That's a really good explanation (as always!) The oscilloscope analogy is perfect because, I have to admit, I learnt the hard way. Back in the early 90s I made the mistake of "live" working on a valve amp. Didn't use a mains isolation transformer as I thought I was careful. No shock fortunately, but 240v straight up the test lead into a £399 HP 'scope didn't end well. Amp survived, scope wasn't so lucky. It's amazing how quickly you learn to always use an isotran after watching your prize measuring tool emit it's magic smoke!
I started working on the so-called All American 5 vintage radios. These have one side of the AC line connected to the metal chassis & the tube/valve heaters in series. And since old US plugs weren't polarized, connecting one way would make the chassis hot/live, even when the radio is switched off. So, I made myself an "iso-xfrmr" using a pair of doorbell transformers connected back-to-back.
Hi John, You have responded to a few of my comments over the past while, I suspect, in hindsight, that some of my questions were trivial to you but you took the time to respond. Since then I have been pottering away with and oscilloscope, isolation transformer, signal generator, variac, etc, etc. Thanks to you I understand some of these things a bit more and I am grateful for your help. Owen
Thanks for your videos John. I’m an electrician but haven’t been to college in 12 years and want to brush up on my knowledge. Your ‘sparking’ my interest again !.
Isolation transformers have a few other uses as well that are a little less safety related. Very handy devices when used correctly. One example is when measuring RF and you have rather long runs of cable between the transformer providing power to your equipment it is possible to have conducted noise, an isolating transformer breaks the ground current path and can remove undesirable noise.
Thank you for this video! i'm helping a group setting up a electronics workshop, and it have been such a long time since i thought about isolation that i couldn't remember clearly why you need it.
When I started working on live circuits it was required to have item you work on behind an isolating transformer AND have measurement devices like scopes on a separate isolating transformer. Υes, you need two isolating transformers for that (with no ground connection!) ! The reasoning was that if you have a grounded scope connected to device you're working on, connecting ground lead makes it earth referenced. Now if you touch anything on the device by accident, you will get shocked by the now mains referenced device (even if it is behind an isolating transformer). Having the scope chassis also isolated makes it safe to touch any single point in device even with scope ground lead connected.
Another issue with isolating the scope instead of the device under test is that the scope BNCs could become live simply by connecting the probe's ground lead where it shouldn't be. I need an isolating transformer, might make one by taking the primary winding of an MOT and replacing the secondary of another with it, so it has two 120v windings.
11:00 There is in fact an even more dangerous situation that could happen if you cut off the earth pin from the oscilloscope. Poking the clip from the probe into anything at line potential *will* result in the metal casing of the oscilloscope (and possibly your workbench) being at full mains voltage, even if there is no fault in either in the scope or in whatever device you're probing.
I remember many years ago when I was carrying out PAT testing at an aircraft factory near Gloucester. I found an oscilloscope that had no earth continuity because someone had foolishly disconnected the earth conductor in the mains plug. They were not happy when I reconnected it. I had no alternative other than to submit a formal report to the firm's safety officer.
Bear in mind there are also 240v output 'yellow box' transformers around that are NOT isolating transformers. Like the 120v building site transformer they are center-tap earthed. If one of these finds its way onto the testbench it's a potentially hazardous situation. Not only does it not isolate, but if there are also standard 240v supplies on the bench you could have 360vac between these and one side of the transformer output. Potentially a very nasty shock, or (as I saw happen once) a mighty great BZZZAP! if they get interconnected. You can generally convert this type into an isolating transformer by disconnecting the center tap so long as it has separate primary and secondary windings. Just don't use it as is, though.
Hi John,In the video, the earth crocodile clip of the scope was shown to clip onto the neutral of the non isolated device under test. Will that not create a parallel path for the current from the live wire in the device under test to flow to the neutral as well as the earth of the clip?. There will be an imbalance between live and neutral in the scope circuit and/or device under test circuit. In that case, if there is a RCD, will the RCD trip?Thanks.
Hi John; enjoy your videos. It seems most folks recommend using isolation transformers when repairing old radios and such, especially the transformerless type. Is there any advantage to using one when working on modern tube guitar amplifiers such as Fender or Vox--or could one be in more danger plugging such amps into the isolating transformer?
You can get metal framed/cased isolating transformers with the casing earthed to the mains. If you have a PMR mains supply this means that the neutral carries through to the secondary neutral of the isolating transformers.
Years ago, I made an isolation device from two identical transformers, 240 in, 12v out - 12v in 240v out. It was so well buffered, you could get a nice spark from the 240v out, without blowing anything. I suppose you could call it an inverter of sorts.
Neutral (blue or black ) is connected to 'the earth' and transformer windings, Earth (green or green and yellow) is only connected to the 'earth' but NOT the transformer so protects you by providing a preferential 'path to earth'.
Enjoyed this video.i have 2 site transformers 240_110v and another 240_110v.I need 240 _240v mains isolated to check an audio amp fault.if I connected both transformers back to back and removed earth from second transformers would I be able to use my oscolscope with caution.engineer 50 years electrical.tks Michael
Theoretically yes, however in reality this would require modifications to one of the transformers, as if using it in reverse the output would be the exposed plug pins and the input a socket. There may also be other problems if the two transformers are not exactly matched, output voltage may not be what you expect and there could be further issues with inrush current.
Really really interesting. I'm retired now but always had an interest in electronics and want to buy an oscilloscope and am rather concerned about blowing it up. This video explains a lot but wonder if the scope is earthed and if I'm using a bench power supply which has a max output of 30 volts 3 amps do I still need an isolating transformer between the power supply and circuit board? Or is there not enough power there to worry about? (when using scope)
I happy you clarified to always keep to scope to mains but make sure your broken device is on the floating power. Quick extra question. - The earth pin of the isolation transformer will be earthed back to mains earth and neutral at power board. SO is there any way your ground clip touching anything powered in the broken circuit can flow back to ground eventhough the 240V is floating? AND whats the point of then connection that earth pin up instead of just supplying 2 connected pins on the output so you know you only dealing with the live and N floating voltage and thus TOTALLY isolated?
Isolation transformers were at one time very common in some patient care areas of hospitals (Canada) where mains powered equipment was connected to patients for any given time.. They also had to be monitored for leakage current in microamps, and fault location (L1 or L2) ... Now in new installs these are not common as most medical equipment is effectively isolated by a transformer within the device or within the cart it is attached in or on.
Hi Jonh, I found myself here trying to sort out a problem with my Off grid, inverter and battery setup. Basically I've got a 3kva Victron easy solar inverter/charger a 48v battery bank (8×12v 160AH in series/parallel) and a 10kva kubota/ linz generator, no solar panels yet! that's the set up, now the problem, I connected a washing machine with the generator running 2kw going to the battery charger, the washing machine does nothing just flashing fault light, turn the generator off, the washing machine runs perfectly and draws 1800w. I've can run various tools a kettle and Hoover, 6kw or more no problem from the generator, I've noticed the frequency has gone up to 52.2 Htz was 51.5 Htz possibly due to a fuel filter change, it's a capacitor controlled generator not automatic voltage control, after some research I've found this type of genny isn't the best for electronics, and washing machines very often don't like them. I know enough about electrics to know I'm out of my depth, not sure if the machine doesn't like the voltage, frequency, noise or something else. sorry for the long post.
Isolating transformer type 120 volt bathroom shaver sockets used to be used widely in Canada as well, although they have been largely replaced by GFCI (RCD) sockets.
Why is it so difficult to find a small mains isolation transformer for one appliance that does not step down or up the voltage? Thanks for the video. I've been all over the internet and cannot find one. The BNC - connection on my signal generator leaks about 128 vac.
Very correct to connect your test circuit to the isolation transformer and not float the scope. However, as soon as you connect your little black scope ground probe to the test circuit, you effectively have reintroduced reference to mains ground, which means you can receive a deadly shock by only touching one wrong place in the test circuit. That can be avoiding by using active differential probes on your scope in addition to plugging the item you are working on into the isolation transformer.
Yes very good info that even l could understand. I have been trying to buy one and done searches in several very large electronics suppliers (in UK) for isolation transformers but have yet to find one listed. 240v to 240v
Sorry for commenting 5 years too late, but why anyone not get a shock from the secondary side of an isolating transformer if they came into contact with a live conductor while their feet were on the ground (true earth) ?? In this instance would the current not flow through the person connected to live (via the body and feet) through to the general mass of earth ???
