The Rigol scope is only rated for Category I measurements. This means that connecting to any Mains powered circuit is beyond it’s rated capacity and extremely dangerous for both the operator and the oscilloscope.
Please do not connect the Rigol to any AC powered circuits unless you are using an isolation transformer to power the device under test.
(From the Rigol User Guide, page VIII ):
Measurement Category
MSO1000Z/DS1000Z series digital oscilloscopes can make measurements in Measurement Category I. WARNING
This oscilloscope can only be used for measurements within its specified measurement categories. Measurement Category Definitions
Measurement category I is for measurements performed on circuits not directly connected to MAINS. Examples are measurements on circuits not derived from MAINS, and specially protected (internal) MAINS derived circuits. In the latter case, transient stresses are variable. For that reason, the transient withstand capability of the equipment is made known to the user.
Connecting the Rigol scope to the output of a car charger or other simple
rectifier is also NOT allowed. These are often broken without and load
protection.
Oscilloscope’s frame is connected to GROUND so To probe power circuits you need to at least use isolation transformer. Some people say that running scope like that is dangerous, but 120V AC prolly won’t kill you but anything above 48V considered to be dangerous and it hurts like hell. So when running scope on isolation transformer you have to be alert all the time and that is at least inconvenient.
Second, more correct solution is to use High Voltage Differential probe. - Wikipedia and internet, Ebay is your friend here. Some $200 Chinese units are OK.
The first is destruction by ground loops. This video addresses the first problem.
The second is VOLTAGE! which may be in excess of hundreds of volts. This can happened with load dump circuits and AC main power.
You can get high voltage probes or make a resistor divider.
The best way to approach this is to make a resistor divider box.
This is a simple resistor divider housed in a safe non-conductive
project box. Connect the top and bottom of the resistor divider to a
line cord with a correctly polarized plug. Then bring the bottom of the
divider and the center tap of the divider out to 5-way binding posts or
banana jacks. Also route the Earth Ground lead of the line cord to
another banana jack, or 5-way, on the enclosure.
Select a resistor divider ratio so you get an output voltage which is
both safe to touch and suitable for your scope’s input range. Also,
select the resistor values so they have a low enough impedance to not
affect your scope’s accuracy, but are high enough that you are not
burning up too much power in the upper resistor and creating a lot of
unnecessary heat.
As you will be multiplying all of your scope readings by the inverse
of this ratio, choose a ratio which is easy to manipulate mentally -
e.g. 10:1, 15:1, 20:1 - but still provides a safe-to-touch voltage level
on the output jacks. ( Not that you will be purpously & routinely
touching the output terminals, but accidents and slip-ups do happen. )
Make sure you construct this box in such a way and seal it up so
there is NO CHANCE of accidentally touching the Hot Wire. You might also
include a pilot light to indicate the box is plugged into the mains.
You can’t be too careful when messing with the power mains!
Finally got around to printing this out. It’s taped to the bottom of the scope. Can be flipped back underneath when you are using the scope. Leave it out when finished.
Feel free to (re)move it if you don’t like the sign or where it is attached.
I figured this sign should be enough to encourage less educated scope users to ask for the proper way to do what they wish to do if it involves AC voltages.
High voltage is not what folks here are as concerned about. That’s a bit more obvious and hopefully less likely for well-minded individuals to run into.
The problem is that the scope’s BNC inputs are not isolated and the shells are connected to mains earth. When working on a faulty or poorly understood mains-powered circuit, it’s easy to connect the probe ground to something that’s not at earth potential, and poof you have 15+A of fault current until the breaker or GFCI trips, or some trace on the scope PCB burns up. This is a risk on any internally mains powered gear, even if you’re not working on the high voltage power supply section, because not everything uses an isolated power supply, though if it has a two-prong plug it should be.
Okay, so I’d read through this whole thread and seen the flowchart and the no-ac sign, and I still failed to understand what the rule was. The reason for my failure to understand is because when I read something like “connecting to any Mains powered circuit is beyond it’s rated capacity and extremely dangerous for both the operator and the oscilloscope,” I’m left with significant cognitive dissonance to resolve. My thought process goes something like:
Okay, this doesn’t make sense… Probing a circuit on wall power is beyond its rated capacity? Isn’t that what oscilloscopes are used for? How else are you expected to power it? Batteries only? But nobody does that!
And it’s extremely dangerous? But people probe circuits powered by wall warts, or their laptops, or the bench top power supplies all the time! I’ve never seen anything go wrong and nobody makes a fuss about it.
I must be thinking about this wrong. When they say “no connecting to any mains-powered circuit,” they must mean something more specific, like connecting to mains power directly, or to high voltage AC or to an AC power supply. Yeah, that makes sense. That certainly sounds dangerous.
I hope I’ve painted a clear enough picture that, despite some of the prior posts being probably very precise information, it was easier for me to conclude that this information is being miscommunicated or misunderstood, rather than to take it as literal truth.
Thanks @ktims for clarifying and correcting my misunderstanding. I don’t think I’ve watched the EEVblog video posted above by @tdwebste, so I’ll watch it and hopefully come back with a better understanding of it all.
Most laptops are powered from some form of wall wart so one should be protected there too. I am guessing that most computer power supplies are built in some way to isolate the mains from the output DC voltage.
I am guessing that bench top power supplies have some form of isolation transformer built in otherwise we’d likely see a lot more accidents
Sooooo, just to ask the question here… why would anyone want to open up a mains cable and test this? I mean, aside from because you can, is there any legitimate reason to ever test the mains?