OScope Safety Question - Page 1
Forum section: electronics / beginners

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 17, 2018, 04:48:07 pm
content: Hi, I'm not quite a newb, but I am when it comes to oscilloscopes. I've read a lot of mixed opinions regarding safety, and I would like to see if my setup makes sense, and whether or not I should bother with a differential probe.I have a DS1054Z connected on a surge protector to a GFCI outlet. I have an isolated transformer (Toroid ISB-060W) with a verified floating neutral, also connected on the same surge protector to the same GFCI outlet.Any DUT goes to the IT, and I don't use any adapters to defeat the earth. I'm generally only testing audio equipment like tube guitar amplifiers, and microphones. I may test guitar pedals too, but those are low voltage and I can battery supply them. I also have a Tekpower TP3005P bench supply for DC but I haven't tested that yet to see if it's isolated or not.Generally speaking, I probably designed and built 99% of the amps or mics or pedals I'm testing with my scope. I'm obviously very family with the circuits and I'm not going to connect the ground clip to B+ and blow anything up.All that said, what do you think? Is my setup sufficient / does it make sense? Should I bother getting a differential probe, and if so, what models should I look at?Thanks,JoshPS. Yes, I've seen the "How NOT To Blow Up Your Oscilloscope!" video.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2018, 12:35:34 am
content: That is how I do it. The idea is that with the isolation transformer, the power line ground can be moved to where the oscilloscope's probe ground is attached.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 18, 2018, 12:33:02 pm
content: Thanks!

author: bhishmar
author_rank: Contributor
timestamp: September 03, 2018, 05:09:59 pm
content: Quote from: David Hess on August 18, 2018, 12:35:34 amWould you please care to elaborate on the above point?I have read carefully both KungFuJosh query & ur reply.But I am missing the point on "power ground being moved to DSO-ground"

author: rstofer
author_rank: Super Contributor
timestamp: September 03, 2018, 06:03:46 pm
content: Quote from: bhishmar on September 03, 2018, 05:09:59 pmAs soon as you clip the scope probe ground to some point in the circuit, that point is now earth ground by way of the BNC connector and the fact that the scope is grounded. That is the way it's going to turn out whether intended or not.Note that GFCI protection does not extend beyond the isolation transformer because it is looking for the current balance between the hot and neutral wires that only reach the primary of the transformer.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2018, 06:52:01 pm
content: Quote from: bhishmar on September 03, 2018, 05:09:59 pmWith power from a three pin wall socket which includes ground, neutral, and hot, ground is attached to neutral at the circuit breaker box. Any current through neutral, which should be all of it, raises the potential of neutral compared to ground so if any ground from the same circuit breaker box like the ground lead on the oscilloscope's probe is attached, it creates a ground loop and diverts current away from neutral which at best will corrupt the measurement and at worst break things.The isolation transformer allows moving the ground point from the circuit breaker box to any point on the isolated side of the transformer with some limitations due to capacitance coupling across the transformer and capacitive coupling into the environment. Now the ground lead on the oscilloscope's probe can be attached at that point relatively safely without creating a ground loop.The ground connection on a differential probe would also be attached at that point to maximize common mode rejection. Note that a differential probe has a ground connection whether it is brought out or not and it relies on that ground connection to control its common mode voltage. If it is not brought out as a separate connection, then it is relying on the ground connection through the power cord.This is why differential probes should not be used with oscilloscopes which have isolated inputs and for the same reason, they should not be used with isolation transformers unless a separate ground connection is made to the isolated output of the transformer somewhere. Without the ground connection, the common mode voltage into the differential probe can drift all over the place.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 08:52:18 pm
content: Quote from: David Hess on September 03, 2018, 06:52:01 pmYour IT info sounds incomplete or assumptive/misleading unless I'm reading it wrong. In my case, my IT has an isolated floating neutral but still uses common ground (3 prong). My DUT chassis ground is still earthed through this.Your description sounds like it would be more accurate with an IT and a non-grounded DUT (for example a 3 to 2 prong grounding adapter on the DUT plug).

author: tautech
author_rank: Super Contributor
timestamp: September 03, 2018, 09:01:34 pm
content: Quote from: KungFuJosh on September 03, 2018, 08:52:18 pmJust confusion and seeing it from a different angle.A recent thread might offer some additional clarity:https://www.eevblog.com/forum/beginners/question-isolation-transformer-ground-floating-or-ground-shorted-to-neutral/

