DIY build of an Isolation Transformer - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 11:34:43 am
content: Hi all,My supplier ishttps://export.farnell.com/and they do not have any isolation transformers pre-packaged so I'm happy to build my own.Any thoughts on whether I should go for this toroid:https://export.farnell.com/hammond/del-182n117/transformer-234v-2-x-117v-500va/dp/4607442or this Variac:https://export.farnell.com/multicomp/mc2f-1/transformer-variable-2a/dp/1086028As for safety precautions I will install:Panel mount 1.5A fuse2A circuit breaker30mA RCD (GFCI) breakerEarth line will be bonded to the chassis. Only the "isolated output" will be isolated!Thoughts? Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: September 10, 2020, 11:51:33 am
content: 99.99% of all Variacs do not *NOT* provide any isolation whatsoever. Not unexpectedly your link is a direct-from-Line (i.e non-isolated) autotransformer.A GFCI (RCD) is incompatible with an isolation transformer. The transformer prevents any imbalance at the output tripping it, as the current into one end of any single transformer winding *MUST* must always equal the current out of the other (neglecting the tiny leakage current due to interwinding capacitance).Bringing the Ground through to the output socket of your isolation transformer makes it unsuitable for electronics repair bench use, as it compromises safety by introducing a ground in the supposedly isolated zone.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 12:17:52 pm
content: Quote from: Ian.M on September 10, 2020, 11:51:33 amYes, that is correct. This is nicely demonstrated by Martin in this video. I thought it wouldn't hurt to include the GFCI though.Quote from: Ian.M on September 10, 2020, 11:51:33 amSorry, not sure where I made you think I'd do that. The point of being isolated is so there is no return to mains-earth reference. Grounding the chassis is a precaution though.The output socket will simply be Live/Neutral from the Isolation transformer's secondary. Right so the variac is not a good idea - thanks for that.Can I use the Toroid then?https://export.farnell.com/hammond/del-182n117/transformer-234v-2-x-117v-500va/dp/4607442

author: Ian.M
author_rank: Super Contributor
timestamp: September 10, 2020, 12:23:20 pm
content: The GFCI gives a false sense of security and does nothing for safety. Do *NOT* fit one to your isolating transformer build as electric shock is not alleviated by the placebo effect.Its rated for 4KV isolation, so why not? I've used similar toroidal transformers for bench isolation.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 12:30:39 pm
content: Quote from: Ian.M on September 10, 2020, 12:23:20 pmGood point, will drop the GFCI. Anything else I should consider for safety precautions?Thanks!

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 12:55:39 pm
content: Hi Ian,I was thinking about the doc notes on the Toroid, re. high inrush current. I could use HRC fuses. However, even fuses take a while to react to any current spikes, especially if the AC end sees a short at the device under test (DUT).Martin has shared a build of his where he has done this (please note I've adapted this for 240VAC, Martin has used this for 110VAC):(1) Used a chinese chepo 240VAC -> 24VDC 200W LED Power supply as the input end:https://smile.amazon.com/gp/product/B07RHLFFYT/ref=ox_sc_act_title_3?smid=A2DYIB4IPW7T3M&psc=1(2) This is wired to a digital buck converter PSU. The advantage here is we can program OCP protection any time. This can be set to drive an output of 15VDC.https://smile.amazon.com/gp/product/B0744BT79M/ref=ox_sc_act_title_4?smid=AFHAE9RJVUMB&psc=1(3) I would order this inverter from Farnell; it's the only one supporting 240VAC output for the lowest costhttps://export.farnell.com/ea-elektro-automatik/ea-srx-pb-300-12/inverter-dc-ac-12vdc-230vac/dp/1335838(4) Feed the 240VAC from the inverter into the Toroid via HRC fuses.(5) Install Toroid inside a metal casing, with the casing grounded to earth.(6) IEC plug connected to Toroid only has the isolated Live/neutral supply. No earth or any connection to mains-earth reference.Thoughts?

