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I connected the oscilloscope probe to the mains supply but i find its waveform not excactly pure sine wave, So im wondering if there are problem in my scope or something else.

enter image description here

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    \$\begingroup\$ I suspect that you're using a 10X attenuator probe. Did you check its specs to see if it could handle such high voltage? Some probes would be overloaded and could be destroyed. \$\endgroup\$
    – glen_geek
    Dec 22, 2017 at 18:14

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The sinusoidal waveform of the mains supply is very rarely (if ever) perfect. It is affected by transmission line impedance, as well as the parasitic inductance, capacitance, and resistance of the system. Additionally the machinery used to generate the mains power is often imperfect and will lead to imperfect waveforms. The signal your scope shows appears fine to me.

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    \$\begingroup\$ Extra info: For a lab assignment, I had to measure the THD (Total Harmonic Distortion) of the mains voltage. It was somewhere between 1 to 10%. So I agree with your answer. Looks fine. \$\endgroup\$ Dec 22, 2017 at 19:07
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To expand on Dan Mills' answer, devices with rectifiers - and this includes switched mode power-supplies used on many domestic devices from computers, TVs, oscilloscopes, etc. - draw a pulse of current as the mains voltage rises above the capacitor voltage. If the power supply is not very "stiff" (low impedance) then the AC voltage will droop with the load.

enter image description here

Figure 1. Notice that the rectifier current in this half-wave example peaks before the AC peaks and is nearly turned off at mains peak.

This would result in a mains voltage distortion similar to that seen in your scope.

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Using a standard scope probe on the mains is a very bad idea from a safety perspective, but that said....

What you are seeing there a bit of flat topping, this is usually caused by crude power supplies doing the bridge rectifier into a cap thing and only drawing current near the mains peaks, but it does not look too bad, I have seen far worse.

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  • \$\begingroup\$ Why is it a bad idea? \$\endgroup\$
    – Edw590
    Aug 5, 2021 at 0:36
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    \$\begingroup\$ Because the mains can provide a LOT of current and scope probes are typically not designed to fail safe if something goes wrong. The mains might be nominally 230V, but it has spikes on it at random that can routinely hit several kV, and if something flashes over in response to such a spike the main current can then pass kA currents thru that path. There is a reason isolated differential probes exist in Cat III and Cat IV classes, and it is for mains measurement. There is also a reason they cost what they do. \$\endgroup\$
    – Dan Mills
    Aug 5, 2021 at 14:24
  • \$\begingroup\$ Interesting. Thank you! \$\endgroup\$
    – Edw590
    Aug 5, 2021 at 23:11
  • \$\begingroup\$ @DanMills kA and kV? Surely not in a residential setting? \$\endgroup\$
    – Dagelf
    Apr 5 at 19:00
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    \$\begingroup\$ @Dagelf Yep, we get away with 6kA interrupt rating breakers in the consumer unit, on the assumption that a fault in the outgoing cable is unlikely in the first two meters, but you can easily hit a few kA if you have a short close to the consumer unit (And that consumer unit is close to the distribution transformer). Here nist.gov/system/files/documents/pml/div684/… is a 1967 paper showing 1200V surges routinely (on a 120V network) and the highest thing they recorded was 5600V. \$\endgroup\$
    – Dan Mills
    Apr 6 at 15:19

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