Isolation Transformers - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: February 11, 2014, 09:19:06 pm
content: I'm having trouble understanding the point of an isolation transformer. I've read articles and watched youtube videos but their purpose eludes me. The two uses commonly cited are safety and DC isolation.Apparently it's safer because there's no wired connection between the primary and secondary sides. So if we're talking mains power, the primary side is sitting at ~170V (peak) relative to earth ground at 60Hz. The coils are 1:1 so the same voltage/current should ideally come out the secondary. If I touch the lead off the secondary coil and I'm grounded I should get shocked since we have the same voltage/current reproduced from the primary and I just provided a ground reference. So I'm not really seeing the safety.As for DC isolation, I don't see how this is specific to a transformer with a 1:1 ratio. Since a transformer requires an alternating current to create a changing magnetic field to induce a voltage/current in the secondary, wouldn't a constant DC voltage source on the secondary side "do nothing" to the primary side since it's constant? Regardless of the windings ratio?

author: Rudane
author_rank: Regular Contributor
timestamp: February 11, 2014, 09:26:59 pm
content: Well, with regard to the DC isolation. If you send DC into the primary, nothing comes out of the secondary. So you have isolated the DC from the secondary. For the safety thing, I don't think it's about human safety. It's about isolating from the grid do you don't cause issues on the grid by doing something foolish.

author: Simon
author_rank: Unknown
timestamp: February 11, 2014, 09:29:07 pm
content: isolating transformers work with AC only. your household mains is referenced to earth, this means that at the substation the N is burried in the ground and that is earth, that is why you have an earth stake at your house too so that you have a "ground reference", if you touch L your automatically get a shock because it goes to earth and back to the N in the substation.When you have an isolation transformer that earth is lost because neither of your new L and N are referenced to ground you cannot complete a circuit to earth by touching only one, you have to touch both to get a shock from this new floating power source.

author: Legion
author_rank: Frequent Contributor
timestamp: February 11, 2014, 09:31:41 pm
content: Quote from: Simon on February 11, 2014, 09:29:07 pmWhat are L and N?

author: Simon
author_rank: Unknown
timestamp: February 11, 2014, 09:35:26 pm
content: live and neutral

author: grumpydoc
author_rank: Super Contributor
timestamp: February 11, 2014, 09:41:35 pm
content: QuoteIf you grab hold ofoneof the secondary outputs you won't be shocked because the secondaryisn'tground referenced.Of course if you get hold of both then you will still get a nasty and potentially fatal shock.Edit: In pictures

author: KDC
author_rank: Contributor
timestamp: February 11, 2014, 09:49:30 pm
content: Actually, with a isolation transformer with 2 terminals, you do not have a neutral. to have a neutral you'd need it to be centre tapped.Quote

author: woodchips
author_rank: Frequent Contributor
timestamp: February 11, 2014, 09:52:48 pm
content: An isolation transformer is used to isolate the mains from your piece of equipment. If you have to repair a switch mode PSU then you need a scope. You can't just attach the scope ground lead to the PSU and start poking around because of the difference in the earth potential and the neutral potential. The isolating transformer achieves this required isolation. There is also the problem of getting live and neutral swapped, so you try to earth the live wire with your scope probe, not recommended.One problem is that you are no longer protected by the house wiring residual current breaker, so putting one on the isolating transformer secondary is a good idea. This protects you if you touch the live and earth at the same time.If you are fault finding a switch mode PSU then a Variac to bring the voltage up slowly is pretty essential. This is connected to the isolating transformer secondary but be aware that you start to lose the protection of the residual current breaker as you drop the voltage.Must say that the DC isolation confuses me too, there isn't any DC in the mains supply, and if there was then the transformer would saturate and cause all sorts of problems.

author: Kjelt
author_rank: Super Contributor
timestamp: February 11, 2014, 10:15:34 pm
content: QuoteAn rcd won't work after an isolation transformer. The secondary is totally isolated from earth so all current is flowing from the one pole of the transformer through the body to the other pole of the transformer, the rcd won,t see any currentloss so won't trip.This looks dangerous because 35mA is enough to stop your heart, but the same goes for dc power supplies if the dc voltage is high enough. Rule nr1 for any electronics hobbieist never ever touch any live pcb / apparatus with both hands.

author: Legion
author_rank: Frequent Contributor
timestamp: February 11, 2014, 10:15:51 pm
content: Here's a picture to better illustrate my confusion.The zero line for the primary side is both ground for the primary and true earth ground. The secondary side has a zero line that acts as ground for the secondary side circuit but is not the same as earth ground. Since the secondary is floating, what is 0 volts or ground in the secondary circuit could be any voltage relative to true earth ground. It could be a 50 volt difference or a 5000 volt difference. So if I touch the floating secondary circuit and I'm grounded to true earth, why won't I act as a short to true earth ground and get shocked?

