Thanks for all the replies, i spent some time to read them all carefully. For some months i read plenty of other threads on osciloscopes and mains and appreciate its not a yes/no question…

i dont often need to do this and perfect accuracy is also not really necessary; my uses are usually on 240v mains at 50hz,

reading all the safety issues, i therefore thought the simple transformer solution was safe and good enough. I will probably get a 100x probe and compare to a transformer readings. I need to debug and check waveforms from a tig welder at different settiings so i think waveform and approx readings are fine which i will cross check with a multimeter…

i may try to get a 400v transformer to improve things or two 240 transformers primary in series think also works? With similar secondary voltage in parallel or something like that

Secondaries in series, not in parallel.
By the way, I did found such a "measuring transformer" in an faulty old UPS, I guess, it was APC UPS. It's size was 4x4x5cm approximatelly . Many turns in the primary, and my multimeter indicates 6 000 Ohm or so

 

Thanks for all the replies, i spent some time to read them all carefully. For some months i read plenty of other threads on osciloscopes and mains and appreciate its not a yes/no question…

i dont often need to do this and perfect accuracy is also not really necessary; my uses are usually on 240v mains at 50hz,

reading all the safety issues, i therefore thought the simple transformer solution was safe and good enough. I will probably get a 100x probe and compare to a transformer readings. I need to debug and check waveforms from a tig welder at different settiings so i think waveform and approx readings are fine which i will cross check with a multimeter…

i may try to get a 400v transformer to improve things or two 240 transformers primary in series think also works? With similar secondary voltage in parallel or something like that

Ensure you get a high voltage probe , not just a 100:1 probe.

 

Keep in mind that isolating the entire oscilloscope avoids adding anything to the signal path. Thus it does not change the accuracy at all.

 

isolating the entire oscilloscope

Many engineers I know have their scope plugged into the PC via USB cable. Many scopes now days use the PC as a display. Many scopes are connected to the network.
What is the IT person going to say when they find your PC's case connected to 220Vac.

 

Many engineers I know have their scope plugged into the PC via USB cable. Many scopes now days use the PC as a display. Many scopes are connected to the network.
What is the IT person going to say when they find your PC's case connected to 220Vac.

Ouch?*

The internet is galvanically isolated, but USB isn't so if you were to do something so daft as to connect your pc to live mains via its USB connection, the internet connection would survive!

*[Edit] I forgot. IT staff aren't rated to such a high voltage as engineers.

 

I was ONLY referencing an actual, physical, oscilloscope, NOT an "oscilloscope adapter attachment for a PC.
Although such attachments, at least some of them, can provide very impressive functions, they are adapters, no matter how great the capabilities they provide.
My suggestion is ONLY APPLICABLE a single unit oscilloscope that can be completely galvanicly isolated from mains power. Such adapters, unless they offer such isolation, are not capable of being useful, other than, possibly, if they were used with a battery operated PC. BUT that combination is not one that I would choose to use on a test bench arrangement..