I'm curious about how socket grounding testers like these work: https://www.amazon.com/Amprobe-ST-102B-Socket-Tester-GFCI/dp/B008E07HM2/.

I'm also curious about what kind of false positives they could give. Could they for example mistakenly show grounding as present while in reality there is no real grounding? For example if the grounding is not going to earth but just to a bunch of other sockets (and their connected devices)?

  • \$\begingroup\$ Meters like this usually measure electric potential elsewhere, i.e. hot to neutral, so not necessarily hot to earth ground. \$\endgroup\$
    – Colin
    Commented Apr 8 at 15:21
  • \$\begingroup\$ find a schematic diagram \$\endgroup\$
    – jsotola
    Commented Apr 8 at 15:36
  • \$\begingroup\$ There are many tear-downs of these simple testers on the Web with schematics. They are not foolproof and best used on new installations to test mains outlets without any other loads connected anywhere in the installation. It is also handy to have a voltmeter to check for proper voltages. In new installations (commercial & home) walk-throughs, I have come across wrong wiring and outlet styles. \$\endgroup\$
    – qrk
    Commented Apr 8 at 22:22

1 Answer 1


Most have aome kind of lamps and resistors between each pair of terminals.

Basically there could be a lamp between live and neutral, live and earth, and just to be sure, between neutral and earth to indicate swap between live and neutral for systems that are polarized.

They might have been neon bulbs but can be LEDs these days. The GFCI/RCD test button just creates leakage from live to earth to trigger it.


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