Be aware, don't put the isolating transformer on the scope unless you fully understand what risks that creates.
Put it on the device under test instead.
If you put it on the scope you may make the entire outer case of the scope live!
I concur completely with the "understand the risks" - no matter WHAT METHOD you use.
But.. I'm not quite as convinced that the traditional "don't float the scope, float the DUT" is as applicable today as it used to be.
Sure... when scopes were big metal boxes, you would absolutely NOT want to float it. But modern scopes are not like that.
They are well insulated cases and knobs, if you cap off the BNC's and calibration output, and don't go putting your fingers into the network or USB connectors, a lot of modern scopes will be safer to float then the DUT. (Still not something I recommend doing!)
The problem is that "floating" the DUT does not make it safe! but does tend to give people a dangerous false sense of security because "I did it the safe way".
As soon as you clip the ground lead from your grounded scope to some high voltage point on the DUT, it is NO LONGER FLOATING! - in fact, it is now grounded at a place where the manufacturer never intended it to be grounded. That means lots of spots which were left exposed because "they're grounded" are now sitting at a large offset from ground. Things like that grounded heatsink you've gotten in the habit of touching to gauge the power dissipation level... or the grounded device case you sometimes rest your fingers on while holding the probe in a difficult spot...
There are much "safer" ways to look at HV components in a mains connected system, such as:
- Use the math functions of your scope to see the differential between two probes (no extra cost option)..
- Use a HV differential probe
- Use a properly insulated battery powered scopemeter
The reason "safer" is in quotes above is because just because you have the right tools doesn't make you safe. Mains isn't something to screw around with.
IMHO the only good advice you can give a newbie who wants to scope something directly connected to mains is "DON'T - wait till you're not a newbie and REALLY know what you are doing".
Having said all that - if you just want to see a power grid sine wave, looking at the output of a low voltage power transformer (no connection to mains other than induction) is a good way to do it.
Dave