Isolation transformer for safety - Page 1
Forum section: electronics / beginners

author: metebalci
author_rank: Frequent Contributor
timestamp: April 30, 2023, 06:28:33 pm
content: I always work with low voltage circuits so nothing much to worry about. I am thinking to work on something with live mains voltage (nothing too complicated, but I need to measure some voltages), and looking for safety information. I come across with isolation transformer, and I wonder how common is to have an isolation transformer for such work. Is it something like a must-have (why not as it provides some additional safety measure) or is it used only in some settings ?

author: tggzzz
author_rank: Super Contributor
timestamp: April 30, 2023, 06:57:43 pm
content: Quote from: metebalci on April 30, 2023, 06:28:33 pmAll tools help in some respects and hinder in other, have some advantages and some disadvantages.There is no substitute for understanding how a tool works, and how that applies and doesn't apply to your specific use cases.Statements from random people on bulletin boards can only be the starting point for your understanding.Let's start with "mains voltage". I have one instrument with an exposed terminal having up to 500V, and another instrument with exposed terminals which can also have 500V - i.e. > "mains voltage". One I am happy to touch. The other scares me sufficiently that I continually put off trying to get it workingAnother example: I taught my daughter that electricity could be painful by jolting her standing in the middle of a room and in the middle of a field. One involved touching one conductor (only), the other didn't have any conductor other than her body.In other words "mains voltage" is not a sufficient characterisation of the potential (ho ho ho) dangers.I suggest you consider starting with the last of these:define the measurements you wish to makeandthe conditions under which they need to be madeunderstand that safety isn't the use of one tool, it is aprocessandway of thinking. Part of that involves the selection of the appropriatesetof toolsenumerate and understand the many differentclassesof tool and process. Isolation transformer is one class of tool, high voltage probes are several others, RCDs are another, etc. Tying one hand behind your back is one example of a non-toolNo, I am not going to offer advice about which tool to use. Partly that's because I don't know what you want to achieve, partly it is because any abbreviated advice I spouted might be insufficient and thus lead to damage in ways that I didn't anticipate. One example: I damn near electrocuted myself in circumstances in which an isolation transformer wouldn't have helped.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 30, 2023, 07:00:40 pm
content: The occasion where an isolation transformer (or alternative) becomes essential is when you need to connect the AC mains side of a DUT to measuring equipment which has a grounded input or output. Scopes and signal generators are often such equipment.If only measuring voltages with an appropriate "safe" multimeter then the transformer is not essential though care is. Even with an isolation transformer, due care is still needed.For some equipment such as switch mode power supplies an alternative to the transformer may be a UPS that is unplugged from AC mains and is running on battery power. Other options might include a differential input probe for 'scope input.

author: metebalci
author_rank: Frequent Contributor
timestamp: April 30, 2023, 07:19:08 pm
content: I never thought UPS can be useful like this, interesting point.To give some context, the DUT gets 230VAC from the wall, has a linear power supply inside, so a transformer and some RC power filtering stages. I need to measure voltages at some points, the voltage is always less than 230VAC, there is no step-up inside, but they will be I think >100Vdc at least. I do not need to use scope or something like that, I will only use a handheld DMM. So my understanding is isolation transformer is not something always used when high voltages are involved, this I was wondering. I already have other obvious precautions, proper DMM and probes, RCD etc.

author: tggzzz
author_rank: Super Contributor
timestamp: April 30, 2023, 08:29:29 pm
content: Quote from: wasedadoc on April 30, 2023, 07:00:40 pmStill has lethal voltages present.Think about how an RCD/GFCI would/wouldn't protect you.If completely floating, what is the voltage relative to your fingertip? Don't forget the consequences of "parasitic/stray" capacitances.QuoteHV differential, not any old differential

author: dr_harika
author_rank: Newbie
timestamp: May 17, 2023, 01:48:32 am
content: rehashing this post.1) I use a Dim Bulb Light Tester in series. See this for example:2) I am interested in buying a isolation transformer, both for safety, and for future work with a scope (yet to purchase).Where is the best place to buy an isolation 240v--> 240v transformer for TV Repairs in Australia?NOTE for others - typically these dont transfer earth (floating earth) so wont trip an RCD if you have issues.

author: .RC.
author_rank: Frequent Contributor
timestamp: May 17, 2023, 02:41:33 am
content: If I was using an isolation transformer, I would also add an RCD after the transformer via a portable RCD.I am not up to working on HV equipment yet though.