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 11, 2014, 10:52:59 pm
content: If the secondary is truly galvanically isolated (as any good isolation transformer would be), the potential between the secondary and earth ground is zero. There is no electrical connection between the secondary and earth ground. If you touch one or the other of the output terminals on the secondary, you, assuming you are grounded, will establish a ground reference on that terminal (albeit a relatively high resistance one through your body), but no current will flow, because there is no circuit. The other terminal is floating. Of course if you were to touch the other terminal at the same time, electrocution would ensue....

author: Legion
author_rank: Frequent Contributor
timestamp: February 11, 2014, 11:55:37 pm
content: Quote from: commongrounder on February 11, 2014, 10:52:59 pmThat was my problem. I had misunderstood the requirement for a closed loop and assumed current would flow from the secondary through you to ground indefinitely if there was a voltage between earth ground and the secondary circuit's ground.Thanks!

author: Steve Gare
author_rank: Newbie
timestamp: February 13, 2014, 03:44:56 pm
content: For what it's worth. I've just built a power supply isolation transformer test box using TWO dead large ups transformers back to back giving me 240v in 230v out...i'm in the UK so obviously using 240v. Dead UPS's are cheap if you have a source.ie...240v in----48v out (into) 48v in-----230v out.Some people have used microwave oven transformers in pairs as isolation transformers or even broke the core and swopped in the secondry with a primary from an identical unit but this involved splitting the core which isn't good. I did pick up a very large 240v/110v power tool isolation transformer from the carboot. This is 55v centre tapped on the secondry. I'ts an old 'oxford' unit in a metal box and by the looks of it it would be easy to wind an additional matching secondry which was my intention. This will give you oodles of power if you need to isolate something 'mega'If you intend to use the scope testing amplifiers for example then 'float' the unit under test not the scope. See Daves video on blowing the scope up lol it's ace.I've also built a stereo 'dummy' load 8ohm per channel using resistors. I've house this in an old dead smoke machine case with push/pull fans. I've used 2 x 240v/24v transformer for scope isolation ie 10:1 ratio, one on each chanel for cross talk seporation. from the main outputs.Don't you just love transformers?? they are nearly as good as VFD'sHave fun

author: Kjelt
author_rank: Super Contributor
timestamp: February 13, 2014, 04:43:02 pm
content: Quote from: Steve Gare on February 13, 2014, 03:44:56 pmI would advise to use two isolation transformers, one for the D.U.T. and one for your equipment (scope) or use a differential probe. Esp. with audio, removing or placing earth somewhere in the circuit might cause hum or oscillation.

author: Steve Gare
author_rank: Newbie
timestamp: February 13, 2014, 05:00:45 pm
content: I've been using an isolation transformer for the D.U.T and 10:1 transformer connected to the dummy load to test my audio. I'm still hunting round for a scope probe but damm they are pricey.... Let's hope I survive the wait

author: Shock
author_rank: Super Contributor
timestamp: February 14, 2014, 02:36:46 pm
content: Quote from: Kjelt on February 13, 2014, 04:43:02 pmIsolating a scope is bad from what I have read (unless it's designed as a insulated portable model). I stand to be corrected if I'm wrong.This is what Tektronix says about floating your oscilloscope.http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 14, 2014, 02:51:12 pm
content: Quote from: grumpydoc on February 11, 2014, 09:41:35 pmNice pictures, show exactly why I think isolation transformers are "false safety"Picture shows someone happyly touching a lead. Giving the impression "Its OK to do so, because I am on an isolation transformer"However, it you make that into a habit, than the chance to accidentally also touch the second lead is about equal to someone working life touching the first lead. Only this time there is no RCD to protect you.So an isolation transformer as a "trick" to make your Scope "differential": Maybe, yes.But for reasons of safety? Unwise (imuho). Better of with a RCD/GFCI and awareness to never touch any wire, and if you do, your mistake is clearly pointed out to you (by suddenly sitting in the dark)(grumpydoc did mention that touching both leads is still dangerous, but I felt with too little emphasis)

author: grumpydoc
author_rank: Super Contributor
timestamp: February 14, 2014, 04:03:03 pm
content: QuoteYes, you're probably correct about the emphasis - you really need to remember that isolation transofrmers help to avoid blowing up test equipment. They are not nearly so good at stopping you killing yourself if you blindly think of them as a safety device.Suppose that you have a DUT which has some point at a high potential relative to ground, which you need to use as "0V" for a measurement. Connecting an isolation transformer will allow this point to be connected to 'scope ground without blowing up the 'scope.Butdoing so establishes a ground reference which could mean the case of the DUT is now sitting an equally large voltage from ground and touching it represents a shock hazard.The safest thing to do is to set up the 'scope connections with the power off and then power up the DUTwithout touching the DUT with the power onn, even if connected through an isolation transformer.

