, doing dumb stuff like removing the fuse from the holder with the thing still plugged in, dodgy wiring, etc.. When you think you know enough, you usually don't, lol.
There's also a case to be made here for the cheap handheld / battery powered scopes. Nice to have on hand for quick testing of questionably wired devices, and no worries about probe grounds connected to service earth.
For those on a limited budget, you can make your own isolation transformer with two old filament transformers. For example, take two 24volt, 4amp transformers and connect them back to back, that being connect the two secondaries together, phasing is not important in this application. The primary of the second transformer is now your isolated 120v AC. But keep in mind that power is power, the product of volts times amps. So (24volts* 4amps) = 96 watts. So the safe limit for this example is only about 90 watts of isolated 120vac due to losses in the transformers. You don't get 120v at 4 amps just because the transformer secondaries are rated at 4 amps. The VA or watt capacity of your kludge isolation transformer is only as high as the VA rating of the one of the transformers.
I got hit early in my career. Using a screw driver I was undoing a 240V connection not checking to see if it was live first. Touched the return wire at the same time and was blown 10 feet across the shop floor and now everything above 25V or with a lot of energy behind it gets much respect from me. Sometimes you need a good scare to get your head right.
That’s a pretty rad setup. I know what you mean about using a massive transformer. My bench area sounds like a warehouse with the buzzing of that massive iron! But like you said, the price was mos def right
I remember talking to a friend who was a tv engineer in the late sixties and all the outdoor techs were issued with isolation transformers but very few used them. With the rental sets, if there was an issue they would just swap it out but of course that meant carrying a really heavy tv set up and down three or four flights of stairs. My mate being young and wise always tried to fix the sets in situ and even carrying a transformer up stairs wasn’t going to happen. So he was in the back of the sets, no transformer with live chassis doing repairs in customers houses. He got a few belts but survived!
31:20 Yes. Mr Carlson has an eye opening description of when he was electrocuted - always treat high voltages with respect. When I was a teenager I was very cavalier with devices having mains voltage input (240VRMS in the UK) and I lost count of the number of times I received an electric shock😳
Excellent video Tony, another issue to consider are the USB or RS232 connections of the equipment, if the cpu is connected before the isolation transformer, automatically all equipment are grounded.
It is common practice to isolate the mains and earth (chassis) ground in a bench custom designed troubleshooting AC power supply. Many experienced electronic technicians eventually build their own troubleshooting AC power supply to get the features they want. Also common is to have a traditional factory ladder logic control scheme to operate the AC power supply control circuit, but usually operating from 12VDC which is very common for relay coil control and other control devices found in an industrial control cabinet. In a ladder logic scheme you will have a low voltage control approach controlling high voltage (120VAC) at high currents, where the two systems are electrically isolated (either magnetically or optically or both). A common feature is a series interlock where if any alarm or fault is tripped (open) the unit will shut down. These cabinets are recognizable by having a Green start push button switch (with latching relay) and a Red stop push button switch. As such it is relatively easy to include a transformer surge current limiter using a large resistor, which is then shorted out by a relay at some time after the end of the magnetization current worse case (which would include a downstream variac). I like the resistor approach because it does not use heat to operate like a thermistor, and is ready whenever the power is quickly cycled. A lot of the jitter hysteresis ON/OFF function is usually included in the 12VDC time domain relays. You can also delay when output power is available to make sure the system is stable incase there is a load on the output. It is very handy to have two analog AC ammeters in two ranges (like 10A and 2A for example) along with an output AC analog voltmeter. The analog meter movements are highly damped which can provide a clue as to the failure by watching the indicator movement upon power up. If you work on the same type of gear all of the time, like at a company repair depot, you can often know the failure just by observing the current movement behavior on power up. Setup the meter so you start with the 10A and then can switch to the 2A with a make before break switch. If you work on a lot of gear I find you only really need one current limiting device (other than several different CBs in parallel 1A, 3A, 10A) because the failure mode is almost always a direct short or almost a direct short. I am using using a 220 ohm automatically cooled low inductance 600W power resistor as my current limiting device, it is put in or out of with a shorting switch. It is very handy to have a large power toggle switch to enable power to the output isolated AC receptacles. You can buy a Hospital Grade Toroidal Isolation transformer stand alone without the case and connect it as you like. For a large transformer, features include an automatic reset thermal cutout and a separate shield. For large company repair depots, there is a troubleshooting bench and a separate outgoing test/calibration bench. The calibration bench will have a pure sinewave AC power supply where the earth ground is not isolated but the mains are, mainly for personal safety. The instrumentation test pure sinewave AC power supply can include an AC power analyzer which will give many different real time measurements which can be very specialized, like power factor, AC voltage distortion (THD), AC current distortion (THD), as well as have a large operating range for testing universal (switching or selectable) equipment. The outgoing test procedures can include calibration and testing at specific AC voltages (if it is a power supply this would include line and load regulation). It is possible that a three phase Y (with neutral) as wired for a street of homes may not string the neutral to the single phase transformer along with two phase connections. In this case the two terminal transformer primary will not include the neutral. As such the pole transformer neutral for lightning protection will be derived from the secondary neutral (includes the transformer case through a jumper connection). The Y neutral will be available along the primary main backbone feed (which includes the neutral connection to ground at each backbone pole for lightning protection), but not necessarily the street feeds having only two high voltage feeds. If you had three identical streets each would use two phases in sequence such that the load is balanced per street. It is interesting how different power companies come up with creative solutions for high voltage distribution, which at times can get very creative in how transformers are connected as derived from three phase Y or Delta.
Very interesting, thank you! I just checked an my isolation transformer has a 3-prong isolated outlet but only has a 2-prong polarized cord where it plugs into mains so it isn't physically possible for it to be connected to earth ground.
I am in a couple of Facebook groups for various facets of electronics. There is no other topic then isolation transformers where people have such wildly varying differences of opinion; especially carrying the ground lead to the output socket. It gets almost as hot when folks ask about adding a grounded cord to old test equipment that came with a two-wire cord.
