Best practice for protecting a scope that's probing mains-attached devices? - Page 1
Forum section: electronics / testgear

author: skyhawk
author_rank: Contributor
timestamp: April 23, 2015, 06:26:43 am
content: tl;dr - Assuming the person operating the scope is a moron, what's the best bet for keeping his stupidity from blowing it up?I apologize in advance for any faulty understanding on my part - I'm a PC nerd who's trying to expand his horizons, so I'm not totally up to speed on the electrical side of things.I've got a new DS1054Z coming to me, and I expect to be doing a fair amount of probing on devices that are attached to or incorporate a mains-attached switching power supply. As per Dave's warning, I'd rather not let the magic smoke out of my new oscilloscope with a careless ground lead connection.Background: I'm Canadian, so 120V 60Hz, no GFCI as part of standard building wiring.I scored a 500W isolation transformer from Kijiji, aHammond 171E. This unit appears to be an older revision; it has no switch/breaker, and a duplex receptacle with two outlets rather than a cord hanging off the back. In addition to the apparent lack of over-current protection, the ground lead on the secondary side is connected to the ground lead on the primary side, which is ultimately shorted to neutral at the service entrance to the building. Should I be worried about this?I was planning to build two GFCI outlet-boxes (Can I buy super-short GFCI extension cords at the hardware store? I haven't even looked yet), one to plug the scope into, and another to plug either the isolation transformer or the device-under-test behind the isolation transformer into, with the idea being that should any kind of ground-loop form either one or both of the GFCIs would sense the hot-neutral imbalance at either device and trip, hopefully keeping the seal on the magic smoke intact.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 23, 2015, 06:29:15 am
content: Get an isolated probe:

author: pascal_sweden
author_rank: Super Contributor
timestamp: April 23, 2015, 06:39:22 am
content: What is the bandwidth of such an Isolated Front End module?Siglent has such an ISFE module as well:http://www.tequipment.net/Siglent/ISFE/General-Accessories/But it seems the bandwidth is very limited, as in the manual (see attachment).The manual says BW <= 1 MHz. What does that mean? =)I am looking for an ISFE to provide isolation to the 4 channels of my DS1054Z, without sacrificing the input bandwidth. Of course I could use 2 ISFE's to get up to 4 channels, but the ISFE needs to pass through 50-100 MHz signals.

author: Dago
author_rank: Frequent Contributor
timestamp: April 23, 2015, 06:45:17 am
content: You need a high-voltage differential probe like Ice-Tea said. You can get them at least up to 150 MHz but they of course won't be cheap. For examplehttp://www.ebay.com/itm/281667278397

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2015, 07:01:11 am
content: Study recent thread:https://www.eevblog.com/forum/testgear/reasonably-priced-differential-probes/

author: john_p_wi
author_rank: Regular Contributor
timestamp: April 23, 2015, 01:45:40 pm
content: Guys, I may be obtuse - it seems that the OP is asking how to probe mains connecteddevices- not necessarily taking mains, HV or differential readings.First he must understand where the power section is and understand bench safety.Knowing the signal or logic ground etc and where the ground clip should go (or not go) can the OP not probe low voltage circuits all day to his hearts content with the supplied probes??The key is he needs to know what he is probing and how, ie - not using the scope probe and ground clip the way he would use a voltmeter to probe across a resistor or other device...BUT, if he is interested in HV supplies or or looking at SMPS etc, then yes the precautions mentioned above are necessary. I think he is more curious of the fundamentals and knowledge (what and why) which will ultimately be his best tool.John

author: electr_peter
author_rank: Unknown
timestamp: April 23, 2015, 01:58:39 pm
content: Proper way to measure mains connected devices is to use differential isolated probes connected when device is turned off. Also, install RCD/GFCI if not installed.And DO NOT use isolation transformer with oscilloscope because from your post it is obvious you do not know what you are doing and implications of such setup. Isolation transformer adds significant danger in such case.Although it was not directly asked, please do not try to measure mains directly with a scope. 1) WTH would you do that? What gain will you get from this measurement? 2) It is dangerous for you, measurement equipment and DUT.