author: Kjelt
author_rank: Super Contributor
timestamp: February 14, 2014, 07:12:58 pm
content: Quote from: Shock on February 14, 2014, 02:36:46 pmWell at my company we have all laboratory tables fully isolated, DUTs have their own isolation transformer or Chroma AC PSU, which is important so the scope is also floating in regard to the DUT. We have more then 60 LeCroy scopes at pricetags around $10000, none ever died in the last 15 years due to the fact they were isolated. Some died because the engineer did not use a highvoltage probe while measuring a couple of kV's and some died because LeCroys are sometimes LeCraps. For ESD you need to have some high ohms connection to earth, this could be 2Mohms or somewhat more with multiple high voltage resistors.Anyway, you always have to know what you are doing but that is also the case with unisolated connections as Dave showed in his video's.For safety rcd´s can also fail and if you have highvoltages they can be too slow. If you have the money I would still advise a differential probe for Voltages higher then 50VAC RMS or 60VDC better safe then sorry.

author: Kjelt
author_rank: Super Contributor
timestamp: February 14, 2014, 07:20:08 pm
content: Quote from: grumpydoc on February 14, 2014, 04:03:03 pmThat is why the DUT should have their own isolation transformer. The DUT is then floating and the labgear is floating, you are always safe to touch one point, never two points. But this is also the case without isolation transformer, touching the mains: live and neutral with two hands and wearing good isolating sneakers can also be the end of your life, 35mAmps is enough to stop your heart. Electricity is one of the most underestimated dangers out there, you can not smell it or see it, and touching can be lethal. Always keep that in mind and if you are sleepy do not experiment with voltages >50VAC or 60VDC, go to bed

author: woodchips
author_rank: Frequent Contributor
timestamp: February 14, 2014, 07:39:13 pm
content: Still seems to be some confusion, and the diagrams don't help.There is no direct connection between earth and neutral for the incoming mains. They are connected, but might be just outside, might be at the nearest pole. In any case there is always volts of difference, and it varies. Even if you connected the scope 0V to this neutral you wouldn't get any sensible waveforms.What the transformer does is allow the neutral to be connected to earth, so the diagram must show this. It then makes more sense in that the neutral line is the same as the 0V scope connection, and measurements can then be made.As I said, the secondary of the transformer needs an RCD, and they will disconnect fast enough to stop any danger, remarkable things really for the piffling amount they cost.The scope doesn't need an isolating transformer, it's internal PSU does that for you.

author: Kjelt
author_rank: Super Contributor
timestamp: February 14, 2014, 10:04:10 pm
content: Quote from: woodchips on February 14, 2014, 07:39:13 pmThis might depend on the country but in our country it is connected where it enters the house.If the neutral and earth were NOT connected, so lets say >5Mohms resistance between neutral and earth, the mains would be floating and an rcd won't work since no current can flow from mains to earth anyway, even when you short it. So since your installation does have an rcd this means that the neutral and earth are connected.QuoteThe secondary of an isolation transformer is totally floating and no current will flow to earth, so no current will get "lost" so the rcd will do nothing.If you want to install an rcd on the secondary of the transformer you need to connect one of the secondary outputs to earth. That will also kill all the benefits of the isolation transformer since it is no longer isolated, you then turned it into an autotrafo.QuoteNot all SMPS are fully isolated and I would not bet my life on it.