In Germany, houses are usually supplied with three-phase electricity. There is a transformer on the street that reduces the voltage from 10kV or 20kV to 230V/400V. Between a phase and neutral or earth it is 230V. Between two phases 400V. Four power lines lead to the house: 3 phases and a combined neutral + earth conductor. In the first box in the house, neutral and ground are split and connected to the ground rod. Neutral blue and earth yellow-green may no longer be connected afterwards. The normal consumers get one phase, neutral and earth, i.e. 230V. The consumers are distributed across the three phases. Larger motors and the stove gets three phases, neutral and earth, so 400V. Electric cars can also be charged using 230V or 400V. Some even heat the house with 400V But I warn, 230V to ground can bite you very well, 400V will definitely ruin your week, 10kV is certainly fatal
Hi Tony 76yrs old Jeffrey from England. When you are able, could you Pleeeeeease, do a Vid on Transformer in rush current. And how to select the correct type, and rating for an input Fuse. Thanks.
Sometimes you need to zap yourself with high voltage just to not forget its power. I just did it while playing my new isolation transformer, immediately tought that's why I'm building it. Smaller 100-200W transformator with 2-3A breaker is good If you don't want to burn your sockets and switches, some PC power supply doesn't start trough light bulb, the bulb just blinks, but the powersupply works correctly from the mains without the bulb, and sometimes it has no protection at all and burns out half of the PCB and only the mains 25A breaker stops its madness. One of my basic trick if I don't feel confortable working on something I just use one arm, so even if I get shocked doesn't go trough my heart.
I have a medical grade isolation transformer where I have also disconnected the ground wire from the secondary. I also included the option to reconnect the ground thru a 1M resistor. I did this thru a lighted switch so I absolutely know when I have a solid ground or a 1M ground. I have my oscilloscope connected to this isolation transformer. I understand the danger, but I don't want any surprises or possible damage to my scopes. I have a second isolation transformer I can use for my DUT, that is totally isolated, should I need to be extra cautious. I'd appreciate your thoughts on this. By the way, a great video. Very educational for any novice.
Something I've never understood about US/CA mains wiring is the black wire being "hot" and the white being neutral. This makes absolutely no sense, as one would assume black to be either negative or neutral. In 'Stray'a (Australia) and the UK I believe, we use brown for active (live/line/hot) and in the 1980's it used to be red, blue for neutral (and again in the 1980's it used to be black), and green (with a white stripe) for ground/earth.
@@AstrosElectronicsLab 🙂 who's got a random 13A fuse? Fuses should always be appropriate, but you're right that there's nothing to stop someone taking the right fuse out and replacing with a 13A.
Well I think the Brown/Blue European standard is also backwards. The earth is brown and the sky is blue. So why is the brown the hot and the blue neutral?
Goof info! I would like to add that old transformer-less radios and TVs are not the only item to be careful with. Switch mode power supplies are also direct operation on the primary AC power side. To be safe, always use an isolation transformer when troubleshooting an SMPS or even working on a device that uses an open exposed SMPS. Note that the DC output of most SMPS's is isolated. But the on board transformer is operating at 50khz or so. The primary oscillator or chopper circuit is directly connected to the AC line. I have a master switch on my bench with a GFCI outlet. Yes the GFCI sometimes nuisance trips upon bench power up but it may save your life some day. GFCI Theory (for those who want to know more): A GFCI works on the principal that all current must return to it's source A GFCI passes the hot and neutral wires through the center of a small toriod transformer. The winding of that transformer is connect to a relay or re-settable circuit breaker coil via some simple circuitry. If all the current being used by the connected device is returned on the neutral, it will cancel at the small transformer and no voltage will be induced to the secondary coil. If however some current, over 5-10ma, is finding an alternate path to ground, like through you, voltage will now be induced into the small transformer secondary and trip the relay or internal breaker. Note that even though GFCI outlets all have a ground connection and three prong outlet, they don't need a ground connection to work nor do they care about the ground wire. They simply monitor the current flowing through the hot wire coming back on the neutral wire. Any difference exceeding 5-10ma indicates a ground fault situation. This is also why they sometimes nuisance trip when a device has power cord noise reduction capacitors tied to the ground wire, which as we know is tied to the neutral at the service panel. The current charging those small value capacitors sometimes exceeds the GFCI threshold and trips it.
nice overkill on that transformer as well as good recycling. It is always belt and suspenders when it comes to safety precautions. before I built a similar setup I have seen the blinding flash of an unexpected path to ground. I added a double pole single through switch for the device under test that disconnects both neutral and hot so I do not have to unplug every time. I still have tp pay attention just the same.
I’m glad I stopped to read this. In school I learned of the danger but was taught to use a Variac and an ‘adapter’ for the scope that allowed one to convert the three prong connection of the oscilloscope into a two prong connection. Essentially to float the test equipment, not the unit under test. I seem to have it all wrong. The thing that concerns me is not only to protect the scope, but how do I protect myself? Should I be using a so-called ‘ground fault circuit interrupter’ (GFCI) outlet at the wall ahead of the isolation transformer or Variac? Is a GFCI receptacle still effective at the output? My understanding is that the GFCI wouldn’t trip at the output without the ground wire as a reference. Finally, if I’m NOT floating the scope, but floating the equipment under test, will I be protected as long as I have the scope plugged into a GFCI and only touch the probes, and not the chassis or live connectors of the unit under test? I have, and maintain, a healthy respect for the ability of electricity to reach out and bite (hard) unexpectedly. Edit: I appreciate you sharing the design of your dim bulb tester. I have an older Universal Power Supply (UPS) that came with a largish transformer that I thought I could use to build my own isolation transformer, assuming that there is at least one set of secondary taps that don’t step the voltage up or down.
I followed your approach, disconnected the secondary ground on my isolation transformer, truly isolated. Now I am working on AA5 radio (hot chassis) for alignment, the radio alignment procedure requires that the negative lead of signal generator needs to connect the radio chassis while the positive lead of signal generator is connecting 12AT7 pin #7. My question is do I need to plug both the radio and signal generator to the same isolation transformer to do the alignment?
