Isolation transformer - why? And lab type vs hifi - Page 1
Forum section: electronics / beginners

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 25, 2021, 11:33:56 am
content: I'm looking for an isolation transformer. Not that I actually understandhowthis can protect me. I think I get it when you have a difference between live and neutral, but here in Europe we don't -- at least where I live, they're the same but opposite phases (when using ground as a reference). That said, even though there is galvanic isolation, AC is AC - so if I get electricity through it, I can get shocked by it - so what's different? Is it to avoid any DC component?And when getting an isolation transformer, is there any practical difference between those you buy for your lab, and those you buy for your stereo? The latter is easier to find used at reasonable prices.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: March 25, 2021, 12:18:24 pm
content: >I'm looking for an isolation transformer. Not that I actually understand how this can protect meHonestly, this is a HUGE red flag. If you don't understand why an isolating transformer is a good idea then you really shouldn't be doing anything that requires the use of one.Normal mains is referenced to Earth. The neutral wire is connected to the Earth. So if you touch a live conductor while standing on the ground, you will complete the circuit and receive a shock.If you use an isolating transformer, you will have a supply that is not connected to Earth at all. So if you accidentally touch any part of that circuit, nothing bad will happen. To get a shock you need to make a (hopefully) extremely unlikely situation where one part of your body is touching a Live conductor and another part of your body is touching a Neutral conductor.

author: PKTKS
author_rank: Super Contributor
timestamp: March 25, 2021, 01:52:29 pm
content: Quote from: mvno_subscriber on March 25, 2021, 11:33:56 amAs said above... you better pile some required knowledge first...YES THERE IS AN ENORMOUS DIFFERENCE BETWEEN BENCH ISOLATION DEVICES...and ordinary transformers.. YOU *SHOULD* BUY A SPECIFIC DEDICATED TYPE..with proper rated expected power...Please refer to same (recurrent) discussion herehttps://www.eevblog.com/forum/repair/earthing-safety-with-an-inherited-isolation-transformer/IMHO (and the post will tell you why).. BENCH SERVICE REQUIRES THAT..Mains reason being not zapping *you* but instead.. dead shorting yourinstruments by hooking some clips to an unexpected LIVE MAINS path..it happens.. but WITH THE ISOLATION transformer.. IT WILL NOT HAPPEN.One minus fact to deal with.. nevertheless the list is still long..Paul

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: March 25, 2021, 02:09:27 pm
content: Yes, I assumed I would get a beating by posting this. Mains power is not my speciality, but even though I've tried searching and reading about it I still don't get it. Yes, I'm laying my head on the guilotine, I know. But before you dispose of my wretched corpse, if you could tell me in a way I understand, I'd be eternally grateful.If a voltage is induced through a transformer, how does that differ from being physically connected?(And even then, ground is still ground, and connected all the way through. So we still don't have galvanic isolation? This seems to be the case for both hifi andlabprofessional equipment as far as I've seen.)

author: themadhippy
author_rank: Super Contributor
timestamp: March 25, 2021, 02:23:34 pm
content: How does any isolating transformer work? it electrically isolates the primary from the secondry.In most single phase electrical distribution systems one side of the supply (neutral) is connected to mother earth so if you grab the other side of the supply (live) whilst also being in contact with mother earth the circuit is complete with you as the load and the current flows.with an isolating transformer both sides of the supply on the secondary are isolated from the primary so theirs no connecting to neutral and so no connection to mother earth

author: PKTKS
author_rank: Super Contributor
timestamp: March 25, 2021, 02:27:31 pm
content: The ISOLATION device will put all your equipment attachedto it in a so called "FLOATING REFERENCE".Then.. your floating devices will be attached on the benchforming a common ground in which you have no physicalpath to the "real" mains..The ISOLATION device **SHOULD** have as well someinherent protections ( I USE resettable fuses attachedto the inputs) ..In addition to the ISOLATION device I use cascaded LINE CONDITIONERSwhich are devices made for the purpose of preventing and conformingsurges in the inputs.**YOU** should wear gloves whenever you deal with ratingsabove 40V .. in particular touching LIVE devices ISOLATED or not.The ISOLATION functions to protect your equipment from unexpectedor by accident dead shorts..BENCH SERVICE IS NOT RISK FREE..and requires proper gradual training..Paul

