1
\$\begingroup\$

The other day I was experimenting with a multimeter and checked the voltage in one of the electric plug at home. Without surprise I measured around 230V between phase and neutral and also 230V between phase and earth (By earth, I mean the earth wire, the "3rd hole").

Then I tried the measure the voltage between phase and my body by sticking one probe in the phase hole and holding the other in my hand (Might not be the best idea if the meter as a failure or is set to measure current instead of voltage... did it nevertheless...). There I measured around 8V AC. Why ? The voltage between me and the earth should be almost 0V hence the potential difference between me and the phase should be around what I measure between phase and earth wire (230V). Which make sense since if I stick my finger in the plug I'm sure I'll get a nice full 230V shock. So why does the meter indicate such low voltage ?

My guess is that it has something to do with capacitance since Earth (big 'E') capacitance is much higher than my body capacitance and would therefore allow higher alternating current to flow in it than into my body ? (Really fuzzy guess sorry).

Thanks for you help :)

\$\endgroup\$
  • 2
    \$\begingroup\$ Please stop! Your multimeter does not provide insulation! \$\endgroup\$ – winny Aug 7 '18 at 20:11
  • \$\begingroup\$ Do you know what a voltage divider is? \$\endgroup\$ – PlasmaHH Aug 7 '18 at 20:11
  • \$\begingroup\$ @PlasmaHH In a nutshell yes, but I cannot see how it relates to the question here. \$\endgroup\$ – Zubzub Aug 7 '18 at 20:19
  • 4
    \$\begingroup\$ You need to stop playing with electricity NOW! \$\endgroup\$ – Solar Mike Aug 7 '18 at 20:28
  • 2
    \$\begingroup\$ You know you're just a mistake away from death, right? \$\endgroup\$ – Spehro Pefhany Aug 7 '18 at 21:04
5
\$\begingroup\$

Mains neutral/earth is tied to the general mass of the earth.

Digital multimeters normally have an input impedance of 10MΩ. So a reading of 8V means that 0.8uA was flowing between the mains and your body through the multimeter.

At the time you made your measurement the impedance between your body and the general mass of the earth was much higher than the input impedance of your multimeter. So most of the voltage was dropped between your body and the general mass of the earth rather than across the multimeter.

It follows that if nothing had changed since you made the measurement and you touched the mains live conductor you would not have recieved a shock.

Of course it is all too easy to end up inadvertantly grounding your body, so I strongly reccomend avoiding touching live mains.

\$\endgroup\$
  • \$\begingroup\$ Yeah I get edgy working on live stuff even with electrical rated boots on. \$\endgroup\$ – K H Aug 7 '18 at 21:50
0
\$\begingroup\$

The voltage between you and Earth may not be anywhere near 0V. Carpets can be very good insulators, and even dry wooden floors are poor conductors.

Have you ever walked across a carpeted floor on a dry day, and then got zapped when you touched something metal? If so, there would have been several thousand volts between you and Earth at that point.

If you put enough insulation between yourself and Earth, then you wouldn't get a significant shock if you touched the phase wire. But I really don't recommend you try it, as a mistake could prove fatal.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.