Floating the DUT

[ArcticPhoenix0]

Hello all. Please forgive the ignorance you're about to read; there's probably a lot of it.

I'm very new to Oscilloscopes and EE in general and it's safe to say that I know just enough to be exceptionally dangerous to everybody and everything involved.

With that said, I think I understand why it's necessary to float the DUT to prevent ground loops and making my probes into fireworks. I'm also pretty sure I understand why the scope should never float but always be grounded.

It seems to me that cheater plugs would be a cheaper, albeit probably not safer alternative to the newbie probing around an mains AC+Grounded DUT.  Of course an isolated 1:1 transformer is the safest way to to do this but is it at least feasible? I'd rather just plug the thing into a 3/2 plug instead of carry around a transformer provided it's as safe.

Secondly, would it not be a good idea to simply omit use of the grounding clip in mains AC+grounded devices? Seems like the best way to avoid ground loops altogether to this very inexperienced and ignorant hobbyist EE.

Anyway, I'm not interested in blowing up anything in my $500 scope; however pretty such an event may be.

I plan on sticking with the battery powered stuff or at least the non-grounded stuff until I understand this better.

Thanks in advance.

[james_s]

No, it's not safe, end of story. You MUST either use an isolation transformer or a HV differential probe when probing the primary side of line powered equipment.

How do you expect to get a measurement without grounding the probe at the DUT? You have to have a complete circuit, without the probe grounded all you will measure is noise.

[BravoV]

Anyway, I'm not interested in blowing up anything in my $500 scope; however pretty such an event may be.

Forget about the scope, ask your self, while in safe low voltage probings, how many times in your life you made mistakes in those probings ? Say 99.9% you've done it right, cool, impressive number.   

Now, when it come lethal HV or mains, a single mistake at 0.1%, or just once, once only, is enough to toast your limb or even lost your life, not worth it.

As you can afford the scope at $500, HV differential probe is within your reach.

An example of the HV diff. probe in action, the scope was measuring the mains line at 220 VAC that was powering it self.

[EEEnthusiast]

I would recommend to use a transformer along with the probe. That should provide enough isolation from the mains. You could use a regular step down transformer with 20:1 turns ratio.

[Brumby]

I'm very new to Oscilloscopes and EE in general and it's safe to say that I know just enough to be exceptionally dangerous to everybody and everything involved.

It is good you acknowledge your limitations and even better that you present yourself here with such an introduction.

In regards to working with mains - whether it's 120v, 240v or whatever - always consider it lethal and always consider it capable of delivering chassis melting power.  This is basically because that is exactly what it can do.

I have been an electronics hobbyist for over 50 years and I have yet to probe a mains circuit with a scope.  I prefer a properly rated multimeter, some observations and exercising the grey matter.  So far, I have been able to avoid such a risk, despite being sorely tempted on one occasion.  Such is my recommendation: stay away from mains wherever possible.  (It's a lot harder to zap equipment or yourself if you don't come into contact.)

Having said that, I will concede there are times when you need to know what's going on.  In those instances, I would strongly recommend the differential probe solution.  These are designed for the express purpose of dealing with this type of situation, providing the safest option to both person and property.  Yes, they can be pricey - but they are still cheaper than a funeral.

IMO, an isolation transformer should not be considered a safety measure.  Its purpose is to ensure operation of a device is unaffected by the circuit in which it is placed (well, as much as possible).  A side effect is that you have less constraints when attaching a scope - but, at the same time, you must be aware how you connect the scope to the DUT.  Attaching the "ground" connection of the scope to a point on the DUT which is at a significant voltage could render the chassis energised with the same potential (but opposite sign).  This creates a far more dangerous scenario.


Of course, it is possible to deal with the dangers directly - but taking the steps to avoid them will yield greater safety, especially when you have those "oops" moments.

Take care.

[GeorgeOfTheJungle]

the scope should never float but always be grounded

A thousand scopes all floating
https://duckduckgo.com/?q=micsig&iar=images

The only rule is never poke your probe where you shouldn't

[tggzzz]

Hello all. Please forgive the ignorance you're about to read; there's probably a lot of it.

I'm very new to Oscilloscopes and EE in general and it's safe to say that I know just enough to be exceptionally dangerous to everybody and everything involved.

That's an excellent starting point; I wish more people were like that

As others have said, there are many dangers with mains voltages, some obvious, some subtle. I'll just reiterate two points:

• 1:1 mains isolation transformers don't make things safe. The full mains mains is still present at the transformer output. They will defeat GFCI/RCDs since the live/neutral currents will be balanced
• do use a high voltage differential probe. They are expensive, but less expensive than paying for someone to spoonfeed you for the rest of your life because you have fried your arms

But before that, in fact before making any measurement: define what you are trying to find out and then what equipment to use to do it in a safe way. For example, measuring voltage/current is better done with a multimeter, looking at harmonics is better done by probing the output of a mains:12V isolating transformer, etc.

As for most low voltage circuits, power them from a bench power supply. They are cheap and flexible, and setting the appropriate current limit can be very helpful. Be cautious of consumer wall-wart PSUs; some completely ignore safety regulations.