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 09:16:40 pm
content: Quote from: tautech on September 03, 2018, 09:01:34 pmI guess I'm a little confused by this:QuoteIt's all still connected on the same ground. The only difference is the Neutral is separated from the ground. So what's the difference with connecting the ground?The thing that confuses me is that it reminds me of a well-known "genius"who actually suggested using a 3 to 2 grounding adapter on an IT to connect regular probes anywhere in the circuit like you would with appropriate differential probes.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2018, 09:24:34 pm
content: Quote from: KungFuJosh on September 03, 2018, 08:52:18 pmNo, it still applies because for the same reason you cannot safely attach the oscilloscope probe's ground to the line side circuit anywhere, you cannot attach the power line ground either. The ground connected to the chassis is not connected to neutral except of course back at the circuit breaker box.The isolation transformer allows connecting the oscilloscope probe's ground to any part of the line side with some limitations duplicating the situation where neutral and ground are tied together at the circuit breaker box. For safety however, the AC ground connection should also be attached to that point.Quote from: KungFuJosh on September 03, 2018, 09:16:40 pmThe difference is that with current flowing through neutral, neutral and ground at the device are at different potentials so there is no safe point to attach ground.QuoteThat does not make any sense unless you mean using a 3 to 2 grounding adapter on the oscilloscope. That does work for small or negative values of "work" and it even used to be recommended but it is definitely not safe for the oscilloscope or user.

author: tautech
author_rank: Super Contributor
timestamp: September 03, 2018, 09:30:29 pm
content: Quote from: KungFuJosh on September 03, 2018, 09:16:40 pmI presume you mean on the isolated side, in which case there is no Neutral. There is only Live and Live WRT each other.Isolation transformers isolate the current/voltage supply from mains ground and thereby reduce the hazard of phase (live) to neutral and/or ground shock risk.Mains ground retained to the DUT is always preferable.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 10:19:22 pm
content: Quote from: David Hess on September 03, 2018, 09:24:34 pmNope. The "genius" uses ungrounded DUTs on ITs. Exactly what I wrote. He literally has a video on youtube where he demonstrates doing that.Sorry if the quotes on "genius" didn't convey my sarcasm strongly enough.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 10:23:14 pm
content: Quote from: tautech on September 03, 2018, 09:30:29 pmCorrect, and I agree. I don't like the guy's stupid video spreading stupidity, but I'm not trying to start a social media war, it would just make me look bad.I get what you're saying, but when you wire a plug, or look at the IEC socket, it still says N on it.

author: ogden
author_rank: Super Contributor
timestamp: September 03, 2018, 10:43:45 pm
content: When you connect scope ground clip to your device somewhere, you effectively ground that particular point of circuit to scope ground AND chassis of device. I do not find this safe, especially for your scope. Slightest error of yours or problem with equipment under test and least of your problems could be shopping for new scope.You definitely shall use probe that is safe w/o isolation transformer.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 10:46:50 pm
content: Quote from: ogden on September 03, 2018, 10:43:45 pmI'm personally not an idiot (most of the time). I only connect the probe ground to the chassis ground.QuoteI think something is lost in translation here. What do you mean?

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2018, 10:49:49 pm
content: Quote from: KungFuJosh on September 03, 2018, 10:19:22 pmAh, well I do not know why one would bother doing that except in special circumstances. One might for instance want to disconnect the chassis ground so that one does not inadvertently ground oneself or something else while poking about inside while the DUT is energized. I might also want to use a different ground in place of the AC power ground if I thought electrostatic coupling was causing a problem.Quote from: ogden on September 03, 2018, 10:43:45 pmThat is why I mentioned attaching a real ground connection to the point where the oscilloscope's probe ground is connected. At least then if there is a fault, some or most of the current will be diverted away from the oscilloscope. It is not always feasible to avoid using a probe ground with differential probes although newer ones usually do not provide the option to do so even if you wanted to.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 03, 2018, 11:26:26 pm
content: Quote from: David Hess on September 03, 2018, 10:49:49 pmWell, in my case and his, we're talking about tube guitar amplifiers. There's always a metal chassis, and whether or not it's grounded is kinda a big deal. I know people can get away with doing that, but it's...so...stupid.QuoteMy differential scope is powered by USB on the oscilloscope. Should I not do that, or is it cool?

author: ogden
author_rank: Super Contributor
timestamp: September 03, 2018, 11:38:24 pm
content: Quote from: KungFuJosh on September 03, 2018, 10:46:50 pmYes, it's late here. Ignore that. If every voltage you measure in tube amp is chassis-referenced, then all you need ishi-voltage 1:100 probe, or two. Original probe is 150V rated in 1:1 and 300V rated in 1:10, I would not use such in 400..500VDC (or whatever voltages you have there) tube amp, especially knowing how sloppy that switch is.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2018, 11:44:36 pm
content: Quote from: KungFuJosh on September 03, 2018, 11:26:26 pmIt does not matter either way. Just use it as designed.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 04, 2018, 01:03:03 am
content: Quote from: ogden on September 03, 2018, 11:38:24 pmI've changed my setup, but I'm well covered. I returned the DS1054Z and bought a Siglent SDS1104X-E instead. I have Pico TA131 250MHz probes, which are rated 600V in 10X, and I have a BK PR2000B 100X probe that's rated 2000V. I did also buy a differential probe, but that's 1300V at 500X, which isn't always useful.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 01:29:38 am
content: Quote from: KungFuJosh on September 04, 2018, 01:03:03 amKeep in mind that the voltage rating of those probes only applies when the oscilloscope is set to DC coupling. If AC coupling is used, then the full input voltage is applied to the oscilloscope input which is probably only rated to 400 volts.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 04, 2018, 01:46:26 am
content: Quote from: David Hess on September 04, 2018, 01:29:38 amYup, 400. One of the benefits over the Rigol which only could handle 300. But even when I eventually figure out what the benefit of changing the coupling type isthen it's all good because most of my circuits don't go above 350.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 02:11:26 am
content: Quote from: KungFuJosh on September 04, 2018, 01:46:26 amYou might want to use your x10 or x100 probe to measure low ripples voltages which would require AC coupling to work at a higher vertical sensitivity.They make, or used to make, special x10 and x100 probes which included an internal shunt allowing them to be used with DC or AC coupling without limitations. x1000 probes always include this shunt.

