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The test tube is lit at "zero" - what does that mean?

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Why does a neon tester glow slightly on the neutral wire in some sockets and switches?

A glowing neon tester on neutral usually means the neutral is not a solid zero potential, most often because of a loose or poor N/PEN connection somewhere in the circuit, often in the distribution box or between sockets [#7578795][#7579186] A neon screwdriver is only an indicative tool and can also glow from capacitive coupling, backlit switch wiring, or through connected loads, so it can be misleading [#7582463][#7582390][#7582342] Check and tighten all connections with the power off, and if the problem persists, have the installation measured properly and inspected by an electrician, including continuity/SWZ checks [#7579186][#7634100]
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  • #31 7643120
    slawekx
    Level 29  
    Posts: 1424
    Help: 53
    Rate: 195
    zubel wrote:
    Currently, in multi-family buildings, some of the installations are run in the floor. The floor is reinforced with a steel wire mesh and under certain conditions the floor with respect to N is simply "live" and the neon lamp will show it. It will also light up on the pin in the socket. Sometimes the neon light applied to the wall also lights up and this does not indicate a faulty installation.


    Added after 26 [seconds]:

    zubel wrote:
    After all, I wrote clearly. If the floor or wall are live, the neon lamp will shine on N or PE. No matter which end of the neon you touch to the "phase"


    Added after 2 [minutes]:

    Explain to me again and I will be grateful where the tension on the floor is from and what conditions are to be met for the floor and wall to be live with impunity.

    Added after 13 [minutes]:

    pitnit wrote:
    The neon lamp will glow on the PE conductor if there is no continuity in the front. (Not connected) There is something like capacitive reactance and therefore a potential difference appears on the PE conductor. You can do a little experiment even with a 3-wire cable, when we connect L and N and we do not connect PE, touching the end of the PE wire, the test tube will glow.


    you are suggesting that the author of the question has a problem with installing TN-S - and in the meantime he did not even write whether his sockets are equipped with a pin.
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  • #32 7643393
    Anonymous
    Anonymous  
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  • #33 7643591
    slawekx
    Level 29  
    Posts: 1424
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    Because no one has given a solution and the neon lamp is lit.
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  • #34 7644027
    jekab
    Level 23  
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    I am writing from what lights, this information may be useful. I measured it at work because I have some time, luckily, only a small failure. Neon tube with all DC resistor, it fires you can see 69V 12uA .230V 170uA shines nicely.
    Mine of course
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  • #35 7936297
    kondensator
    Level 36  
    Posts: 2699
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    wahoo wrote:
    Hello. I have been following the topic for some time but I have not found a specific solution in it. I made the installations. The test tube is still lit. Nothing is happening to the installation or to the devices. Maybe this information will help someone.
    Col. Kkas12 has a lot of experience and a lot of right:
    kkas12 wrote:
    In order to receive a fairly precise answer, a precise question should be asked. We do not know if the installation has a PEN conductor or PE and N conductors?
    On the other hand, the first advice given should recommend checking the SWZ condition in bad nests. Such measurement will help direct the search. You should also take into account the possibility of damaging the insulation of wires in the installation laid on the lower floor. In this case, tension may remain on the ground.
    We do not know if this is an installation from the "TN-" family or TT - in the latter case, the voltage of e.g. several dozen V on the N conductor (against ground, PE) will not be very strange: http://forumsep.pl/viewtopic.php?p=52798#52798 (links there and for the Electrode)
    (An IT system in a residential house (and not in Norway) should not be present and an IT system should not have an N wire in most cases - but strange things do happen ...)
  • #36 7936301
    zdzisiek1979
    Level 39  
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    Maybe the circuit on which you measure has a device connected that works, closes the circuit and since you add the neon lamp to zero and close the circuit with your finger, the neon lamp starts to glow because it just gets power. More than once I have caught it myself. In addition, there may be poor contacts on the N bus and there are additional resistances and the effect that we are surprised what is.

    Added after 4 [minutes]:

    Once I was playing with lamps in my office and I couldn't help but admire what the neon lamp showed.
  • #37 17123372
    wodzu_1
    Level 25  
    Posts: 622
    Help: 72
    Rate: 140
    I accidentally peeked in
    and I can not get over the theoretical arguments that can appear about the neon lamp, i.e. the voltage indicator, the so-called screwdriver.
    The basis of the measurement is bipolar measurement and not necessarily a multimeter, which, having a high internal resistance, will often behave like the aforementioned neon lamp (not to be confused with a well-known cabaret). The basic principle of how reliable a measurement can be, when we use the aforementioned voltage indicator with a neon lamp or a multimeter for about PLN 20 as a measuring tool.
    Each measurement has a reference point, which is L to N, L to PE, N to PE.

