Confusion around probing isolated circuits. Is this a safe practice?

[AmeliaBuns]

Hello.

I was trying to probe my circuit powered by my garbo Amazon power supply.
when doing so, I noticed some very very light buzzing/humming from my PSU when my ground clip is attached. I also once saw a slight spark when removing the clip. It made me worried so I stopped and investigated.

I found 4.8Vppk on both terminals of my power supply relative to ground. To check the impedance/current, I tried creating a shunt using a 20k and 2.2k resistor.
The 20k resistor collapsed the 4.8Vpp to 1.38Vpp and the 2.2k collapsed the 4.8Vppk to 0.740Vpp. This is obviously a leakage voltage from either the transformer or other components, maybe the Y capacitor is letting 60hz pass through too.  My calculations tell me it'd be 96uA if I'm correct. (50,000ohm~ leakage impedance)


 This makes me concerned that the currents flowing to the ground terminal of my scope, although very small could potentially damage it. Is probing isolated circuits like this a good/common practice? Is this simply nothing to worry about and normal? What is the best solution for this? Is it to just make sure all my circuits are earth-references? Is it to buy a better Lab bench power supply (The good ones are sadly upwards of 500$ and lower wattage, an insane price for me)? I've been told that to avoid the need for an expensive (500CAD minimum...) differential probe, isolating the circuit is a common practice. Is this wrong/bad?

My other issue with differential probe is not just the cost, but the noise, 20mv Peak to peak is a great amount of noise, not to mention the very low bandwidths of most common budget probes. I have been debating trying a DIY approach, but I have extreme doubts about being able to create the same quality probe for cheaper.

I knnow that just by existing near electricity, our bodies act as antennas and pass small currents through it, same must already be happning to the scope (Although I assume it'd be in the nano amp range, not micro amps that I calculated, 96uA)

I'm also going to order a GFCI in the future because I can be clumsy/forgetful...

[thm_w]

GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.

uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.

What is the specific PSU?

[AmeliaBuns]

GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.

uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.

What is the specific PSU?

Are the power strips with GFCI any good? I live in a rental sadly :/

oh  so im paranoid I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)

This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)

[AVGresponding]

Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).

If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.


EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.

[watchmaker]

GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.

uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.

What is the specific PSU?

Are the power strips with GFCI any good? I live in a rental sadly :/

oh  so im paranoid I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)

This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)

There may well be a reason why it is discontinued.  In the meantime, would an isolation transformer between the switching PSU and the 120 help?  A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

[KungFuJosh]

A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

Those can be dangerous too, if the user doesn't understand what they're doing with it.

[AmeliaBuns]

Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).

If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.


EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.

huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.

The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.

[AmeliaBuns]

GFCI is a good idea if you are working on a mains device or have questionable quality equipment. You can either install in the wall or buy a junction box, wire a standard plug to it, and use it as an extension.

uA of current is not going to damage the scope, if you ground 120V with your scope probe, its going to blow something up though. Possibly damage the scope, definitely the probe. Probably y cap leakage as you say.

What is the specific PSU?

Are the power strips with GFCI any good? I live in a rental sadly :/

oh  so im paranoid I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)

This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)

There may well be a reason why it is discontinued.  In the meantime, would an isolation transformer between the switching PSU and the 120 help?  A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

I don't own an isolation transformer and tbh i'm a bit scared of using them. I feel too clumsy and distractable to mess with mains, as much as I'd love to. do they transformers cause any issues on the power supply in terms of noise/wave forms? I don't see why they should personally.

[AmeliaBuns]

I tore the power supply down! I don't understand ac/dc design so  can't say much, but the design is strange...
the markings for the mains and low level is half there  and it stops.
the DC/DC side has no ground planes?
The cut-offs at high voltages are only present in some areas.
I did not find a y capacitor on the main circuit, but the small little PSU powering the display has a y capacitor (I think?) that might be the culprit....
I also found 4 EI transformers. I'm wondering why that is. I thought I'd just find one (aside from the small separate PSU on the side). I'm assuming the funky one near the  plug is a common's mode choke.

also I don't think the shunt has a Kelvin connection.

[tautech]

I tore the power supply down! I don't understand ac/dc design so  can't say much, but the design is strange...
the markings for the mains and low level is half there  and it stops.
the DC/DC side has no ground planes?
The cut-offs at high voltages are only present in some areas.
I did not find a y capacitor on the main circuit, but the small little PSU powering the display has a y capacitor (I think?) that might be the culprit....
I also found 4 EI transformers. I'm wondering why that is. I thought I'd just find one (aside from the small separate PSU on the side). I'm assuming the funky one near the  plug is a common's mode choke.

also I don't think the shunt has a Kelvin connection.

Yes a bit weird in using so many SMPS transformers. 