author: Berni
author_rank: Super Contributor
timestamp: May 17, 2023, 05:39:18 am
content: RCD placed after a isolation transformer will NEVER trip.In order to make a RCD work you have to tie N to PE before it. If you do that on the output of a isolation transformer then the transformer is no longer an isolation transformer.Isolation transformers only add a layer of safety when you know what you are doing. Respect for high voltage is still very much required as it only removes one of the ways you can get shocked. But as long as you only touch things with one hand you are fine in most cases (This includes connected test equipment!). You can still stick your fingers across L and N on the isolated side and get a heck of a zap, but if those fingers are on the same hand no current will flow up your arm and trough your heart. So while very painful it would not kill you.The more useful thing for working on mains is current limiting. Making a short circuit on your wall outlet could cause >1000A to flow, making a bright flash, bang, vaporize metal into black marks all over, draw arcs, blow components apart...etc. For that you can just have a lightbulb or heating element in series. But a small isolation transformer also does this job since there is a limit to how many amps it can provide (just don't leave it shorted for long or it will fry itself).

author: .RC.
author_rank: Frequent Contributor
timestamp: May 17, 2023, 06:07:21 am
content: How do you work out a RCD would not trip.As far as I know it works like thisAn RCD works by Kischoffs current law. All current entering a node equals all current exiting it.As such the RCD works by detecting current going in and current going out (via sensing coils), any imbalance like current flowing elsewhere and it trips. Of course if you were to grab the live and neutral and just be a resistive load and leak no electricity anywhere else, then you are in for a shocking experience and no RCD would break the load.

author: dr_harika
author_rank: Newbie
timestamp: May 17, 2023, 06:18:07 am
content: For what its worth, there is a mob that has RCD Isolation transformers. The website has a description of where this is required:https://www.tortech.com.au/product/uncategorised/rcd-isolation-transformer/

author: tggzzz
author_rank: Super Contributor
timestamp: May 17, 2023, 06:31:01 am
content: Quote from: .RC. on May 17, 2023, 06:07:21 amWhen in doubt, test and verify. In this case buy a "socket tester" that also has an RCD check. FFIhttps://uk.rs-online.com/web/content/discovery/ideas-and-advice/socket-testers-guideIf fiddling with mains, you really ought to have such a tester and preferably one that also measures the earth loop impedance. Such testers are cheap and easily available.Thought experiment:use a battery (or cell) as an analogue of an isolation transformer secondaryconnect a load to the battery, and measure the current exiting the +ve and -veconnect the battery -ve or +ve to PME. Will either current change?

author: .RC.
author_rank: Frequent Contributor
timestamp: May 17, 2023, 08:04:24 am
content: I suppose to do it really safely. You should get a job as a conductor, then give away all your money and assets.Then you would be a "very poor conductor"..... Get it.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2023, 07:47:57 am
content: Quote from: .RC. on May 17, 2023, 08:04:24 amI would prefer you thought about the issues and tell me/us where you don't believe what I write. That may help everybody.

author: .RC.
author_rank: Frequent Contributor
timestamp: May 18, 2023, 08:52:24 am
content: I am not sure where I said I did not believe what you wrote.All I did was write a modified old joke about the poor conductor.https://www.ba-bamail.com/jokes/tag/bad-conductor/I make no claim at being smart or know much about working on HV equipment or the safe way to do it, but I did watch a big clive video and he showed how a RCD worked, and it made sense when I watched the eevblog kirtchoff current law video. I also have some experience with a home generator I built and it was wired up floating, so like an isolation transformer and the reasons why it was wired that way, and the risks involved in using.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2023, 11:03:36 am
content: Quote from: .RC. on May 18, 2023, 08:52:24 amBut youdid make a claimabout how to work on HV equipment.I have questioned that claim, with suggestions about how to validate/invalidate your claim.QuoteI'm not going to spend time finding and watching videos that aren't referenced. Even then, most youtube videos are a waste of time - I can read articles/blogs/posts much muchmuchfaster than people can speak.Key question: did those videos show the technique you suggested: RCD after the isolation transformer.