author: sotos
author_rank: Frequent Contributor
timestamp: April 23, 2015, 02:11:42 pm
content: Just like me, put your hand deep in your pocket and buy something like this or similar.http://export.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=71&urlRequestType=Base&partNumber=1667343&storeId=10152

author: ebastler
author_rank: Super Contributor
timestamp: April 23, 2015, 02:27:37 pm
content: Would like to re-iterate John's point here: I understand that the intent is to measure inside mains-powered devices -- i.e. devices where the ground is typically not floating, but tied to the mains protective earth. But to stay safely away from any mains voltages or derived high voltages in the power supply, and only measure circuits on the low-voltage secondary side -- right?In that case, there is no danger to the operator. There is of course the risk of doing damage to the device under test and/or the oscilloscope, if the scope's ground is connected to some other (low) voltage in the device under test. My recommendation would simply be:Be really careful and thoughtful when attaching your ground clip. Make sure that the connection point in the circuit is really at ground potential, don't touch anything else while connecting, and attach the clip securely so it does not slip off later and touches something else. Afterwards, poking around with your probe tip at your heart's content does not imply any risk to the oscilloscope. (Assuming that there really is no high voltage in the circuit, of course!). You could still damage the device under test by touching two points at once and short-circuiting them with the tip, of course...Buying dedicated isolated probes etc. is not justified for this application in my view.Regards, Juergen

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2015, 02:28:15 pm
content: Quote from: skyhawk on April 23, 2015, 06:26:43 amassuming the moron does not want to buy another addon fancy gears, and the mains is normal household mains.... put the ground lead away, set the probe to 10X attenuation and keep one hand on the arse. and wear goggle.

author: schopi68
author_rank: Regular Contributor
timestamp: April 23, 2015, 03:53:57 pm
content: tektronix says "no" to isolation transformershttp://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdfBut they are talking about floating the oscilloscope. A better (and much less unsafe) solution would be to to float the device under test.

author: dom0
author_rank: Super Contributor
timestamp: April 23, 2015, 04:23:13 pm
content: Quote from: schopi68 on April 23, 2015, 03:53:57 pmB/s. They say "no" to using isolation transformersto float the 'scope.QuoteThey arenot. They have a huge box on page two essentially saying "never float your earthed scope!".In table 1 they explicitly refer to the limit of 30 Vrms between a battery powered scope with exposed metal parts and safety earth. 30 Vrms = 42 Vp. That's the maximum allowable voltage on any exposed metal part a device is allowed to carry. They have like twenty huge red warning boxes about doing this on page three.The only techniques declared sane by Tek in the articles are those, that are sane.That is, scopes with isolated inputs and isolation/diff amps.

author: mikerj
author_rank: Super Contributor
timestamp: April 23, 2015, 05:36:17 pm
content: Quote from: dom0 on April 23, 2015, 04:23:13 pmThat is exactly what schopi68 was saying, I don't understand why you are disagreeing with him?

author: skyhawk
author_rank: Contributor
timestamp: April 23, 2015, 06:41:33 pm
content: I see some clarification is needed.Yes, I'm intending to probe only low-voltage DC devices, albeit ones powered by a mains-attached power supply. I intend to keep the scope well away from anything that might be high or line-voltage AC. I'm aware the ground lead is only supposed to attach to the ground plane on any given circuit, but I'm worried about mistakes and moments of stupidity.I bought the isolation transformer intending to float the device-under-test, not the scope itself.I hate being that guy, but can we discuss best practice that doesn't involve spending hundreds of dollars per channel on additional hardware? If I ever feel the need to probe around the internals of a mains-connected power supply I know I'll need more hardware, but for now all I want is inexpensive protection against ground loops.My main questions are whether I should take apart the transformer and remove that earth ground on the secondary side, and where the GFCI breakers should be. One between the scope and mains, obviously. Should the second one be between mains and the transformer, or between the transformer and the device-under-test? (I expect the correct position is on the secondary side, especially with the earth ground present) Are there any other relatively inexpensive precautions that would make sense?Does Rigol include a decent manual with their scopes, either physically present or a PDF somewhere? If there is a readable manual, I intend to study it thoroughly before using the scope. (Yes, I'm one of those people who actually reads the manufacturer's suggestions)