@@redfields5070The usual way with hot and neutral lines. Any sensed imbalance occurs if there were to be a leakage path elsewhere, perhaps from the hot chassis to a grounded test instrument.
Yep, I have a couple of Powervar isolation transformers that had the neutral bonded to earth (as well as earth bonded across both sides), which I had to remove.
Thanks Tony. Yes UK is very different. We rarely have transformers on poles in residential areas. We have local sub stations and cables go underground to each house. We Have Live, Neutral and Earth NO center tap. Every UK House will have several safety devices. an RCD (residual current device) to detect an earth faults above 30mA and MCB (miniature curcuit breaker) for overcurrent protection. We don't use the term "Hot" we say Live. Exposing a live conductor in UK is in fact Illegal except for testing by a qualified engineer. All equipment in UK installations have finger proof terminals so if you remove an outlet from the wall with live conductors connected it is impossible to touch the live conductors. We still use isolation transformers in some situation.
What I would like to see is someone explain how to TEST an isolation transformer....🤔 (just verify the isolated ground is not connected to "house ground" or the transformer case???) I'd love to know what you use as an inrush limiter...mine tends to pop the house circuit breaker on occasion....
So long as neither side of the secondary of the isolation xformer, the secondary remains isolated You may measure a ‘ghost voltage‘ between either side of the secondary and ground, but that is capacitively coupled. It is not “real” voltage, and carries no current capacity. You are doing exactly the same thing as the ground wire in the secondary of the commercial isolation xformer when you connect the ground wire of your oscilloscope, or other powered test equipment when you connect it.
@jeffoldbean... i'm looking for the same information... I have a 2500 W 120V 1:1 transformer and I'm not quite sure what value in-rush current limiter to use, if Tony would've explain the value of his I could've used it as a reference...
Very good info..another GREAT service to keep your audience safe! I am always using one with the receivers , but my question is SHOULD I plug the scope into it as well ?? or even a second one ( because mine is a 500va ) I could get a second.. and also wondering if a dedicatedGFI station/circuit would be sufficient for the scope instead.
You shouldn't need to isolate the test equipment. The main goal is to lift the DUT from earth potential so you don't blow your test equipment up because of an earth loop. This is really only a problem with high-voltage equipment (with say a live chassis), such as valve gear. It kind of gets expensive to repair/replace your oscilloscope because the negative of the probe is connected to mains earth! Earth a live chassis through your oscilloscope, you're up for big dollars 😵
So in other word, you create like they say in the soundsystem a DI-box that you can lift from the ground. In Belgium we have L1/L2/L3/N and ground at 400 volt @ home. Between L1 and N, you have 230 volt. Between L1 and Earth you have 230 volt. I once lifted the the Ground from a mixer because of buzzing sound from the light dimmers in the soundsystem. I guess because of a fase problem in the N and Earth.
Might it be cheaper to get an inverter and a battery or power supply to run it, especially if you have the inverter for other purposes? Server PSUs are really cheap secondhand.
I have a question. Your oscilloscope has three prongs and is plugged into an outlet with earth ground. Won't the device under test be earth grounded when you connect the ground lead of the oscilloscope probe to the chassis of the device under test? What am i missing?
That's correct, once you connect the scope probe ground to the chassis or device ground, you will be forcing it to Earth ground. And that's fine for most modern devices even if the device only has a two wire plug. That's because a power transformer or switch mode power supply* performs the same function as an isolation transformer. The problem is vintage radios, some low cost stereos, and TVs, from 1940s through the 1970s that don't have a power transformer for cost reasons. Here one side of the AC line is connected to the chassis. Depending on how the two prong plug is inserted, the hot side of the AC line will now be be connected to the chassis. The scope probe ground is ultimately connected to the neutral wire at the service panel. Now you have a direct 120v short circuit. * see my warning about switch mode power supplies above.
If you have an isolation transformer, you DON'T need to float the scope, nor do you need to plug it into the isolation transformer. Yes, you will provide a path to earth ground through the scope probe, but it will still be isolated from the live connection from the mains power source. The reason I remove the ground connection from the AC outlet of the isolation transformer is because when you are working with certain equipment, especially RF or high frequency equipment, it's better to use the scope (or your test equipment) as the central ground point. This will minimize the potential (no pun intended) for ground loops, which can show up as noise on the scope/test gear. If you look on most scopes, they have a connection on the front, usually close to the probe connections, for a ground lead. You can connect a lead from there to the ground of the device and make the test equipment the star ground point, minimizing ground loop noise. Although I've done this in some of my videos, I really didn't mention it. Maybe I should have....
I will have to rewatch the video. Maybe I'm over analyzing the situation. I have a tendency to do that. It makes me crazy. Lol. My apologies if I sound stupid. If I understand this correctly, with the device under test plugged into an isolation transformer, it is okay to attach the probe ground of the unisolated scope (test equipment) to the device under test so long as the device under test has a transformer and a two prong plug. Is that correct or am I still missing something?
Thanks for this video. Tony you explained the DUT is isolated, but do you also have your scope connected to the same isolated output? Could one also use a separate smaller isolation transformer just for the scope?
The goal is more to isolate from each other rather than from the mains. If you think of you and your test gear on one level and the DUT on an isolated level. The test equipment being isolated from the mains isn't really required unless it's not practical to isolate the DUT due to size/supply voltage/current draw etc. in which event you'd definitely isolate the test gear and be aware that you are not isolated from the DUT.
You don't need to. The idea is to separate the DUT from the ground/earth of the mains, so you don't blow expensive test equipment up because of an "earth loop". Mains powered oscilloscopes suffer badly for this.
The problem with floating the ground on the test equipment (scope) is you can have high voltage on the chassis using conventional probe of the test equipment which could shock you. I hope I'm telling you correctly.
Hey Tony...do you have the isolated outlet set off the case front on insulators/standoffs?? If not wouldn't your ground pin be connected to the other ground pin on the non-isolated outlet? I used a plastic snap in plug to avoid the mounting screws on the metal case....
