We've all seen AC line testers, with three lamps that diagnose the wiring of an electrical outlet (well at least in Canada/US).

I'm (somewhat) used to the type that use neon bulbs, so I have two related questions:

1) How do the more modern sorts with LEDs work?

2) How do any of them avoid tripping ground fault interruption protection?

I want to build a special power bar for my repair work and I think it would be useful to build this diagnostic feature into that power bar.

  • \$\begingroup\$ I don't know what it takes to trip a GFCI breaker, but those neon indicator lamps draw less than one milliampere. \$\endgroup\$ Jul 17, 2018 at 14:56
  • \$\begingroup\$ I had the same question that you did, so it's too bad you have not yet gotten an answer on how it works inside. If nobody answers you, you could do a tear-down on a few of them to get the answer yourself, because my thought is to be as safe as I can, so I'm with you on that score, and that means making sure that wiring is proper. I was thinking, if I mod a store-bought circuit, and replace the LED's with optoisolators, I could detect proper wiring with a little additional circuitry and a microcontroller, an Arduino, or a PIC or something, tied. Some checking is better than none at all. \$\endgroup\$ Jan 14, 2019 at 12:56
  • \$\begingroup\$ I have to add a disclaimer and a suggestion to my previous comment. What you're thinking of doing is a one-off design that allows you to work with house current more safely. It is BECAUSE house current is so dangerous that you should probably buy the solution, somehow, though if it doesn't exist, maybe somebody else would buy it, so there's a product idea for someone. I'd much rather buy it with all the certifications and a company and an engineering team behind it, if I'm messing with high voltage + current. Regarding my previous idea on HOW to do it, I actually advise against it. \$\endgroup\$ Jan 14, 2019 at 13:10

1 Answer 1


US GFCIs are specified to trip at around 5ma (Wikipedia). Most NE-2 neon bulbs take less than 1ma (datasheet) so do not trip the GFCI.

Many LEDs, while specified for max current of 20ma, will produce adequate light at 2ma so can also not trip the GFCI.


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