Below was my setup:

enter image description here The moment I connected the probe the RCD tripped.

My scope was earth grounded. Does that mean I shorted the neutral and the earth ground? I know that neutral and earth are connected somewhere outside the building or at the utility transformer. But I completely forgot about it.

And what can I do to prevent that that? Which one is a safe solution: using an unearthed plug for the scope or a 1:1 isolation transformer? Any ideas?

1:1 isolation transformer might not be okay for all cases and might corrupt the original waveform?

  • \$\begingroup\$ Im sorry it was 100k resistor I will update. \$\endgroup\$ – user16307 Nov 2 '18 at 15:47
  • \$\begingroup\$ Google for "how not to blow up your oscilloscope". When you connect your gnd clip to something, you connect it to GND offering current a possible alternative path which can blow up your scope. If unsure check for connectivity between the desired gnd clip point and the earthing of your device. \$\endgroup\$ – PlasmaHH Nov 2 '18 at 15:51
  • \$\begingroup\$ You are lucky that you didn't damage scope or probe! There are isolated probes available for doing this. Basically this is a high-voltage differential measurement. \$\endgroup\$ – mkeith Nov 2 '18 at 15:55
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    \$\begingroup\$ Chalk one up for the RCD. That probably just saved you from bigger problems. \$\endgroup\$ – JRE Nov 2 '18 at 15:56
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    \$\begingroup\$ We used high voltage differential probes. Or, voltage permitting, two 100x (or 1000x) probes connected to EARTH... remember neutral is NOT earth... and then maths function to determine the voltage difference \$\endgroup\$ – JonRB Nov 2 '18 at 16:14

If you can afford the wait but not much money, order this probe and do a differential measurement.

Sorry for posting it as an answer; I wouldn't have done it if I had enough reputation to write it as a comment.

  • \$\begingroup\$ It was even cheaper from EU seller: batterfly.com/shop/micsig ,perhaps it worths keeping an eye for some special action. \$\endgroup\$ – Marko Buršič Nov 2 '18 at 19:22
  • \$\begingroup\$ Personally, I would be very hesitant to attach that between my oscilloscope and mains voltage. Also, I would be hesitant to touch it if it was connected to mains voltage. That is why I linked to a name brand rather than an Ali Express part. Rigol is budget, but at least it is a name brand, and the insulation ratings are probably not fake. \$\endgroup\$ – mkeith Nov 3 '18 at 5:04

Looking at your circuit what you are doing is a really bad idea. Measuring mains is fraught with problems hence the CAT rating on test gear. Use a simple voltage transformer (see ZMPT101B or similar). They will give you isolation without distorting the waveform too much and quite likely save your life let alone your oscilloscope! If you are considering this for a long term measurement, add a TVS and a good 250V Fuse as well to help with power surges.


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