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I am doing some measurement on Scope ( non-isolated) , my set up is like this -Scope is connected to 230V line mains with only two pin plug ( Earth or ground not used) -channel is connected with voltage probe -None of probe is hooked to any measurement point yet.

I observer when I touched any probe positive lead, I saw some high voltage waveform on screen,

a) First case :- I am reading 240Vac peak to peak and 82 Vrms on scope also can see sine wave on screen . I was sitting on chair , any I wore shoes and my shoes touching to ground.

b) First case :- I am reading 170Vac peak to peak and 50 Vrms on scope also can see sine wave on screen . I was sitting on chair , any I lift my legs form ground.

Q1) Is everything is right with setup

Q2) is it true voltage , then why i am not getting shock by touching probe lead

Q3) Why voltage shown when I am touching the lead, does I am generating the voltage from my body.


marked as duplicate by pipe, Bimpelrekkie, Bence Kaulics, Daniel Grillo, Vladimir Cravero Aug 24 '16 at 11:33

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You are acting like a (very bad) antenna, picking up some of the EM field from the mains running in the walls around you.

The scope inputs are usually in the M\$\Omega\$ range, with a 10:1 you go up to 10M\$\Omega\$: such a great impedance means that when you connect something that can source even a tiny current, you see the corresponding waveform on screen.

Try to touch the probe tip and the probe ground, you will see that the sine wave is almost gone.

You do not get any shock because this current is very tiny. 240V peak on 10megs means 240uA, well below the 20/30mA that start to be dangerous, and somewhat below the "feeling" threshold.

I think that there's nothing wrong with your setup, I am quite surprised that your scope does not need earthing, every scope I have ever seen is connected to the mains with a three terminals plug. Using a three terminals plug can possibly mitigate this problem, anyway it is perfectly normal to see AC if you touch an high impedance terminal.

  • \$\begingroup\$ Thanks , Scope has three plug ,but I bypass the earth wire. I seen lot people do this to measure floating signal. that's what i think. \$\endgroup\$ – Transformer Aug 24 '16 at 7:46
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    \$\begingroup\$ Lots of people are stupid. Earth is there to protect the user, and wheter you connect it or not the absolute maximum ratings of the scope still apply. Maybe you get something off of it because the line transformer can go somewhat floating, but it is dangerous for you and the instrument. Please, please, please leave earth connected if you are not extremely sure of what you are doing. \$\endgroup\$ – Vladimir Cravero Aug 24 '16 at 7:48
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    \$\begingroup\$ ^^^^ A thousand upvotes for this comment. Leave the earth ground on you scope connected. ALWAYS. \$\endgroup\$ – JRE Aug 24 '16 at 7:58
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    \$\begingroup\$ @Transformer: Never ever float the scope unless you really really know what you are doing (i.e. don't have to ask about anything here). Float the DUT. There are some questions about this here, and also some youtube videos like from eevblog that you should study \$\endgroup\$ – PlasmaHH Aug 24 '16 at 7:59
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    \$\begingroup\$ Or use something like a Fluke Scopemeter (large multimeter style) which has a high-voltage insulated plastic case. This is floating and runs from an isolating battery charger, if required, so it can safely be connected anywhere on the circuit. \$\endgroup\$ – Transistor Aug 24 '16 at 11:29

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