Earthing safety with an inherited isolation transformer - Page 1
Forum section: electronics / repair

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: March 06, 2021, 10:00:20 am
content: Inherited a 240V, 2.5A Isolation transformer - old ATCO OGTE90D602-01. Pulled the lid off for inspection and safety test (photo attached). All the insulation tests looked good (~400MOhm on my cheap Chinese megger set to 250V). Looks to work fine. However 2ndry earth is connected directly to 1ry earth. Found this quote from Ian M here (apologies- I haven't worked out how to link to specific thread yet):"Finally, you need to determine how the earth pin on the secondary's output socket is wired. As this is effectively an independent supply, it should be open circuit (or tied to output neutral), but it is not uncommon to find its tied to input earth which is extremely dangerous for bench work. If that's the case, you'll need a custom output lead that disconnects E which must never be used for other equipment so should be secured with a cable clip under one of the lid mounting screws."This suggests I should get/make up an output lead with earth pin disconnected or routed to 2ry neutral, which would go from round earth pin outlet on xformer secondary to a standard Australian flat earth pin plug. That's easily enough done - but would it be safe to simply disconnect the 2ry side earth as an alternative. Have tested from 2ry side outputs to 1ry side earth and all read ~400MOhm. I want it only for bench use when chasing faults.

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 06, 2021, 06:07:20 pm
content: Make sure the primary earth is used.If you're not going to use the earth on secondary you can leave it as-is and just wire a 2 pole socket on secondary.If you feel it's useful to have an earth on the secondary make sure it stays floating from the mains earth or it defeats the purpose of having an isolation transformer. In that case, might as well do it the right way and add a secondary-side GFCI/RCD. The secondary terminal you use for your earth reference becomes the local neutral.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: March 07, 2021, 08:01:27 am
content: Thanks for the advice. Why would they have built it this way? Perhaps for an audio setup to reduce noise somehow?

author: PKTKS
author_rank: Super Contributor
timestamp: March 07, 2021, 05:45:47 pm
content: Quote from: Lorenzo_1 on March 07, 2021, 08:01:27 amProbably not...Several folks assume that EARTH should be propagated fromthe primary to the secondary...These folks are right in the strict sense in which the "EARTH"would NEVER EVER be connected to the NEUTRAL...... but mostly and very dangerously the NEUTRAL is INDEED connectedto the EARTH somewhere ... vast majority os cases ...in these cases the "ISOLATION" serves absolute nothing...I have my DUT and my instruments isolated with dedicatedtransformers.. to achieve this kind of bench setup it is imperativethat the NEUTRAL SHOULD NOT BE CONNECTED TO EARTH by any means.There is just one possible way.. no EARTH on either side.. 100% ISOLATED.100% FLOATING networks.You need to attach your devices with that in mind bearing thatthey are floating and you are connecting isolated networks.It works as expected.. and NO I have not killed myself so farin several decades of TUBE service.I do have a differential probe AND a handheld scope..and neither one still prevents my setup being 100% ISOLATED.Paul

author: 2N3055
author_rank: Super Contributor
timestamp: March 07, 2021, 08:34:49 pm
content: Quote from: PKTKS on March 07, 2021, 05:45:47 pmNo it is actually a law in many countries.Quote from: PKTKS on March 07, 2021, 05:45:47 pmYou show shocking (pun intended) misunderstanding of the topic. What you're saying is wrong and bad advice, unless I don't understand what you're saying.Could you please draw a quick schematic to explain how do you connect trafos and device?Why would devices have neutral and ground connected unless they are malfunctioning ?

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 08, 2021, 12:33:06 am
content: The proper use of powering an electronic instrument being tested through an isolation transformer is to isolate what would be the neutral from earth.Correctly, the chassis/frame should still be earth grounded, for safety.By isolating the AC neutral from earth , any point of reference in the primary power supply (for instance the negative terminal of a full wave bridge rectified power input) can be connected to a properly grounded oscilloscope/meter, etc. without safety problems.Alternatively, an isolated, battery powered scope (Tek 212) or DVM can be used.

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 12:41:05 pm
content: Quote from: 2N3055 on March 07, 2021, 08:34:49 pmYes I am aware of that. I am not looking legal aspectsas that can change even next month..But still the reality may be other.The reality here is that - ALTHOUGH THE JACKS ARE NOW 3PINS -vast majority of the mains distribution do not have a EARTH readywhatsoever..To worsen conditions ... is very common to attach the EARTH pinto the NEUTRAL in several (if not most) of main input panels.All my system was isolated due to good reasons including thisnasty one... in which not having a REAL EARTH.. I do not wantthe EARTH pinned to NEUTRAL by any chance.Attached my setup in which I was forced to plan and drawreason being it is not simple to deal with..This is a simplified sketch I use to organize myself..Photos included..Mains is 120V but my ISOLATED VARIAC is RATED 380 VACfor DUT repair on the bench.There are 2 instrument classes available: WITH UPS and WO it.ALL THE SOLDER BENCH STUFF IS TOTALLY SEPARATEDINTO ANOTHER DEDICATED PHASE OF THE MAINS...HIGH POWERED THERMO MAGNETICS and FAST FUSED** ALL THE JACKS AND FUSES WERE MODIFIED BY MYSELF **they are **SOLDERED** internally with high capacity fuse holdersand very good connections...this **IS NOT RETAIL STUFF ** be warned..Paul