author: C
author_rank: Super Contributor
timestamp: February 15, 2014, 12:51:50 am
content: A good RCD does not need a ground to function!http://en.wikipedia.org/wiki/Residual-current_deviceThe basics of a RCD is to measure the difference in current flows. If the flows are equal then there is no Residual-current and no error.This can be done for DC as well as AC.For AC the easy way is to use a transformer. Look at the drawing on wikimediaYou have two wires with current flowing in opposite directions(L & N). When the currents are equal then their fields cancel, When this is not the case then you have a signal in the winding #2 in the drawing. At some signal level on winding #2 the RCD trips.The RCD transformer does not care what caused the difference, just that there is a difference!It does not care how the difference was created or where that current difference went.Then an RCD is connected to the secondary of an Isolation transformer it could trip just do to the difference created when only one of the wires it is protecting is connected. This would be due to the current flow required to bring isolated winding to the electric potential of that one connected wire which takes some current flow.The electric potential of that isolated winding could also be constantly changing with some valid test setups in which case the RCD would be constantly trying to trip.So yes a RCD could save someone when it can function as intended.In many test setups with an isolation transformer expect the connected RCD trip a lot.For some test setups, you would need a bypass switch for the RCD.This does not say that a RCD that was built for the AC power line will function correctly when connected to the secondary side of an isolation transformer. It could fail due to too many trips.Or one of the many problems listed on the wikipedia link.C

author: aroby
author_rank: Regular Contributor
timestamp: February 15, 2014, 05:39:15 am
content: I understand the isolation transformer rationale and theory. I'm not doing mains work, but just bought a scope and want to be safe. From Dave's video on the subject, it seemed to imply that bench power supplies are effectively isolation transformers / devices. Is that correct? And, if so, I assume that best practice would then be to power all circuits under test from a bench supply vs. an arbitrary mains power pack supply?Anthony

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 15, 2014, 06:45:47 am
content: The bench power supply I have has a removable bridge between the black post (GND, 0V) and green post (earth). So it came non-isolated but you can easily choose to remove the bridge and isolate it. I have it permanent in the isolated mode. So indeed no risk to my scope.The additional 12V/5V PC power supply I modded originally was not isolated, but I opened it and added some nylon washers between PCB and case, so now it is.Edit: added picture. Not a high end power supply, so as you can see that 'bridge' is following the KISS principle.

author: C
author_rank: Super Contributor
timestamp: February 15, 2014, 06:53:50 am
content: arobyLook at the specifications for the scope you bought for the max safe voltage on input.if you are using a 10x or 100x probe, the probe will allow a higher safe voltage.If you are sure that the circuit that you want to probe is less than the safe limit with that probe,You can use the scope to see if there is a AC difference and | or a DC difference between the scope's ground and the circuit's ground.Start with the scope to highest volts/div setting.you can then touch just the probe tip to the circuit and get a display.Note that you have to work down from scope's Max volts/div setting or you risk harm to scope.You can then get a display reading to decide if it's safe to connect the scope's ground to the circuit.C

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 15, 2014, 07:05:38 am
content: The video refers to discusses damage by accidentally shorting out ground. So input voltage levels are not really relevant to that.Mind you: I did short out ground once accidentally (long ago). Blown a fuse in the mains, and no harm done to the scope. Not that I advise people to try it, but I think you need quite some power before you melt one of the ground connections inside the scope.

author: Kjelt
author_rank: Super Contributor
timestamp: February 15, 2014, 09:07:11 am
content: Quote from: C on February 15, 2014, 12:51:50 amAgain, if the mains is not connected to earth/ground, or if your secondary of the isolation transformer is not connected to earth/ground the outgoing Voltages live and neutral or + and - are floating in regard to earth/ground, do we agree on that?You can view this as using a big battery, which is fully isolated from earth, if you connect one of the poles to the ground NO current will flow because there is no closed circuit.So next step I agree that the rcd will trip if a small current (typ. 35mA) is going out but not going back in.So where in an isolated setup would that 35mA possibly flow to? Nowhere, in an isolated setup al outgoing current will always go back in because there is no other possible closed circuit in regard to any of it surroundings. Ergo the rcd will never trip or it should be on false positives. You can test this if you like by using a fully isolated isolation transformer (not an autotransformer, you hopefully do know the big difference because an autotransformer is NOT isolated and could use a rcd but has no real safety advantage so do not use that) placing a rcd in its secondaries and try to trip it by placing a proper resistor to ground. Good luck, you won't succeed.In a normal mains connected to earth/ground setup the rcd will trip if there is another closed circuit to ground/earth, namely a human body that is always somehow connected to earth/ground. What happens is that an earth/grounded human will touch the live and more then 35mA will flow from the live through his body to earth/ground which somewhere is connected to the neutral. Then the rcd will trip.If that same human touches both poles and is pretty well isolated from earth/ground the rcd will not trip and the human could possibly die.

