GFCI plus 2-prong plug adapter as poor man's isolation for Oscilloscope? - Page 1
Forum section: electronics / testgear

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 06:07:53 am
content: I often find myself using my oscilloscope to troubleshoot mains-earth referenced circuits... and inevitably I forget that or I want to measure a signal with respect to a different reference point... which leads to a ground loop that "fries" the circuit under test...I know that the "right" (recommended) solution is to either use a differential probe or to use an isolation transformer to isolate the circuit under test (or I guess to use a battery-powered scope).I also know that people often "shortcut" by removing the grounding pin from the oscilloscope to force the scope ground to float which can be dangerous if there is a ground fault in the oscilloscope.But what about removing the grounding pin from the oscilloscope (e.,g., by using a 3-to-2 pin plug adapter) and then plugging the two-pronged plug into a GFCI? (in practice since my workbench is in the basement, all my outlets are GFCI protected anyway).Since even old 2-pin electrical outlets are code-legal with a GFCI since they protect against shock, would this be a reasonable compromise solution that effectively "floats" my oscilloscope while still protecting against shock?

author: Jeroen3
author_rank: Super Contributor
timestamp: September 10, 2020, 06:57:31 am
content: The GFCI has nothing to do with you frying your DUT with equalization currents.Basically what happens when you pull ground in you scope is that your probe ground will be a capacitance to mains wherever you attach it in your circuit.I unknowingly had a scope like this as a beginner and you'll fry many power supply circuits and reset many chips by attaching probe grounds....

author: capt bullshot
author_rank: Super Contributor
timestamp: September 10, 2020, 07:00:09 am
content: The GFCI doesn't make any significant difference in this setup.Your personal hazard is the same as w/o the GFCI, since the GFCI cannot detect if the scope's GND potential is floating at a dangerous voltage due to a fault in your DUT, or if you connect the GND clip to a dangerous potential. That's the main issue with using a floated scope.If there's a ground fault in the scope, the GFCI will work as intended (as soon as you touch the scope GND and you'll receive a shock from that, a properly grounded scope will trip the GFCI at the very moment the fault happens, before you touch it), but IMO there's less risk for a fault in the scope than with your setup / DUT that you're probing.Otherwise, most modern scopes have SMPS that will have EMC capacitors across its isolation barrier and / or to line earth, so these will cause some line (and higher) frequency currents from the scope through your GND clip through your DUT to ground that might irritate your measurements.

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 01:40:00 pm
content: Quote from: Jeroen3 on September 10, 2020, 06:57:31 amCould you help me understand what these "equalization currents" are and how/why they originate and then fry the DUT?Quote from: Jeroen3 on September 10, 2020, 06:57:31 amHow does this happen? Is there a capacitor in the scope power supply between ground and neutral? If so this would create a huge issue in house circuits with neutral and hot reversed.Really would like to understand this all better...

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 01:43:00 pm
content: Quote from: capt bullshot on September 10, 2020, 07:00:09 amWouldn't this be solved by having the DUT also on a GFCI, especially if it's an independent GFCI from the scope.Quote from: capt bullshot on September 10, 2020, 07:00:09 amBut is this dangerous? Is this common? Do I care so long as I know they might be there and then ignore/avoid them?

author: capt bullshot
author_rank: Super Contributor
timestamp: September 10, 2020, 02:05:19 pm
content: Quote from: puterboy on September 10, 2020, 01:43:00 pmNot in general. Depends on your setup. A GFCI protects you only if it is used as it is intended to, so normally with non-modified and properly grounded stuff. If you're using a scope (floated or not) on a DUT, there's many ways the intended GFCI operation can be defeated, especially if you use isolation transformers and / or floating by cutting the ground. So no, in general neither the GFCI for the scope nor the GFCI for the DUT will protect you from receiving shocks. That's the only safe thing to assume. You might be able to setup your stuff in a way the GFCI may work. though, but that depends on your knowledge and understanding of the matter. Often one just uses these techniques to get around a ground loop problem or similar, but one needs the knowledge and experience to know what one is doing and what is the risk involved. You can't rely on protection through GFCI and other stuff if you're doing so.QuoteWell, that depends. The range of possible interference is from nothing to blown DUT. Often there's just a spike or ripple on your signal that doesn't have its origin in the DUT, but is a result of the grounding technique and leakage. Anyway, as with the GFCI, there's no general rule to apply.