@@jwflame Thanks for response, im still finding it difficult to understand how if there is 230v on secondary side of the transformer, then why this cannot flow to earth (via a persons body & feet) and give them a shock. In normal operation, on the secondary side, current will flow out through line and back through the neutral, so similarly why not out through line and to true earth (via a person). ???
It seems that these isolation transformers usually has a connected earth wire on their outlet sides. Is this neccesary? In case there's an earth fault in the device under test, wouldn't it cancel the benefit of using an isolation transformer?
At 12:45 we see that the isolation transformer, itself is grounded via a three prong plug. However the device under test connects to the transformer via a three prong plug as well but I assume that this is for compatibility reasons as the ground plug on the secondary is not connected to mains earth?
Hey John great video, found it searching for isolating transformers in regards to use on a leasure boat. I'm an electrician my dad has recently bought a sail boat and has asked me to install an electrical connection for 230v use, it seems that research in to marine electric has opened up a can of worms just wondering if you have any help or guidance you could give me to ensure a safe installation. Kind regards Scott
John, I bought an A.C. rheostat but was advised to connect this to an isolation transformer, which I did. I also connected my D.C. power supplies to this transformer too (it has several "kettle" type outputs). I attached the oscilloscope to this transformer but now I see that it was a bad idea. As a rule, should I just attach all my lab equipment to the regular mains and ONLY the test item(s) to one or more isolation transformers?
Grounding can be interesting from place to place. Japan tends to use dedicated grounding posts rather than having ground connections in the sockets. In Florida, we have to paranoid about the matter due to absurdly common lightning strikes.
+IrishHitman79 Yes, double pole MCB should be used on the output. Larger transformers with multiple outlets such as those used on building sites usually have DP MCBs fitted as standard.
Thanks John great presentation. How about if one were to fit an RCD on the secondary of the safety transformer? Then presumably it would offer live and neutral protection too so much safer? Thanks
Thanks for your explanation of possible mains earth current paths. I was probing signals on a microprocessor. It was powered by a PC USB supply. I had the oscilloscope ground connected to the negative side of the USB supply and at some point the microprocessor exploded and everything connected including the USB port vaporised and the house RCD breaker tripped. My first thought was that the negative rail of the USB was not at ground potential but surely it can only be plus or minus a few volts with respect to ground? I still can’t quite understand what happened. Perhaps it is impossible to say from this information. Needless to say it was a very expensive catastrophe.
I like the home conversion of the isolating transformer. It looks like a converted 110v builders' transformer very confusing mix of standardised colours, blue, yellow 110v or 240v. A builders' site transformer is designed to output 110v only.
Quiz for newbies. What about all the current flowing thru neutral as the normal course of events. What is the resistance of the "earth", from your house to the power pole and back to the generating station.
Great vid as always John. But gonna make myself look stupid here but still don’t get how you don’t get a shock of touching live? Surely touching a live on isolation transformer will still pass through body/into ground and back to sub station then back up to isolation transformer and conducted through into person to make circuit? Sorry for stupid question!!
There is no electrical path through the isolating transformer, it's only a magnetic field that links the primary and secondary side. Current only flows in the secondary if something is connected to both of the output terminals, connecting to only one of them doesn't complete any circuit so no current flows and no shock.
@@jwflame Thanks. All I could see in my head is that the circuit would still be made through magnetic field down output then through person then through ground then back up input to make circuit
This is probably a daft question.. Is the main earth lead still connected to ground at each individual property or is the earth now taken back to the local substations? Years ago I remember seeing the earth on some old places used to look like a bit of iron re-bar knocked into the ground with the earth wires attached outside the property's, looking back, wouldn't this become almost next to useless? I only ask because it's something I've not seen these days.
some houses still have an earth rod where for various reasons the network operator cannot supply an earth. but in most cases now new supplies have the house earth tapped off the neutral in the service head, since the neutral itself is earthed at the substation. there are several situations where this is undesirable for example petrol stations, where outdoor machinery and metal pipework is used, animal barns, garden sheds. this is because there can be a difference between the ground potential on the neutral wire and the local ground potential, so it is inadvisable to use an exported earth outdoors. in these cases a local earth rod should be installed and tested and the network earth abandoned for the outdoor installation.
+Steve Craft Where you from steve ? Here in Belgium it's either a rod into the ground in renovation work. And in new buildings we have a earth loop around the house few meters deep.
That's fine as long as you do not have a ground pin on the plug. In the USA most earth ground is directly connected to neutral in the fuse panel. This can be very dangerous.
Thanks. Very informative. I picked up a RS safety isolating transformer second hand. It looks like yours and is UK made. Inside it, the earth is passed-through from the input to the output sides directly, via a bolt on the transformer housing. It designed to be disconnected if desired and they even provide a little screw terminal to put it in if disconnected! The earth is not strapped to either terminal of the output side of the transformer. It just goes straight through via bolt on the transformer housing. I tested that there is no continuity between the earth and either of the pins of the transformer's output socket. I did this when it was plugged into the wall (obviously not when turned on). I am struggling to understand why they have made deliberate provision for the earth to be disconnected and even suggested how to do it on a sticker on the lid! If I leave it as is, the earth on the output is effectively linked straight back to the neutral at the fuseboard of the house. Does this mean it is still 'isolated' and therefore safer to use with my oscilloscope? Sorry for the long question!
The output is still isolated regardless of whether the earth is connected or not. For many things it doesn't matter if the earth is connected, but it may be relevant in some situations, all depends on the equipment connected to it. Could be useful to have earth connected for some types of electronics to cover ESD issues, and older items such as radios or televisions may need the earth reference to operate correctly. However caution is required as connecting an item that contained a fault to earth would then result in the isolated output being referenced to earth, defeating the isolated aspect of it.
@@jwflame Thanks very much for the detailed response. I think I will leave the earth connected for now. Somebody on WhatsApp scares me a bit by saying that mains filters on the item under test could be an issue if I disconnected the earth. Thanks again for helping. All the best.
Simple question but... If the incoming neutral is connected to earth, why do we have a separate earth running through the circuit, especially when the earth is connected to the neutral when it enters the house (our old house anyway). Never been clear on that. Thanks John.
The reason for that is somewhat complicated but I will try to explain my take on it. In the case of GFCI outlets (other countries call them other names) all of the power goes from the live and into the neutral, and in theory, the current measured through each terminal should be the same. However, if current leaks from live to ground, such as a faulty motor winding or a wet piece of equipment, the RCD will notice that more current is going through the live terminal than the neutral (because live is leaking to earth) and disconnect that piece of equipment. The earth helps to make damn sure that someone didn't mix the live and neutral around, because if there was just live and neutral and the chassis was connected to neutral instead of earth, if someone jams the plug in backwards or the electrician had too many drinks, all of the sudden the appliance becomes live. Here in america, we had non polarized sockets where you can mix the live and neutral around without any difficulty, these were mainly used for older lamps and stuff. I know this is not a great explanation, but I hope it helps :D
+Colin Herbert A single connection for earth and neutral used to be common in many places, the building where I work is still wired that way. You need to provide local GFI/RCD protection as needed like Luke described. As you say it is now only common on a distribution level, and split up where the power enters the house. This way you can still have a lower earth resistance and better lightning protection without the associated danger of unreliable connections in your house wiring.
+Colin Herbert Here in Belgium we still run 3phase 230VAC (3X230VAC) in some places and so that means that there is no neutral whatsoever. These are the older lines, and are replaced on street level with 3x400+N, with each domestic house tapping into one phase randomly or controlled, to spread the load. But in the case of 3x230 you obviously need a separate earth tap. Also in some cases the power companies fuckup hard and your voltage will be higher between phase and PE than phase and N. This means that your own earth will protect you more than the N. It all stacks up. And i do have a question for some people here. In Belgium we require the PE tap to be below 30ohms for domestic purpose. What is value in some other places ?
In the UK there is a supply system called TN-CS which in essence only has the two wires entering the premises, the line and the combined earth/neutral. The reason we then separate the earth and neutral, apart from RCD issues, is reliability. If you have a combined earth and neutral in your final circuits and the combined e+n became detached then you can end up with full mains voltage on the exposed metalwork on anything downstream. It is an illegal practice in the UK for a consumer to combine the earth and neutral in the same conductor. For a similar reason, the TN-CS earthing system is banned in caravan sites and marinas as the risk is of an open circuit neutral and earth from the supply is considered too great.