author: woodchips
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:22:08 am
content: As I said, the incoming neutral an earth are connected, but the exact location means that there will be a voltage difference between them. Similarly the secondary of the isolating transformer needs connecting to earth. Then the secondary of the isolating transformer needs an RCD. Note that the RCD must be connected after the earth connection, so if any earth leakage currents from you then they don't go through the RCD causing the imbalance to trip it.Anything with a transformer such as a bench PSU, scope etc is effectively isolated from the incoming mains, so no extra isolation is needed. You will need to recognise an autotransformer, best if you just never use them.Haven't you learnt the trick of gently rubbing the BACK of a finger against a piece of metal to feel if there is any tingling, saying that it is live? Use the back of the finger so if it is more than just leakage currents then the muscle spasm will pull the finger away. Similarly the trick of keeping one hand in the pocket, and poke around with the other. The floor is insulating so even if you do touch a high voltage there is no circuit to cause danger, and the tingling will tell you to be more careful next time.

author: Kjelt
author_rank: Super Contributor
timestamp: February 15, 2014, 11:48:34 am
content: Quote from: woodchips on February 15, 2014, 11:22:08 amNo if you want to use it as a safety precaution this is absolutely forbidden, if you do this you create the same problem as with an autotransformer which is aboslutely not safe anymore, the secondary isnotfloating in regard to earth/ground. You then indeed do need an rcd for safety but you can just as well not use a transformer at all since the whole point of using one is gone? Except for some noise cancelation. My point is using two isolation transformers to be safe without rcd because there is no path to earth/ground anymore.Or if you look at the pictures with the person and the isolation transformer above which are correct, if you connect earth/ground to one the secondary outputs of the isolation transformer in the second picture, touching the other output will shock you! So the whole (safety) point of using an isolation transformer is gone.

author: Kjelt
author_rank: Super Contributor
timestamp: February 15, 2014, 12:24:54 pm
content: I do see that there are applications out there that do ground the isolation transformer on the secondary side, however these applications are not for safety reasons for electronic engineers to protect them from accidental electrocution.Realize that electronic engineers working and measuring on live DUTs is a special case that should not be taken lightly.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 15, 2014, 12:54:23 pm
content: @Kjelt: I see the same thing as you, but arrive at the opposing conclusion:Indeed, if you ground the secondary side, you need a RCD and may as well not use a transformer anymore. Conclusion: For safety, drop the isolation transformer, and use a proper RCD. An isolation transformer will just give you a false feeling of safety, teach you the habit of touching wires, meaning one day you will touch two wires.A RCD will warn you every time you touched something wrong, thus teach you not to touch anything, and even if you touch both of them, chances are it is unbalanced in time and/or current so even then the RCD will have protected you.As an hybrid solution, how about thisconfigurable centre tapped 'safety' transformer.Setting Normal: Normal (or Neutral) earthed mode with RCDSetting Isolated: Isolated mode, no RCD, but maybe useful for special measurementsSetting Safety: Safety (or Split) earthed mode, half voltage, with RCDPS: need to allow java to run, and wait a few seconds for java to startupPPS: to protect equipment, also add a regular (automated) fuse.PPPS: if you want a variac version, placing a cheaper autotransformer variac on theprimaryside with with all 3 modes, even correctly spitting the voltage.

author: Kjelt
author_rank: Super Contributor
timestamp: February 15, 2014, 01:33:27 pm
content: Quote from: cybermaus on February 15, 2014, 12:54:23 pmThen you could skip the expensive isolation transformer. Still, how about high voltages? I know from the not so long days ago that every repair person working on CRT tv's used an isolation transformer since gaps of a cm could be bridged, and I am afraid the RCD could be too slow to prevent this.

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 15, 2014, 03:25:47 pm
content: So. Where, then, are the commercial isolation transformers with Ground Fault Protection included, or center taps or other similar safety devices? I don't see any. I think this is because an isolation transformer, used by a technician with the best test instrument, the brain, is a safe and effective way to do testing on equipment that requires work on a hot portion of the DUT. The iso-transformer allows the technician to establish a new ground reference, and it is up to the tech to recognize and understand what they are doing and not come in contact with a point in the circuit that has dangerous potential relative to that ground. I don't believe it is possible to make this type of servicing or testing "idiot proof" and that there really is any need to. You must understand what you are doing before attempting work on the mains powered side of a device. I have personally used a variable isolation transformer (Sencore PR570) for many years and have yet to have had a problem with shorts or shocks related to leakage or grounding issues.My apologies for the rant. It feels very important that we emphasizeknowledgeas the best safety device.

author: SeanB
author_rank: Super Contributor
timestamp: February 15, 2014, 03:34:32 pm
content: Centre tapped isolation transformers with fault warning are normally used in medical applications to reduce the risk of shock. Mostly used on equipment connected to the patient directly. Generally the big heavy white box in the bottom of the crash cart that everything else is plugged into.