@@xraytonyb I rewatched...seems I made mine a bit different by grounding the transformer case and isolating the DUT outlet... same thing only different huh!?!?! ;-)
so to avoid so much confusion, don't use an isolation transformer, and put a 3 to 2 prong cheater on your oscilloscope? How can ground be connected to neutral without a short at the buss bar in the breaker box? Boy am I confused, I may not touch a light switch again.
There is never a reason to float your scope. You either put the device you're testing on an isolation transformer, or you use differential mode or a differential probe on the scope. You NEVER want a situation where the chassis of the scope can be live with respect to earth ground. As for ground and neutral, they are always (all the way back to the power company) bonded to one another. This means one side of the line is always at ground and the other is live.
@@xraytonyb Well I can think of one instance of a ground lifter on the scope that I actually used last year. Tracing my old X10 home automation problem. I wanted to see the X10 pulse train at the problem outlets. I specifically did not want to use a small filament transformer to measure across as that would attenuate the signal and not give me an accurate reading. But I know what I am doing. I would not suggest someone do this who does not fully understand the dangers. (problem was the X10 phase bridge and booster unit)
I've just been discussing this somebody who had a buzz on his turntable and he disconnected the earth, and he pointed out it was connected to the water pipe, he says that is how it's done in his area the ground saying that's under code, this is in America i think or Canada. no I told him that's not correct it should be at the metre have you not got on earth in your house he didn't seem to have one. a water pipe is not going to give you good ground especially modern buildings in the last probably 40 years where they've put plastic pipe in the ground. there is a potential problem if you have too many grounds outside you can get issues from neighbours and lightning strikes. in my country you have to be min 40 m away from any buildings or pole for the earth connection.
I'm not sure about the code for this today, but there used to be a regulation for using the water pipe for earth ground. First, it could only be the cold water pipe. Second, it had to be copper, all the way to the ground point. Third, if it is installed anywhere after the entry point to the building, you have to install a meter bypass wire, using proper gauge copper and the proper terminal clamps. That said, it is best to use a ground rod, driven into the ground. There are regulations for how far down the rod must go into the earth and you may need multiple rods, depending on the amount of current the service can provide (100 amp, 200 amp, etc.).
Just information, for the sake of safety... In the earlier portion of the video, you said that earth ground and neutral are not tied after the breaker box. This is not correct. Your major appliances may tie earth and neutral. This is particularly true for older electric stove/ovens. Hope that keeps somebody safe.
What gets confusing for many weekend electricians (and even some licensed) is sub panels. In a sub panel you DO NOT tie the neutral to ground. Most panels come with a green screw that can bond the neutral buss to the ground buss or panel case. In a sub panel installation you leave this out. Yes, older 3 wire ranges and dryers did in fact bond the neutral to the ground at the appliance. It posed a rather minor safety hazard but the NEC now requires four wire connections in new construction for these 240v appliances that need a neutral wire. I don't recall which 3 year electrical code cycle that was adopted.
Tony - Did I hear you correctly? You said when using 240 volts the two 120 lines (L1 & L2) are "tied together" ? ?? *&^$? Can you clarify? When my dryer or stove was wired, the two hots were NOT wired together! They are out of phase, right? Both L1 and L2 go to the appliance.
I'm not sure what part of the video you are referring to. In a split phase system, you either have a single 240 volt secondary winding with a center tap (which is the neutral), or you have two 120 volt secondary windings that are connected in series in-phase. the connection point becomes the center tap and is also the neutral. In the case of my isolation transformer, the two secondary windings are tied in PARALLEL in-phase, which will output 120 volts at higher current.
Hi Tony, I actually heard you say that L1 and L2 will be tied together at 8:07, still a great video! Like in the UK, things are very different in the Netherlands. Where I live, there are no poles, no wires above ground to be seen, except for the transmission lines at 150 kV, all the rest is underground. Then there are substations with transformers for 150kV to 10 (or 20) kV. Those feed local 10 kV local split ring circuits, feeding 10 kV to 400V transformers. Something like 200 to 300 houses will be connected to a single 630 kVA transformer, via a 400/230V 3 phase local low voltage grid. All houses get the 3 phases and neutral into their connection box. We get either a single 35A fuse or three 25A fuses and a smart meter. Like in the UK, all circuits have a RCD. Fun fact, in many houses, the neutral is not connected to the ground pole at the house, only at the transformer, so typically there can be 300 - 400 mV between ground and neutral.
What I said was the the breakers are tied together mechanically. If one leg overloads with respect to neutral OR to the other leg, the two pole breaker will trip and disconnect BOTH L1 AND L2. That's what I was talking about. Sorry about the confusion.
Tony, with the comments you get you may live to regret making this video! 😀 Isolation transformers are a religion. Question about the red receptacle. Is the common (as well as the ground) insulated from the metal tabs?
Good one! BTW, in your isolation transformer diagram the connection on the left is darken for male/plug while the connection on right side is light signalling an outlet/female connection.Always enjoy!
Also, your closing summary. I agree, do NOT get complacent around electricity. I live in a 240V country and I've lost count of the number of times I've come in to contact with both active and neutral at the same time and got a shock. Across uninsulated fuse holders, doing dumb stuff like removing the fuse from the holder with the thing still plugged in, dodgy wiring, etc.. When you think you know enough, you usually don't, lol.
Very important topic, Tony, thank you! There's a lot of people who underestimate the danger.
Thanks Tony! Very informative and useful as always. UA-cam should allow hitting the like button more than once for videos as this one.
I grew up working on tube equipment and CRTs... Worked a lot with one hand in my pocket. 😁😁😁
There's also a case to be made here for the cheap handheld / battery powered scopes. Nice to have on hand for quick testing of questionably wired devices, and no worries about probe grounds connected to service earth.
For those on a limited budget, you can make your own isolation transformer with two old filament transformers. For example, take two 24volt, 4amp transformers and connect them back to back, that being connect the two secondaries together, phasing is not important in this application. The primary of the second transformer is now your isolated 120v AC. But keep in mind that power is power, the product of volts times amps. So (24volts* 4amps) = 96 watts. So the safe limit for this example is only about 90 watts of isolated 120vac due to losses in the transformers. You don't get 120v at 4 amps just because the transformer secondaries are rated at 4 amps. The VA or watt capacity of your kludge isolation transformer is only as high as the VA rating of the one of the transformers.