Hi John. I am wanting to get a Isolation transformer in the UK, could you tell me the best place to look for second hand one, would it be very hard to build on ?? Regards
HI John, I'm curious what the earth pin on the plug of the thing that's plugged into one of these is connected to? So if there is a fault to frame of the equipment that's plugged in will it cause a current to flow and trip a beaker?
John.....thanks again for a comprehensive explanation. Can you explain when you say at 3:55 there is "no path to ground"? If the resistance of a human body is (say)1000 ohms and a person touches the Live wire then at 240 volts surely 0.24 amps will flow through the body? Is that not enough to kill a human?
John , figured it out I think. The fault passes through the man, through the ground to the Neutral of the primary winding, to the Live of the primary winding but the loop is broken here - no physical link between the primary Live winding and the secondary Live winding - so no complete circuit. This may help others if I'm correct.
John, hope you can help. Based on your videos I was looking to put together an isolation transformer setup. I have two beefy transformers, 1) Legrand 44147 (PRI 240, SEC 110 CT, 6A, 500VA) and 2) Legrand 44143 (PRI 240, SEC 110 CT, 2A, 200VA). When I connect them back to back with a mains voltage of 247V, I get 245V out. So, I thought this seems reasonable. Couple of questions...I would like to have a couple of 120V out sockets and a couple of 240V ones. I am presuming the large transformer goes first, then splits out to provide the 120V sockets, but also feeds the second transformer, which then provides the two 240V sockets. Is this reasonable? I am totally stuck as to where I should put switches and fuses in this flow, and which rating of fuses should be used at which points. Any info you have much appreciated. Cheers, Clive
+Simon Howroyd Yes it is redundant and some standards specifically require that the earth on isolation transformer outputs must not be connected. Depending on the situation the earth after an isolation transformer can increase or decrease safety in case of a fault, so this is a contentious point.
Yes, in Finland Isolating transformers do not provide any earth connection as one use is to use an unearthed device in an earthed room. www.trafox.fi/wp-content/uploads/2015/12/KLT2000_230.jpg
In addition to the video: There are also versions available where you could adjust the output voltage. Here's a picture of one: www.amplifier.cd/Technische_Berichte/Amplifier_Trenntrafo/images/Grundig_Trenntrafo_RT5A.jpg The text is in german: adjustable isolation transformer, for dry rooms only, protective contact (literally translated, mean earth or ground) not connected. Netz is "mains", manufacturer was Grundig. This model is about 30 or 40 years old. The symbols are harmonised I think: Galvanic isolation (two rings with the line between) and class 2 appliance (double insulation, quadrat inside of a quadrat).
This line made me laugh spontaneously: "It's an additional safety feature useful [..] when working with stuff from other countries... I'm sure you can guess which one."
Pls can you explain about power generator, how do someone get shock when touch one of the cable neutral or live. Because it doesn't have any connection to ground.
I'm curious, in the UK what is the tap limit for service transformers? In the US it's six for utility owned transformers, and ONE for the same exact transformer if it's customer owned.
I have the same transformer as yourself John. Am a little puzzled whether the Earth should be carried through to the secondary connection, which is the case in mine. Should it be disconnected? Your help will be much appreciated especially for safety reasons. Great videos :).
I just bought one for my workshop. It's a Carroll and meynell one. But I was surprised to find that the earth is connected all the way through. Also connected to the transformer and to he output earth pin. How can this be isolated??
@1:18, why don't the electrons go directly to ground through the earth wire? Why do they choose to instead continue on along the neutral wire...and eventually find themselves on the hot side again? Is it not less work for them to go to earth?
In the United States we have 240 coming off the transformer but we also have a center tap which is grounded that we refer to neutral. So all of our lighting and recpticals are 120 volts. Stoves and water heaters use both legs for 240, so my question is do all your lighting and recpticals use 240? Is there anything within the appliance that drops the voltage down?
Everything in the UK and Europe is 240, there is no center tap, and no appliances or anything else that uses 120V. All appliances are designed for 240V, including lighting. Two wires from the transformer with 240V between them, one of those is grounded and becomes the neutral.
Most electrical suppliers have them such as www.superlecdirect.com/6111-carroll-meynell-isolation-transformer-750va-230230v/ cheaper used ones can be found on ebay fairly often, such as www.ebay.co.uk/itm/234727374514
Thanks for the great vids.. if you use the black earth lead from the scope arnt you linking it back to earth? Still a bit confused hope someone can explain to a novice. Many thanks.
@Darren Tipple In Finland isolating transformers actually have an unearthed socket on the output. The only legal way to use an unearthed class 0 equipment in a room with earthed sockets is actually an isolating transformer. www.trafox.fi/wp-content/uploads/2015/12/KLT2000_230.jpg If course most people just take a knife and cut the plug to fit a schuko socket. Nothing is more comfortable than using a computer resting your feet on the grounded case and operating an old hot chassis radio with your left hand.
As the fool that liked the challenge of repairing all and every kind of switchmode powersupply that came to my bench in the 80s, i chose to build a heavy duty bench shelf come cabinet for the old Tek scope out of thick acrylic. It was labelled up with brightly colored lightning icons and an ornate skull & crossbones... with the message "Danger... all surfaces likely to be LIVE to MAINS"!... The scope had its electrical Earth removed and even the IEC power lead had the internal earth conductor disconnected and exposed on purpose at the plug end. I fault found hundreds of SMPS with the kit and yes the scope would go hot with full 240VAC on its outer case, often! Pete's floating scope it was nicknamed.... Then oneday came the new OOSH rules and my employer banned 'my' scope safety compliance they said & i came in one mooring my scope was gone ! replaced with a new top end digital thing plus a couple of HV Differential Probes..... I swear though, the old analog Tek let me see weird patterns in a trace due to a fault but the digital scope would never show it.... I think back at the risks i took & yeah... nah not a good way to do your job.
Hi John, I have finally purchased a Carroll & Meynell isolation transformer. Having a peek inside, I see that the transformer core is earthed and the two outlets also earthed. I can understand the transformer core being earthed but should the outlets be earthed also? Measuring the socket with my trusty fluke I get 180 Volts to the earth pin. This is not enough to light a 60 watt lamp. Is this just noise I am measuring or does this need to go back to the supplier? Many thanks.
That is normal - the core is earthed so that if a fault occurs between the winding and the core, it will trip the circuit breaker or fuse. The 180 volts is due to capacitance between the core and the windings. If you connect the 60W lamp and then measure the voltage, it will be zero. Only appears with a high impedance connection such as a multimeter. The outlet is usually earthed as well, although it's not referenced to the isolated output.
Can you please explain to me why I cannot use a $15 RCD, residual current device, which in 30 m/s and at 15 mA shuts off power to the scope and saves my life? If I put a pin in my 240 vac outlet, the RCD trips and everything is off until I reset the RCD. Please explain, thanks.
You said that if you have two pieces under test that you should use two isolation transformers. But some isolation transformers have two plug sockets available to use. Would this be considered two transformers ? Let's leave the question of ground aside. Let's assume we're using an isolation transformer made in the 1960's, before grounding plugs.
Why is everyone so concerned about isolation transformers.its not a mystery and the AA5 is only one use. In the 1980's TV, started using hot grounds and cold grounds. Supply voltages came off the fly back along with an isolated ground.
It isn't, unlikely there would be any need for that unless there was some obscure device that required it's metal chassis to be referenced to the supply. Doing so would also increase the risk of shock when working on the device.
If you’re working on an amplifier with its own power transformer, isn’t the circuit after the power transformer effectively isolated from the mains power other than the earth ground?
John Ward looks like on my VoxPathfinder15R amplifiers, the secondary is center tapped to chassis ground, which connects to earth ground through the the 3 prong plug. So if I lift the ground (assuming power transformer isn’t shorted between primary and secondary). It should be safe to probe around with oscilloscope leads?
Quick question for you, in European countries such as France and Germany have 'normal' socket outlets in their bathrooms, however are these bathroom socket outlets fed from an isolating transformer? Not necessarily behind the outlet as the faceplate of the socket looks the same as all the others....reason I ask is that a recent visit to Germany in a hotel, using a plug in type voltmeter revealed that all the usual outlets provided around 234v, but the bathroom socket was giving 236v, slightly higher....didn't have any equipment to test between line and earth to either confirm or reject my suspicion..any ideas?
No, it's only the UK which is obsessed with having transformers and useless shaver outlets in bathrooms. Everyone else has normal outlets. Bathroom may have been on a different circuit, or the voltage changed between measuring the various outlets - 2V is insignificant.