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 15, 2014, 03:42:08 pm
content: SeanB:Fair enough. But you wouldn't really see anyone using such a device for electronic servicing or testing. Obviously if the center tap is permanent, it is not really a true isolation transformer, but more like a balanced AC power supply. Lower line noise for the medical diagnostic equipment, etc. There are similar devices used in the Home Theater industry that have center tap grounded secondaries for the good common mode noise rejection performance.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 15, 2014, 07:50:16 pm
content: It would not be isolation, but it does have the advantage that any touchable voltages (and thus body current) are only half of what they were before, while it is still able to use fault detection.

author: C
author_rank: Super Contributor
timestamp: February 15, 2014, 09:04:58 pm
content: KjeltYou say is true, but not complete.What you are forgetting is that the isolation winding will have some capacitance.If you connect just one lead from that isolation winding to a different electric potential you will have a current flow in just that one wire while charging or discharging that winding capacitance to that new electric potential.C

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 15, 2014, 09:56:50 pm
content: cybermaus:Wouldn't that beg the question of what we are after here, which is a method of servicing/testing the mains portions of the DUT using grounded 'scopes, etc. without causing faults? What would be the usefulness of protection for the technician, if they are unable to do any testing without causing a current flow to ground through that 'scope (either full or half voltage).C:Practically speaking, a good (i.e. safety rated) isolation transformer uses split bobbins in the core which physically separate the primary and secondary windings and reduce interwinding capacitance to a miniscule amount.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:24:38 pm
content: Well, somewhat.I could answer that is why the 'configurable' proposal has an isolation setting on its switch.However, I think that if you already know DUT has a wiring fault causing chassis to be hot, then the thing to do is not to plug it in and use the voltmeter, but instead keep it unplugged and use the ohmmeter (or megger). After all, you are going to need to unscrew stuff to find out which components causes the fault. So in principle, even when troubleshooting a ground fault, you should not need to keep the ground fault flowing.Of course, life is not always so theoretically correct, not everyone has a megger, maybe the fault only appears after relays are activated, etc, etc. Or indeed you do want to turn your scope into a cheap diffirential probe. So I do accept the occasional need for the isolation setting. Hence the option on the switch. But not as a first choice of safety, but as a later one. (BTW the RCD is not the first choice either. Common sense should be your first safety device).What I see this whole transformer/RCD thing for more is measuring other issues on DUT, therefore having it open and being near the hot part. Maybe even working on the mains part of it, but looking for other errors then GF errors and using the scope ground references, as is normal. And then having both pins at half-volt together with a working RCD is a nice combination, I think a better combination then 100% isolated or 100% line voltage.

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 16, 2014, 01:05:27 am
content: Hmmm. Well, your system is an interesting concept. I think it does ultimately come down to, whichever method you use, there has to be a total understanding of where the vulnerabilities are. I do own and use both differential probes and isolated input 'scopes in my work with switching power supplies and bridged audio power amplifiers. There is usually more than one way to successfully troubleshoot and test a device. Sometimes creating a ground reference via a 'scope probe ground, after an isolation transformer, is the only way to get a clean signal for diagnosis.In any case, knowing what you are doing is paramount.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 16, 2014, 08:18:31 am
content: And of course, in your country you do this at 120 mains (not 240 dryer) so half voltage is only 60V. Thats barely worth the label hot.BTW: just to make it clear: I do not have this setup. In the past I relied purely on RCD/GFCI (not that I trip them often. Whole years go by without me tripping one). I only recently was inspired byUK 'site transformers', when I learned this stuff is required for UK construction sites and I realized its extra safety.But looking into it: Those 1:1 centre tapped transformers sure are pricey, if you want one with a little umph in it.

author: aroby
author_rank: Regular Contributor
timestamp: February 16, 2014, 04:29:28 pm
content: Quote from: C on February 15, 2014, 06:53:50 amThanks for the tip!