I got hit early in my career. Using a screw driver I was undoing a 240V connection not checking to see if it was live first. Touched the return wire at the same time and was blown 10 feet across the shop floor and now everything above 25V or with a lot of energy behind it gets much respect from me. Sometimes you need a good scare to get your head right.
That’s a pretty rad setup. I know what you mean about using a massive transformer. My bench area sounds like a warehouse with the buzzing of that massive iron! But like you said, the price was mos def right
I remember talking to a friend who was a tv engineer in the late sixties and all the outdoor techs were issued with isolation transformers but very few used them. With the rental sets, if there was an issue they would just swap it out but of course that meant carrying a really heavy tv set up and down three or four flights of stairs.
My mate being young and wise always tried to fix the sets in situ and even carrying a transformer up stairs wasn’t going to happen.
So he was in the back of the sets, no transformer with live chassis doing repairs in customers houses. He got a few belts but survived!
31:20 Yes. Mr Carlson has an eye opening description of when he was electrocuted - always treat high voltages with respect. When I was a teenager I was very cavalier with devices having mains voltage input (240VRMS in the UK) and I lost count of the number of times I received an electric shock😳
Very good Tony, I think you should have talked about variacs as some people think they are isolation transformers.
Excellent video Tony, another issue to consider are the USB or RS232 connections of the equipment, if the cpu is connected before the isolation transformer, automatically all equipment are grounded.
Great description thanks👍👍
Great video, thanks much
It is common practice to isolate the mains and earth (chassis) ground in a bench custom designed troubleshooting AC power supply. Many experienced electronic technicians eventually build their own troubleshooting AC power supply to get the features they want.
Also common is to have a traditional factory ladder logic control scheme to operate the AC power supply control circuit, but usually operating from 12VDC which is very common for relay coil control and other control devices found in an industrial control cabinet. In a ladder logic scheme you will have a low voltage control approach controlling high voltage (120VAC) at high currents, where the two systems are electrically isolated (either magnetically or optically or both). A common feature is a series interlock where if any alarm or fault is tripped (open) the unit will shut down. These cabinets are recognizable by having a Green start push button switch (with latching relay) and a Red stop push button switch. As such it is relatively easy to include a transformer surge current limiter using a large resistor, which is then shorted out by a relay at some time after the end of the magnetization current worse case (which would include a downstream variac). I like the resistor approach because it does not use heat to operate like a thermistor, and is ready whenever the power is quickly cycled. A lot of the jitter hysteresis ON/OFF function is usually included in the 12VDC time domain relays. You can also delay when output power is available to make sure the system is stable incase there is a load on the output.
It is very handy to have two analog AC ammeters in two ranges (like 10A and 2A for example) along with an output AC analog voltmeter. The analog meter movements are highly damped which can provide a clue as to the failure by watching the indicator movement upon power up. If you work on the same type of gear all of the time, like at a company repair depot, you can often know the failure just by observing the current movement behavior on power up. Setup the meter so you start with the 10A and then can switch to the 2A with a make before break switch.
If you work on a lot of gear I find you only really need one current limiting device (other than several different CBs in parallel 1A, 3A, 10A) because the failure mode is almost always a direct short or almost a direct short. I am using using a 220 ohm automatically cooled low inductance 600W power resistor as my current limiting device, it is put in or out of with a shorting switch.
It is very handy to have a large power toggle switch to enable power to the output isolated AC receptacles.
You can buy a Hospital Grade Toroidal Isolation transformer stand alone without the case and connect it as you like. For a large transformer, features include an automatic reset thermal cutout and a separate shield.
For large company repair depots, there is a troubleshooting bench and a separate outgoing test/calibration bench. The calibration bench will have a pure sinewave AC power supply where the earth ground is not isolated but the mains are, mainly for personal safety. The instrumentation test pure sinewave AC power supply can include an AC power analyzer which will give many different real time measurements which can be very specialized, like power factor, AC voltage distortion (THD), AC current distortion (THD), as well as have a large operating range for testing universal (switching or selectable) equipment. The outgoing test procedures can include calibration and testing at specific AC voltages (if it is a power supply this would include line and load regulation).
It is possible that a three phase Y (with neutral) as wired for a street of homes may not string the neutral to the single phase transformer along with two phase connections. In this case the two terminal transformer primary will not include the neutral. As such the pole transformer neutral for lightning protection will be derived from the secondary neutral (includes the transformer case through a jumper connection). The Y neutral will be available along the primary main backbone feed (which includes the neutral connection to ground at each backbone pole for lightning protection), but not necessarily the street feeds having only two high voltage feeds. If you had three identical streets each would use two phases in sequence such that the load is balanced per street. It is interesting how different power companies come up with creative solutions for high voltage distribution, which at times can get very creative in how transformers are connected as derived from three phase Y or Delta.
Very interesting, thank you! I just checked an my isolation transformer has a 3-prong isolated outlet but only has a 2-prong polarized cord where it plugs into mains so it isn't physically possible for it to be connected to earth ground.
I am in a couple of Facebook groups for various facets of electronics. There is no other topic then isolation transformers where people have such wildly varying differences of opinion; especially carrying the ground lead to the output socket.
It gets almost as hot when folks ask about adding a grounded cord to old test equipment that came with a two-wire cord.
Yes! Isolation transformers should be added to the no religion, no politics conversation list! 😂
In Germany, houses are usually supplied with three-phase electricity. There is a transformer on the street that reduces the voltage from 10kV or 20kV to 230V/400V. Between a phase and neutral or earth it is 230V. Between two phases 400V.
Four power lines lead to the house: 3 phases and a combined neutral + earth conductor.
In the first box in the house, neutral and ground are split and connected to the ground rod.
Neutral blue and earth yellow-green may no longer be connected afterwards.
The normal consumers get one phase, neutral and earth, i.e. 230V. The consumers are distributed across the three phases. Larger motors and the stove gets three phases, neutral and earth, so 400V.