author: .RC.
author_rank: Frequent Contributor
timestamp: May 18, 2023, 11:49:34 am
content: Quote from: tggzzz on May 18, 2023, 11:03:36 amNo not really. I did say "As far as I know it works like this"QuoteI did not suggest it, I said one thing I would also do if I had to work with one.I do not know about others, but I am not going to be going to a single internet forum and just believe what one person says, or a group for that matter. Surely you would be cross referencing and researching all over the place before doing something that has the potential for making you go poof. You have made me sorry now I posted it since it has caused such a problem.As for Big Clive

author: wasedadoc
author_rank: Super Contributor
timestamp: May 18, 2023, 12:14:49 pm
content: An RCD works by sensing the current difference in the two wires. If all current is going only where it should the two currents are equal and opposite. No difference and the RCD does not act. When some current is going somewhere else, eg through you to ground, there is a difference in the two wires that the RCD is monitoring. If that difference is more than about 30 mA the RCD trips.Now consider the secondary of an isolating transformer. Its output is fully floating. If any current is flowing out of one end of the secondary it must be flowing back into the other end. If you put an RCD on the secondary it will always sense zero difference and never trip. If you touch one end of the secondary side, before or after the RCD, no significant current flows through you to ground because there is no path from ground back to the other end of the secondary. If you touch both ends of the secondary, even after the RCD, current flows through you (and might kill you) but the RCD senses zero difference and does not trip.So putting an RCD on the secondary side of an isolating transformer is pointless and provides no added protection.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2023, 04:15:25 pm
content: Quote from: .RC. on May 18, 2023, 11:49:34 amUnfortunately beginners probably won't know which of the many sources and claims to believe.It is always a problem posting safety information on a public forum.If you are not an expert you may make an honest unwitting mistake.If you are an expert then you have probably won't be able to see the issues from the beginner's perspective, and omit vital information that is obvious to you.In either case you can't tell what the beginner has/hasn't understood, and won't spot the bizarre mistake they are about to make next. Nor will you have time to put together a fully reasoned lesson.Hence I often choose not to give advice, but point them towards relevant sources where the authors are expert and have had time to create decent educational material. Surprisingly often the vintage material is very good; it was expensive and difficult to create and distribute, so a lot of thought was put into the subject. Now it is too fast, easy, and cheap, so every nutter does it. (And leaves the "ums" and "ahs" in a spiel given by a talking head that doesn't make use of the moving pictures)

author: EPAIII
author_rank: Super Contributor
timestamp: May 20, 2023, 08:14:56 am
content: I have worked with Voltages from one or two Volts to 40,000 Volts or more over many years. I do not think ANY single precaution will completely erase the possibility of getting shocked. Nor is there any single precaution that will avoid shocks in all cases. I guess the one, single precaution that would come closest to that would be to only work with Voltages under 50 Volts or so. But even there, in some circumstances, like using electrodes under the skin, it would still be possible to induce shocks. Perhaps even life threatening shocks from those very low Voltages.In my humble opinion, the most necessary and effective thing you can employ to avoid electric shocks is your BRAIN. Know what you are working with. Know how it is connected/arranged. And if you don't know FOR SURE that something is safe, then DON'T DO IT.I can tell stories. First one was personal. Sitting at a workbench, working on a CRT TV monitor in the HV section. Using only ONE HAND, so I was safe, right? NO! The HV found it's way around or through the insulated probe I held, out my foot, through my shoe sole, and into the grounded metal bench my foot was resting on. That smarted!Another one: a TV personality was lecturing on air on how a complete circuit was needed in order to get shocked. So he grabbed a live wire while staying away from the neutral and ground in the circuit. And got a heck of a shock. He couldn't figure it out until one of the TV engineers told him he was holding a metal microphone in his other hand, which was grounded. He thought he knew he wasn't grounded. But he was. I looked for the video but couldn't find it.I have seen circuits that operated on really low Voltages, like 5 or 12 Volts. They had transistors and other components with maximum ratings of 50 Volts or lower. But the entire circuit was elevated to a level above 1000 Volts. If you went around probing those circuit with a poorly insulated meter or scope probe you could easily get a nasty shock.You must KNOW what you are working with. And you must KNOW how it is connected. And not just the connections you can easily see. A path may extend to a fuse or breaker box in another part of the building or even out to a transformer on a nearby electric company pole.As for isolation transformers, I think of them as a way of isolating grounds when using test equipment or in some cases of powering equipment without a ground connection between different pieces of that equipment. That does not mean that I operate equipment without any ground connection, just not that particular one. And in a long professional career, I have only worked in one or two places which even had an isolation transformer. I can't remember ever using one. I wasn't the one who purchased them and they just sat on the shelves in case they were needed. There was always better ways to prevent shocks to my person. Including primarily that one that resides between my ears.And RCDs? They are an excellent way of preventing shocks in many situations. A mixer being used in a kitchen has an internal short to it's external, metal part(s) and the user also touches a ground. The RCD trips and that chef is safe. In a garage shop a hand held saw or drill has similar problem and the user is also grounded. The RCD trips and that user is saved. Bravo.BUT, most electronic equipment has a transformer as the first element that the incoming power sees. OK, fuse, switch, then transformer. My point is there is a very limited number of components that the mains power flows to or through. And most electronic equipment has a METAL chassis or box which is connected to the building power GROUND right where the power cord enters that box or chassis. An short to that chassis or box is going to trip the fuse or breaker very quickly. I have designed a number of facilities that used many electronic devices. They were mounted in metal racks which were grounded. They had power strips that they were plugged into with the power grounds. They were fed by regular circuit breakers. NO RCDs were ever used. There was very little that they could prevent and they could only add an additional point of failure in an already complicated system. NO ONE ever got shocked from this equipment in normal use. And when it was "benched" it was again powered from a strip or wall outlet with the power ground. Only a problem in that very limited, primary power circuit would place anyone in a situation where an RCD would help. Most problems were in the lower Voltage circuitry, PAST that transformer so any stray currents there would not register in the RCD and it would be of NO value.I am not saying that it is not possible to find examples in electronic work where an RCD would be of value. I am sure they can be found. I may have even been there a few times myself. But overall, they are of very little value in providing added safety in this kind of work. And using one may even produce a false sense of security.Again, use that GRAY MATTER.