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 23, 2015, 06:50:32 pm
content: A big problem here is that the problem definition is almost impossibly broad.A "mains-powered" device could be something powered from a low-voltage "wall-wart" (or "line-lump") external "power supply"Or it could be a "double-insulated" gadget that is designed to isolate rather well from mains voltage.Or it could be a conventional proper power-supply device with a traditional linear or newer SMPS and chassis and circuit ground bonded to mains green-wire safety ground.Or it could be a dangerous "hot-chassis" device of some kind ranging from modern gadgets back to 60-year old domestic radio and TV receivers, etc.There are different precautions for each of those situations. It is almost impossible to answer this as a single, generic question.

author: dom0
author_rank: Super Contributor
timestamp: April 23, 2015, 06:55:31 pm
content: You only need one GFCI for the entire circuit.A GFCI between an isolation transformer and the connected device will never trip, because PE simply isn't connected to anything with an isolation transformer.Regarding your Hammond transformer, could you draw up a schematic to avoid misconceptions? It sounds like the secondary winding is connected to PE, which would be very strange.General best practices that match most stuff pretty well- DUTs are always powered by an isolation transformer for multiple reasons:a) it gives you an off switch that is completely independent of the DUT, so if things go haywire you always have that switch that's always safe to touch, which might not be the case with the DUTb) if you accidentally touch the isolated mains nothing happens with a single error. For lethal currents to flow you need to touch both poles, but touching the case and then isolated mains will not harm you, except when it's a double error (i.e. iso failure in the DUT that shorts case to the other pole of the isolated mains)c) Isolation transformer has higher impedance and lower fuse rating than normal mains ; things tend to explode much less violently.- For low power DUTs I usually connect a 40 W or 60 W light bulb in series- When working with exposed mains voltages, never touch, move or connect anything while the transformer is ON. That is, you make the meas connections etc., you remove all your body part(s) (hands, feet, head) from the test area. You turn the transformer ON, note your measurements or whatever, you turn it off. Yes, you will make exceptions to the rule, but it's a good rule nonetheless that prevents some levity, at least.- Be always aware of the potentials in the DUT. 400 V caps in a SMPS might stay charged for a while if they don't have proper bleeders. HV supplies can keep substantial charges over hours, sometimes days.

author: skyhawk
author_rank: Contributor
timestamp: April 23, 2015, 07:02:00 pm
content: Quote from: Richard Crowley on April 23, 2015, 06:50:32 pmFair enough. Perhaps we should break this down to each category?1) External power supply, only positive/negative or hot/neutral connections to the device under test - unlikely I'll ever poke at something with an AC power supply, but DC devices are quite likely.2) Chassis with power supply and ground connection - it's quite likely I will be probing at devices like this at some time.3) Double-insulated jobbies - aren't those mostly power tools? I can't say never, but that doesn't seem like the kind of thing I'll take a fancy to poking at.4) What modern gadgets are hot-chassis? I thought that kind of thing went away back in the 70s. I'm not planning to poke at anything over 20 years old.

author: skyhawk
author_rank: Contributor
timestamp: April 23, 2015, 07:05:47 pm
content: Quote from: dom0 on April 23, 2015, 06:55:31 pmHere's a link. That diagram is accurate except that my unit has a duplex receptacle rather than an output cord, and does not appear to have a switch or over-current protection device.

author: dom0
author_rank: Super Contributor
timestamp: April 23, 2015, 07:35:16 pm
content: Really looks like the PE of the output is connected to mains PE. That shouldn't be the case with a good and proper isolation transformer.Regarding over current protection, maybe it's integrated in the switch?

author: dadler
author_rank: Unknown
timestamp: April 23, 2015, 07:40:11 pm
content: The cheapest way to measure differential voltages (and not short your circuit to ground), is to clip two ground leads together (optional step) and then use two channels plus your scope math functions (one channel inverted+sum, or math subtraction). This has downsides: it wastes channels, does not let you use the front end amplifier of the scope to its full potential, and won't let you use XY mode or other math functions across channels.