Electric cars can also be charged using 230V or 400V.
Some even heat the house with 400V
But I warn, 230V to ground can bite you very well, 400V will definitely ruin your week, 10kV is certainly fatal
Nice setup. I’m looking to build something similar, and with what I have on hand as well. Much appreciated!
Thanks for this video! 👍
Hi Tony 76yrs old Jeffrey from England. When you are able, could you Pleeeeeease, do a Vid on Transformer in rush current. And how to select the correct type, and rating for an input Fuse. Thanks.
Sometimes you need to zap yourself with high voltage just to not forget its power. I just did it while playing my new isolation transformer, immediately tought that's why I'm building it.
Smaller 100-200W transformator with 2-3A breaker is good If you don't want to burn your sockets and switches, some PC power supply doesn't start trough light bulb, the bulb just blinks, but the powersupply works correctly from the mains without the bulb, and sometimes it has no protection at all and burns out half of the PCB and only the mains 25A breaker stops its madness.
One of my basic trick if I don't feel confortable working on something I just use one arm, so even if I get shocked doesn't go trough my heart.
I have a medical grade isolation transformer where I have also disconnected the ground wire from the secondary.
I also included the option to reconnect the ground thru a 1M resistor. I did this thru a lighted switch so I absolutely know when I have a solid ground or a 1M ground.
I have my oscilloscope connected to this isolation transformer.
I understand the danger, but I don't want any surprises or possible damage to my scopes.
I have a second isolation transformer I can use for my DUT, that is totally isolated, should I need to be extra cautious.
I'd appreciate your thoughts on this.
By the way, a great video. Very educational for any novice.
Very useful information .Thank you very much
Something I've never understood about US/CA mains wiring is the black wire being "hot" and the white being neutral. This makes absolutely no sense, as one would assume black to be either negative or neutral. In 'Stray'a (Australia) and the UK I believe, we use brown for active (live/line/hot) and in the 1980's it used to be red, blue for neutral (and again in the 1980's it used to be black), and green (with a white stripe) for ground/earth.
Yeah the colours are strange, but I think the non-polarised plug sockets are far stranger! 😀 Gotta love our UK style plugs!
@@dughuff8825 except for the random 13A fuse in the plug...
@@AstrosElectronicsLab 🙂 who's got a random 13A fuse? Fuses should always be appropriate, but you're right that there's nothing to stop someone taking the right fuse out and replacing with a 13A.
Well I think the Brown/Blue European standard is also backwards. The earth is brown and the sky is blue. So why is the brown the hot and the blue neutral?
@@andydelle4509 because the earth is green
My father's voice in my head... black to brass to save your ass. Lol
Another one; Wide is White!
Goof info! I would like to add that old transformer-less radios and TVs are not the only item to be careful with. Switch mode power supplies are also direct operation on the primary AC power side. To be safe, always use an isolation transformer when troubleshooting an SMPS or even working on a device that uses an open exposed SMPS. Note that the DC output of most SMPS's is isolated. But the on board transformer is operating at 50khz or so. The primary oscillator or chopper circuit is directly connected to the AC line.
I have a master switch on my bench with a GFCI outlet. Yes the GFCI sometimes nuisance trips upon bench power up but it may save your life some day.
GFCI Theory (for those who want to know more):
A GFCI works on the principal that all current must return to it's source A GFCI passes the hot and neutral wires through the center of a small toriod transformer. The winding of that transformer is connect to a relay or re-settable circuit breaker coil via some simple circuitry. If all the current being used by the connected device is returned on the neutral, it will cancel at the small transformer and no voltage will be induced to the secondary coil. If however some current, over 5-10ma, is finding an alternate path to ground, like through you, voltage will now be induced into the small transformer secondary and trip the relay or internal breaker. Note that even though GFCI outlets all have a ground connection and three prong outlet, they don't need a ground connection to work nor do they care about the ground wire. They simply monitor the current flowing through the hot wire coming back on the neutral wire. Any difference exceeding 5-10ma indicates a ground fault situation. This is also why they sometimes nuisance trip when a device has power cord noise reduction capacitors tied to the ground wire, which as we know is tied to the neutral at the service panel. The current charging those small value capacitors sometimes exceeds the GFCI threshold and trips it.
nice overkill on that transformer as well as good recycling. It is always belt and suspenders when it comes to safety precautions.
before I built a similar setup I have seen the blinding flash of an unexpected path to ground. I added a double pole single through switch for the device under test that disconnects both neutral and hot so I do not have to unplug every time. I still have tp pay attention just the same.
I’m glad I stopped to read this. In school I learned of the danger but was taught to use a Variac and an ‘adapter’ for the scope that allowed one to convert the three prong connection of the oscilloscope into a two prong connection. Essentially to float the test equipment, not the unit under test. I seem to have it all wrong.
The thing that concerns me is not only to protect the scope, but how do I protect myself? Should I be using a so-called ‘ground fault circuit interrupter’ (GFCI) outlet at the wall ahead of the isolation transformer or Variac? Is a GFCI receptacle still effective at the output?
My understanding is that the GFCI wouldn’t trip at the output without the ground wire as a reference.
Finally, if I’m NOT floating the scope, but floating the equipment under test, will I be protected as long as I have the scope plugged into a GFCI and only touch the probes, and not the chassis or live connectors of the unit under test?
I have, and maintain, a healthy respect for the ability of electricity to reach out and bite (hard) unexpectedly.
Edit: I appreciate you sharing the design of your dim bulb tester. I have an older Universal Power Supply (UPS) that came with a largish transformer that I thought I could use to build my own isolation transformer, assuming that there is at least one set of secondary taps that don’t step the voltage up or down.
your logo in the right bottom corner does funky things ;D
I followed your approach, disconnected the secondary ground on my isolation transformer, truly isolated. Now I am working on AA5 radio (hot chassis) for alignment, the radio alignment procedure requires that the negative lead of signal generator needs to connect the radio chassis while the positive lead of signal generator is connecting 12AT7 pin #7. My question is do I need to plug both the radio and signal generator to the same isolation transformer to do the alignment?
same thing i created mine. i use microwave transformer e-core 2 pieces connected each other with 4 primary coils in series each two.