No, it is just a normal earthed socket. The different voltage could be for example that it gets a different phase. In several European counties the standard used to be that on wet rooms sockets were earthed and on dry rooms unearthed. On modern installations everything is earthed. This also means that you cannot plug in an unearthed lamp that you inherited from your grand mother. AN isolating transformer is the only way to connect it legally and safely unless you have an electrician earth it. My bathroom socket says: "Only for a washing machine". Probably so that nobody puts a radio or something like that on it.
@@okaro6595 "This also means that you cannot plug in an unearthed lamp that you inherited from your grand mother. " Nonsense, even today new lamps aren't earthed. That's a classic example of a double insulated device, of which there are millions of sold every year.
@@kyle8952 Unearthed (class 0) and double insulated (class II) are different things. An unearthed device has no extra layer of protection. The safety is based on the idea that there is no ground potential nearby. In Finland unearthed lamps were sold still in the 90s (which is idiotic, they should have been banned a long ago). I think they were only banned in 1997 when grounding in all rooms became mandatory. They still are a problem when people move to new homes. Many have incentive to cut the plug or even change it. That is strictly prohibited. 3.bp.blogspot.com/-5r8ISaXrPTY/WhFmIIM3WTI/AAAAAAAANp8/kbOs6HlQoDAsIV6OkyKwQsMHAzfeakJ2gCLcBGAs/s1600/pistotulpat.JPG In the 70s we had tile floor on the TV corner. That mandated a grounded socket. TVs at the time were class 0 (they became double insulated in the late 70s). That is the plug did not fit. One of my parents solved this by making an extension cord that had a grounded pug and an ungrounded socket (strictly illegal, in fact any wiring as banned at the time). Many others did the same. When the cord became unnecessary they removed it and later reused it as a normal extension cord without realizing how dangerous it was. Such cords have caused fatalities. The general attitude was that grounding was a hindrance.
John -- can you describe -- what choices you make in terms of the earth contact on the output socket of your isolation transformers ? Floating vs grounded vs otherwise? W/ thanks,
+sbusweb +John Ward (bump!) -- Please answer above comment John - lots of people seem to be asking similar/related questions in this videos' comments ... NB: I also notice some confusion happening between isolation transformers and CTE 110v transformers, too, have to be careful to segregate the concepts...
Hi John, i would like to know could i get an electrical shock if i touches either one of terminal of a cheap portable non inverter generator set output terminal ? Another question, will neon test screw driver light up if i poke either 1 or the gen set output terminal ?
Can someone please answer why you can get a shock on the first one , but not the second? I understand it’s an isolated transformer with no earth but the drawing of the man touching live is the same on both examples, both have contact with earth. How when you are touching 230v and also touching earth at zero potential do you not get a shock? Even if it’s an isolating transformer Thanks
Earth isn't a zero potential, or any potential as it's not connected to the output of the transformer. Voltage is relative between two points in the same circuit, there is no voltage between two unconnected points. Touching one of the transformer output terminals is equivalent to touching one terminal of a battery.
@@jwflame just trying to understand is at college we’ve been told that voltage flows when there’s a different potential. So in my original question , how do you not get a shock touching a live part from a shaver socket if you’re other hand is touching a tap. But you would get a shock if you touch a live in a switch while you’re stood on ground or touching s tap ?
In electronic power supplies, there is always a Y-cap between input and output (for EMC). So there is always a tiny leakage current. And nothing is perfect isolated from the ground. SELV output is limited to 120 Vdc or 50 Vac. I wouldn't dare to touch anything higher despite that is isolated. A perfect floating system doesn't exist.
Hi John, I have been testing a mobile welfare unit which has three 13A sockets supplied off of an inverter through a single radial circuit. Could this cause an issue such as the scenario you describe around 11:25, ie if a fault develops we could see 230v between the exposed conductive parts of both pieces of kit?
I am wanting to buy an auto transformer and understand the non isolated risks when using one. I want to run an idea past you that I am thinking I could use for my safety. I have already an in-line RCD a RS 3 terminal fused connection block, two 160VA 240v - 24v-0v-24v transformers. I already have the in-line RCD connected between the mains and connection block. I realise this will offer no protection to me if I am seen as part of the circuit. However, I hoped to connect the two transformers back to back. From the 3 terminal block, 240v to lower voltage to lower voltage to 240v to a fused socket and plug in auto transformer. I could to minimise eliminate the RCD and terminal block and connect to a fused plug, these can be looked at as convenience, or not. Am I over looking a problem. Any advice is very much appreciated.
why don't you touch the secondary wire and ground with your hands instead of the lightbulb to demonstrate the safety of an isolation transformer. that would impress more .
4:00 Why don't you get a shock from the transformer? If one of these leads is at a higher potential and you are its path to ground shouldn't you get shocked? I'm just struggling to get my head around that.......... I've just re watched it after posting my comment and I've got it now. (I think).
Think of the secondary of the transformer as like its own completely independent power source, like a battery (it's like a battery driven by magentic induction). It only wants to move electrons from one of its wires to the other. If you only touch one wire at a time (either one) there's no path (through you) to the other wire; so nothing happens. Neither side of the secondary is connected to ground (it's 'floating') so it doesn't care about it.
I will pick two of these up, but just wondering could you not plug in an extension lead into isolated transformer to allow more devices. I have no intention of doing this just wondering.
Connecting multiple items will compromise safety, as it's far more likely to have multiple faults when multiple items are connected. One item with a fault to the case = no danger as you can only touch one of the connections. Two items both with faults = 50% chance of 240V on exposed parts between the two items. Three or more just makes it even more likely.
I do electrical work in homes, but I've never understood ground. How do I get shocked wearing rubber soled boots with cotton socks on standing on carpet that's on padding that's on concrete. None of these are at all good conductors. How do I get shocked? If it's that easy to transfer current, why do we use copper and not rubber? Seems not to bother the electricity going through my body and my boots.
That's a really good explanation (as always!)
The oscilloscope analogy is perfect because, I have to admit, I learnt the hard way. Back in the early 90s I made the mistake of "live" working on a valve amp. Didn't use a mains isolation transformer as I thought I was careful.
No shock fortunately, but 240v straight up the test lead into a £399 HP 'scope didn't end well. Amp survived, scope wasn't so lucky.
It's amazing how quickly you learn to always use an isotran after watching your prize measuring tool emit it's magic smoke!
John, you're a superb instructor, i've been through your videos and you've got a great way of explaining everything. Thank you
...and a wonderfully dry sense of humour to match.
I started working on the so-called All American 5 vintage radios. These have one side of the AC line connected to the metal chassis & the tube/valve heaters in series. And since old US plugs weren't polarized, connecting one way would make the chassis hot/live, even when the radio is switched off. So, I made myself an "iso-xfrmr" using a pair of doorbell transformers connected back-to-back.
"Not particularly desirable!" Very funny but true observation. Nice video.
Hi John,
You have responded to a few of my comments over the past while, I suspect, in hindsight, that some of my questions were trivial to you but you took the time to respond. Since then I have been pottering away with and oscilloscope, isolation transformer, signal generator, variac, etc, etc. Thanks to you I understand some of these things a bit more and I am grateful for your help. Owen
Thanks for your videos John. I’m an electrician but haven’t been to college in 12 years and want to brush up on my knowledge. Your ‘sparking’ my interest again !.
Isolation transformers have a few other uses as well that are a little less safety related. Very handy devices when used correctly.
One example is when measuring RF and you have rather long runs of cable between the transformer providing power to your equipment it is possible to have conducted noise, an isolating transformer breaks the ground current path and can remove undesirable noise.
Thank you for this video! i'm helping a group setting up a electronics workshop, and it have been such a long time since i thought about isolation that i couldn't remember clearly why you need it.
When I started working on live circuits it was required to have item you work on behind an isolating transformer AND have measurement devices like scopes on a separate isolating transformer. Υes, you need two isolating transformers for that (with no ground connection!) !
The reasoning was that if you have a grounded scope connected to device you're working on, connecting ground lead makes it earth referenced. Now if you touch anything on the device by accident, you will get shocked by the now mains referenced device (even if it is behind an isolating transformer). Having the scope chassis also isolated makes it safe to touch any single point in device even with scope ground lead connected.
