From reading all of the above comments, I also want to add my 0.02 Eurocents to that...
- In todays world, every live circuit (especially in domestic use) should have a RCD present. Here in germany the usual RCD for the normal wall outlets is 30 mA by 230 VAC nominal voltage. This means that the tripping range according to VDE standards is between 15mA...30mA, and tripping time is allowed to be as high as 0.2 sec- usually within 20-30 ms with fresh gear.
- As I understand, a GFCI protected outlet according to UL is required to have 5mA +/- 1mA tripping current- which would loosely correspond with the german 10mA RCD (tripping between 5mA and 10mA). As in the USA approx. 110 VAC are used, and the resistance of the human body about 1000-2000 Ohms, depending on path of the current is constant, a lower voltage would drive lower currents through the human body.
(Please corrrect me, if these data I had to look up are not correct)
- above 10mA you really feel the current flowing to you, so even if 30mA is not directly and immediately lethal within short exposure, it can do other nasty things like blood clogging up and causing thrombosis some hours later
- If you have a live circuit and touch L-N, it really depends on the circumstances of enough current is able to flow through your body, shoes and floor to ground/earth connection to make a RCD trip. a well insulated floor (wood etc.) and shoes with thick rubber soles probably would make sure the current is not high enough to cause a standard RCD (30 mA usually, so in reality between 20mA and 25mA range) to trip
This said, I acquired myself some used isolation transformer some days ago, that needs some minor repairs- and given the situation that I am currently doing some repair work on a lab power supply and want in the near future to be able to do some ripple measurements with my oscilloscope, an isolation transformer is one way to go and deal with the situation.
(As I do not have a differential probe for it- and with the price point of this entry level scope a decent quality differential probe probably would be more expensive than my scope...)
And most important: As previously stated, knowledge can (and should) not only be acquired through internet forums- the whole situation on-site has to be taken into account to prevent disasters and accidents. Also some other good practices can and should be employed to minimize the risk of getting accidentally zapped.
These can include working on wooden benches, wearing rubber gloves, clothes that cover the arms and are quite tight, or standing on a rubber mat- and making sure that the workbench or table in the hobby-room is nowhere near where central heating or plumbing is in the reach with grounded metal pipes...
Depending on knowledge and preferences there are the options to ground everything in the hope(s) of having the grounding and RCD/GFCI getting into place when something goes south, and leading the currents away from the body, or isolating the whole enchilada...
Isolation is in most cases a bit easier to do and check, whereas grounding has to be frequently checked to ensure everything is working well...