author: Ultron81
author_rank: Contributor
timestamp: September 10, 2020, 03:32:03 pm
content: I believe if you want an isolation transformer, you can just wire 2 of the same transformers back to back. Wire line voltage to Transformer # 1 Primary, wire #1 Secondary to # 2 secondary, then use # 2 Primary as your isolated output. Just make sure to use a transformer with a suitable VA rating for your project.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2020, 04:11:40 pm
content: Quote from: Ian.M on September 10, 2020, 11:51:33 amAnd yet somehow devices that are built this way are UL-approved, CE-marked and used widely for many years by service technicians and educational laboratories worldwide. You should alert them all to their folly.This topic has been beaten to death, but there simply is no magical 'isolated zone' that protects unthinking bumblers from their mistakes. Like any device, you have to know what it does and how to use it. The 'false sense of security' arguments make my blood boil, especially when GFCI is mentioned in the next breath. Perhaps idiots deserve a good zap to wake them up.Omitting the ground pin connection is an option best reserved for advanced users. It also has little to do with isolation transformers--you can 'float' most deviceswithoutan isolation transformer by just breaking the ground connection somewhere--as long as you don't float them very high. The problem with doing it routinely is that it is mostly pointless--you reintroduce the ground reference when you connect your non-isolated scope or whatever.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2020, 04:39:14 pm
content: Quote from: bsodmike on September 10, 2020, 11:34:43 amI'll give you my take and let others disagree as they like. FWIW, I've been using a setup like this for years with nary a shock or incident other than popping the breaker occasionally.Splurge and buy both the variac and a transformer. Get a case big enough for both. Wire it up so that the variac is first, the transformer is the output. I don't like your transformer selection as it doesn't appear to be rated for isolation and I don't see specs for that. I think this is better.https://export.farnell.com/schneider-electric/abl6ts40u/transformer-400va-1-x-230v/dp/2070202Ideally you want a circuit breaker on the input to the variac and a slightly larger fuse on the output of the variac. In this case a 2A breaker and a 2.5A fuse might be appropriate.GFCI before the transformer will be useless unless you are poking around in the device itself, and on the output it might technically function in some odd cases. The only real protection would be in the case of an internal malfunction of the isolation device itself, like if you poured water into it and then touched an output. It won't actually help you in any realistic scenario. So skip it.By all means run the ground through. That's how it is done. If you want an ungrounded DUT, think it through and then use an adapter. The downside of having a ground is that you can easily end up creating a ground reference that makes a part of your internal circuitry 'live'--it will shock you--despite the isolation. The downside ofnothaving a ground is that you can connect test devices to your DUT that will create a ground referencethat will make the chassis and case of your DUT 'live'. You tell me which is worse.So, two things to remember. First, there is no foolproof safety zone that will allow you to poke around without thinking about it--unless you stick to things powered by 9V batteries. Second, if you build what I've described, the safety feature that will be the first to save you from a mistake won't be the isolation, it will be the circuit breaker!

author: bsodmike
author_rank: Regular Contributor
timestamp: September 11, 2020, 01:40:25 pm
content: Quote from: bdunham7 on September 10, 2020, 04:11:40 pmYou'd use a differential probe in that case though??