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 02:30:42 pm
content: Would adding a GFCI on the secondary of an isolation transformer (with no grounding connection between primary and secondary loads) add safety?Specifically, if my scope ground is connected to say the secondary neutral, it it useful that the GFCI on the secondary now adds protection in case current goes from the secondary hot through the scope.Said another way, does isolation transformer plus GFCI on the secondary allow me to eat my cake and have it too in that my secondary load is now isolated (though not grounded internally) but the GFCI gives back the protection of a grounded outlet?

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2020, 02:35:08 pm
content: Quote from: puterboy on September 10, 2020, 06:07:53 amSee what I have done with the title of this reply ?QuoteNo never.Do yourself and us a big favor and get the right tool for the job !Something like the reasonably priced Pintek DP-25 will keep you safe and posting here for years to come.

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 02:48:33 pm
content: Quote from: tautech on September 10, 2020, 02:35:08 pmI understand why putting the scope and DUT each under a (separate) GFCI could still lead to distorting currents or a technically "hot" scope or a "fried" scope -- but not sure why this would lead to a "dead" man unless I make the double oopsie of having my body between hot and neutral (which could kill you under any scenario).With the scope and DUT on GFCI's any current through my body to earth ground should trip the GFCI before killing meNow there may be exceptions in very high voltage/high frequency circuits where even low current passing through me could hurt me, but on mains level voltage and frequency, I don't see why I wouldn't be personally protected.

author: Monkeh
author_rank: Super Contributor
timestamp: September 10, 2020, 02:49:49 pm
content: GFCIs are not a primary protective measure. They are fallible and unreliable.Use the appropriate tooling or stay away from dangerous voltages.

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 03:15:06 pm
content: Thanks for all the input here... very helpful.So short of using a differential probe or battery-powered scope, would the following be reasonably safe:1. Mains -> GFCI#1 -> Scope2. Mains -> Isolation Transformer -> GFCI#2 -> DUTWould this generally protect me (first and foremost) and secondarily also the DUT and the scope equipment from disastrous ground loops that could injure either me or the equipment?Of course there are always "gotchas" that can't be prevented such as touching hot and neutral... but is the above setup as good as one can do short of a differential probe or battery-powered scope?

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2020, 03:15:17 pm
content: Quote from: puterboy on September 10, 2020, 02:48:33 pmFFS, for a few 100 $ it's not worth the risk.Yes I have floated scopes in the past at mains potential and lived to tell the tale and would advise no one to do it.Today we can access cheap and safe differential probes whereas once they were only available to the more fortunate and wealthy.As this is an international electronic forum read by all skill and knowledge levels we would be negligent not to recommend safe practice for all.Those in the States might feel their mains voltage is not of high risk however most of the world is 220+VAC which is an entirely different risk level, not to mention 3 phase work @ 415VAC/phase.Play safe !

author: capt bullshot
author_rank: Super Contributor
timestamp: September 10, 2020, 03:33:56 pm
content: Quote from: puterboy on September 10, 2020, 03:15:06 pmThis GFCI won't do anything. You're protected through the isolation transformer for a certain class of faults, and the GFCI can't detect any fault in this setup.You'll be perfectly safe without any GFCI as long as you know what you're doing. And to think a GFCI will come to your rescue if you're not knowing what the GFCI can do for you and what not will kill you.Anyway, your flag shows USA, so you're most probably using 120V mains with one end grounded, this won't kill you under typical lab circumstances. E.g. you're not sitting in a filled bath tub while operating your DUT and scope

author: Monkeh
author_rank: Super Contributor
timestamp: September 10, 2020, 03:35:48 pm
content: Quote from: capt bullshot on September 10, 2020, 03:33:56 pmPlease don't say daft things like that, it's quite adequate to kill you with two points of contact.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 10, 2020, 03:42:52 pm
content: Quote from: Monkeh on September 10, 2020, 03:35:48 pmAnd the GFCI won't safe you if done correctly. I'm talking about accidental touching a single point with a finger while observing the usual wariness in a lab setup, not electric chairs.