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2014, 06:33:47 pm
content: This RCD vs isolation transformer debate often crops up. The thing one needs to bear in mind is that isolation transformers and RCDs are used for different purposes and each one has unique advantages/disadvantages over the other. For electrical safety, an RCD will quickly disconnect the circuit when an earth leakage occurs, which is good for safety but it can be a nuisance and one should never forget that an RCD is a mechanical device which can fail. A transformer is passive, without any moving parts, so is more reliable. Of course if one side becomes earthed, the protective benefits are lost but the system can be designed to avoid this or a device can be included to indicate this state.Here in the UK small isolation transformers are commonly fitted in bathrooms because they won't fail like an RCD can. Centre tapped to earth isolation transformers are also used on building sites to provide 55V-0-55V, often with an RCD so even if it fails to trip, the potential for shock is much lower.As far as bench testing is concerned. An isolation transformer is good for debugging mains devices. It also has the advantage of enabling an oscilloscope to be connected to the circuit, without causing a fuse to blow or RCD to trip. This isn't fool proof of course, one should always be aware that once the 'scope is connected to a device, the circuit then becomes referenced to earth. If you're worried about this, you could add an indicator to warn you when one side of the secondary is earthed.

author: Kjelt
author_rank: Super Contributor
timestamp: February 16, 2014, 06:56:30 pm
content: Quote from: Hero999 on February 16, 2014, 06:33:47 pmInteresting, do you have an URL or maker+partnumber or a schematic of such a device?

author: M0BSW
author_rank: Frequent Contributor
timestamp: February 16, 2014, 08:08:59 pm
content: Quote from: Simon on February 11, 2014, 09:29:07 pmSo would you have to get a 240v to 240 volt transformer for the UK or if you got say a 2 x 240 in and 100 v out transformers wired them up 1st 240 to 100 - turned the 2nd around 100 to 240v would that work as an isolation transformer.

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2014, 08:22:55 pm
content: Quote from: Kjelt on February 16, 2014, 06:56:30 pmHere's an example of a part which will do the job:http://docs-europe.electrocomponents.com/webdocs/025a/0900766b8025a716.pdfFor hobby use, it's possible to build a simple earth fault detector from a couple of high voltage resistors and a neon lamp. That way you get a nice warning when the secondary is earthed.

author: made2hack
author_rank: Frequent Contributor
timestamp: February 16, 2014, 08:35:21 pm
content: When building a AC to DC power supply, should one use an isolation transformer on the AC side? Say a 230V to 12V isolation transformer? Or no? Just a regular transformer for this?My question is somewhat not related.

author: C
author_rank: Super Contributor
timestamp: February 16, 2014, 08:38:03 pm
content: RCDI was just trying to point out two things.1. You can have current flows to an isolated device or circuit.2. A good RCD does not need a Ground or Earth connection to function correctly.The RCD should be measuring just the differences of protected wires.One thing an RCD is suppose to protect you from is a two wire device failure. One where the insulation has failed and you touch that water pipe or faucet that is grounded. That current path does not go back to the RCD.QuoteIn my thinking if an RCD has to have a Ground or Earth connection to function it is a poor design!Knowing your safety equipment is important.Knowing how your safety equipment functions is important. What it can do and what it can NOT do.When designing in a fuse to a circuit, You should know how and what that fuse will add protection for and what it will not add protection for.Same is true for an RCDThe people working very high voltage do not trust their safety equipment to function properly, They test the safety equipment to confirm that it is functioning.If you are counting on safety equipment to function, not a bad idea to test it before it may be needed.C

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2014, 08:48:42 pm
content: Quote from: C on February 16, 2014, 08:38:03 pmAll RCDs need an earth ground connection to work. The neutral wire has to be connected to earth in order for any RCD to work.If the neutral wire is not connected to earth than an RCD will not work.

author: C
author_rank: Super Contributor
timestamp: February 16, 2014, 09:16:48 pm
content: Quote from: Hero999 on February 16, 2014, 08:48:42 pmQuoteWhere is the ground?If #1 needs more power than supplied by #2 the Current transformer secondary winding, it should come from L & N.Nothing in that circuit requires or has to have a ground.C

author: cellularmitosis
author_rank: Unknown
timestamp: February 16, 2014, 09:39:31 pm
content: Another use for isolation transformers is replacing the battery in an HP 3478A bench multimeter.These meters store their calibration offsets in RAM, so that RAM has to remain powered up at all times, so there is a lithium battery in these units which lasts maybe 20 years.The trick is that these batteries are soldered in-place, and the tip of your soldering iron is typically grounded. So as soon as you try to desolder the positive terminal of the battery, bang, you just lost your calibration data (and re calibration is typically $100+).If you power your soldering iron with an isolation transformer, you don't ground the positive battery terminal as you desolder it.The other solution is to heat up your soldering iron, unplug it, and quickly desolder the battery using the thermal capacity of your soldering iron.This guy made a great video about all of this:http://youtu.be/r8fXZWvO2yU

author: Kjelt
author_rank: Super Contributor
timestamp: February 16, 2014, 09:46:03 pm
content: QuoteIn theory you are right, in practice there is only one way current can "leak" and that is what is everywhere around us, earth-ground.If the neutral is nowhere connected to ground then you can touch mains and earth and no current would flow. In practice every installation has neutral connected to earth somewhere and you get a shock. To prevent this current flowing through your body to earth is where the rcd comes in and the only reason why it is working is that somewhere before the rcd the neutral is connected to earth-ground. Else it will never ever trip. If you touch mains phase and neutral together you get shocked but the rcd will not trip since all outgoing current is going in.