author: bsodmike
author_rank: Regular Contributor
timestamp: September 11, 2020, 02:05:52 pm
content: Quote from: bdunham7 on September 10, 2020, 04:39:14 pmThe datasheet referred to a Hi-pot test, so I guess insulation is decent between the primary/secondary:https://export.farnell.com/hammond/del-182n117/transformer-234v-2-x-117v-500va/dp/4607442Hi-Pot test of 4,000V RMS between primary & secondaryQuote from: bdunham7 on September 10, 2020, 04:39:14 pmHa, brilliant! I quite like that ideaQuote from: bdunham7 on September 10, 2020, 04:39:14 pmEek no, I would always keep the DUT chassis grounded as I've received mild shocks before when seeing poor grounding. I've even had Apple laptops give slight shocks when to see some idiot has used an adaptor leaving out the earth pinQuote from: bdunham7 on September 10, 2020, 04:39:14 pmYes indeed. Also by powering the Variac input (or the Toroid directly) via the programmable DC power-supply and inverter, I can set the OCP protection on the DC power-supply. If you check Martin's video he shows how the OCP protection kicks in much much faster than the 0.5 Amp breaker takes to trip.The final arrangement will be:240VAC Mains --->> 3x breakers wired in parallel: 0.5A, 1A and 1.5A; at least (1x) breaker must be enabled for current to flow> LED PSU with 24VDC 200W output> Input of 24VDC converted to 15VDC output, operating as a DC "buck" programmable supply.> 15VDC input into the EA-SRX PB 300-12 300VA inverter, providing 240VAC output.At this point, we either power the Variac > Toroid or Toroid directly.Probably go with 2A HRC fuses to start. Here are some high quality ones from BUSSMANNhttps://export.farnell.com/eaton-bussmann-series/nitd2a/fuse-hrc-2a/dp/1202355Thanks a lot for your input, I particularly enjoyed your forthcoming candour.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2020, 02:12:45 pm
content: Quote from: bsodmike on September 11, 2020, 01:40:25 pmIf you are using a differential probe then floating is still pointless because you don't need it. In fact, DPs still need some ground reference to operate. As for how I would do it, that is driven by what the DUT is and what I have handy. For a DUT that uses up to 4 amps, I just routinely use the iso-variac setup because it is already on the bench. Then depending on what the DUT is, I might start with an isolated scope, presuming that I need a scope at all. In certain cases I might use an adapter that breaks the ground and also allows me to measure ground leakage directly. On a DUT that draws more than 4 amps, I might float it with a cheater plug, but the only reason I can recall doing that is to repair a device with a ground fault that was tripping the GFCI.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2020, 02:18:56 pm
content: Quote from: bsodmike on September 11, 2020, 02:05:52 pmThe Hi-pot test is good for most purposes--and this does appear to be a high quality transformer--but isolation transformers are also made to minimize capacitive coupling and leakage current. Typically nowadays that means dual plastic bobbins and lead separation. Depending on the winding arrangement of the toroid, it may have greater leakage current than is ideal for an isolation transformer, but perhaps good enough. And if you are driving it with that offline inverter system, there may be no issue at all.

author: andy3055
author_rank: Super Contributor
timestamp: September 11, 2020, 04:34:17 pm
content: Quote from: bsodmike on September 10, 2020, 11:34:43 amYou are in Sri Lanka? Go to Silvertones at: 132, Vauxhall street, Colombo 02. They will make any transformer to your specs. I am not sure if old Mr. Kulasinghe is still there. He was a genius and specialized in transformers!http://www.silvertonesonline.com/

author: bsodmike
author_rank: Regular Contributor
timestamp: September 12, 2020, 08:38:09 am
content: Thanks for the tip Andy - sadly the staff there were terribly rude a good 20 years ago. I called them up re. the transformer. A polite receptionist answered and I got transferred over to technical. Uh oh.I asked "could I kindly get your good name please?", then I was greeted with a typical gruff grunt and a pause (yup, he's not so great speaking English). So this time I broke into my not so great Sinhala and by the time I mentioned transformer I was speaking in English. In any case, I mentioned that I was after an isolation transformer and whether he had any that could handle 1:1 240VAC with ~400VA apparent power output.Then he asks "what capacity?". I responded, "around 400VA"."WHAT CAPACITY?!?"At which point I recalled why I rarely deal with SilvertonesThe local Farnell agent charges a fair bit more but at least I get what's on the sticker. I've got a really efficient sales agent and I email him BOMs as spreadsheets and the order is typically placed within the day and delivered with an ~2-week delivery cycle.

author: andy3055
author_rank: Super Contributor
timestamp: September 12, 2020, 05:41:12 pm
content: That is disappointing! Old Mr. Kulasinghe was in his 60 s at the time about 20 years back. Obviously he is not there anymore. It was a family business but I forget his son's name. Probably the business has changed hands as well. There was a guy by the name Sunil in his 30s who was the winder. Even he may not be there anymore. If you are in Colombo, it may be worth a visit for fact finding.Good luck with your project.

author: kallek
author_rank: Regular Contributor
timestamp: September 13, 2020, 07:40:34 am
content: I have done isolation transformer and variac combination myself.Wiring:1. fuse2. Double pole main switch3. Two bulb sockets in parallel each others for adjustable current limit, and bypass switch4. variac5. Second fuse on wiper!6. isolation transformer7. meters8. Wall outlet without earth

author: bsodmike
author_rank: Regular Contributor
timestamp: September 14, 2020, 02:21:31 pm
content: Awesome - thanks for sharing your build. That looks great!Questions:What gauge (AWG) wiring did you use?Did you go for 2A fuses? fast blow or HRC?The variac I ordered is identical to yours. I'm going with an IEC jack to make sure that this unit is "safer" and no one at home would suddenly decide to go plug something in.You won't believe the number of times I've found workmen/cleaners just plug into _ANY_ socket they can get their hands on, without thinking at all.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 14, 2020, 03:34:34 pm
content: Quote from: kallek on September 13, 2020, 07:40:34 amPlease explain how the bulbs help to limit the output current?

