Safety of measuring the circuit powered by switching power supply by osciloscope

[marekpasek]

Hi, I have ordered some switching power supplies from china (originally supposed to supply LED strips) and I want to make a power supply from them, but I measured that the ground of the 12v output of the supply has a voltage about 90v referentially to The Ground (meant the ground which the most of oscilloscopes is grounded to) but when shorted with The Ground ,only few hundreds of miliamperes flows (I think it was about 300mA at most). And I have also watched Dave's video called "How to not blow up oscilloscope". There he said that the measured circuit should be isolated from the scope otherwise the scope may blow up , when falsely connected. So I have a question, is it safe to measure circuit connected to the power supply I am gonna make by oscilloscope when attaching the ground aligator clip to the ground of the supply having 90v to The Ground?
Thanks a lot (I would like to not blow up my new scope immediately )

[Zero999]

It's normal for a switched mode power supply to float at around half the mains voltage, due to a tiny current coupled by the Y capacitor which is required to suppress radio frequency interference. However 300mA is far to higher current to measure when it's short circuited to ground.

Are you certain you've not made a measurement error? Could you have got the decimal point wrong and it's really 300µA? Connect a small mains voltage incandescent lamp, between the 0V side of output of the switched mode power supply and earth: does it glow dimly?

If the answer is no, now try connecting a 100k resistor between the 0V and earth and measuring with your multimeter: is the voltage still 90V or has it dropped to something safer?

[marekpasek]

Thanks a lot. I will try what you suggested. Currentwise I am quite sure with 300mA because I have 25-years-old Metex multimeter which I inherited and which has no uA range (but why not to assure myself by checking it ).