There will probably a mains input filter on the scope. These cause some leakage to the earth wire on the scope. There is normally a capacitor from live an neutral to earth. The one between neutral and earth may be a larger value than the one between live and earth. Just as an example a 100 nF capacitor has a reactance of about 33K at 50 HZ The capacitors in the filter are probably about 4.7nF
Les
Most dual channel scopes have a provision for displaying a single trace that is (Channel1-Channel2). To use it, you have to have matched probes and set the vertical gain of both channels to the same Volts/Div.
This effectively makes a differential measurement using two probes so can keep the scope frame grounded through its line cord. You will have to look at the manual on your scope to find out what the maximum common-mode input voltage is limited to... (usually +-500V if using 10X probes).
Anybody that cuts off the third prong on the line cord of a scope (or uses a two-prong adapter) to isolate the frame of a scope is an idiot. I have fired a technician for that...
I can understand some leakage from one of the lines to ground, but I don´t understand why ground input is ground output at scope, is that normal? I think it should be isolated, if not any measure you do on mains AC is tripping the earth leakage protection.
You set scope to dual trace, invert channel B and look for a ADD mode switch.
Yes that's normal, and that's how earthing works.
Officially, and for those who don't fully understand the implications, the device under test should be powered from a mains isolation transformer.
However, pretty well the entire service trade, and those who full understand the reasons for doing so, commonly disconnect the scope earth.
Earthing only makes things 'safer' under certain specific circumstances, in many other circumstances it makes it more dangerous - and pretty well all servicing falls in the 'more dangerous' category.
It's on often discussed subject on these forums - but unless you know EXACTLY why an item should be earthed or not, then leave it earthed (and the fact you've started this thread shows you don't understand it).
I would suggest you obtain a mains isolation transformer, it's a common requirement in service departments anyway.
Both of my scopes (An Advance OS240 and a Tektronix 2465B) have all the input grounds connected to earth. The reason the floating earth on the scope is at some potential between live and neutral is because the capacitors in the filter form a potential divider. IF the filter had been designed with two equal value capacitors. One between earth and neutral and one between live and neutral then The floating earth on the scope would read half the mains voltage. Filters often do not have equal value capacitors so this explains why the floating earth is not at half the mains voltage. I would expect the lower reading to be between neutral and the floating earth.
Edit.
To add to Nigel's post. Consider working on the primary side of a switched mode power supply. Your scope's earth would probably be connected to the negative of the bridge rectifier. This point would be at a lethal potential with respect to true earth. If you now touch the case of the scope at the same time as some item that is at true earth potential. (Which is connected to neutral at the substation transformer star point or where the mains cable enters your building.) you will receive a lethal shock. Even if you are thinking about this danger you only need to forget once.
Les.
Both of mine are unearthed, one doesnt have an earth on the power lead (philips), and the other I disconnected.
When I was daydreaming a while back I managed to destroy my laptop, I'm still not sure what happened, but I think the earth on my old 'scope was the cause.
I've also seen more than once wall warts that have a y rated cap from neutral to the o/p, including one where the mains in connector didnt have polarizing tabs so you could have it to the the live, the psu in your device probably has a cap to neutral.
As mentioned a isolation transformer is a really good idea, if like me you dont want to part with the cash, use 2 cheap transfomers back to back, then it doesnt really matter what the secondarys are, mine are control panel 110v transformers.
Can anybody explain in detail how to measure mains 220vac with the scope when earth input is connected to ground output?
it will be the first time I use this method, I´m not sure if I understand point number 6, both probes should be connected at two points in the circuit, so in example, one to 220Vac and one to 5Vdc, then I have 220Vac + 5Vdc in the screen?
Is like this the scope using a floating ground?
the water pipes was at 120 volts anything that was grounded stove ac was at 120 volt.
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