author: bdunham7
author_rank: Super Contributor
timestamp: September 14, 2020, 03:48:20 pm
content: Quote from: kallek on September 13, 2020, 07:40:34 amVery nice build! Two questions:1. Where did you get those meters?2. Why are the bulbs lit with nothing connected to the outputs?

author: kallek
author_rank: Regular Contributor
timestamp: September 14, 2020, 06:30:14 pm
content: Quote from: bsodmike on September 14, 2020, 02:21:31 pmThank you! I got the idea from MrCarlson's Lab:Wiring is 1,0mm2so it's around 17 AWG.Fuses are 20x5mm slow ceramic fuses. I don't remember value of primary fuse but second fuse is 1.5A because I haven't needed more current. 2A is OK too.That's maybe good idea.Quote from: bsodmike on September 14, 2020, 03:34:34 pmBulbs are in series with variac, so when current increases, bulbs starts light up and reduce voltage.Good luck with your project!

author: kallek
author_rank: Regular Contributor
timestamp: September 14, 2020, 06:36:07 pm
content: Quote from: bdunham7 on September 14, 2020, 03:48:20 pmThank you!Meters are these from TME:https://www.tme.eu/en/details/eqn72-250v/voltmeters/gossen-muller-weigert/eqn72-250v/https://www.tme.eu/en/details/eqn72-1.5_3a/ammeters/gossen-muller-weigert/eqn72-1-5-3a/Bulbs lit because transformers no-load losses.

author: bdunham7
author_rank: Super Contributor
timestamp: September 14, 2020, 07:39:27 pm
content: Quote from: kallek on September 14, 2020, 06:36:07 pmWhat was the rationale for not putting the bulbs at the output?

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2020, 08:33:34 pm
content: Quote from: bdunham7 on September 14, 2020, 07:39:27 pmIndeed, that's what I was wondering...Usually the dim-bulb(s) are directly in series only with your load, right at the output.

author: kallek
author_rank: Regular Contributor
timestamp: September 15, 2020, 09:45:04 am
content: At the output of what?That unit can be used as an dim bulb tester, variac or combination of both.If output is shorted, in input side bulbs glows always at full brightness, no matter what output voltage is.

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 09:52:43 am
content: Quote from: kallek on September 15, 2020, 09:45:04 amYes, but usually the dim-bulb part isafterthe Variac, in series only with your output socket to the attached load, not before an isolation transformer or Variac or whatever. It's usually the last thing in the chain so it lights only with actual load, helping you limit current to, and monitor what is actually going on with the current drawn only from your "device under test."

author: kallek
author_rank: Regular Contributor
timestamp: September 15, 2020, 09:59:40 am
content: I don't know. I did it as it was made in video i posted earlier. In my unit the bulbs are glowing very little, so I can see all changes in current that DUT takes.

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 10:14:39 am
content: Quote from: kallek on September 15, 2020, 09:59:40 amThe only reason that Paul (Mr. Carlson) did it that way was because he wanted to be able to use either the 120V or the 240V outputs on his salvaged "isolation" transformer to run 120V or 240V devices off the output without having to change light bulbs (or switch them to be in series, or whatever) to go to 240V from 120V.He points that out in the video at1:45Normally, you would put the light bulbs on the output side.You're not using a 120/240V center-tapped, dual-winding, or dual output isolation transformer, are you?

author: kallek
author_rank: Regular Contributor
timestamp: September 15, 2020, 10:18:11 am
content: No I'm not. Thank you for information!

author: kallek
author_rank: Regular Contributor
timestamp: September 15, 2020, 10:29:26 am
content: Still one think I don't understand. If it's better to put DBT after variac, why Mr. Carlson didn't put it between variac and isolation transformer? Isn't it 120V too?