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2021, 02:38:38 pm
content: Not grounding the isolating transformer's secondary side socket's PE pin is for safety. If you are using an isolating transformer for electronics bench repair work, odds are you are going to introduce a ground to some part of the DUT (device under test) by connecting the ground clip of your scope probe, fully knowing that doing so on the primary side of its PSU, or anywhere in a DUT with other high voltage circuits looses you the personal safety benefits of isolation.As such, your bench environment should be carefully assessed for safety when working on a live DUT, one of the most critical issues being the risk of coming into contact with a live conductor and ground at the same time resulting in electric shock or electrocution. As long as you aren't in contact with ground, a single point of contact with most live circuits wont shock you.(I exclude high voltage pulse, and high power RF circuits that can shook you through your body's capacities coupling to ground.)As many who didn't have repair trade training back in the '80s will be unfamiliar with a safe(ish)#workspace for working on live DUTs I'll briefly detail it.(I'm not licenced to *teach* the old TV and entertainment equipment repair course and it was a looooong time ago so I may have missed something)To minimize the risk of ground contact, the flooring should be non-conductive*, the bench *MUST* *NOT* have a metal top, any metalwork under the bench surface or along its front you may come into contact with should be isolated from ground, and any grounded metal fixtures in the room within 2m of your work position should be insulated to prevent contact.Test equipment should be divided into two categories, #1: designed for floating operation, and #2: inherently grounded (e.g. most scopes), and non-essential items in #2 placed somewhere you'd have to move away from the DUT to access. The remaining essential #2 items present a risk of personal grounding so should be in one specific area of your bench as far from the DUT as reasonably practicable, and you must obsessively develop the habit of *NEVER* reaching for them while your other hand is near the DUT. That means *NEVER* adjust the scope while probing with a hand held probe - either find something to clip the probe to or STOP, power down, and solder in a small loop of wire to clip the probe to.Beware of grounds from unexpected sources e.g aerial cables or signal cables to monitors etc. Aerial cables should have dedicated isolated sockets, and if there is a preamp or LNB that requires power, a power injector should be used on the aerial side of the socket. Its rare to need to work on the primary side of a DUT while connected to a monitor, as its usually sufficient to simply check the secondary side voltages. If you do, the monitor should be dedicated (i.e. not shared with your presumably grounded bench PC or other grounded signal source) and designed for floating operation (i.e. either battery powered, or with a factory-fitted two pin mains connector).While you are working on a live DUT, all colleagues, family members etc. *MUST* know and understand not to approach you, and should know how to safely isolate power to your bench in the event that you do receive a shock and require assistance. Also, young children and pets should *NOT* be permitted access to your bench area* Concrete, quarry tile, and dirt floors, can be sufficiently conductive to ground you. If you have such a floor, use a thick rubber or plastic insulating mat extending as a minimum the full width of your bench, under it and 2m from its front edge. Sub-grade basements with permanently damp floors are unsuitable for use as an electronics workshop. Carpet can also be problematic as any spills may make it conductive, and you probably have no idea of the location of any grounded pipes, wiring and other metal objects under the floor. Again use a mat, but on carpet you can get away with simple flooring vinyl, as if you keep the carpet under the mat dry and reasonably clean it will also contribute insulation. It has the side benefit of making dropped small parts findable!# Such a workspace relies on the user avoiding ground contact, which requires training, continued concentration and no distractions. As such in the 2020s OSHA, HSE and other national safety bodies would probably classify it as an unsafe workspace!

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2021, 02:52:19 pm
content: Personally, if I was modding the O.P's isolating transformer for electronics testbench use, I'd disconnect the secondary side mains socket PE (ground) and fit a row of shrouded 4mm jacks bringing out the primary side PE, and the secondary side mains socket PE, Neutral and Line conductors (in parrelel with the mains socket. Normally you'd use it with nothing in the 4mm jacks, but if you are using it on a Variac you can simply plug your multimeter in to monitor the voltage, and if you need a secondary side ground you can jumper the primary and secondary side grounds. You can also jumper PE to neutral so that down-stream RCDs (GFCIs) will trip in the event of leakage from a live conductor to chassis. *DON'T* use jumpers with it unless you fully understand the supplyearthing systemconfiguration you are setting up and its hazards!

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 02:54:17 pm
content: Just care about yourself and your equipo...Should be fine after careful consideration...Paul

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2021, 03:02:59 pm
content: Quote from: Lorenzo_1 on March 07, 2021, 08:01:27 amIt's a standard way to build an isolation transformer. Isolation transformers used areas other than bench testing--medical devices, for example. The secondaries are isolated, which means not referenced to ground and thus reducing shock hazard and current leakage. The ground is passed through and made available where it is needed. There are times you want the device grounded, times you don't want it grounded and very rare occasions where you might deliberately tie the output neutral to ground. Don't do the last one unless you fully understand why you are doing it and of course, be aware that this reintroduces a shock hazard.Your local mains wiring setup has a lot to do with your choices here regarding 'ground', so don't rely on advice from someone who doesn't know and hasn't asked exactly what your local setup is. My neutral is bonded to the ground wire at the service panel and is then connected with a heavy copper wire to both the plumbing and to a ground rod driven into the ground. IOW, ground is really earth.