Another issue with isolating the scope instead of the device under test is that the scope BNCs could become live simply by connecting the probe's ground lead where it shouldn't be. I need an isolating transformer, might make one by taking the primary winding of an MOT and replacing the secondary of another with it, so it has two 120v windings.
Thats good but you missed out common mode rejection which is a great reason to buy one, especially for hifi or recording.
Well done.
(I would suggest using large felt markers or highlighter, because the thin colored lines are not very visible.)
11:00 There is in fact an even more dangerous situation that could happen if you cut off the earth pin from the oscilloscope. Poking the clip from the probe into anything at line potential *will* result in the metal casing of the oscilloscope (and possibly your workbench) being at full mains voltage, even if there is no fault in either in the scope or in whatever device you're probing.
I remember many years ago when I was carrying out PAT testing at an aircraft factory near Gloucester. I found an oscilloscope that had no earth continuity because someone had foolishly disconnected the earth conductor in the mains plug. They were not happy when I reconnected it. I had no alternative other than to submit a formal report to the firm's safety officer.
Neat video. I am purchasing an isolation transformer and was going to use with my scope. Using on the IUT makes more sense. Thanks.
Bear in mind there are also 240v output 'yellow box' transformers around that are NOT isolating transformers. Like the 120v building site transformer they are center-tap earthed. If one of these finds its way onto the testbench it's a potentially hazardous situation.
Not only does it not isolate, but if there are also standard 240v supplies on the bench you could have 360vac between these and one side of the transformer output. Potentially a very nasty shock, or (as I saw happen once) a mighty great BZZZAP! if they get interconnected.
You can generally convert this type into an isolating transformer by disconnecting the center tap so long as it has separate primary and secondary windings. Just don't use it as is, though.
Hi John,In the video, the earth crocodile clip of the scope was shown to clip onto the neutral of the non isolated device under test. Will that not create a parallel path for the current from the live wire in the device under test to flow to the neutral as well as the earth of the clip?. There will be an imbalance between live and neutral in the scope circuit and/or device under test circuit. In that case, if there is a RCD, will the RCD trip?Thanks.
+istoreit Yes it would. The RCD might trip, depending on how much current passed via the test lead.
Hi John; enjoy your videos. It seems most folks recommend using isolation transformers when repairing old radios and such, especially the transformerless type. Is there any advantage to using one when working on modern tube guitar amplifiers such as Fender or Vox--or could one be in more danger plugging such amps into the isolating transformer?
John I love your clear & very precise delivery on your video's great job.
You can get metal framed/cased isolating transformers with the casing earthed to the mains. If you have a PMR mains supply this means that the neutral carries through to the secondary neutral of the isolating transformers.
Years ago, I made an isolation device from two identical transformers, 240 in, 12v out - 12v in 240v out. It was so well buffered, you could get a nice spark from the 240v out, without blowing anything. I suppose you could call it an inverter of sorts.
Hello John,
Thank you for youre good information, but a "Mains Isolation Transformers" still has an earth wire inside? please can you explean?
Neutral (blue or black ) is connected to 'the earth' and transformer windings, Earth (green or green and yellow) is only connected to the 'earth' but NOT the transformer so protects you by providing a preferential 'path to earth'.
Enjoyed this video.i have 2 site transformers 240_110v and another 240_110v.I need 240 _240v mains isolated to check an audio amp fault.if I connected both transformers back to back and removed earth from second transformers would I be able to use my oscolscope with caution.engineer 50 years electrical.tks Michael
Theoretically yes, however in reality this would require modifications to one of the transformers, as if using it in reverse the output would be the exposed plug pins and the input a socket. There may also be other problems if the two transformers are not exactly matched, output voltage may not be what you expect and there could be further issues with inrush current.
Really really interesting. I'm retired now but always had an interest in electronics and want to buy an oscilloscope and am rather concerned about blowing it up. This video explains a lot but wonder if the scope is earthed and if I'm using a bench power supply which has a max output of 30 volts 3 amps do I still need an isolating transformer between the power supply and circuit board? Or is there not enough power there to worry about? (when using scope)
I happy you clarified to always keep to scope to mains but make sure your broken device is on the floating power.
Quick extra question. - The earth pin of the isolation transformer will be earthed back to mains earth and neutral at power board. SO is there any way your ground clip touching anything powered in the broken circuit can flow back to ground eventhough the 240V is floating? AND whats the point of then connection that earth pin up instead of just supplying 2 connected pins on the output so you know you only dealing with the live and N floating voltage and thus TOTALLY isolated?
Isolation transformers were at one time very common in some patient care areas of hospitals (Canada) where mains powered equipment was connected to patients for any given time.. They also had to be monitored for leakage current in microamps, and fault location (L1 or L2) ... Now in new installs these are not common as most medical equipment is effectively isolated by a transformer within the device or within the cart it is attached in or on.
You mean this type of isolation transformer ? ruclips.net/video/5SjVm-rF39A/видео.html
Hi Jonh, I found myself here trying to sort out a problem with my Off grid, inverter and battery setup. Basically I've got a 3kva Victron easy solar inverter/charger a 48v battery bank (8×12v 160AH in series/parallel) and a 10kva kubota/ linz generator, no solar panels yet! that's the set up, now the problem, I connected a washing machine with the generator running 2kw going to the battery charger, the washing machine does nothing just flashing fault light, turn the generator off, the washing machine runs perfectly and draws 1800w. I've can run various tools a kettle and Hoover, 6kw or more no problem from the generator, I've noticed the frequency has gone up to 52.2 Htz was 51.5 Htz possibly due to a fuel filter change, it's a capacitor controlled generator not automatic voltage control, after some research I've found this type of genny isn't the best for electronics, and washing machines very often don't like them.
I know enough about electrics to know I'm out of my depth, not sure if the machine doesn't like the voltage, frequency, noise or something else.
sorry for the long post.
Isolating transformer type 120 volt bathroom shaver sockets used to be used widely in Canada as well, although they have been largely replaced by GFCI (RCD) sockets.
Why is it so difficult to find a small mains isolation transformer for one appliance that does not step down or up the voltage? Thanks for the video. I've been all over the internet and cannot find one. The BNC - connection on my signal generator leaks about 128 vac.
Very correct to connect your test circuit to the isolation transformer and not float the scope.
However, as soon as you connect your little black scope ground probe to the test circuit, you effectively have reintroduced reference to mains ground, which means you can receive a deadly shock by only touching one wrong place in the test circuit. That can be avoiding by using active differential probes on your scope in addition to plugging the item you are working on into the isolation transformer.
Fabulous explanation John. I have been looking for an isolation transformer for a long time but alas can not find one at a reasonable price.
Yes very good info that even l could understand. I have been trying to buy one and done searches in several very large electronics suppliers (in UK) for isolation transformers but have yet to find one listed. 240v to 240v
As usual, this video made me understand something for the first time. Thanks
Sorry for commenting 5 years too late, but why anyone not get a shock from the secondary side of an isolating transformer if they came into contact with a live conductor while their feet were on the ground (true earth) ?? In this instance would the current not flow through the person connected to live (via the body and feet) through to the general mass of earth ???
The secondary is not connected to Earth, so there is no circuit for current to flow.
@@jwflame Thanks for response, im still finding it difficult to understand how if there is 230v on secondary side of the transformer, then why this cannot flow to earth (via a persons body & feet) and give them a shock. In normal operation, on the secondary side, current will flow out through line and back through the neutral, so similarly why not out through line and to true earth (via a person). ???
It seems that these isolation transformers usually has a connected earth wire on their outlet sides. Is this neccesary? In case there's an earth fault in the device under test, wouldn't it cancel the benefit of using an isolation transformer?
At 12:45 we see that the isolation transformer, itself is grounded via a three prong plug. However the device under test connects to the transformer via a three prong plug as well but I assume that this is for compatibility reasons as the ground plug on the secondary is not connected to mains earth?
Hey John great video, found it searching for isolating transformers in regards to use on a leasure boat. I'm an electrician my dad has recently bought a sail boat and has asked me to install an electrical connection for 230v use, it seems that research in to marine electric has opened up a can of worms just wondering if you have any help or guidance you could give me to ensure a safe installation.
Kind regards
Scott
John, I bought an A.C. rheostat but was advised to connect this to an isolation transformer, which I did. I also connected my D.C. power supplies to this transformer too (it has several "kettle" type outputs). I attached the oscilloscope to this transformer but now I see that it was a bad idea. As a rule, should I just attach all my lab equipment to the regular mains and ONLY the test item(s) to one or more isolation transformers?