author: bhishmar
author_rank: Contributor
timestamp: September 04, 2018, 01:29:44 pm
content: Part of the earlier confusion here is created by non-standard or arbitrary terminology used in the posts here, by different poster's I think.Same is the case with the other post referred by TauTech.So let me define/ re-define some symbols , & put forward a topology here, as I understood from earlier posts, to aid in any further discussion (if any).==================================================================IT : Isolation TransformerDUT : Device under Test.E1 : Earth-Pin (3rd pin) of Mains Power receptacleL : Phase (Live) point of Power receptacleN : Neutral point of Power receptacle.L1 : Output-Line-1 of IT Output Power receptacle.L2 : Output-Line-2 of IT Output Power receptacle.E2 : Earth-Pin (3rd pin) of IT-output receptacle.** I am not claiming above is standard terminology. Just that some terminology aids in streamlining discussion & help in better understanding.==================================================================Standard wiring configuration inside Isolation Transformer (IT)L & N wired to primary winding of IT (obviously)L1 & L2 wired to secondary winding of IT (obviously)E1 & E2 both wired to Chassis of IT. (so E1 & E2 are shorted inside IT)==================================================Typical Wiring outside IT(Topology):-DUT is powered from a Bench power supply /adapter which is powered from L1 & L2 of IT.E2 is connected to the chassis of Bench PowerSupply.(or adapter if it has a 3rd pin Power-connector), thru power-cordBench Power-Supply chassis may or may not be connected to its DC-supply output -ve terminal (& hence DUT Circuit Gnd) - Tester's option.If using a DSO directly powered from mains (not thru IT), its BNC-Gnd and any DUT-point touched by black-alligator clip will be at Mains-Earth (E1) point.If DSO is powered thru IT, then BNC-Gnd will be same as E2.But since E1 & E2 are shorted, ultimately BNC-Gnd (DSO-Gnd) will be E1, in both cases.==================================================I am assuming in earlier posts "Power-Gnd" meant E1,& "DSO-Gnd" meant BNC-Gnd.So "Power-Gnd" (E1) & DSO-Gnd (BNC-Gnd) will always be shorted & same, irrespective of DSO is powered directly or thru IT !!

author: bhishmar
author_rank: Contributor
timestamp: September 04, 2018, 01:50:26 pm
content: Quote from: David Hess on September 03, 2018, 06:52:01 pmWould you care to explain the above in the context of above terminology & topology?I understand there are 3 separate issues to contend with.1. Ground loop creation by DSO-Gnd & associated noise introduction.2. Some Ground continuity path thru a hi-impedance for differential-probe's (internal ground), to limit common-mode voltage at differential amplifier inputs.3. Saftety

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 04:59:43 pm
content: Quote from: bhishmar on September 04, 2018, 01:50:26 pmNeutral is nominally at the same potential as ground because they are connected together back at the distribution panel so it seems like you can connect the probe ground to neutral without getting in trouble. The problem is that the current going back through neutral raises its potential compared to ground and adding the probe ground just makes another path for the neutral current to return to the distribution panel. That may not cause damage, but the ground going through the test instrument is now carrying part of the neutral current so it has different voltages across it.The safe thing is to treat neutral the same as hot. Nobody would ever ground hot, right? RIGHT?QuoteThe differential probe has three inputs; two signal inputs and a ground (more properly called common but whatever) whether it is made available or not. Modern differential probes are grounded through their connection to the test instrument. But if they are connected to an isolated input on the test instrument, then this ground connection is missing and the common mode voltage may drift anywhere within the limits of the high impedance signal inputs. If the common mode voltage drifts out of range, then the probe will not work correctly and may be damaged.This becomes a significant hazard to the user if the isolated different probe is used to make a measurement on something which has a high common mode voltage. For instance if the differential probe is used to make a high side measurement on rectified 240 volts AC which will be 340 volts DC or slightly higher, then the differential probe will charge to 340 volts DC without its ground connection through an oscilloscope which has an isolated input. 340 volts storied in the common mode capacitance of the differential probe and across the input impedance of the differential probe is not much but imagine the same situation with 1000s of volts of common mode voltage which high voltage differential probes may be specified for.QuoteIt comes down to just being a bad idea to connect any low impedance point to the non-isolated DUT. Treat neutral the same as hot.The differential probe issue with isolated inputs is not commonly encountered but it is important to be aware of it just like using AC coupling with common x10 and x100 probes.