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 25, 2021, 04:37:23 pm
content: Why are you looking for an isolation transformer?By removing the earth reference, the isolation transformer allows you to substitute your own wherever you want. Generally this is used to allow safe connection of earth referenced test gear to mains referenced circuitry, oscilloscopes, signal generators, etc.The trap there of course being that as soon as you make that earth connection to the DUT, the DUT is again earth referenced, and again a shock hazard and hazard to connecting additional equipment.

author: Benta
author_rank: Super Contributor
timestamp: March 25, 2021, 04:52:45 pm
content: In defense of the OP, Norway is a bit of a special case. Installations normally do not have protective earth, as the earth connection is miserably bad. Most of the country is granite, which is a really bad conductor.This is why the distribution network is semi-floating.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: March 25, 2021, 05:00:00 pm
content: so, what means 'semi-floating' here? they have to have some kind of installation type if it's now TN, TT or IT.let's talk facts please

author: helius
author_rank: Super Contributor
timestamp: March 25, 2021, 06:09:16 pm
content: In theIT system, the step-down transformer from distribution voltage to the entrypoint has a floating secondary; unlike other common systems, there is no connection to ground. This means that any ground pin on a socket is not a Protective Earth: it can'tprotectagainst insulation faults since there is no electrical path through which it could trigger a disconnect device.This is a potential fire hazard. However, there are some safety benefits of the IT system: since the current-carrying conductors are not referenced to ground, there is no inherent touch hazard from them. Feel free to use this:It will be painful, but not that dangerous. Another benefit is that less RFI is produced.Moderator note: we do not recomend you try this, treat the advice as a "theoretical" demonstration. You are responsible for your own safety.Something similar happens to a DUT powered through an isolation transformer. The live wires are not referenced to ground, making them less hazardous if touched. This isrequiredwhen the DUT is a "live-chassis" appliance. The common practice of using isolation to "substitute your own ground reference" is a bad idea: most often a differential probe setup is easier and safer.

author: themadhippy
author_rank: Super Contributor
timestamp: March 25, 2021, 07:42:04 pm
content: QuoteIs that picture putting the 2 fingers across live and neutral? if so its going to hurt and is very dangourus

author: ogden
author_rank: Super Contributor
timestamp: March 25, 2021, 07:59:04 pm
content: You don't learn skydiving in the library nor in the public forum where anyone can say anything. The same about electrical safety.

author: pqass
author_rank: Super Contributor
timestamp: March 25, 2021, 08:04:39 pm
content: Quote from: ogden on March 25, 2021, 07:59:04 pmHowever, electical safety is self-selecting.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 25, 2021, 08:41:59 pm
content: What an isolation transformer does:

author: mvs
author_rank: Frequent Contributor
timestamp: March 25, 2021, 09:07:59 pm
content: Quote from: themadhippy on March 25, 2021, 07:42:04 pmIt is quite obvious speculation in IT-Network, that as long as the fingers are on the same hand, the current should not go over the rest of the body.This should save one from cardiac arrest and most probably death.The problem with IT-Networks is that they may have an unnoticed connection to ground. So even in such countries as Norway, one can get a benefit from using isolation transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: March 25, 2021, 09:18:19 pm
content: Quote from: pqass on March 25, 2021, 08:04:39 pmSo's Skydiving!....<splat>-------------*NEVER* take up a dangerous sport for the first time while on holiday in a 3rd world country! Get enough training in your home country so you can recognize and avoid unsafe tourist-trap operators.