author: Kjelt
author_rank: Super Contributor
timestamp: February 16, 2014, 09:51:36 pm
content: Quote from: cellularmitosis on February 16, 2014, 09:39:31 pmModern soldering stations let you choose/install the grounding: hard to earth, or open(floating) or what I always do for esd protection is high ohms connected (1M ohm typ.).Also a lot of older stations are usually already fully isolated (24V transformer) and have a seperate connector where you can do the same.

author: C
author_rank: Super Contributor
timestamp: February 16, 2014, 10:32:34 pm
content: Quote from: Kjelt on February 16, 2014, 09:46:03 pmYour in practice just makes it the most likely place, not the only place.C

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2014, 11:49:38 pm
content: Quote from: C on February 16, 2014, 09:16:48 pmThat schematic doesn't show the full story. The ground is normally connected to the other end of the neutral wire, either at the substation or entrance to the building. If the neutral wire is not connected to ground, the RCD won't work.

author: C
author_rank: Super Contributor
timestamp: February 17, 2014, 03:37:27 am
content: Hero999It is very simple to prove that a ground or earth is not needed for a good RCD to function.Connect the isolation winding of your isolation transformer to one side of the RCD.Leave any ground or earth connections of the RCD disconnected.The RCD should now have any power it needs to add for #1 in the circuit to function.The test button will function now function.It would not function if what you say is true.The test button should still function if you connect the isolation winding to the wrong side of the RCD.There are many ways to create the same unbalance that the test button does.With a greater current flow through the L & N wires the easer it will be to create the unbalanced condition that will trip the RCD.The poorer the isolation transformer is the easer this will be from just the other side of the RCD.C

author: Monkeh
author_rank: Super Contributor
timestamp: February 17, 2014, 03:54:08 am
content: I think you're missing the point.

author: C
author_rank: Super Contributor
timestamp: February 17, 2014, 04:17:22 am
content: Quote from: Monkeh on February 17, 2014, 03:54:08 amIf you are replying to me.Which of the following is more correct.1. An RCD will trip for any path that causes ___ current flow difference in the protected wires.2. An RCD will trip for ground path that causes ___ current flow difference in the protected wires.For Safety it is a good idea to be exact!C

author: Monkeh
author_rank: Super Contributor
timestamp: February 17, 2014, 04:20:07 am
content: Quote from: C on February 17, 2014, 04:17:22 amNumber 3: An RCCB will trip for any current imbalance which falls within the time/current curve for the device.You did say to be exact.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 17, 2014, 04:41:19 am
content: Guys, I think you are arguing different things:A ground wire is not neededlocallyto make an RCD work.You do not have to have a ground wire present at the RCD, or at the plug/socket of your device, anywhere near your workbench, or even anywhere in your apartment.This is what I believe C is trying to argue. And within the right frame of reference correct,But a ground wire is neededsomewhere. As mentioned usually at the local power substation and/or the power entrance to your building. Otherwise there can be no leakage and thus no imbalance to measure.This is what the others are arguing. And important to note within the frame of reference of transformers, as that means to make the RCD work you indeed do need tore-wire ground locally, as the power substation's wire is on the wrong side of the transformer. (but if you do so it is no longer an isolation transformer)

author: C
author_rank: Super Contributor
timestamp: February 17, 2014, 06:00:28 am
content: Quote from: Monkeh on February 17, 2014, 04:20:07 amYes I like your wording is much better than what I had.To get the current imbalance in your #3 you just need a leakage path or something that will cause the difference.A ground is not required or needed for #3C

author: Monkeh
author_rank: Super Contributor
timestamp: February 17, 2014, 06:22:28 am
content: Quote from: C on February 17, 2014, 06:00:28 amAnd in practical usage, a ground referenced conductor is which creates the potential for an imbalance.

author: quarks
author_rank: Frequent Contributor
timestamp: February 17, 2014, 06:46:58 am
content: Maybe this is interesting/helpful informationhttp://www.iso-puck.com/contents/en-us/Leakage%20currents.pdf