author: EPAIII
author_rank: Super Contributor
timestamp: May 20, 2023, 08:50:48 am
content: I touched on meter and scope probes in my post above. For many years, even though the meters and scopes we used did have maximum Voltage ratings, those ratings were not emphasized. And we all used instruments and probes with relatively low Voltage ratings.Somewhere along the line the electric industry got serious about electric shocks and new rules were created. Today meters and at least some scopes will have a "CAT" rating which indicates that they are safely insulated from insulation breakdown or arcs up to a given Voltage. These are one good way to judge a meter and it's probes for safety. But remember I said "one", certainly not the only one.https://www.digikey.com/en/blog/what-are-multimeter-cat-safety-ratings

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2023, 09:23:37 am
content: Quote from: EPAIII on May 20, 2023, 08:14:56 amI don't disagree, but...Unfortunately, as per the examples you provide, it is too easy to be "sure it is safe" simply because either the connections aren't as you think they are, or because something slips/falls, or because your understanding/knowledge is faulty. Beginners are prone to the last of those, but everybody is prone to the the first two.Often obscure and/or poorly described/understood rules and regulations are there because people have been injured/killed in "strange" and/or "rare" ways that are not obvious from the rule itself.

author: m k
author_rank: Super Contributor
timestamp: May 20, 2023, 02:03:47 pm
content: There are two kind of UPS devices, original UPS and newer cost reduced IPS.Their newer names are on-line and off-line UPS.Mains disconnected UPS is obviously always isolated.Real UPS is isolated by nature so mains can remain and time is not an issue.Real UPS is also cleaning the juice since it's constantly rectifier/inverter style, and battery in the middle.Once happened so that a noob disconnected a live CRT HV by hand.He was doing final tests for deflection/flyback modules, after that he did something else.Luckily the system was insulated enough and the route of electricity was through one side of the body only.The route back to metal framed table and insulated circuit was assumed to be a key in key ring hanging in trouser's belt loop.You can try it your self by shorting disconnected live HV of isolated VT220 to ground.Sometimes people are making their own isolator transformers but are adding complexity and so forgetting the original purpose of safety.One aspect of safety with isolator transformers is that their use is not regular.Regular in context yes, but not out of it, so when not using it the situation in it self is also adding some safety.It's obviously possible to do a complex isolator transformer correctly.But then it's not a single item, it's a single environment.Larger shops do stuff like that where the whole area is isolated.Nowadays it's actually quite easy, if antistatic is left out.All serious test equipment makers do only isolated stuff, tool makers also.So the only places left are a DUT, lights and those leftover wall jacks around the room.

author: strawberry
author_rank: Super Contributor
timestamp: May 21, 2023, 04:08:09 pm
content: non-isolated (grounded) test equipment will short circuit outlet live wireisolation wont give safety (still 230V)question is not to isolate but where you want current to flow (in worst case)

author: metebalci
author_rank: Frequent Contributor
timestamp: May 30, 2023, 06:06:11 am
content: Just an update. I did what I had to do. I used an RCD, better/safety shoes and isolated alligator clips for DMM. I never repositioned the DMM probes on live circuit and only worked when I had enough focus with no distractions around.