author: john_p_wi
author_rank: Regular Contributor
timestamp: April 23, 2015, 08:06:24 pm
content: Protective Earth, PE, is carried through isolation transformer, from input to output and the ground integrity is intact. The primary and secondary are wound 1 to 1, it is the winding impedance that offers the "isolation" from the mains. The iso transformer I use is 1000 VA - I certainly would not assume that its' fuse would blow first or it is safe to touch a "pole" as mentioned above. Safety guys.

author: Laura
author_rank: Contributor
timestamp: April 23, 2015, 09:38:14 pm
content: Quote from: skyhawk on April 23, 2015, 07:02:00 pmAppliances: The MCU controlling a washing machine or clothes dryer might very well be "hot".Light dimmers: The control circuitry is almost certainly hot.Motion controlled lights: Hot just like the dimmers.

author: dom0
author_rank: Super Contributor
timestamp: April 24, 2015, 09:17:17 am
content: Quote from: Laura on April 23, 2015, 09:38:14 pmThe MCU in a washing machine sitting on top of mains makes the chassis hot? I don't think so. Same for the light dimmer. Yes, the triac and control circuitry rides on top of mains, but they have safety insulation, you can't touch it. Note that many typical triacs come in TO-220, but their metal tab is still insulated.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2015, 12:52:35 pm
content: Quote from: dom0 on April 24, 2015, 09:17:17 amYou don't understand what "hot-chassis" means. It DOES NOT mean that it has exposed metal parts that are dangerous to touch in normal use. It means that the reference INSIDE the protective covers is connected directly to the mains and is inherently unsafe to probe with ground-referenced test gear (like oscilloscopes, etc.).

author: dom0
author_rank: Super Contributor
timestamp: April 24, 2015, 01:06:23 pm
content: My bad, I assumed that hot-chassis means the same as allstromgerät, in those the chassis/case becomes "hot" (connected to mains phase) in certain situations. Strange wording in my eyes, to call something hot-chassis when there is no hot chassis...

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2015, 01:54:02 pm
content: "Hot-chassis" is a term from 50~60 years ago in the American consumer goods marketplace. Millions of low-cost consumer radio and TV receivers were made to designs that used direct rectification of the mains voltage and no power transformer. And because of the symmetrical mains plug, you had a 50% chance that the chassis/reference was connected to the hot/live side of the mains power. So the chassis was completely contained within insulating (wood/plastic) enclosures and with insulating knobs, etc. And TV antenna connections were capacitively coupled with high-voltage caps.

author: Galenbo
author_rank: Super Contributor
timestamp: April 24, 2015, 02:05:07 pm
content: Quote from: schopi68 on April 23, 2015, 03:53:57 pmAnother possible way to do it safe, can be by using an isolation transformer for the DUT, and choose 1 side of the voltage you want to measure, as ground.But do attention for exceptions. Don't connect that usb port that now is on 230V. Look at the details.

author: skyhawk
author_rank: Contributor
timestamp: April 24, 2015, 02:26:42 pm
content: Quote from: Richard Crowley on April 24, 2015, 01:54:02 pmIn that case I don't understand how any modern device could be hot-chassis. Any appliance worth it's salt will have a polarized 3-prong plug, so the designers know where Hot should be, and in-wall devices like dimmers have colored wiring, so unless they're installed or connected incorrectly the designers also know where Hot should be. I don't think I'll be doing any probing on any super-compact or primitive mains devices unless I get the fancy probes you guys are recommending, though.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2015, 03:19:28 pm
content: Quote from: skyhawk on April 24, 2015, 02:26:42 pmI hope you have good life insurance and keep one hand in your pocket.I am NOT willing to bet my life that the minimum-wage electrician wired the mains outlet correctly.