author: james_s
author_rank: Super Contributor
timestamp: March 25, 2021, 11:36:40 pm
content: Quote from: ogden on March 25, 2021, 07:59:04 pmYou can certainly pick up the basics there though, before moving on to other sources. The library (or internet) is rarely a bad place to start.

author: nicknails
author_rank: Regular Contributor
timestamp: March 26, 2021, 12:47:05 pm
content: There's this guy named Dave Jones that made a video about this topic. Might want to check it out.

author: Simon
author_rank: Unknown
timestamp: March 26, 2021, 01:07:05 pm
content: Floating gear should be done with caution. It can harm you or your equipment. The point of grounding gear is that is works in conjunction with an RCD which will trip the supply as soon as there is a leak to earth. This could be triggered by a fault to the chassis before it get's to be a person.

author: helius
author_rank: Super Contributor
timestamp: March 26, 2021, 05:22:54 pm
content: Quote from: mvs on March 25, 2021, 09:07:59 pmIn theory. It would always be instructive to compare accidental LV electrocutions in countries with different earthing systems. If there is such a study, I'm unaware of it.It is not recommended to test this on yourself—the body is not a uniform bulk conductor, and the current through the chest may not be as predicted. Fibrillation is caused by microamps in the affected area; the question is always how best to minimize danger while working to an acceptable level. The "one hand in back pocket" advice is also just a guideline, since current does not need to travel between both arms for microamps to pass through the heart.QuoteIf the line is truly floating, there should be the same apparent voltage between both poles of a load and the earth. You need to test on a working load to remove "ghost voltage" effects, and probably also need a low-Z voltmeter to make the reading stable in case it is truly floating. This is a hidden hazard different from those seen in TT and TN systems.

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 26, 2021, 05:44:56 pm
content: Quote from: HB9EVI on March 25, 2021, 05:00:00 pmAll three are actually in use in Norway, 230 V L-L IT (most common, older installations) 230 V L-L TT in some places, and newer installations are often 400 V TN-C-S.In the case of IT, each line will measure around 133 V to GND in the balanced case, and if somebody on the same distribution transformer has a hard ground fault, two of the phase conductors could be as high as 230 V relative to ground, while the last one would be close to zero.I have actually heard of cases where an inductive ground fault (typically in a magnetic ballast) can resonate with the distribution network earth capacitance and bring up the line voltage beyond 230 V to earth, but I'm not sure how credible this is.

author: tooki
author_rank: Super Contributor
timestamp: April 02, 2021, 12:02:38 pm
content: Quote from: mvno_subscriber on March 25, 2021, 02:09:27 pmNobody wants to give you a “beating”. No, it’s that we want to make sure your heart literally keeps beating. The reason the forum is so vocal on the topic of mains AC is that we don’t want people to get hurt. If we see someone indicating that theyintend to engage in activitiesthat involve dangers they don’t understand, the most important, loudest message you’ll get is “Stop now. Learn, understand, and respect the dangers, and then go back to do the thing you want to do.” (If you even still want to). It’s not to dance atop your “wretched” corpse, it’s to make sure you don’t become one to begin with.In this case, it’s not your how-and-why questions that raise the red flag, it’s your stated intent to get an isolation transformer before understanding what the isolation transformer does and doesn’t do. It’s critical to understand that, lest you get lulled into false security about the safety of what you’re doing. (And as others have said, isolating a device introduces hazards that weren’t there before, so understanding is critical.)

author: Ian.M
author_rank: Super Contributor
timestamp: April 02, 2021, 12:14:56 pm
content: Also, experienced forum members offering advice on mains and other high voltage or high energy circuits almost always have in the back of their mind the thought that an inexperienced newb lurker may read their answer and take it as affirmation for an ill-considered course of action without giving us the chance to step in and sayQuotebecause they never even asked a question!Therefore the consensus of advice on such issues tends to be heavily weighed towards practical safety.One major issue with the use of mains isolation transformers is that they prevent upstream RCDs (GFCIs) tripping due to faults downstream of the transformer, so disable the one item of safety equipment that is most likely to save your life!