Typically a small cap is across the main transformer but I don't see one unless the big brown one is it.

Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).

If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.


EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.

huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.

The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.

Yes and normal for a good linear PSU.
The stepdown transformer gives you full isolation from mains and PE when done correctly.
SPD1168X does offer a front panel PE which you can link to an output when required.....maybe when your DUT has a power rail connected to PE.

[bson]

The mains earth connects to the chassis.
The green center jack seems to connect to the chassis.  Hard to tell.
C44 and C45 look like Y caps, connecting the two outputs to the chassis for EMI suppression. (Edit: bottom right in center pic, near where the red & black wires attach to the PCB.)

When probing inside it, I'd connect the scope ground to the earth/chassis ground (center jack) since both outputs are floating.

When using it to power a DUT don't expect the negative (black) jack to be grounded, and in fact to be floating.  The earth is for EMI suppression and a safety ground for the PSU, nothing else.  It's not a common ground.

[thm_w]

Are the power strips with GFCI any good? I live in a rental sadly :/

Didn't know they existed, they look to be a good option, I just made my own as I have the spare outlets laying around.
https://www.homedepot.ca/product/southwire-shock-shield-gfci-tri-tap-outlet-2-ft-12-3-120v-15a/1001013005

oh  so im paranoid I have foun da 50ma or less as a number online but it wasn't based on anything. this is way way lower. the spark does worry me, I wonder if it's forming capacitance with something and then parasitic inductance is causing a spark? do you think a TVS to protective earth would work? as if it's safe, I'd love to still keep doing this to prevent needing a 1000CAD probe (which still isn't fast and it's too noisy)

A 12V or 24V TVS might be of some benefit to reduce the sparking. Or shorting the earth connector to Gnd.
The risk with that is now your circuit is referenced to earth so clipping the probe gnd to something other than gnd will short it (as shown in daves vid). So TVS or a resistor could be a compromise.

This is the PSU I have, I have no idea if this leakage is normal or too much or even good. I bought this 3 years ago when I knew little about electronics.
https://www.amazon.ca/dp/B0CBM2D3DW?ref_=ppx_hzod_title_dt_b_fed_asin_title_0_0
to get even remotely close from a reputable brand, it seems to cost 500CAD. that's with paralleled inputs (which I thought was a horrible idea? maybe if the voltages are very low noise/tightly regulated and current limited well it'll be fine?)

Probably normal.
It is difficult to get a high power/current linear PSU. In future you could consider using one noisy switch mode PSU for high currents, and something linear for low current stuff.

[AmeliaBuns]

I sadly can’t afford one, maybe one day….
Also the augments seem to also have a big flaw?
On turn on, they short the output together. Unlikely to cause a big issue, but someone had their battery explode because they turned it on while it was connected. A bad practice regardless is this issue but it’s strange and scares me….

500CAD is so much. Those Chinese clones and power supplies are slightly tempting now

I assume linear, If fully linear has less leakage? Although the 60hz leakage would be the exact t same wouldn’t it?

Another option is a DIY PSU using a medical grade power supply. I could also purchase a cheap dc to dc isolated buck converter or make one as a second layer of isolation. That’s probably make it less than 30uA, but 96ua that I measured is already pretty great afaik. The medical ones were rated less than 300ua.

For now, I’ll either stick a TVS or even simpler use a 20k resistor across it to reduce that, while keeping my clumsy self safe from blowing up or damaging my scope ;P

I wonder if 20k is big enough for me to just treat the circuit as fully isolated and probe it differentially? Might need a TVS for that instead.

[watchmaker]

A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

Those can be dangerous too, if the user doesn't understand what they're doing with it.

Josh,

Respectfully,  HUH?   

What piece of test equipment IS safe if the user does not know how to use it?  In this case, the issue was using a SMPS and getting leakage when the scope was grounded.  More dangerous than removing the cover of any power supply without understanding AC/DC?

Would not putting an Isoxformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?

[AVGresponding]

Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).

If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.


EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.

huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.

The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.

Yep:  https://www.eevblog.com/forum/buysellwanted/

Although the leakage current (probably) isn't dangerous to you (I have no idea if you have a medical condition that increases your sensitivity), it will add noise to your measurements, and every time you connect to your scope, the Y capacitor will dump all its charge through your scope ground, and every time you disconnect, you'll have a tiny arc, which has the potential to damage your scope input. Connecting the PSU -VE to GND on the front panel literally short-circuits this problem, protecting your scope front end, and reducing the noise of your measurements.

In the context of what you are doing, your PSU stops being isolated as soon as you connect your scope probe ground, so you may as well do it at the source, and save the extra stress to the scope.