author: skyhawk
author_rank: Contributor
timestamp: April 24, 2015, 03:32:42 pm
content: So here are the safety knobs I have to play with:a) in-line GFCI breakersb) isolation transformerb1) using an extension cord on the secondary side that doesn't carry the protective earth lead throughb2) using an extension cord on the secondary side that doesn't carry the protective earth lead through, and bonds the ground pin to neutralb3) using an extension cord on the secondary side that doesn't carry the protective earth lead though, bonds the ground pin to neutral, and has a GFCI breaker on the endIn the case of (b3) why wouldn't a GFCI breaker work? If the DUT malfunctions such that a short-to-ground is created, current could still flow through the ground pin and a GFCI should sense a hot/neutral imbalance.I should re-iterate that I expect to stay away from the high-voltage stuff, only probing the low-voltage side of the electronics. If we're talking about a dimmer switch or other device that doesn't really have much "low-voltage" side I'm hopefully not going to be stupid enough to poke at it. This means I mostly care about eliminating potential ground-loops, and care less about operator safety.An earlier poster said that only one GFCI breaker is necessary for the setup, which I don't understand. If GFCIs sense imbalance between hot and neutral current, and if both the scope and DUT are connected on the same GFCI, then a ground loop could potentially form between the two devices, causing current to flow in smoke-lettie-outie ways, but the total current moving on hot and neutral through the breaker would remain the same? As a measure of paranoia I was intending to put the DUT and the scope each on their own GFCI. I think this may be a shortcoming in fundamental knowledge on my end.Broadly speaking, we have the following probing scenarios:1) External power supply, only positive/negative or hot/neutral connections to the device under test - some of these, like laptop power supplies, may bond their ground plane to PE internally, making them more like point 2.2) Chassis with power supply and ground connection3) Double-insulated devices4) Hot-chassis devicesI think in all cases I'm taking the same precautions - Scope behind GFCI, DUT behind isolation transformer with PE removed.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 24, 2015, 03:37:17 pm
content: A GFCI (or iso transformer) may reduce the possibility of being electrocuted, but it doesn't really protect our expensive oscilloscopes, etc. if we fail to practice cognitive analysis of the circuit before applying the power and probing that gadget.

author: skyhawk
author_rank: Contributor
timestamp: April 24, 2015, 03:39:19 pm
content: Quote from: Richard Crowley on April 24, 2015, 03:19:28 pmOne hand in pocket around mains voltage is always a good idea, but trusting the electrician isn't necessary - devices like these are cheap:https://en.wikipedia.org/wiki/Receptacle_testerIn my particular case, this wiring is recent and was done by a licensed electrician I know, so I'm pretty confident the wiring is correct.

author: john_p_wi
author_rank: Regular Contributor
timestamp: April 24, 2015, 05:52:46 pm
content: Skyhawk,Why would you not carry the PE through? Any fault to the chassis will make a hot chassis or be carried through the ground wire on your scope probe, only limited in current by the impedance of the isolation transformer or current capability of the probe ground wire, NEVER defeat PE. Period. You guys are playing with fire here... Sorry for the rant, PE is your friend, don't defeat it. Isolation xformers are for limiting line current, not defeating PE.

author: tautech
author_rank: Super Contributor
timestamp: April 24, 2015, 07:38:11 pm
content: Quote from: Richard Crowley on April 24, 2015, 03:37:17 pmElegantly put.@ OP ^^^^^^This also might be achieved with a simple check with a DMM between DUT's intended probe reference connection point and scope BNC.If only a few mV (DMM measuring background EMI) it will be safe to connect reference (GND) lead.

author: dom0
author_rank: Super Contributor
timestamp: April 24, 2015, 08:38:31 pm
content: john_p_wi: I don't know what your thing is with PE behind an isolation transformer, but unless the rules in your country are very different from the European / German ones the PE should not be connected through. Neither should one pole of the secondary be grounded, which completely defeats the purpose of the transformer in the first place.If you don't believe me you can buy the appropriate standard (EN 61558 2-4 I think) or look at commercial products from trustworthy manufacturers:- Statron: output sockets don't even have a PE contact- Ismet: their line of transformers is mainly for construction use (KSTF/KSTG etc), very rugged, extremely reliable (has to be!). Don't even have a PE contact on the primary, they're class II appliances and only have a two prong mains connector.There are isolation transformers which have a PE on both sides, and they're connected. These are not suitable for lab use. If I remember correctly, which I am rather sure of, the EN/VDE standards forbid lab use for those. They are, at the same time, the only ones where it is allowed to connect more than one device ; it is (hopefully everyone knew that already!) not allowed and in fact very unsafe to connect more than one device to a PE-less isolation transformer. There are some special rules for qualified personnel that still allow this, but only with a local (un-earthed) "PE".What happens if you grounded one pole of the shown isolation transformer? You'd die, unless there is a GFCI, which may or may not save your life.