author: Terry Bites
author_rank: Super Contributor
timestamp: April 02, 2021, 04:13:42 pm
content: "Floating" probably translates very badly... All transformers (except autotransformers) are there to provide some degree of isolation as well as stepping up or down the voltage. Isolation transformers remove the ground/neutralreferenceso that there is no current path from the secondary side to ground. This saves you and your kit from going up in a fireball if you accidentally connect the isolated supply to ground. These transformers usually feature reinforced insulation and have low capacitance between the windings. Don't work on live appliances without one.https://en.wikipedia.org/wiki/Earthing_systemNorway gets mention. That's rare!

author: Simon
author_rank: Unknown
timestamp: April 02, 2021, 06:27:06 pm
content: Isolation works under the conditions it was meant for. It does not always protect from electric shock. Your normal mains could have been made isolated but there was a conscious decision to earth it and the whole world does that. I would prefer that my washing machine chassis was grounded and that a loose wire trips the RCD than me trip the RCD or worse get electrocuted by the live chassis. A 30 mA leak to ground is easy to pick out, detect and use to trigger a cut out. Another 30 mA on top of the many amps flowing in and out of your home that are going through you and killing you are not distinguishable from the rest of the load current.

author: calzap
author_rank: Frequent Contributor
timestamp: April 02, 2021, 06:59:17 pm
content: Folks should be aware though that some transformers advertised as isolation transformers aren't in the sense discussed here. They may have one of the secondary output lines connected to the case and mains ground wire of the primary power cable. This is usually stated in the specs. Read the fine print! An example is the Tripp-Lite Isolator series. These transformers are designed for protecting communication, medical and scientific equipment from electrical noise in the power source.A test of true electrical isolation is to apply power and check for voltage between the output contacts and the contacts of a mains power socket that is on the same circuit powering the transformer.Mike in California

author: wizard69
author_rank: Super Contributor
timestamp: April 02, 2021, 07:20:58 pm
content: As you may have found out already, reasonable people will try to guide you away from work on mains systems until you have a more solid understanding of what is happening. In general it is not possible to know if you are communicating completely with a newbie about electrical safety when working on high voltage systems. This leaves many reluctant to say much at al to encourage and rather try to discourage people with limited knowledge from getting even close to high voltage systems. As noted these systems can be lethal, and frankly it only takes seconds.As for isolation transformers they are of limited usefulness in my opinion but are required to achieve safety from shorting to ground. That is if your body completes a circuit from one leg of an isolated supply to ground you are not instantly at risk. However what you need to keep in mind is that the isolation transformer still has 120 VAC (or whatever the local standard is) across the secondary, thus lethal voltages. This is why I consider the safety offered by an isolation transformer as marginal especially for new people. Yes you can solve the issue of circuit completion to a grounded conductor but you still have to be careful and understand what is operating at lethal voltages in the device under test.Note this is a discussion about the use of isolation transformers for personal safety. There are other uses for isolation transformers in the T&M realm. That should be left for another discussion. I just wanted to point out that an isolation transformer doesn't make high voltages safe, it just protects from one type of error or mistake one can make. This is important to understand because as you point out yourself you don't get it and frankly a lot of people don't.In the industrial realm we often have to debug when equipment is hot. Sometimes it is the only way to find faults. These systems can have all sorts of high voltage supplies or lines in them and no isolation transformers (in most cases). Generally these are all ground referenced in some manner or another. This requires good safety practice, one simple consideration is to wear your gloves. It would be foolish to work on these sorts of systems without your PPE. You also need the discipline to shut off stuff when actually repairing the machine. In general don't move forward unless you have had some training.So that wraps it up, don't move forward until you grasp the risks and understand what an isolation transformer can and can not do for you. There ability to save your life is limited but that doesn't mean you can ignore them. You mention audio here and frankly if you are talking tube circuits, you really need to understand the risks before working on that type of hardware.Quote from: mvno_subscriber on March 25, 2021, 11:33:56 am

author: Simon
author_rank: Unknown
timestamp: April 02, 2021, 07:30:11 pm
content: Why would anyone use an isolation transformer for a stereo? no a bloody audiophool thing I hope.