All PSUs that are even half decent will have the option to connect to ground; my Siglent SPD3303X does, my HP and Agilent ones do, my TTi EX354D does, and the various EP PSUs I use for HV stuff come with -VE linked to GND by default.

[rf-messkopf]

Would not putting an Isoxformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?

If a device of protection class I (basic isolation + protective earth ground) is connected and operated via an isolating transformer, the transformer replaces the measure "protective earthing" by the measure "protective isolation" that the transformer provides. Therefore, the protective earth (PE) contact of the device on the secondary side must not be connected to the earth of the supplying network or to any other potential with ground reference. Otherwise the additional protective isolation that the transformer provides is defeated.

With the PE on the secondary side grounded, an insulation fault in the connected class I device would remain undetected, because no residual current would be flowing to earth and trip a breaker or RCD, even though you'd still be safe. But a second fault in another place could lead to a large current that is possibly not detected by the breaker or RCD in your installation, therefore this is not allowed according to DIN VDE 0100-410 for regular operation.

But more importantly for lab power supply, if you run it isolated and connect the output to another circuit that has a large potential with respect to local earth, that potential could be transferred to the housing of the power supply, which can be dangerous.

So never put your test equipment on an isolation transformer (unless you know exactly what you are doing). If you want to make isolated measurements on a mains supplied device under test (DUT), always connect the DUT to the isolation transformer. And be aware that connecting the DUT to another potential (e.g. your scope ground) defeats the protective isolation. This is not for regular operation.

A small leakage current between a lab power supply output and PE is not unusual and generally harmless. I measure about 500µArms between PE and the outputs of my Rohde & Schwarz HMP4040 (topology: toroidal mains transformer --> switchmode converter --> filter --> linear post-regulator), and no current for my Agilent E3646 (purely linear).

[Wrenches of Death]

There may well be a reason why it is discontinued.  In the meantime, would an isolation transformer between the switching PSU and the 120 help?  A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

I own a number of isolation transformers, mainly ONEAC, but I've never owned or used a medical rated one. I'm curious about your views on not using one. I would have thought that they'd have even lower leakage and better shielding between the primary and secondary.

I generally isolate the neutrel of the output from ground. The grounds I leave intact.

I do check all of mine for leakage from time to time with a Simpson 228 and a Simpson 229-2 leakage tester.


Thanks in advance!

WoD

[AmeliaBuns]

A band aid, but an isolation transformer (not medical) is a handy thing to have anyway.

Those can be dangerous too, if the user doesn't understand what they're doing with it.

Josh,

Respectfully,  HUH?   

What piece of test equipment IS safe if the user does not know how to use it?  In this case, the issue was using a SMPS and getting leakage when the scope was grounded.  More dangerous than removing the cover of any power supply without understanding AC/DC?

Would not putting an Isolation transformer between the SMPS and the mains solve the issue at modest cost ($100 for a small one that is not varable) while adding to the toolbox?

to be fare, I'd be cautious recommending an isolation transformer to a beginner too. All tools are dangerous but mains is a bit... "directly" dangerous ya know?
my Philosophy is that awareness protects users, so the recommendation is good with some warnings alongside it. I don't like how some spaces "ban" these topics. it's just more likely for people to fool around on their own without knowledge.
Funny enough, I've seen this effect a lot in the country I grew up in. Most subjects are taboo in my country, and as a result, a lot of people fool around and find out the hard way because they weren't made aware of the dangers or HOW to do it safely .
Personally. I'd much prefer to put a DC/DC isolation converter in front of the PSU rather than on the mains side.

[AmeliaBuns]

Looking at the PSU you have, I would check the continuity between the earth pin on the power plug, and the green "GND" post on the front. If it's good (low ohms, less than 1), I would short the -ve (black) and GND (green) binding posts together. If it's not good (more than a couple of ohms), I would get a different PSU, assuming the internal GND connections are not loose (I have seen this).

If you have a capacitance meter, you could remove the Y capacitor and check it's not failing, or swap it for a smaller capacitance Y cap. A smaller capacitance value would mean a lower leakage current. There's also a non-zero chance that the "Y" cap in the PSU isn't Y rated, in which case, replace it with one that is.


EDIT: You might stalk the Buy/Sell/Wanted section of this forum, to see if you can find a decent used PSU in your area.

huh there's a buy sell section?
Is this even dangerous? as I was mentioning to another poster, the currents are only 96uA. tho they can add up.

The ground pin is probably good, but it comes as isolated by default? I'm assuming that they wanted to give you the option to earth it or not on your end? Siglent SPD1168X is also floating(you can just short the green and black). If using an isolated power supply as a substitute for a dif probe is an okay practice, I'd much rather keep my lab bench PSU isolated. I'm actually wondering if those siglent ones are even isolated or not.