You know can use GFI outlets for your outputs. No ground is required to make it work. It will trip with an imbalance and will keep you safe.
How would they sense a transformer -less radio? Such as he was talking about being plugged into it.
@@redfields5070The usual way with hot and neutral lines. Any sensed imbalance occurs if there were to be a leakage path elsewhere, perhaps from the hot chassis to a grounded test instrument.
Yep, I have a couple of Powervar isolation transformers that had the neutral bonded to earth (as well as earth bonded across both sides), which I had to remove.
I have an older RCA WP-26A with 2-slot outlets.
That automatically solves the ground problem.
Thanks Tony. Yes UK is very different. We rarely have transformers on poles in residential areas. We have local sub stations and cables go underground to each house. We Have Live, Neutral and Earth NO center tap. Every UK House will have several safety devices. an RCD (residual current device) to detect an earth faults above 30mA and MCB (miniature curcuit breaker) for overcurrent protection. We don't use the term "Hot" we say Live. Exposing a live conductor in UK is in fact Illegal except for testing by a qualified engineer.
All equipment in UK installations have finger proof terminals so if you remove an outlet from the wall with live conductors connected it is impossible to touch the live conductors. We still use isolation transformers in some situation.
I believe that the 110V transformers used with portable power tools in the UK do use the centre tap (55-0-55) as earth.
What I would like to see is someone explain how to TEST an isolation transformer....🤔
(just verify the isolated ground is not connected to "house ground" or the transformer case???)
I'd love to know what you use as an inrush limiter...mine tends to pop the house circuit breaker on occasion....
Many thanks for this vid. 👍🎶😊
So long as neither side of the secondary of the isolation xformer, the secondary remains isolated You may measure a ‘ghost voltage‘ between either side of the secondary and ground, but that is capacitively coupled. It is not “real” voltage, and carries no current capacity. You are doing exactly the same thing as the ground wire in the secondary of the commercial isolation xformer when you connect the ground wire of your oscilloscope, or other powered test equipment when you connect it.
@jeffoldbean... i'm looking for the same information... I have a 2500 W 120V 1:1 transformer and I'm not quite sure what value in-rush current limiter to use, if Tony would've explain the value of his I could've used it as a reference...
Very good info..another GREAT service to keep your audience safe! I am always using one with the receivers , but my question is SHOULD I plug the scope into it as well ?? or even a second one ( because mine is a 500va ) I could get a second.. and also wondering if a dedicatedGFI station/circuit would be sufficient for the scope instead.
You shouldn't need to isolate the test equipment. The main goal is to lift the DUT from earth potential so you don't blow your test equipment up because of an earth loop. This is really only a problem with high-voltage equipment (with say a live chassis), such as valve gear. It kind of gets expensive to repair/replace your oscilloscope because the negative of the probe is connected to mains earth! Earth a live chassis through your oscilloscope, you're up for big dollars 😵
Would knowing which prong on a non-polarized plug goes to hot and which one goes to chassis eliminate the danger?
So in other word, you create like they say in the soundsystem a DI-box that you can lift from the ground. In Belgium we have L1/L2/L3/N and ground at 400 volt @ home. Between L1 and N, you have 230 volt. Between L1 and Earth you have 230 volt. I once lifted the the Ground from a mixer because of buzzing sound from the light dimmers in the soundsystem. I guess because of a fase problem in the N and Earth.
Might it be cheaper to get an inverter and a battery or power supply to run it, especially if you have the inverter for other purposes? Server PSUs are really cheap secondhand.
They are unreliable and low power.
I have a question. Your oscilloscope has three prongs and is plugged into an outlet with earth ground. Won't the device under test be earth grounded when you connect the ground lead of the oscilloscope probe to the chassis of the device under test? What am i missing?
That's correct, once you connect the scope probe ground to the chassis or device ground, you will be forcing it to Earth ground. And that's fine for most modern devices even if the device only has a two wire plug. That's because a power transformer or switch mode power supply* performs the same function as an isolation transformer. The problem is vintage radios, some low cost stereos, and TVs, from 1940s through the 1970s that don't have a power transformer for cost reasons. Here one side of the AC line is connected to the chassis. Depending on how the two prong plug is inserted, the hot side of the AC line will now be be connected to the chassis. The scope probe ground is ultimately connected to the neutral wire at the service panel. Now you have a direct 120v short circuit.
* see my warning about switch mode power supplies above.
Sparks will fly, in the least; the oscilloscope needs to be isolated also.
@@atmylab so always use a cheater 3 to 2 prong plug on the oscilloscope?
If you have an isolation transformer, you DON'T need to float the scope, nor do you need to plug it into the isolation transformer. Yes, you will provide a path to earth ground through the scope probe, but it will still be isolated from the live connection from the mains power source. The reason I remove the ground connection from the AC outlet of the isolation transformer is because when you are working with certain equipment, especially RF or high frequency equipment, it's better to use the scope (or your test equipment) as the central ground point. This will minimize the potential (no pun intended) for ground loops, which can show up as noise on the scope/test gear. If you look on most scopes, they have a connection on the front, usually close to the probe connections, for a ground lead. You can connect a lead from there to the ground of the device and make the test equipment the star ground point, minimizing ground loop noise. Although I've done this in some of my videos, I really didn't mention it. Maybe I should have....
I will have to rewatch the video. Maybe I'm over analyzing the situation. I have a tendency to do that. It makes me crazy. Lol. My apologies if I sound stupid.
If I understand this correctly, with the device under test plugged into an isolation transformer, it is okay to attach the probe ground of the unisolated scope (test equipment) to the device under test so long as the device under test has a transformer and a two prong plug. Is that correct or am I still missing something?
Thanks for this video. Tony you explained the DUT is isolated, but do you also have your scope connected to the same isolated output? Could one also use a separate smaller isolation transformer just for the scope?
The goal is more to isolate from each other rather than from the mains. If you think of you and your test gear on one level and the DUT on an isolated level. The test equipment being isolated from the mains isn't really required unless it's not practical to isolate the DUT due to size/supply voltage/current draw etc. in which event you'd definitely isolate the test gear and be aware that you are not isolated from the DUT.