author: electr_peter
author_rank: Unknown
timestamp: April 24, 2015, 09:39:14 pm
content: Quote from: dom0 on April 24, 2015, 08:38:31 pmYou will also die when DUT is powered from isolation transformer and you connect grounded scope to it - even GFCI/RCD won't save you in such case. See this post with a detailed diagram and explanation:https://www.eevblog.com/forum/projects/isolation-transformer-and-oscilloscope/msg590315/#msg590315I am getting tired of such topics. Somebody with no clue tries to do something very dangerous (measure mains or mains operated circuit) and treats isolation transformer as a guardian angel (which is not).Do not know WTH you are doing -> do not do it.

author: tautech
author_rank: Super Contributor
timestamp: April 24, 2015, 10:11:29 pm
content: Quote from: electr_peter on April 24, 2015, 09:39:14 pm+1Trouble is, with such cheap scopes now available, everybody thinks they are EE's.It's not the equipment damage that concerns me, (we've all had to hard learn lessons) but the risk that noobs expose themselves to.Understanding circuit theory and schematics is the key to keeping youself safe.

author: dom0
author_rank: Super Contributor
timestamp: April 24, 2015, 10:12:57 pm
content: Isolation transformers are not a guardian angel, no one ever said that (at least, I didn't).I think everyone is clear on the fact that if you connect the 'scope's ground clip to the output of an isolation transformer then said output is grounded.Things like these:Quote from: nctnico on January 18, 2015, 01:45:19 amAre also very ... not so smart. The correct/safest way is to use an isolation transformer to power the DUT and probe it with diff probesonly. In that case anysinglefailurecannotkill you. As opposed to "nayyyyy isolation transformers, yayyyy diff probes" where a single failure can kill you. (Although it won't blow up your scope. Yayyyy)

author: MiataMuc
author_rank: Regular Contributor
timestamp: April 24, 2015, 11:26:31 pm
content: Quote from: skyhawk on April 24, 2015, 02:26:42 pmthere are some countries where you don't have a polarized plug.domO / Richard Crowley: you both have the same thing in mind. I have an old tube radio, which is "hot", depending on the way you plug in the plug... and my washing machine has the electronics board floating. No problem, as long as don't open the them. If you intend to measure someting inside with an oscilloscope, you have to be aware of mains voltage.

author: Keylectric
author_rank: Contributor
timestamp: April 25, 2015, 12:28:21 am
content: Quote from: MiataMuc on April 24, 2015, 11:26:31 pmYes, big parts of Europe for example.

author: john_p_wi
author_rank: Regular Contributor
timestamp: April 25, 2015, 03:59:17 am
content: Quote from: dom0 on April 24, 2015, 08:38:31 pmWhy? I would suggest that one understands Medical isolation requirements of IEC 60601, which PE is carried through. The attached photo of a medical isolation transformer clearly shows the PE carried through, FYI the output is upper right. Anything else is floating and foolish and oh, by the way, this one is wired for Europe at 230v....

author: john_p_wi
author_rank: Regular Contributor
timestamp: April 25, 2015, 04:28:03 am
content: Quote from: dom0 on April 24, 2015, 10:12:57 pmWould you trust your cheap Chinese probes to provide protective earth? I wouldn't with the voltages and current I play with (500 vdc and 500 ma)... Additionally, I don't know about you, but I have had too many ground clips come undone. I'm tired of this as was said above. The best tool the OP can have is to understand how and what to probe.