Grounding can be interesting from place to place. Japan tends to use dedicated grounding posts rather than having ground connections in the sockets. In Florida, we have to paranoid about the matter due to absurdly common lightning strikes.
I would like to see a video on step-down transformers using say Japanese or US products with their different voltages.
Excellent Vid, Should you have a double pole MCB will using a 230v C.T.E traffo?
+IrishHitman79 Yes, double pole MCB should be used on the output.
Larger transformers with multiple outlets such as those used on building sites usually have DP MCBs fitted as standard.
Thanks John great presentation. How about if one were to fit an RCD on the secondary of the safety transformer? Then presumably it would offer live and neutral protection too so much safer? Thanks
An RCD there would only work if the output was referenced to Earth - which would make the isolating transformer pointless.
Thanks for your explanation of possible mains earth current paths. I was probing signals on a microprocessor. It was powered by a PC USB supply. I had the oscilloscope ground connected to the negative side of the USB supply and at some point the microprocessor exploded and everything connected including the USB port vaporised and the house RCD breaker tripped. My first thought was that the negative rail of the USB was not at ground potential but surely it can only be plus or minus a few volts with respect to ground? I still can’t quite understand what happened. Perhaps it is impossible to say from this information. Needless to say it was a very expensive catastrophe.
I like the home conversion of the isolating transformer. It looks like a converted 110v builders' transformer very confusing mix of standardised colours, blue, yellow 110v or 240v. A builders' site transformer is designed to output 110v only.
Might seem a daft question but could you use a old shaver socket for low amp mains supply appliances testing?
Yes, but they are limited to 200mA so it would only be suitable for tiny loads.
Quiz for newbies. What about all the current flowing thru neutral as the normal course of events. What is the resistance of the "earth", from your house to the power pole and back to the generating station.
Great vid as always John. But gonna make myself look stupid here but still don’t get how you don’t get a shock of touching live? Surely touching a live on isolation transformer will still pass through body/into ground and back to sub station then back up to isolation transformer and conducted through into person to make circuit? Sorry for stupid question!!
There is no electrical path through the isolating transformer, it's only a magnetic field that links the primary and secondary side.
Current only flows in the secondary if something is connected to both of the output terminals, connecting to only one of them doesn't complete any circuit so no current flows and no shock.
@@jwflame Thanks. All I could see in my head is that the circuit would still be made through magnetic field down output then through person then through ground then back up input to make circuit
Is the P.D. between the top and bottom of the primary coil simply created by having the bottom wire grounded?
This is probably a daft question..
Is the main earth lead still connected to ground at each individual property or is the earth now taken back to the local substations? Years ago I remember seeing the earth on some old places used to look like a bit of iron re-bar knocked into the ground with the earth wires attached outside the property's, looking back, wouldn't this become almost next to useless?
I only ask because it's something I've not seen these days.
some houses still have an earth rod where for various reasons the network operator cannot supply an earth. but in most cases now new supplies have the house earth tapped off the neutral in the service head, since the neutral itself is earthed at the substation. there are several situations where this is undesirable for example petrol stations, where outdoor machinery and metal pipework is used, animal barns, garden sheds. this is because there can be a difference between the ground potential on the neutral wire and the local ground potential, so it is inadvisable to use an exported earth outdoors. in these cases a local earth rod should be installed and tested and the network earth abandoned for the outdoor installation.
+Steve Craft Where you from steve ?
Here in Belgium it's either a rod into the ground in renovation work. And in new buildings we have a earth loop around the house few meters deep.
+Brian Boterman Sorry i should have made it clear, I'm from the uk.
+Brian Boterman
Is that TN-C-S with auxiliary earth or TT system?
Sounds like TT to me.
That's fine as long as you do not have a ground pin on the plug. In the USA most earth ground is directly connected to neutral in the fuse panel. This can be very dangerous.
ruclips.net/video/XBsQ3sZ45Fk/видео.html
Thanks. Very informative. I picked up a RS safety isolating transformer second hand. It looks like yours and is UK made. Inside it, the earth is passed-through from the input to the output sides directly, via a bolt on the transformer housing. It designed to be disconnected if desired and they even provide a little screw terminal to put it in if disconnected!
The earth is not strapped to either terminal of the output side of the transformer. It just goes straight through via bolt on the transformer housing.
I tested that there is no continuity between the earth and either of the pins of the transformer's output socket. I did this when it was plugged into the wall (obviously not when turned on).
I am struggling to understand why they have made deliberate provision for the earth to be disconnected and even suggested how to do it on a sticker on the lid!
If I leave it as is, the earth on the output is effectively linked straight back to the neutral at the fuseboard of the house. Does this mean it is still 'isolated' and therefore safer to use with my oscilloscope?
Sorry for the long question!
The output is still isolated regardless of whether the earth is connected or not.
For many things it doesn't matter if the earth is connected, but it may be relevant in some situations, all depends on the equipment connected to it. Could be useful to have earth connected for some types of electronics to cover ESD issues, and older items such as radios or televisions may need the earth reference to operate correctly.
However caution is required as connecting an item that contained a fault to earth would then result in the isolated output being referenced to earth, defeating the isolated aspect of it.
@@jwflame Thanks very much for the detailed response. I think I will leave the earth connected for now. Somebody on WhatsApp scares me a bit by saying that mains filters on the item under test could be an issue if I disconnected the earth. Thanks again for helping. All the best.
Simple question but... If the incoming neutral is connected to earth, why do we have a separate earth running through the circuit, especially when the earth is connected to the neutral when it enters the house (our old house anyway). Never been clear on that. Thanks John.
The reason for that is somewhat complicated but I will try to explain my take on it. In the case of GFCI outlets (other countries call them other names) all of the power goes from the live and into the neutral, and in theory, the current measured through each terminal should be the same. However, if current leaks from live to ground, such as a faulty motor winding or a wet piece of equipment, the RCD will notice that more current is going through the live terminal than the neutral (because live is leaking to earth) and disconnect that piece of equipment. The earth helps to make damn sure that someone didn't mix the live and neutral around, because if there was just live and neutral and the chassis was connected to neutral instead of earth, if someone jams the plug in backwards or the electrician had too many drinks, all of the sudden the appliance becomes live. Here in america, we had non polarized sockets where you can mix the live and neutral around without any difficulty, these were mainly used for older lamps and stuff. I know this is not a great explanation, but I hope it helps :D
Thank you that helps.
+Colin Herbert A single connection for earth and neutral used to be common in many places, the building where I work is still wired that way. You need to provide local GFI/RCD protection as needed like Luke described.
As you say it is now only common on a distribution level, and split up where the power enters the house. This way you can still have a lower earth resistance and better lightning protection without the associated danger of unreliable connections in your house wiring.
+Colin Herbert Here in Belgium we still run 3phase 230VAC (3X230VAC) in some places and so that means that there is no neutral whatsoever. These are the older lines, and are replaced on street level with 3x400+N, with each domestic house tapping into one phase randomly or controlled, to spread the load. But in the case of 3x230 you obviously need a separate earth tap.
Also in some cases the power companies fuckup hard and your voltage will be higher between phase and PE than phase and N. This means that your own earth will protect you more than the N. It all stacks up.
And i do have a question for some people here. In Belgium we require the PE tap to be below 30ohms for domestic purpose. What is value in some other places ?
In the UK there is a supply system called TN-CS which in essence only has the two wires entering the premises, the line and the combined earth/neutral. The reason we then separate the earth and neutral, apart from RCD issues, is reliability. If you have a combined earth and neutral in your final circuits and the combined e+n became detached then you can end up with full mains voltage on the exposed metalwork on anything downstream. It is an illegal practice in the UK for a consumer to combine the earth and neutral in the same conductor. For a similar reason, the TN-CS earthing system is banned in caravan sites and marinas as the risk is of an open circuit neutral and earth from the supply is considered too great.
Hi John. I am wanting to get a Isolation transformer in the UK, could you tell me the best place to look for second hand one, would it be very hard to build on ??
Regards
Thanks J.W,Do you know why electricians read the daily paper ?In order to keep current with the times.
HI John, I'm curious what the earth pin on the plug of the thing that's plugged into one of these is connected to? So if there is a fault to frame of the equipment that's plugged in will it cause a current to flow and trip a beaker?
John.....thanks again for a comprehensive explanation. Can you explain when you say at 3:55 there is "no path to ground"? If the resistance of a human body is (say)1000 ohms and a person touches the Live wire then at 240 volts surely 0.24 amps will flow through the body? Is that not enough to kill a human?