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 03:07:30 pm
content: Quote from: Ian.M on March 08, 2021, 02:38:38 pmThat is *WHY* all my bench equip. is isolated...My scopes have no "EARTH" and when I need to hook the clipI consider if doing so with 100x probe or a differential...... still will not prevent bad days when you hook yoursecondary GND probing the primary...But will not allow you to hook the LIVE MAINS... EVER.Paul

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2021, 03:20:59 pm
content: Quote from: PKTKS on March 08, 2021, 03:07:30 pmThat's great until you energize the exposed portions of the scope that are supposed to be grounded.

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 03:28:22 pm
content: Quote from: bdunham7 on March 08, 2021, 03:20:59 pmThat is why I use gloves when dealing with energy enoughwhich makes me wonder the risk...The bench uses a proper mat the scope a differential probe if required.I just touch energized stuff with those funky gloves ...already tested with a suitable ohm meterthey put me way above the risk...A compromise among several choices ... and risks..Bench stuff is not risk less ..Paul

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 03:38:33 pm
content: TBH and fair... today I am **orders of magnitude** more concernedadding *USB* stuff on my bench than any EARTH consideration...*USB* is some sort of misplaced transgender tech in which you just"HOOK" **ALL YOUR USB DEVICES** into a single GROUND network..Considering the absurd number of USB GIZMOS that is a nightmareto hook into the bench safely..Not draw on my simple sketch but I have 2 dedicated ISOLATED USB HUBSjust to put these nasty gizmos..And.. I still do not feel comfy hooking their GND ad hoc..ETHERNET based instruments are much more safe due to thesimple fact they are ISOLATED by RJ45 by default..but not USB...Paul

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2021, 03:54:04 pm
content: Quote from: bdunham7 on March 08, 2021, 03:20:59 pmIt used to be *OBVIOUS* that floating a scope was dangerous, as vintage CROs had metal cases and large areas of front panel metalwork. Nowadays with plastic cases and front panels its far from obvious. It all *looks* insulated, apart from the BNC connectors, but nasty surprises lurk for the unwary. e.g. grubscrews in knobs on metal shafts or grounded metal switch bodies only a millimetre behind the front panel.*NEVER* float your scopeCompetent engineers who thought they knew what they were doing, and who had taken what they thought to be reasonable precautions against contacting a live part have been killed doing exactly that. e.g.https://groups.io/g/TekScopes/message/10754

author: madires
author_rank: Super Contributor
timestamp: March 08, 2021, 04:05:19 pm
content: Quote from: Ian.M on March 08, 2021, 02:38:38 pmMore food for thought:- The secondary is isolated from mains. How could a PE at the output, bridged to the input's PE, provide any safety?- If you create by chance a connection between the DUT's chassis, grounded via the bridged PE, to the isolated mains, you'd reference the isolation transformer's output to ground and render the whole setup useless.- SMPSUs and other devices may have filter caps between line and PE, and also between neutral and PE. With a bridged PE that would reference the isolation transformer's output weakly to PE, i.e. current limited by the filter caps. And that can impact your measurements when using grounded T&M.

author: madires
author_rank: Super Contributor
timestamp: March 08, 2021, 04:06:45 pm
content: Quote from: PKTKS on March 08, 2021, 03:38:33 pmBeware of shielded plugs!

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 04:11:14 pm
content: Quote from: Ian.M on March 08, 2021, 03:54:04 pmTo complicate things further .... in my case..I HAVE BOTHI have old type scope with ALL METALLIC chassi (with GND screw at rear)... in which the PSU is old school ISOLATED LINEAR -- ALL LINEAR INSIDE.no smps..** IT CAN NOT BE HOOKED TO MY ISOLATED UPS w/FULL LINE CONDITIONER UNIT **I have new PLASTIC STUFF with SMPS those which I hook just fineto the isolated line conditioner...and some USB stuff gizmos...I have been working w/them carefully so far so good wo problemsregarding "EARTH" ... mostly my considerations are w/new stuffsub millimeter SMD ...the old ones w/ old school front ends hardly causes me that worries..and being LINEAR ISOLATED BY DEFAULT.. i NEVER EVER use the GND screw..doing that since...70s ... 80s .. ok so far.Paul

author: PKTKS
author_rank: Super Contributor
timestamp: March 08, 2021, 04:12:19 pm
content: Quote from: madires on March 08, 2021, 04:06:45 pmABSOLUTE TRUE...I DO NOT CAT6 my bench instruments... ALL CAT5ePaul

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: March 11, 2021, 09:21:47 am
content: Thanks for all the clear advice and notes of pitfalls and things to be considered. It gives me a much better understanding. Confident I can now make the changes needed to use this item safely.I know this is a bit of a recurrent topic, but there's evidently a range of views and some of what I read earlier was a bit unclear.