If I place the probe of my Rigol oscilloscope on the metal casing of the outlet box I can see a lot of noise on the scope. The noise definitely coincides with actions of nearby machines with motors running and stopping. The spikes are repetitive in regards to the specific machine, short, from 5 to 27 u sec and from +/- 8V.

Is this something I should be seeing if the ground really goes to Earth ground?

The scope is plugged into the same outlet, so it has a common ground.

  • 2
    \$\begingroup\$ It can be a long ways from where the ground actually connects to earth. \$\endgroup\$
    – DKNguyen
    Nov 14, 2019 at 22:07
  • 1
    \$\begingroup\$ If connected to the same ground, you shouldn't see much difference. Are you connecting the scope probe to ground also? show a diagram of the setup if you can \$\endgroup\$
    – Voltage Spike
    Nov 14, 2019 at 22:57
  • \$\begingroup\$ @VoltageSpike Scope is plugged into the outlet. Probe is connected to channel 1, touching the probe to the metal casing of the outlets electrical box. I see the noise. Connecting the chassis ground of the scope to the box doesn't make a difference here. I'm trying to understand if this is to be expected or if I should see a solid 0V. \$\endgroup\$ Nov 14, 2019 at 23:53
  • \$\begingroup\$ place a 1Kohm resistor between the plate and the probe ground wire. \$\endgroup\$ Nov 15, 2019 at 4:07
  • \$\begingroup\$ This is totally normal. Trying to view power line noise with a scope can end in disaster. \$\endgroup\$
    – EinarA
    Nov 15, 2019 at 5:14

1 Answer 1


The result is not so surprising if you consider what you are actually measuring. If you connect the probe tip to mains earth and the scope itself is earthed via mains plug, then you are just basically measuring the voltage over a loop of wire, basically a coil with a single turn. Changing magnetic fields through the loop induce voltage on the wire which might be what you are measuring. Also between the two measurement points on the earth wire (scope mains plug and probe tip) there can be earth leakage currents of other devices flowing in the wire and these get converted to voltage by the earth wire resistance. The metal box can also act as an antenna so it might receive radiated noise. Or it could be capacitively coupled noise, just like what you see when touching the scope probe tip with a finger.


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