John , figured it out I think. The fault passes through the man, through the ground to the Neutral of the primary winding, to the Live of the primary winding but the loop is broken here - no physical link between the primary Live winding and the secondary Live winding - so no complete circuit. This may help others if I'm correct.
John, hope you can help. Based on your videos I was looking to put together an isolation transformer setup. I have two beefy transformers, 1) Legrand 44147 (PRI 240, SEC 110 CT, 6A, 500VA) and 2) Legrand 44143 (PRI 240, SEC 110 CT, 2A, 200VA). When I connect them back to back with a mains voltage of 247V, I get 245V out. So, I thought this seems reasonable. Couple of questions...I would like to have a couple of 120V out sockets and a couple of 240V ones. I am presuming the large transformer goes first, then splits out to provide the 120V sockets, but also feeds the second transformer, which then provides the two 240V sockets. Is this reasonable? I am totally stuck as to where I should put switches and fuses in this flow, and which rating of fuses should be used at which points. Any info you have much appreciated. Cheers, Clive
Does this mean that the earth downstream of the isolating transformer is electrically redundant as it would not provide a current path?
+Simon Howroyd Yes it is redundant and some standards specifically require that the earth on isolation transformer outputs must not be connected.
Depending on the situation the earth after an isolation transformer can increase or decrease safety in case of a fault, so this is a contentious point.
Yes, in Finland Isolating transformers do not provide any earth connection as one use is to use an unearthed device in an earthed room.
www.trafox.fi/wp-content/uploads/2015/12/KLT2000_230.jpg
Thanks for the explanation. Fantastic clarity. Appreciate it.
In addition to the video:
There are also versions available where you could adjust the output voltage. Here's a picture of one:
www.amplifier.cd/Technische_Berichte/Amplifier_Trenntrafo/images/Grundig_Trenntrafo_RT5A.jpg
The text is in german: adjustable isolation transformer, for dry rooms only, protective contact (literally translated, mean earth or ground) not connected. Netz is "mains", manufacturer was Grundig. This model is about 30 or 40 years old. The symbols are harmonised I think: Galvanic isolation (two rings with the line between) and class 2 appliance (double insulation, quadrat inside of a quadrat).
This line made me laugh spontaneously: "It's an additional safety feature useful [..] when working with stuff from other countries... I'm sure you can guess which one."
Pls can you explain about power generator, how do someone get shock when touch one of the cable neutral or live. Because it doesn't have any connection to ground.
Thanks very much for this, you filled an essential gap in my safety knowledge.
I'm curious, in the UK what is the tap limit for service transformers? In the US it's six for utility owned transformers, and ONE for the same exact transformer if it's customer owned.
In Finland a transformer can serve hundreds of houses. I think it is same in the UK and other 400 volt countries.
I have the same transformer as yourself John. Am a little puzzled whether the Earth should be carried through to the secondary connection, which is the case in mine. Should it be disconnected? Your help will be much appreciated especially for safety reasons. Great videos :).
A little late, but yes it should, there should be no connection to mains Earth at the output socket.
I just bought one for my workshop. It's a Carroll and meynell one. But I was surprised to find that the earth is connected all the way through. Also connected to the transformer and to he output earth pin. How can this be isolated??
@1:18, why don't the electrons go directly to ground through the earth wire? Why do they choose to instead continue on along the neutral wire...and eventually find themselves on the hot side again? Is it not less work for them to go to earth?
Copper wire has a much lower resistance compared to Earth.
@02:20
“Not particularly desirable”. 🤣🤣🤣
Drier than Jack Dee!
In the United States we have 240 coming off the transformer but we also have a center tap which is grounded that we refer to neutral. So all of our lighting and recpticals are 120 volts. Stoves and water heaters use both legs for 240, so my question is do all your lighting and recpticals use 240? Is there anything within the appliance that drops the voltage down?
Everything in the UK and Europe is 240, there is no center tap, and no appliances or anything else that uses 120V. All appliances are designed for 240V, including lighting.
Two wires from the transformer with 240V between them, one of those is grounded and becomes the neutral.
Yes all our lighting and power sockets is 240. All our appliances are designed for 240.
Dear John, the isolating transformer is very useful. Can you advise where I can get one. Thanks!
Most electrical suppliers have them such as www.superlecdirect.com/6111-carroll-meynell-isolation-transformer-750va-230230v/
cheaper used ones can be found on ebay fairly often, such as www.ebay.co.uk/itm/234727374514
@@jwflame Big thanks for your sources!!
Hola. Si uso un Variac, necesito conectarlo al trafo de aislamiento. Para mi es necesario usar el Variac.
So basically you can now choose between whether you think RCD is safer or just don't touch both conductors at the same time.
Your vid has just answered the few questions
I had, on M.I.Ts thanks John.
Thanks for the great vids.. if you use the black earth lead from the scope arnt you linking it back to earth? Still a bit confused hope someone can explain to a novice. Many thanks.
Covered in more detail here: ruclips.net/video/a3ePild_sFs/видео.html
@@jwflame thanks I'll take a look
out of curiosity, what happens to the earth going into and coming out of the isolating transformer?
+TornTech
This has been asked before by "sbusweb" amongst others, hope John Ward will answer that original question eventually =)
@Darren Tipple In Finland isolating transformers actually have an unearthed socket on the output. The only legal way to use an unearthed class 0 equipment in a room with earthed sockets is actually an isolating transformer.
www.trafox.fi/wp-content/uploads/2015/12/KLT2000_230.jpg
If course most people just take a knife and cut the plug to fit a schuko socket. Nothing is more comfortable than using a computer resting your feet on the grounded case and operating an old hot chassis radio with your left hand.
As the fool that liked the challenge of repairing all and every kind of switchmode powersupply that came to my bench in the 80s, i chose to build a heavy duty bench shelf come cabinet for the old Tek scope out of thick acrylic. It was labelled up with brightly colored lightning icons and an ornate skull & crossbones... with the message "Danger... all surfaces likely to be LIVE to MAINS"!... The scope had its electrical Earth removed and even the IEC power lead had the internal earth conductor disconnected and exposed on purpose at the plug end. I fault found hundreds of SMPS with the kit and yes the scope would go hot with full 240VAC on its outer case, often! Pete's floating scope it was nicknamed....
Then oneday came the new OOSH rules and my employer banned 'my' scope safety compliance they said & i came in one mooring my scope was gone ! replaced with a new top end digital thing plus a couple of HV Differential Probes..... I swear though, the old analog Tek let me see weird patterns in a trace due to a fault but the digital scope would never show it.... I think back at the risks i took & yeah... nah not a good way to do your job.
Hi John, I have finally purchased a Carroll & Meynell isolation transformer. Having a peek inside, I see that the transformer core is earthed and the two outlets also earthed. I can understand the transformer core being earthed but should the outlets be earthed also? Measuring the socket with my trusty fluke I get 180 Volts to the earth pin. This is not enough to light a 60 watt lamp. Is this just noise I am measuring or does this need to go back to the supplier? Many thanks.
That is normal - the core is earthed so that if a fault occurs between the winding and the core, it will trip the circuit breaker or fuse.
The 180 volts is due to capacitance between the core and the windings. If you connect the 60W lamp and then measure the voltage, it will be zero. Only appears with a high impedance connection such as a multimeter.
The outlet is usually earthed as well, although it's not referenced to the isolated output.
Can you please explain to me why I cannot use a $15 RCD, residual current device, which in 30 m/s and at 15 mA shuts off power to the scope and saves my life? If I put a pin in my 240 vac outlet, the RCD trips and everything is off until I reset the RCD. Please explain, thanks.
You said that if you have two pieces under test that you should use two isolation transformers.
But some isolation transformers have two plug sockets available to use. Would this be considered two transformers ?
Let's leave the question of ground aside. Let's assume we're using an isolation transformer made in the 1960's, before grounding plugs.
GFCI and differential probes, seems a more elegant solution.
Why is everyone so concerned about isolation transformers.its not a mystery and the AA5 is only one use. In the 1980's TV, started using hot grounds and cold grounds. Supply voltages came off the fly back along with an isolated ground.
I don't suppose the earth pin on that isolated socket is connected to neutral but, having said that, would there be any merit in doing so ?
>
It isn't, unlikely there would be any need for that unless there was some obscure device that required it's metal chassis to be referenced to the supply. Doing so would also increase the risk of shock when working on the device.