author: puterboy
author_rank: Regular Contributor
timestamp: September 10, 2020, 03:52:45 pm
content: Quote from: capt bullshot on September 10, 2020, 03:33:56 pmYes, I just realized this and was about to edit my postBut won't the Scope GFCI at least protect you and potentially the scope in case of the following "ground loop"?Secondary hot -> Scope Ground -> Earth Ground -> "Human" -> Secondary Neutral.Since GFCI's (at least in the US) also trip if there is a ground current (even if the hot and neutral currents are balanced), this fault should trip the scope GFCI protecting you and hopefully also the scope.So, my revised question is whether the following is good belt & suspenders protection:1. Mains -> GFCI#1 -> Scope2. Mains -> Isolation Transformer -> DUT(of course, one shouldn't rely on GFCI's, circuit breakers, luck, etc for primary protection... but we all make mistakes and inadvertently touch things we shouldn't be touchingAnd if I get an isolation transformer with multiple outlets, this could also presumably help in the case of USB cables providing unexpected grounding connections so long as all the circuits are plugged into the isolation transformer.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 10, 2020, 04:43:13 pm
content: Quote from: puterboy on September 10, 2020, 03:52:45 pmNo, at least here in Germany these things trip on unbalanced live/neutral only. This is usually the case if there's an unintended path from live or neutral (or somewhere in between) to earth. That unintended path could be anywhere. From your description, your GFCI also monitors the current in the earth (safety ground) wire and will trip if this isn't zero. I haven't seen such a thing yet, according to I haven't seen any typical GFCI from North America except for some Electroboom (and similar) videos, such a thing may exist and even be in use at your place. I've seen some datasheets and app notes from typical GFCI controller ICs for the American market, monitoring the earth wire wasn't mentioned there.So, in case the GFCI monitors the earth wire, it'll trip if your fault current diverts into the monitored ground wire, I agree. Typically a fault current through your body, aka electrical shock, won't divert into that particular monitored wire but somewhere else to earth. So this will be detected by non-balanced neutral/line only.If you close the fault loop as ("Secondary hot -> Scope Ground -> Earth Ground -> "Human" -> Secondary Neutral") you said, yes a GFCI as you describe should trip. Anyway, don't rely on such thoughts, the fault will find another path to shock you without the GFCI tripping, that's for sure.So, I just don't like this belts and suspenders approach at all, since it never covers all faults, just an increasing amount of faults that someone thought of while doing the fault analysis at an ever increasing amount of equipment, regulations and cost. But there will be always more faults that aren't caught. I prefer less belts and suspenders but rather using your brains, wariness and some simple rules (as "keep one hand in your pocket when messing around with dangerous voltages").QuoteThat's the usual setup if you've got an GFCI employed. Everything is fine (as in normal lab practice) with that. Of course this setup can bear dangers to you, e.g. some elevated potential within the DUT that you might accidentally touch, and as it will be grounded at some point through the scope ground it is dangerous to you. If you're messing around with extra low voltage stuff, not a big deal, but be careful if you debug e.g. the primary side of an SMPS.QuoteUSB ground loops are best dealt with USB isolators.

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2020, 04:59:48 pm
content: Quote from: puterboy on September 10, 2020, 03:52:45 pmNot a universal solution ^.I do a bit of work on invertor and phase control welders where an isolation transformer cannot handle the loads.Therefore there is no way to isolate the DUT and it need be worked on live and ground referenced.Differential probes solve all these safety and ground loop problems.

author: puterboy
author_rank: Regular Contributor
timestamp: September 11, 2020, 12:16:48 am
content: What about running my digital scope (Siglent 1104X-E) off of a UPS running on battery (with AC unplugged)?Wouldn't this give me the equivalent of a (literally) battery-powered scope?Would there be any problems with the lower end UPS's having non-true sine wave (and potentially noisy) output voltages?The scope seems to consume ~33W of power. So, even a small UPS with a 12V 9Ah battery should give almost an hour of runtime. This should be plenty, particularly if I plug/unplug the UPS only when making measurements. Presumably I could install a triple-pole switch to isolate hot/neutral/ground when switching to battery power...

