I used the above GFCI/RCD (up to 250v) socket tester on the 240v black and red live wire of the US split phase ac power system. When I connect the ground, all indicator lights (3 red) got on indicating "Live/Grd Reverse, Missing GRD" although it is connected properly.

What is inside the socket tester? Does it have a live and neutral leg such that it should only be used for ac system with polarity? How about if you use it on the red and black 240v phase to phase? Why would all the indicators light become on (3 red lights)?

The following is the specs of it:

Brand Name: HABOTEST
Color: Black
Material: Flame Retardant ABS
Plug Type: US
Voltage: 48V-250V 45-65Hz
RCD Test: >30mA (EU/UK Plug)
RCD Working Voltage: 220V±20V
GFCI Test: >5mA (US Plug)
GFCI Working Voltage: 110V±20V
Voltage Measurement: 48V~250V / 45~65Hz

Accuracy: ±(2.0%+2)
Working Environment: 0°C ~ 40°C, 20% ~75%RH
Storage Environment: -10°C ~ 50°C, 20% ~80%RH
Safety Rating: EN61010-1,-2-030, EN61326-1,CAT II 300V
Item Size: 62 * 65 * 55mm / 2.4 * 2.6 * 2.2in
Item Weight: UK 64g/2.3oz, EU 59g/2.1oz, US 53g / 1.9oz
Package Size: 7.5 * 7.4 * 7.3cm / 3.0 * 2.9 * 2.9in
Package Weight: UK 90g/3.2oz, EU 86g/3.0oz, US 79g / 2.8oz

  • 1
    Will this magic 8 ball tester work on US voltage? The GFCI test would not work properly as it is probably 48k load , for US test the GFCI function load would be 24k or lower.
    – Ed Beal
    Feb 12, 2019 at 21:17

2 Answers 2


Magic 8-Ball

Harper calls these "magic 8-balls". If you don't really understand them, they are about as useful as a Magic 8-ball.

The original versions of these (and still available) have 3 lights connected across hot-neutral, hot-ground, neutral-ground. The newer testers - like yours - use a microprocessor and some fancy circuitry to simulate the 3 lights and provide additional information.

Wrong circuit!

While this particular device (and many others) can handle a wide range of voltage & frequency (just like a laptop power supply or many other things), it does require a particular combination of wires to work as designed:

  • Hot - called "Live" on this device, also called "Line"
  • Neutral
  • Ground

One key thing to remember is that the electrons moving around (usual metaphor for electricity, though the physicists argue about the details) do not "know" what is supposed to be Hot, Neutral or Ground. They just "go where they can, the fastest/easiest way that they can get there". That is why problems with "lost leg", "lost neutral", "ground fault", etc. can be so dangerous. But it is also why a tester like this is extremely limited in what it can tell you. It can sort of say something is "reversed", but only if everything else is normal.

You connected, TWO HOT WIRES and no neutral. So things won't work right, as you found out the hard way. Among other things, Neutral to Ground is 0V on a Hot/Neutral/Ground cable but you replaced Neutral with another Hot wire, so Hot -> "Neutral" is 240V instead of 120V (that's OK on this particular device) but "Neutral" -> Ground is 120V instead of 0V, which is not OK - and turns on an extra light.

How the lights are SUPPOSED to work

  • Light 1: Hot -> Neutral. Normally ON. Off if Hot or Neutral is open (because the light can't get power from an open circuit) or if Hot and Ground are reversed (because Ground and Neutral are normally bonded so no power flows between them)

  • Light 2: Hot -> Ground. Normally ON. Off if Hot or Ground is open (no power in an open circuit) or if Hot and Neutral are reversed (because then this is Ground and Neutral).

  • Light 3: Neutral -> Ground. Normally OFF. On if Hot and Ground are reversed (because then this is Neutral & Hot) or if Hot and Neutral are reversed (because then this is Hot & Ground).

So each particular light tells you about voltage between two wires. But it doesn't really tell you what is going on. The combination of all 3 lights gives a good indication of what is likely to be going on.

The Problems

There are a lot of possible problems. Let's start with one that is clearly a problem on this device: All lights on == Live/Ground Reverse, Missing Ground

Guess what? There is NO SUCH THING!

My trusty Klein RT105 doesn't have "all lights on" on the label? Why? It does not exist in any normal circuit - and if it exists, the magic 8-ball has no idea what it means!

It can't really mean Live/Ground Reverse AND Missing Ground - if the Ground is missing, it is not "reverse"! They really should not have put that label on there - better to just leave it as "all lights on should never happen, so we'll just pretend it can't happen - and except for someone trying to put in a 2nd hot instead of neutral, it doesn't happen.

All it really means is "enough voltage on all 3 pathways to turn on the lights". In your case, that is:

  • Light 1 - Hot 1 -> Hot 2 = 240V
  • Light 2 - Hot 1 -> Ground = 120V
  • Light 3 - Hot 2 -> Ground = 120V

There are additional possible problems, such as blinking or dim lights, which could mean all kinds of possible problems with your circuits.

The end result is that:

  • These devices are primarily useful on the circuits they are designed to test - Hot/Neutral/Ground.
  • If the display is anything other than "Correct", the lights are a suggestion but not a guarantee of what is/isn't correct in your circuit.

I mainly use these as a double-check. If everything seems OK and the tester shows the correct lights, I'm good to go. If something already seems "wrong", this kind of tester doesn't tell you much of anything. And if I am going to a new (to me) receptacle to check things out, then the tester gives me a very good first indication of correct/incorrect - but if it says incorrect then it is time to open things up and check everything methodically until I figure out the problem.

  • Thanks for the detail explanations. What brand and model is the "original versions of these (and still available) have 3 lights connected across hot-neutral, hot-ground, neutral-ground"?
    – Jtl
    Jan 4, 2019 at 4:47
  • 1
    A typical example (but by no means the only one) is the Klein RT110 Jan 4, 2019 at 4:49

Manassehkatz covers the basics of this kind of tester. You can never tell when electronics intervene, but it looks like this one is at least trying to be faithful to the design of the old neon testers. I.E. The left light lights if Hot-Neutral voltage is present, center if Hot-Ground and right if Neutral-Ground. (Which is not the traditional sequence, normally H-N is in the center and N-G is a different color).

It still murders one of the useful indications from an old style neon tester: that of two lights coming on at half brightness. That indicates the two neon lamps are in series, typically because ground is floating and the two H-G and N-G lamps are thus in series. On the other hand, I would expect a neon light to go POP if you put 240v across it, as you have.

I gather you selected this one for its ability to tolerate 240V across its terminals.

Anyway, you have substituted neutral for another hot, which is indeed 240V from the first hot and 120V from ground. So it seems correct that all 3 lights should light. You are using the unit off-label (a nice way of saying you are not obeying the instructions and labeling) so the label does not apply.

Now let's talk about

How is it even possible to plug that tester in?

I certainly hope you have not wired 240V to a NEMA 5-15 receptacle, for anything other than momentarily testing with this tester.

A lot of these people surf this site years after the question is answered, so bear with me, I kinda "have to do the speech"... From time to time we get people who, just out of the blue, use NEMA 5 because in their logic, "that's what sockets are" and they imagine in their mind's eye that all sockets are the same and people just magically know not to plug 120V appliances into 240V sockets. The issue has never come up for them.

Here is what a 240V socket looks like.

enter image description here for 15 amps, or 20A is enter image description here

You see, it's not a big deal, it's just a different socket, and you use a different plug with it.

  • I thought about How is it even possible to plug that tester in? too. My guess was touching the tester to 3 bare wires, but anything is possible. Jan 4, 2019 at 18:39

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