Thank you for a very thorough and informative explanation!
If you’re working on an amplifier with its own power transformer, isn’t the circuit after the power transformer effectively isolated from the mains power other than the earth ground?
That depends on how it's designed.
John Ward looks like on my VoxPathfinder15R amplifiers, the secondary is center tapped to chassis ground, which connects to earth ground through the the 3 prong plug. So if I lift the ground (assuming power transformer isn’t shorted between primary and secondary). It should be safe to probe around with oscilloscope leads?
Quick question for you, in European countries such as France and Germany have 'normal' socket outlets in their bathrooms, however are these bathroom socket outlets fed from an isolating transformer? Not necessarily behind the outlet as the faceplate of the socket looks the same as all the others....reason I ask is that a recent visit to Germany in a hotel, using a plug in type voltmeter revealed that all the usual outlets provided around 234v, but the bathroom socket was giving 236v, slightly higher....didn't have any equipment to test between line and earth to either confirm or reject my suspicion..any ideas?
No, it's only the UK which is obsessed with having transformers and useless shaver outlets in bathrooms. Everyone else has normal outlets.
Bathroom may have been on a different circuit, or the voltage changed between measuring the various outlets - 2V is insignificant.
No, it is just a normal earthed socket. The different voltage could be for example that it gets a different phase. In several European counties the standard used to be that on wet rooms sockets were earthed and on dry rooms unearthed. On modern installations everything is earthed. This also means that you cannot plug in an unearthed lamp that you inherited from your grand mother. AN isolating transformer is the only way to connect it legally and safely unless you have an electrician earth it.
My bathroom socket says: "Only for a washing machine". Probably so that nobody puts a radio or something like that on it.
@@okaro6595 "This also means that you cannot plug in an unearthed lamp that you inherited from your grand mother. " Nonsense, even today new lamps aren't earthed. That's a classic example of a double insulated device, of which there are millions of sold every year.
@@kyle8952 Unearthed (class 0) and double insulated (class II) are different things. An unearthed device has no extra layer of protection. The safety is based on the idea that there is no ground potential nearby.
In Finland unearthed lamps were sold still in the 90s (which is idiotic, they should have been banned a long ago). I think they were only banned in 1997 when grounding in all rooms became mandatory. They still are a problem when people move to new homes. Many have incentive to cut the plug or even change it. That is strictly prohibited.
3.bp.blogspot.com/-5r8ISaXrPTY/WhFmIIM3WTI/AAAAAAAANp8/kbOs6HlQoDAsIV6OkyKwQsMHAzfeakJ2gCLcBGAs/s1600/pistotulpat.JPG
In the 70s we had tile floor on the TV corner. That mandated a grounded socket. TVs at the time were class 0 (they became double insulated in the late 70s). That is the plug did not fit. One of my parents solved this by making an extension cord that had a grounded pug and an ungrounded socket (strictly illegal, in fact any wiring as banned at the time). Many others did the same. When the cord became unnecessary they removed it and later reused it as a normal extension cord without realizing how dangerous it was. Such cords have caused fatalities. The general attitude was that grounding was a hindrance.
does induction stop load hogging in any way, or does the force transfer magnetically also?
John -- can you describe -- what choices you make in terms of the earth contact on the output socket of your isolation transformers ? Floating vs grounded vs otherwise?
W/ thanks,
+sbusweb
+John Ward
(bump!) -- Please answer above comment John - lots of people seem to be asking similar/related questions in this videos' comments ...
NB: I also notice some confusion happening between isolation transformers and CTE 110v transformers, too, have to be careful to segregate the concepts...
+sbusweb New video covers the earth connections on both types of transformer: ruclips.net/video/fRhofcMyAyk/видео.html
Hi John, i would like to know could i get an electrical shock if i touches either one of terminal of a cheap portable non inverter generator set output terminal ? Another question, will neon test screw driver light up if i poke either 1 or the gen set output terminal ?
Can someone please answer why you can get a shock on the first one , but not the second? I understand it’s an isolated transformer with no earth but the drawing of the man touching live is the same on both examples, both have contact with earth.
How when you are touching 230v and also touching earth at zero potential do you not get a shock? Even if it’s an isolating transformer
Thanks
Earth isn't a zero potential, or any potential as it's not connected to the output of the transformer.
Voltage is relative between two points in the same circuit, there is no voltage between two unconnected points. Touching one of the transformer output terminals is equivalent to touching one terminal of a battery.
Thanks, so why do you normally get a shock touching a live conductor if you’re stood on earth ? Sorry I’m still studying
Appreciate the help
@@jwflame just trying to understand is at college we’ve been told that voltage flows when there’s a different potential. So in my original question , how do you not get a shock touching a live part from a shaver socket if you’re other hand is touching a tap. But you would get a shock if you touch a live in a switch while you’re stood on ground or touching s tap ?
Sir Same principal applicable for welding manchin transformer .
In electronic power supplies, there is always a Y-cap between input and output (for EMC). So there is always a tiny leakage current. And nothing is perfect isolated from the ground. SELV output is limited to 120 Vdc or 50 Vac. I wouldn't dare to touch anything higher despite that is isolated. A perfect floating system doesn't exist.
Dimitri De Rop that is true, so do not touch any metal chassis that is ‘floating ‘.
Hi John, I have been testing a mobile welfare unit which has three 13A sockets supplied off of an inverter through a single radial circuit. Could this cause an issue such as the scenario you describe around 11:25, ie if a fault develops we could see 230v between the exposed conductive parts of both pieces of kit?
I am wanting to buy an auto transformer and understand the non isolated risks when using one. I want to run an idea past you that I am thinking I could use for my safety. I have already an in-line RCD a RS 3 terminal fused connection block, two 160VA 240v - 24v-0v-24v transformers. I already have the in-line RCD connected between the mains and connection block. I realise this will offer no protection to me if I am seen as part of the circuit. However, I hoped to connect the two transformers back to back. From the 3 terminal block, 240v to lower voltage to lower voltage to 240v to a fused socket and plug in auto transformer. I could to minimise eliminate the RCD and terminal block and connect to a fused plug, these can be looked at as convenience, or not. Am I over looking a problem. Any advice is very much appreciated.
You can, provided the maximum rating of the transformers is not exceeded.
***** I will have 3.3 amps 160VA/48V. I will buy a 4 amp auto transformer and fuse both sides, in and out at 3 amps. Thank you.
It's 3.3A only at 48V. On the 240V output, the maximum current is only 0.6A
why don't you touch the secondary wire and ground with your hands instead of the lightbulb to demonstrate the safety of an isolation transformer. that would impress more .
why does the neon tester glows when it is contacted with hot wire in secondary side of isolation transformer?
4:00 Why don't you get a shock from the transformer? If one of these leads is at a higher potential and you are its path to ground shouldn't you get shocked? I'm just struggling to get my head around that.......... I've just re watched it after posting my comment and I've got it now. (I think).
Think of the secondary of the transformer as like its own completely independent power source, like a battery (it's like a battery driven by magentic induction). It only wants to move electrons from one of its wires to the other. If you only touch one wire at a time (either one) there's no path (through you) to the other wire; so nothing happens. Neither side of the secondary is connected to ground (it's 'floating') so it doesn't care about it.
I will pick two of these up, but just wondering could you not plug in an extension lead into isolated transformer to allow more devices. I have no intention of doing this just wondering.
Connecting multiple items will compromise safety, as it's far more likely to have multiple faults when multiple items are connected.
One item with a fault to the case = no danger as you can only touch one of the connections.
Two items both with faults = 50% chance of 240V on exposed parts between the two items.
Three or more just makes it even more likely.
Nice explanation but I wondered why they ground the neutral at transformer why don't they feed us isolated power thanks
An isolated supply wouldn't stay that way for long. Any fault between one of the conductors and earth anywhere and it's now earth referenced.
@@jwflame i am sorry i may sound like an amateur but it can happen to phase also which has to be isolated from ground ,thank you
I do electrical work in homes, but I've never understood ground. How do I get shocked wearing rubber soled boots with cotton socks on standing on carpet that's on padding that's on concrete. None of these are at all good conductors. How do I get shocked? If it's that easy to transfer current, why do we use copper and not rubber? Seems not to bother the electricity going through my body and my boots.
It's AC current, the problem is capacitance, even if you are insulated you basically act like a big capacitor and thus get a shock.
Thank you. Appreciated your video.
Great explanation & presentation.