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 10:31:53 am
content: Quote from: kallek on September 15, 2020, 10:18:11 amWhile it doesn't really make ahugedifference, the main advantage of having the filaments cold when you power up a DUT is that the cold filaments have a lower resistance, therefore allowing a slightly larger startup surge current, which can help when starting some devices (like switching power supplies, for example) when they're going to have appreciable voltage drop across the light bulbs to their input.Obviously, this depends on the device you're testing and the bulb wattage you're using at the time to protect it. That is one of the nice things about using light bulbs in this way, it makes changing the current limit as easy as changing a light bulb. Literally.

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 10:32:45 am
content: Quote from: kallek on September 15, 2020, 10:29:26 amHis bulbsarebetween the Variac and the isolation transformer.Edit:Oh, maybe they're not... Just skipped to his schematic. I haven't actually watched that video for some time, and I'm not sure I even watched it in its entirety in the first place.Again, strictly it really doesn't mattermuch, but in your case there is no reason not to have them at the actual output. It saves a little idling power, as well.

author: bdunham7
author_rank: Super Contributor
timestamp: September 15, 2020, 01:55:12 pm
content: Quote from: kallek on September 15, 2020, 09:59:40 amIf you use it like that, the filaments are already warm and into their resistance 'knee', which means they will be even quicker about limiting current surges, but will have much poorer load regulation. There might be instances where this is what you want, but in most cases you would want consistent output voltage under varying loads within the limits.

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2020, 03:19:16 pm
content: Quote from: bdunham7 on September 15, 2020, 01:55:12 pmOh, for sure... It is true that in some certain situation you might actually want that behavior, but usually the opposite is true, the problem is usually keeping the voltage high enough for proper operation of the DUT, but if you were troubleshooting something small where youknowthere's a short or something, then it might be useful.You can always add some additional dummy load (like another low wattage bulb, maybe like a 7W night-light bulb, to your output to "pre-load it with a little glow" before you plug in your DUT in parallel along with it already running that small load. You cannot "undo" the bulbs being wired on the line side or between Variac and isolation transformer in those cases. Is suppose if you're making it deluxe you could use a DPDT switch to select your bulb position between line/load side of the isolation transformer, but if you're getting into adding bulb switches, my preferred way would be to have an array of bulbs like 25, 40, 60, 100, 150W or whatever and be able to switch multiple bulbs on and off without unscrewing and changing them. Having the capability to have more than two in parallel really increases your startup surge capability with multiple cold filaments in parallel.It really depends on what your usual use case is... That's starting to get a bit over-the-top for most people's regular use, though if you use one every day on a wide variety of devices, say on a repair shop bench, it might be well worth the effort to add half a dozen sockets and switches and have 20 different bulbs available, I suppose.To each his own, YMMV, etc.In any case, having a good assortment of bulbs and atleasttwo sockets gives you a very wide range of available current limiting "curves", even if you just unscrew one part way to turn them "off".

author: intabits
author_rank: Frequent Contributor
timestamp: September 16, 2020, 06:53:41 pm
content: Some of my frolicking in this area may be of interest to you:-https://www.eevblog.com/forum/projects/a-simple-project-to-enhance-a-variac-with-a-voltagecurrentpowerenergy-meter/msg1358361/#msg1358361https://www.eevblog.com/forum/projects/testing-back-to-back-power-transformers-for-use-as-an-isolation-transformer/msg2398320/#msg2398320https://www.eevblog.com/forum/projects/back-to-back-isolation-transformers-now-in-a-box-plus-other-contraptions/msg2783828/#msg2783828

author: bsodmike
author_rank: Regular Contributor
timestamp: September 18, 2020, 07:08:52 am
content: Quote from: intabits on September 16, 2020, 06:53:41 pmAwesome, thanks for sharing! I've subscribed as well. BTW what gauge (AWG) stranded wire were you using? Looked like ~18AWG?Thanks.

author: intabits
author_rank: Frequent Contributor
timestamp: September 23, 2020, 02:33:36 am
content: Quote from: bsodmike on September 18, 2020, 07:08:52 amI'm not sure which wire you mean?Not that I'd know - it just would have been whatever wire I had hanging around that looked like a fair thing.For most of my mains wiring, it was probably stripped out of mains cables...Thanks for the like and subscribe.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 30, 2020, 03:36:01 pm
content: Hi all - a bit more progress on my end. I received this Fuse block with a compatible HRC 2A fuse.I thought of wiring my input end as ---> HRC Fuse 2A --> 2A fast-blow---> rest of the internal equipment.First code is for Farnell:556361 CM32FC EATON BUSSMANN Fuseholder, DIN Rail, 690V, 32A, NITD and NIT Fuse Links1202355 NITD2 EATON BUSSMANN Fuse, High Rupturing Capacity (HRC), NITD Series, 2 A, Bolted Tag, 550 VACThanks!