Yep:  https://www.eevblog.com/forum/buysellwanted/

Although the leakage current (probably) isn't dangerous to you (I have no idea if you have a medical condition that increases your sensitivity), it will add noise to your measurements, and every time you connect to your scope, the Y capacitor will dump all its charge through your scope ground, and every time you disconnect, you'll have a tiny arc, which has the potential to damage your scope input. Connecting the PSU -VE to GND on the front panel literally short-circuits this problem, protecting your scope front end, and reducing the noise of your measurements.

In the context of what you are doing, your PSU stops being isolated as soon as you connect your scope probe ground, so you may as well do it at the source, and save the extra stress to the scope.

All PSUs that are even half decent will have the option to connect to ground; my Siglent SPD3303X does, my HP and Agilent ones do, my TTi EX354D does, and the various EP PSUs I use for HV stuff come with -VE linked to GND by default.

oh nooo! I didn't know it's dangerous. hope I didn't ruin my expensive precious scope D: I'm always scared of that. I spent so much on it. (And it doesn't even have 50 ohm inputs!)
Being honest. I'm more worried about my scope. Wounds heal, scopes won't

Why does the arc happen btw? would a TVS diode trying the negative/positive terminal to earth help prevent this issue?
I've been trying to use this to my advantage to probe across a resistor as a shunt or to measure something relative to VCC not GND. I guess i'll stop doing that? I'm debating DIY-ing one of those differential probes. although I wonder how well they really work....
I also wish I had a power rail probe. those are so much more expensive, more so than even differential probes

[KungFuJosh]

One of the reasons isolation transformers are dangerous is because inexperienced (and sometimes experienced) people will do something stupid with their test setup, at which point, the user may become the path to ground. Ooops.

+1 for the SPD3303X being a great PSU. Fully isolated channels, linear supply. I bought mine used on fb marketplace cheap.

[Martin72]

Ah yes, the classic question when it comes to measurement...
The oscilloscope must remain grounded unless it has isolated inputs.
If the scope is connected to an isolating transformer, dangerously high potentials can build up at the inputs.
If the scope does not have isolated inputs, all inputs have the same potential—as described in the input post, this can lead to very undesirable effects, including destruction of the circuits.
An isolation transformer is only permissible if the test circuit must be decoupled from ground, e.g., to avoid ground-related interference.
In this case, however, it is indispensable.
You are on the safe side if:
- The scope is and remains grounded.
- And differential probes are used for measurements.
These in turn must be grounded(This is taken care of when the scope is grounded), otherwise they will not function properly.

[KungFuJosh]

- The scope is and remains grounded.
- And differential probes are used for measurements.

Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground.

[AmeliaBuns]

One of the reasons isolation transformers are dangerous is because inexperienced (and sometimes experienced) people will do something stupid with their test setup, at which point, the user may become the path to ground. Ooops.

+1 for the SPD3303X being a great PSU. Fully isolated channels, linear supply. I bought mine used on fb marketplace cheap.

Are you able to measure the leakage current to ground just out of curiosity? I hope I find one of those for sale maybe I have so much I want... I have an old weller but it's so weak and it's hard to swap the tips. I've been wanting a geeboon ta305 so bad...
not to mention the probes and a 50 ohm input probe which I probably won't afford LOL

[AmeliaBuns]

Ah yes, the classic question when it comes to measurement...
The oscilloscope must remain grounded unless it has isolated inputs.
If the scope is connected to an isolating transformer, dangerously high potentials can build up at the inputs.
If the scope does not have isolated inputs, all inputs have the same potential—as described in the input post, this can lead to very undesirable effects, including destruction of the circuits.
An isolation transformer is only permissible if the test circuit must be decoupled from ground, e.g., to avoid ground-related interference.
In this case, however, it is indispensable.
You are on the safe side if:
- The scope is and remains grounded.
- And differential probes are used for measurements.
These in turn must be grounded(This is taken care of when the scope is grounded), otherwise they will not function properly.

Hmm so what I'm hearing overall is that even if you're circuit is isolated, just don't probe it as is without a differential probe? and that I should ground my circuits unless for some reason, it must not be grounded:?  I'm a bit confused. the 60-200ua of current going into the ground of my scope is apparently just fine, the tiny spark is the spooky part. (not sure how that's happening)

for me the only realistic option is to give up. maybe eventually buy a DP700 for times where I don't need high badnwith or low noise (17mv RMS is pretty horrible for power supply measurements for example, and 10mhz is pretty bad for rise time etc)

[shapirus]

- The scope is and remains grounded.
- And differential probes are used for measurements.

Exactly. Some idiot on youtube recommended using an isolation transformer so that a standard probe could be used as a diff probe by connecting the ground lead to a different part of the circuit instead of ground.

What's wrong with that?