author: dom0
author_rank: Super Contributor
timestamp: April 25, 2015, 06:33:26 am
content: Quote from: john_p_wi on April 25, 2015, 03:59:17 amisolation transformer for medical safety =/= isolation transformer for general purpose applications (construction, electronics lab etc.). A medical isolation transformer essentially gives you an isole terre (IT) system, which has some specific advantages over other system types (TT/TN), like protection against some faults and higher reliability.Notice how every medical isolation transformer is and will be connected like you have shown (which is absolutely correct!), and how every isolation transformer for construction / lab use will be connected like I described and already shown above (I.e. no PE and only a single socket).I think everything else has been said often enough.

author: skyhawk
author_rank: Contributor
timestamp: April 25, 2015, 09:24:18 am
content: Thanks for all the input guys, and also thanks for your patience with another unedumacated noob.To summarize what I've picked up here:I do need to defeat the PE on this transformer, and never connect more than one device to it while it is in such a state. This doesn't involve modifying the unit, only using a short extension cord on the secondary that doesn't carry the ground pin through.I should always check my intended ground connection point with a voltmeter prior to connection, looking for no voltage difference between the connection point and BNC on the scopeWhile the isolation transformer might save the scope from a careless mistake, or save me from a careless mistake, it can't do both at the same time - once the scope ground is connected the transformer's safety value is largely nullified.If I want to probe anything that's not clearly on the secondary side of a well-defined low-voltage power supply circuit I really, really need differential probes. Also, I should probably refrain from doing so as I don't fully understand all the ramifications of it.No ground means GFCI is not helpful on the transformer secondary. Is there any point making sure the scope or transformer primary is connected to GFCI?

author: dom0
author_rank: Super Contributor
timestamp: April 25, 2015, 10:01:25 am
content: Quote from: skyhawk on April 25, 2015, 09:24:18 amUse a light load with the voltmeter, e.g. measure with 20 ... 100 k? in parallel to the DMM to load capacitively coupled voltages down.Quote from: skyhawk on April 25, 2015, 09:24:18 amYes.Quote from: skyhawk on April 25, 2015, 09:24:18 amYes, the GFCI helps with many sorts of faults. I don't know how this is handled in other countries but having GFCIs for all circuits in a household has become pretty much a standard in Germany.It also avoids some "uncomfortable" fault modes. Example: If you don't have a GFCI and an earth fault occurs a large current will flow (sparks etc.) which will eventually trip the over current protection. If you do have a GFCI and an earth fault occurs a large current will flow only for a very short time (until the GFCI trips).Quote from: skyhawk on April 25, 2015, 09:24:18 amAs I pointed out above there is a difference between medical isolation transformers and those used in labs.The transformersthemselves(I meanthe actual transformer,not the entire device with switches, fuses, sockets and a case here!) are no different and must fulfill essentially the same very high standards regarding isolation etc. so you can use the transformer of a medical isolation transformer to built a lab iso transformer. The difference is in the connections (as mentioned) and certifications of the complete product (although both are very stringent, since both are safety devices directly responsible for safety of humans).So just to be sure I will reiterate the requirements for alabiso xformer- The transformer itself must be suitable (must be in accordance to EN 61558 2-4)- Primary side might be class I or II, does not really matter, usually lab transformers are class I for better shielding, construction ones class II because they're potted and often have plastic cases- Some transformersrequirethat the core must be grounded. See transformer datasheet. If so, they'll usually have a 6.3 mm tab or screw terminal welded to the core.- EU/German connections, don't know/care about the other types of connectors: either round 2 prong socket (no PE contacts) or a usual schuko socket with the PE not connected. The latter is probably not up to code but is easier available for DIY.- SecondaryNOTgrounded- And as mentionedNO PEconnection.-Properly ratedcircuit breaker, fuses are impractical- Two pole switch on the primary, versions with built-in light are much preferableAdhering to these rules you can built yourself a lab iso transformer that is essentially identical to the commercially available units.Never connect more than one device to a lab iso xformer. Only for use by people who know what they're doing. Discussed safety rules apply of.c.