author: LateLesley
author_rank: Frequent Contributor
timestamp: September 30, 2020, 03:54:26 pm
content: Quote from: bsodmike on September 10, 2020, 12:17:52 pmI don't see a reason why you couldn't, apart from cost. I've been looking for a cheap way to do this, and came across these -https://www.screwfix.com/p/carroll-meynell-1-5kva-intermittent-isolation-transformer-230v-230v/752hvIt may be a cheaper way to do it, with a higher current rating. You may need to double-check the output isn't earthed though.

author: dmmartindale
author_rank: Contributor
timestamp: March 23, 2025, 12:00:50 am
content: The BBC has a standard for isolation transformers (EGN 4) that calls for the output receptacle ground terminal to be left unconnected. However, this isNOTto make any exposed metal on the device under test ungrounded. EGN 4 says that there should be a separate ground terminal near the output receptacle, and that this ground should be connected to any exposed metal on the DUT via a separate "wander lead" instead of depending on the ground wire of the device power cord. So the intent is that the DUT will be properly connected to an earth ground - just not using the power cord ground pin.There is an explanation for this and a diagram in the EGN 4 document, and I encourage people to find a copy and read it. But a quick summary is that it is possible for devices to be wired with neutral and ground interchanged (either at the plug, or inside the device) and that such devices work normally, with no sign of problem, when plugged into a normal non-isolated wall outlet (because neutral and ground are connected together at the main panel). But plugging such a device into an isolation transformer outlet, with the ground terminal actually grounded, creates a hazard that might place 230 V on the DUT case (230 V because this is UK line voltage). Using an external "wander lead" wire to connect the device case to a ground binding post ensures that the case is truly grounded, even if neutral and ground are interchanged.

author: Benta
author_rank: Super Contributor
timestamp: March 23, 2025, 12:12:54 am
content: I'm pretty certain that either the OP's circuit has been working for the last 5 years, or he/she was electrocuted back then. Either way, it's moot.

author: dmmartindale
author_rank: Contributor
timestamp: March 23, 2025, 12:59:38 am
content: Probably true, for that particular person.But these discussion threads remain online, and become a general resource for people looking for information. This thread can be found via other people looking for information about isolation transformers as long as the website remains on line. A search of this forum for "isolation transformer" returns hundreds of results. The transformers themselves last for decades too.So I don't think the information is "obsolete", even if it comes too late to help the OP.

author: Shock
author_rank: Super Contributor
timestamp: March 23, 2025, 03:58:42 am
content: In that case then, best to start a new thread and show that schematic to discuss that specific setup in context. This is always best if you feel you have anything new to add or a better way of doing things, rather than bumping old help threads.The BBC EGN4 document from 2010 is posted in the below thread, please provide an updated version in your thread if you have it.https://www.eevblog.com/forum/reviews/isolating-transformer-(uk)/

author: dmmartindale
author_rank: Contributor
timestamp: March 23, 2025, 04:30:49 am
content: I have the same version of EGN 4 as the one pointed to by the thread you mention. The diagram I referred to is "Appendix 1" of that document, showing the difference between an "off the shelf safety isolating transformer" and the "BBC method" when the DUT has interchanged neutral and ground in its power cord.I don't have any new information to add - just old information that no one had mentioned in this thread. I was attempting to reply to "Reply #7" by bdunham7, which said "Omitting the ground pin is an option best reserved for advanced users". EGN 4 is one example where omitting the ground from the output receptacle is arguably a good idea. (Unfortunately, the forum does not seem to provide threading of replies, so you can't tell which posting I was attempting to reply to unless I quoted some part of it).However, you pointed out a 2016 thread (the isolating transformers (UK) one) that I hadn't seen. So I'll go off and read it. (A forum search for "isolating transformer" yields about 1000 hits, and I've only read through the first 10 or so).