(Another?) Isolation question....

[MartolFart]

Hi,
I have a USB scope I've been using for SMPS troubleshooting from a desktop PC.... I'm switching (no pun!) to using a laptop and was planning to run from battery to facilitate probing across front end components which I'm thinking is safe (assuming normal HV precautions). But were I to accidentally leave the laptop connected to the charger, I'm thinking it would not be an issue either? The charging adapter plug in is not polarized with no ground prong. There is no continuity from prongs to tip connectors. Thoughts? Thanks!

[Benta]

Yes. You'll end up killing yourself.
Do not isolate the test equipment, isolate the device under test (DUT).
This means: get hold of a mains isolation transformer.

[Ian.M]

This comes under the general category of 'Floating a Scope not designed or intended for floating use'.  Good engineers have been killed doing that - it only takes ONE small mistake to electrocute you.  Differential HV probes rated for CAT II usage aren't *that* expensive nowadays so there should be no need, or excuse, for floating a scope.

The *ONLY* way I'd ever consider floating a laptop is if it (and all connected instrument pods etc.) are *TOTALLY* inaccessible behind a perspex safety screen, its on battery power with no PSU connected, any need for the user to control it is handled with a wireless keyboard and/or mouse, and probes are only connected/disconnected from the D.U.T. with its mains power disconnected and its reservoir capacitor discharged (i.e. absolutely *NO* hand-held probing).

Don't trust the 'isolation' provided by a non-grounded laptop PSU.   It will have a relatively high capacitance between input and output, and may well suffer insulation breakdown if stressed by high frequency high voltage switching waveforms as are common in SMPSes, power inverters, VFDs etc.

[David Hess]

But were I to accidentally leave the laptop connected to the charger, I'm thinking it would not be an issue either? The charging adapter plug in is not polarized with no ground prong. There is no continuity from prongs to tip connectors. Thoughts?

Internally the charger has a transformer which provides galvanic isolation between the input and output.  However it was designed for and operates with a minimum common mode input to output voltage and they are typically only rated for continuous operation to a fraction of their input voltage between the primary and secondary.

In theory if you want to do this, then an isolation transformer would allow it however as others have already said, this is a hazardous mode of operation and is not recommended.

[Purduephotog]

I was working under a desk in Germany at an AFB. They run on 220. Well... someone dropped a neutral- so I took 110V or 220V across the chest... by touching the USB port on one computer and the Serial port on another computer.

So don't float it. It hurts.

...there was a MASSIVE writeup over the incident, as you'd expect. That could very well have killed me given where and when I was working.

[magic]

assuming normal HV precautions

Yes, the problem is that when you connect unisolated test equipment to an HV circuit, all normal HV precautions also apply to the test equipment now.

And if you further have a laptop connected to that scope, ...

[Benta]

Yes. You'll end up killing yourself.
Do not isolate the test equipment, isolate the device under test (DUT).
This means: get hold of a mains isolation transformer.

And render GFCI useless. This is as dangerous as isolating the scope.

If the scope is correctly earthed (as it should be), the GFCI will work perfectly.

[capt bullshot]

Apart from the plentyfully mentioned personal safety issues, measuring live SMPS components the way you mentioned might disturb or damage your equipment due to high frequency and high voltage common mode signals. Better do not measure switching nodes or nodes significantly affected by the primary switching action with any kind of floated scope, except it is specially made for this purpose (e.g. the TPS20xx, or some of the stuff from Cleverscopes). If not for your personal safety, for the sake of your equipment.

[Benta]

If the scope is correctly earthed (as it should be), the GFCI will work perfectly.

No. The GFCI doesn't sense earth wire current. It senses differential current between L and N.

A fault after isolation trafo, through human body, to ground, back to ground bar, to the scope, then back to the person will not trip the GFCI.

Whether you simultaneously touch L and N on the "live" side or the "floated" side of the isolation transformer makes no difference, neither will trigger the GFCI and you'll be just as dead.
The point is that having 'scope/laptop at live potential is lethal, having them at earth potential is safe (apart from the fact that you're working with 230 VAC, of course).

[Ian.M]

Anyone who doesn't believe that an isolating transformer prevents GFCI (RCD) protection devices from working should consider the following facts:

• GFCIs detect faults by detecting current imbalance between Line and Neutral.
• Neglecting capacitive leakage and non-ideal insulation, by Kirchoff's current law, the current entering any two terminal transformer winding at one terminal must be identical to that leaving it at the other (as any point in the winding may be considered a KCL node).
• Capacitive leakage in a line frequency mains transformer will be far less than even the most sensitive GFCI's trip threshold current.

Therefore it is impossible for a GFCI with either its supply or load terminals directly and only connected to a two terminal transformer winding to trip, unless the circuit insulation or transformer insulation is defective.

To add ground fault detection after an isolating transformer with a floating secondary would require sensing the ground conductor current of all test equipment that may introduce a ground to the D.U.T. and wouldn't protect you if you made contact with a live part of the D.U.T and a grounded test equipment chassis on the load side of the ground current sensing point.  It would also be defeated if anything introduced a parallel ground path that did not pass through the ground current sensor. Unfortunately ground conductor current sensing circuit breakers are no longer readily available.

