With mains power, things get a little more dangerous. It's not about simple voltages any more, it is about serious energy levels. 10KV around a CRT tube will wake you up in a hurry and perhaps fry some types of component, but 230VAC mains can make things explode. Add a transient or two and things can get really dicey, really quickly and unexpectedly. Your very first post indicated you were not really aware of this, as you stated:
Hi, I'm new here, I hope you can help me with this. I already know the answer, so the reason I'm asking for this is because I want to make sure that my house won't burn.
....
The question is: is it safe to measure mains 220AC with A-B operation so probes ground clips are not used? (assume the probe is x10)
This line in particular ...
Again, I know the answer, it is safe. I just want to make sure, you guys are more experienced than me.
... demonstrates you didn't
know. This is the sort of thing that makes a lot of experienced people very anxious for your safety (and why some people bring up the D-K syndrome thing).
To your credit, you did have the presence of mind to bring your question here, before sticking probes into a mains environment. As a result of that, you have, indeed, learned something - and something that has the very distinct potential to save you from injury (or worse) - without exposing yourself to the real risk.
To be fair to those who may have come across as overcritical, nobody here has any real idea of who you are, your background or your capabilities. You've only been a member for a couple of days - and the only thread you have posted on is this one. As time goes by and we have the chance to discuss things with you, we will develop a better understanding - but at this point in time, you are somewhat of an unknown to us.
While there are a lot of things we can help you with, sometimes it is better if advice like the following is given:
If you're not absolutely sure you can test this safely - call an electrician.
Not only does an electrician have the qualification for this sort of work, they also have experience. Sometimes, it is this experience that can interpret the readings and/or follow a testing path that can identify the fault - because faults in the real world are sometimes non-intuitive, until
all the necessary information is at hand (which might take a lot of time, reasoning and measurement) ... or it has been encountered previously.
We are certainly in favour of helping and educating here, but we don't want to encourage wrecklessness - and until we get to know a member a bit better, we are likely to err on the side of caution.
One last word .... there is a wide scope of members here - with different backgrounds, different experience, different cultures - and, of course, different native languages. It's a global community. There are bound to be communications issues.