You don't need to. The idea is to separate the DUT from the ground/earth of the mains, so you don't blow expensive test equipment up because of an "earth loop". Mains powered oscilloscopes suffer badly for this.
At work we use a small iso trans for the test equipment because the DUT is usually large.
@@jamiewykes8585 This is exactly my issue too. Thanks very much for the reply guys.
The problem with floating the ground on the test equipment (scope) is you can have high voltage on the chassis using conventional probe of the test equipment which could shock you. I hope I'm telling you correctly.
Hey Tony...do you have the isolated outlet set off the case front on insulators/standoffs??
If not wouldn't your ground pin be connected to the other ground pin on the non-isolated outlet?
I used a plastic snap in plug to avoid the mounting screws on the metal case....
As I explained in the video, the orange outlets have isolated ground terminals.
@@xraytonyb I rewatched...seems I made mine a bit different by grounding the transformer case and isolating the DUT outlet... same thing only different huh!?!?! ;-)
so to avoid so much confusion, don't use an isolation transformer, and put a 3 to 2 prong cheater on your oscilloscope? How can ground be connected to neutral without a short at the buss bar in the breaker box? Boy am I confused, I may not touch a light switch again.
There is never a reason to float your scope. You either put the device you're testing on an isolation transformer, or you use differential mode or a differential probe on the scope. You NEVER want a situation where the chassis of the scope can be live with respect to earth ground.
As for ground and neutral, they are always (all the way back to the power company) bonded to one another. This means one side of the line is always at ground and the other is live.
@@xraytonyb Well I can think of one instance of a ground lifter on the scope that I actually used last year. Tracing my old X10 home automation problem. I wanted to see the X10 pulse train at the problem outlets. I specifically did not want to use a small filament transformer to measure across as that would attenuate the signal and not give me an accurate reading. But I know what I am doing. I would not suggest someone do this who does not fully understand the dangers. (problem was the X10 phase bridge and booster unit)
I've just been discussing this somebody who had a buzz on his turntable and he disconnected the earth, and he pointed out it was connected to the water pipe, he says that is how it's done in his area the ground saying that's under code, this is in America i think or Canada. no I told him that's not correct it should be at the metre have you not got on earth in your house he didn't seem to have one. a water pipe is not going to give you good ground especially modern buildings in the last probably 40 years where they've put plastic pipe in the ground.
there is a potential problem if you have too many grounds outside you can get issues from neighbours and lightning strikes. in my country you have to be min 40 m away from any buildings or pole for the earth connection.
I'm not sure about the code for this today, but there used to be a regulation for using the water pipe for earth ground. First, it could only be the cold water pipe. Second, it had to be copper, all the way to the ground point. Third, if it is installed anywhere after the entry point to the building, you have to install a meter bypass wire, using proper gauge copper and the proper terminal clamps. That said, it is best to use a ground rod, driven into the ground. There are regulations for how far down the rod must go into the earth and you may need multiple rods, depending on the amount of current the service can provide (100 amp, 200 amp, etc.).
Just information, for the sake of safety... In the earlier portion of the video, you said that earth ground and neutral are not tied after the breaker box. This is not correct. Your major appliances may tie earth and neutral. This is particularly true for older electric stove/ovens. Hope that keeps somebody safe.
What gets confusing for many weekend electricians (and even some licensed) is sub panels. In a sub panel you DO NOT tie the neutral to ground. Most panels come with a green screw that can bond the neutral buss to the ground buss or panel case. In a sub panel installation you leave this out. Yes, older 3 wire ranges and dryers did in fact bond the neutral to the ground at the appliance. It posed a rather minor safety hazard but the NEC now requires four wire connections in new construction for these 240v appliances that need a neutral wire. I don't recall which 3 year electrical code cycle that was adopted.
Tony - Did I hear you correctly? You said when using 240 volts the two 120 lines (L1 & L2) are "tied together" ?
?? *&^$? Can you clarify? When my dryer or stove was wired, the two hots were NOT wired together! They are out of phase, right? Both L1 and L2 go to the appliance.
I'm not sure what part of the video you are referring to. In a split phase system, you either have a single 240 volt secondary winding with a center tap (which is the neutral), or you have two 120 volt secondary windings that are connected in series in-phase. the connection point becomes the center tap and is also the neutral. In the case of my isolation transformer, the two secondary windings are tied in PARALLEL in-phase, which will output 120 volts at higher current.
Hi Tony, I actually heard you say that L1 and L2 will be tied together at 8:07, still a great video!
Like in the UK, things are very different in the Netherlands. Where I live, there are no poles, no wires above ground to be seen, except for the transmission lines at 150 kV, all the rest is underground. Then there are substations with transformers for 150kV to 10 (or 20) kV. Those feed local 10 kV local split ring circuits, feeding 10 kV to 400V transformers. Something like 200 to 300 houses will be connected to a single 630 kVA transformer, via a 400/230V 3 phase local low voltage grid. All houses get the 3 phases and neutral into their connection box. We get either a single 35A fuse or three 25A fuses and a smart meter. Like in the UK, all circuits have a RCD. Fun fact, in many houses, the neutral is not connected to the ground pole at the house, only at the transformer, so typically there can be 300 - 400 mV between ground and neutral.
What I said was the the breakers are tied together mechanically. If one leg overloads with respect to neutral OR to the other leg, the two pole breaker will trip and disconnect BOTH L1 AND L2. That's what I was talking about. Sorry about the confusion.
Tony, with the comments you get you may live to regret making this video! 😀 Isolation transformers are a religion.
Question about the red receptacle. Is the common (as well as the ground) insulated from the metal tabs?
Good one! BTW, in your isolation transformer diagram the connection on the left is darken for male/plug while the connection on right side is light signalling an outlet/female connection.Always enjoy!
Hi Tony
Love your work. What is your email address?
Want to ask you a question regarding Nippon sounds model number SR3400 receiver
I wonder if you didn't know this stuff, should you be playing around with it?