[calzap]

If you are touching the scope probe or attaching the ground clip to an non-isolated DUT, there is the chance than any piece of metal on the computer (case, latch, hinges, connectors, etc.) and anything attached to it might be hot relative to ground or a neutral.  Say it's the hinges.  If you are grounded and touch them, it could be the last thing you touch.

It you are using an isolation transformer for the DUT, use a multimeter to check that it's really isolated before proceeding with tests.  Some transformers that are sold with an isolation label are not really isolated from ground.  They are designed to protect equipment from line noise and surges.  Read the fine print; use a multimeter to check.

Mike in California

[MartolFart]

Thanks to everyone for their comments, concerns and recommendations! Back at it after some domestic distractions!

In summary, ideally I'd have the isolation auto transformer and purpose built differential probe.... When I was in production doing this kind of work frequently I would already have or track down these goodies (usually). But us old dabblers just wanna have some fun without expending a ton of funds! I did take a look at differential probes and didn't see much under $200 (and none too impressive really), maybe I'm missing the deals to be found? Pretty sure I don't need the 10kV 10GHz jobs....

So anyway, I'm doing the two 100x probe (ground leads removed) A-B approach, and based on the sage advice rendered, won't (intentionally) test with the laptop on its charger.

Another idea I had for isolation work around is to use a disconnected from mains UPS to power the DUT for quick tests (under light load and using the light bulb current limiting gimmick)?

Btw I am definitely an adherent of the one hand behind my back HV testing technique (if I can't just hang a probe somehow) and I don't do it enough to get complacent (which is probably the biggest risk right along with faulty design and implementation issues) which is how I've earned my moniker I imagine! I should be okay unless I fall off a ladder....

[magic]

 

If you use standard passive probes without ground leads then you can ground the scope without issues.

Moreover, you probably should, in case one of the probes arbitrarily decides to short out. If the scope is grounded, there will be a bang, smoke and tripped circuit breaker. If it's floating, there will be a surprise HV on the scope.

edit
Honestly, I were to float a scope I would rather connect it to the hot ground, which is the only point of reference that I have ever cared about in an SMPS. That way at least I know the scope is live, stand back and don't touch it. Not sure if you should be doing it, though, given the sort of questions you ask

isolation auto transformer

No such thing, it's either auto or isolation.

[TimFox]

A related thing to watch out for:
If you have a metal-cased computer (or other equipment), and the PE (3rd prong) wire is not connected properly, then the two capacitors in the line filter will pull the metal case to half of the mains voltage, in series with a reasonable but not trivial capacitance.  The capacitor should be chosen such that 60 V or 110 V (depending on the local mains) should not electrocute a healthy person, but you can easily receive a jolt that could knock you off a ladder or other slapstick cause of injury, especially in 220 V lands.  At work, we called this the "Dan effect", as in "it's not easy to shock Dan, but..." after he encountered this phenomenon in the field, where the locals had not bothered to ground the generator properly.

[bdunham7]

Hi,
I have a USB scope I've been using for SMPS troubleshooting from a desktop PC.... I'm switching (no pun!) to using a laptop and was planning to run from battery to facilitate probing across front end components which I'm thinking is safe (assuming normal HV precautions). But were I to accidentally leave the laptop connected to the charger, I'm thinking it would not be an issue either? The charging adapter plug in is not polarized with no ground prong. There is no continuity from prongs to tip connectors. Thoughts? Thanks!

Is your USB scope rated for those voltages and CAT II? 

As for relying on your charging adapter for ground isolation, it may be adequate to keep 120VAC 60HZ at bay, but depending on how you connect everything up, you could be exposing it to something like 400V and 50kHz.  It won't do well at that.  And even before it goes up in smoke, you may have a highly energized laptop.  Make sure your files are backed up and your will is current.

[bdunham7]

No such thing, it's either auto or isolation.

He's referring to a combination unit with both in one box, typically autotransformer first and isolation transformer second.

[MartolFart]

Thanks for the comments!

My pure chinesium USB scope has no stinking IEC 1010 category labeling... It does proudly sport the CE insignia (self tested to the highest standards!). The max input with 1x probe is 35V. The 100X probes don't have cat specs either, just 2KV DC+AC again with CE label.

Per my previous post, I'd already discarded the idea of leaving the charger on the laptop powering the USB scopage gizmo; may as well just run off the battery anyway. I really don't think it was a ridiculously stupid question or idea though.... I didn't see any responses to the other idea of running the DUT off a disconnected from mains UPS? As I mentioned before, if I was doing this kind of work frequently/for a living, I'd prolly break down and buy an isolation transformer, maybe even a fancy one with integrated rheostat...

So another question comes to mind as a result of the test eq cat rating.... If the DUT is powered by "protected electronic equipment", does CAT I suffice?

My reading on the IEC category ratings indicated these were developed to address transient and available energy (essentially short circuit current) as adjunct to the insulation voltage ratings already commonly specified..... My ancient Fluke DMM has no labelling on the leads with max 1000VDC/750AC on the input jack and certainly no CAT rating!

Not having much luck with the SMPS PD so far (I did fix two other smaller ones!), I might have to beg some help when I get data collected, trying to sketch out the discrete chopper schematic is killing my old eyes....)!

[magic]

A related thing to watch out for:
If you have a metal-cased computer (or other equipment), and the PE (3rd prong) wire is not connected properly

I will add yet another: 3 phase motors with the chassis miswired to line