author: calzap
author_rank: Frequent Contributor
timestamp: April 02, 2021, 09:07:06 pm
content: Quote from: Simon on April 02, 2021, 07:30:11 pmThey are used to reduce noise in the incoming power. It doesn't need to be a true isolation transformer but can be something like the Tripp Lite Isolater series I mentioned in an earlier post. Does it make any difference if incoming power is reasonably clean? I doubt it.Mike in California

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: April 02, 2021, 10:16:26 pm
content: Quote from: SilverSolder on March 25, 2021, 08:41:59 pmThis is an excellent (and very amusing) explanation!We talk about "referenced to earth", "isolation", "floating" etc., but to someone who doesn't have the basic introduction to what these terms mean in context, telling them that an isolation transformer removes ground references is completely meaningless.

author: Jwillis
author_rank: Super Contributor
timestamp: April 03, 2021, 05:47:42 am
content: Hope this helps.

author: BrianHG
author_rank: Super Contributor
timestamp: April 03, 2021, 06:13:35 am
content: Quote from: Simon on April 02, 2021, 07:30:11 pmWhen used for some low power end audio equipment, like DAC, CD player, Preamp, it can eliminate the background audible 60/50Hz hum. It tends to be useful when mixing older equipment with newer equipment which may have switching supplies with a huge Y cap or poor mains isolation.I used one with my old Sony 757 HiFi VHS player (damn switching supply instead of linear). Once I did so, + excessive thick ground shielded RCA cable, I got studio grade audio free of the an inherent 60hz buzz down from around the crummy -80db, to a point where it surpassed the player's SNR spec of -92db. This was an audible and measurable difference. Though, I could not place my PC feeding the VCR's audio on an isolation supply due to the huge 500 watt demand compared to the small transformer I used for the VCR's 65watt demand.

author: wizard69
author_rank: Super Contributor
timestamp: April 04, 2021, 05:24:23 pm
content: Quote from: Simon on April 02, 2021, 07:30:11 pmIt can sometimes help.On the other hand you would be a fool to buy some of the isolation or line conditioning systems sold to audiophools. I often visit local thrift shops and one day they had this massive line conditioning unit sitting on the shelf. I was tempted to buy it just to see what was inside as the weight implied some sort of transformer. The thing was huge, in rack mount format and frankly I just couldn't do it. It did have the audiophool branding.Now that you have me thinking, that might have been a source for a real cheap isolation transformer.

author: harerod
author_rank: Frequent Contributor
timestamp: April 04, 2021, 06:46:16 pm
content: @TO: Get professional help.A couple of weeks ago an intern at a customer's didn't understand that concept. He was lucky enough to be able to leave the hospital before Easter. I don't want to go too far into details - this ADULT uni student got creative and changed another person's test build, without consulting anybody beforehand. The original build was a big, isolated, battery powered closet and he introduced 230VAC via extension cord AND then wired some sub-optimal connections...Another place with unusual grounding would be an EMC test lab. Yet another reason why we keep people who only specialize in tying neckties out of those places.

author: Simon
author_rank: Unknown
timestamp: April 04, 2021, 07:05:48 pm
content: Quote from: wizard69 on April 04, 2021, 05:24:23 pmSteady on there. I would not use any transformer that I could not be certain was correct. If isolations transformers are being used for safety and are required to meet certain criteria you want to know they meet the criteria. I see mentions of low capacitance between windings. This makes sense to me as you don't want enough capacitance between the windings that earthed referenced power will cross the transformer capacitively and connect you to ground.