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2020, 12:39:54 am
content: Quote from: puterboy on September 11, 2020, 12:16:48 amThat solves the potential mains ground loop issue however only for 1 channel.Still the DSO will be floating which make every metal part of it a shock risk.It's a risky practice so you mustn't undertake it without knowing so and general advice is not to float a scope.Old application notes listed how to do floats but in recent times these have all been wisely pulled offline.Play safe.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2020, 12:43:21 am
content: Quote from: puterboy on September 10, 2020, 03:52:45 pmNo they don't, at least not one I've ever seen, and I've seen plenty. If you disagree, can you provide an example?Can you provide a concrete example of something you are trying to measure that you think requires floating your scope?

author: puterboy
author_rank: Regular Contributor
timestamp: September 11, 2020, 01:45:37 am
content: Quote from: tautech on September 11, 2020, 12:39:54 amSo would the same go for any battery-powered scope with multiple channels (unless each ground is separated)? Because I have seen multiple videos recommending battery-operated scopes just for this case?

author: tautech
author_rank: Super Contributor
timestamp: September 11, 2020, 02:05:34 am
content: Quote from: puterboy on September 11, 2020, 01:45:37 amYes.Unless a scope specifically has isolated channelsallBNC shells are common. Ext Trig, Pass/Fail too !

author: Monkeh
author_rank: Super Contributor
timestamp: September 11, 2020, 02:16:42 am
content: Quote from: bdunham7 on September 11, 2020, 12:43:21 amIndeed, they absolutely do not trip on ground current - ground does not have to connect to them in any way.They also probably don't trip in the presence of DC or pulsed DC (including half-wave rectified loads). This includes a half-wave fault current through you.

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 11, 2020, 03:27:15 am
content: US GFCI’s operated on the simple principle of differential current. The hot and neutral leads pass through a current transformer, any imbalance in that current results in a trip. Not that you should but you could detach the ground lead and they would still function as previous mentioned.They were primarily designed to lessen (not eliminate) the chance of electrocution near water which is why code requires them anywhere within six feet of a water source or any outdoor location, including within a certain radius of a door leading outside. Haven’t read the code book lately but the rules are always getting more stringent and as they say in aviation, the rules are written in blood. Code always changes as people find new and interesting ways to kill themselves or burn their homes down.Buy a differential problem or a Fluke scopeview. Don’t become a entrant in the Darwin awards.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 11, 2020, 05:40:50 am
content: Quote from: puterboy on September 11, 2020, 12:16:48 amDoing so doesn't make any difference to lifting the ground regarding your personal safety. So if you _really_ want to float your scope and _know_ what you're doing, simply cut the earth wire. Nothing else required, and adding stuff like your UPS setup creates a false feeling of beeing safe.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 11, 2020, 06:13:07 am
content: The problem with pulling the ground wire from your scope is that you create an implicit 50V (us) or 115V (eu) ~1mA AC source with regards to earth, or DUT's with the plug reversed.This means wherever your attach ground in your circuit, if that circuit is connected to mains, there could run a few mA of AC current trough it from power supply to power supply.This is what kills boards and screws up your measurement since the frequency can be 50/60Hz or whatever else is used in those power supplies plus noise on the grid.I hear you thinking, but what if I don't have grounded sockets? Then you use equipotential bonding. This forms automagically when using a grounded terminal strip. All "grounds" are connected, but it isn't "earth". This is a risk in itself, but sometimes it is done. Read some literature about this before you even consider it.Running the scope on battery is the best of the worst methods. But you'd have to keep in mind that all exposed metal is scope ground and can't be touched anymore. Including the USB and GPIB plug and probe cal.And you still don't have inter channel isolation.Can you do it? Yes.Will it work? Maybe, depends on what you measure.Is it safe? No.Does a GFCI make it safe? No.What is safe?HVP70And:Test it!Pull the ground wire. And then take your multimeter in milliamps AC to make the ground connection. What do you see?This will run through your DUT when you pull ground.

author: Fungus
author_rank: Super Contributor
timestamp: September 11, 2020, 06:22:44 am
content: Quote from: puterboy on September 10, 2020, 06:07:53 amNo!The bad electrons don't come along the 'scopes power cable, they come from the DUT when you clip the ground probe onto it. Your GFCI won't make the slightest difference to safety.