[Brumby]

the scope should never float but always be grounded

A thousand scopes all floating
https://duckduckgo.com/?q=micsig&iar=images

Battery powered scopes are quite in the minority, so the warning is valid for the majority of cases where the scope is mains powered.

Those using battery powered scopes will usually be very aware of the capabilities of their instrument, especially considering they tend to be a more significant financial investment ... which tends to NOT be your average beginner.

[ArcticPhoenix0]

Thanks all.
So what I'm hearing is that I'm neither equipped nor experienced enough to be messing with mains AC stuff yet and that's fine. I didn't watch a handful of YouTube videos and suddenly become Nikola Tessla when I woke up this morning.

So in order to get there, eventually, it sounds like a good investment to go with a variable power supply and/or a transformer until I'm experienced enough to be able to successfully navigate higher voltage systems. That about sum it up?

[ArcticPhoenix0]

Oh and differential probes too!

[james_s]

What exactly are you trying to do?

I use a differential probe occasionally when repairing a tricky switchmode power supply or CRT monitors, other than that I've only ever used my scope on low voltage stuff. Outside of a few niche areas you shouldn't really need to be looking at stuff where this is even an issue, the vast majority of electronic circuits operate on low voltage DC.

[fcb]

It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

[tggzzz]

It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

It may be classic, but newbies should be aware of the info in https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 especially the "example fatality".

[magic]

Nope.

If the DUT has no power transformer or you ground anything before the transformer (like an SMPS regulator), is guaranteed regardless of whether the device is connected with a 2 or 3 prong plug. You may disconnect safety earth and float the chassis of the device, but the mains voltage which goes into it doesn't float, it is what it is.

[fcb]

It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

It may be classic, but newbies should be aware of the info in https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 especially the "example fatality".

I should also have said DON'T do it. Ensure your equipment is grounded correctly - including 'scopes.

[tggzzz]

It's certainly classic to 'lift' the ground (disconnect) of a scope in the good old days. It was also frowned upon by management, but widely ignored.  When PAT testing became a thing, you would reconnect the ground, wait for the PAT test to be done and then disconnect again. Times have moved on and scopes have switching supplies in them that leak to ground so wouldn't do it; and we have diff. probes, low cost battery scopes etc.

It may be classic, but newbies should be aware of the info in https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 especially the "example fatality".

I should also have said DON'T do it. Ensure your equipment is grounded correctly - including 'scopes.

Your "don't" was implicit, but for newbies being explicit might be useful

[ArcticPhoenix0]

What exactly are you trying to do?

Right now I'm trying to safely explore and learn how to use my tools and figure out what does what while staying safe. I'm naturally curious about this kinda stuff and I want to make sure I don't get myself into a situation I'm either not equipped or experienced enough to handle.

I realize that I enter an inherently dangerous situation just by removing the cover off certain items. I've been bitten by a few digital cameras so I understand that just because it's "off" doesn't mean it's not charged still.

Going back to my original post, I said that I know just enough to be dangerous. I'm intelligent enough to know that things plugged into the wall can be deadly but at the same time I know that there's a correct way to go about dealing with those things. So, in essence, I'm trying to figure that out with a minimum of collateral damage.

[james_s]

I would say that you should for now avoid anything mains powered entirely if possible. Items that are powered by one of those "wall wart" or inline power adapters are ok as those are isolated and provide the device with low voltage.

Once you know your way around inside something it's generally ok to test mains powered devices as long as you stay on the secondary side of the power supply, but an important caveat here is that there are some kinds of devices that are not isolated. Because of that I consider this to be expert territory, best to just stay out of this stuff entirely, at least keep the scope out of it until you are 100% confident at determining what is safe and what isn't.

[tggzzz]

I would say that you should for now avoid anything mains powered entirely if possible. Items that are powered by one of those "wall wart" or inline power adapters are ok as those are isolated and provide the device with low voltage.

Provided you don't use one of these wall warts...

"New Which? research has uncovered dozens of dangerous USB chargers, travel adapters and power banks listed for sale on AliExpress, Amazon Marketplace, eBay and Wish that could cause fires, give electrical shocks and potentially harm or even kill consumers. In a new investigation, more than three quarters of the 35 products from ‘unknown brands’ – those our experts didn’t recognise – that we purchased from online marketplaces, failed our electrical safety tests.... Eight of eleven lookalike brands failed standard safety tests.... Seven of these products also failed to pass standard electrical strength tests. And for one unbranded charger with three USB ports bought from AliExpress, arcing could be heard – this is where electric current flows through the air.... We tested 12 USB travel adaptors by unknown brands bought from online marketplaces Eleven of the adaptors failed electrical safety tests,"

FFI see https://www.which.co.uk/news/2019/09/killer-chargers-travel-adaptors-and-power-banks-rife-on-online-marketplaces/

For most purposes where you are debugging your own designs, use a bench power supply. They are cheap nowadays, and the current limiting can be highly beneficial.