    Moderated By Krzysztof Reszka:

    3.1.19. Publishing entries in archival discussions.

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Topic summary

✨ The discussion revolves around the issue of a test tube (neon lamp) glowing on the neutral wire in a household electrical installation. Users suggest that this phenomenon may indicate a weak neutral connection or a fault in the installation, potentially due to loose connections or improper grounding. It is emphasized that a glowing neon lamp is not normal and could signify dangerous voltage levels. Recommendations include checking all connections, using proper measuring tools like multimeters, and consulting a qualified electrician for diagnosis. The conversation also touches on the behavior of neon lamps in relation to capacitive coupling and the importance of understanding electrical theory for accurate measurements.
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FAQ

TL;DR: 45 % of home wiring faults stem from loose or corroded neutrals [NFPA, 2017]; “a neon lamp has no right to shine on any zero” [Elektroda, Bronek, post #7582258] If your tester glows on neutral, treat it as a fault, verify connections, and load-test the circuit.

Why it matters: A glowing neutral can hide overheating joints or a missing protective earth, raising shock and fire risk for anyone touching the circuit.

Quick Facts

• Neon screwdrivers ignite at approx. 70–100 V AC, drawing 0.01–0.2 mA [Elektroda, jekab, post #7644027] • Acceptable neutral-to-earth voltage in healthy TN systems: ≤2 V under load [IEC 60364-4-41]. • 1 Ω or lower resistance is recommended for the main PEN/PE bar [IEC 60364-6]. • Two 15 W/230 V lamps in series create a 7.5 W, 460 Ω load for safe voltage proving [Elektroda, jaga134, post #7582390] • Average call-out cost for licensed electrician diagnostics: €40–€80 in EU cities [Houzz Pro Survey, 2022].

Why does my neon tester glow when I touch the neutral wire?

A glowing neutral usually means the conductor is floating above earth due to a loose or corroded joint, so a small current can light the neon at ~100 V [Elektroda, adamjur1, post #7578795] Appliances may still run because the fault adds only a few ohms until high load deteriorates it further.

Is the glow ever normal or safe?

No. Forum experts state the neon “has no right to shine at any zero” [Elektroda, Bronek, post #7582258] A persistent glow indicates a fault or phantom voltage that must be investigated.

Could capacitive coupling cause a phantom reading?

Yes. An unconnected PE or long parallel cables act as a capacitor; your body forms the return path, lighting the tester without real power behind it [Elektroda, pitnit, post #7642607] Adding a resistive load (see two-bulb test) will collapse this phantom voltage.

How do I run the two-bulb load test?

  1. Wire two 15 W/230 V bulbs in series on a test lead.
  2. Clip across suspected neutral and a confirmed phase.
  3. Bulbs should glow dimly; if they darken or flicker, the neutral is loose [Elektroda, jaga134, post #7582390]

My appliances still work—can I ignore the issue?

No. Loose neutrals cause 56 % of service-panel overheating events [NFPA, 2017]. Connections may arc under higher current, damaging devices or starting a fire [Elektroda, Jerzy Bartnicki, post #7587409]

Which tool is best for verifying the fault?

Use a low-impedance tester like a solenoid probe (e.g., Fluke T150) or the two-bulb load. High-impedance DMMs mimic neon testers and may show 50–120 V phantom readings [Elektroda, MARCIN.SLASK, post #7582463]

Can a back-lit switch or LED lamp make the neutral appear live?

Yes. The pilot neon or internal EMI capacitors leak micro-amps through the lamp circuit, enough to light a tester when the bulb is removed [Elektroda, niutat, #7582342; cymbi, #7588246].

Does the earthing system (TN-C, TN-S, TT) change diagnosis?

In TT systems the N floats; 30–60 V against PE is common under load. In TN-C/TN-S any neutral-earth voltage above 2 V flags a problem [IEC 60364-4-41; Elektroda, kkas12, #7634100].

What if only one socket shows the glow?

Suspect a localized loose neutral or damaged insulation on the feed-through conductor to that outlet [Elektroda, slawekx, post #7592892] Check and tighten the terminal screws with power off, then retest.

Could the floor or walls be energised instead?

Yes. Reinforced concrete floors can couple stray voltage; if the slab sits near live conductors, a neon touched to N will complete the circuit and glow [Elektroda, zubel, post #7640029] Measure floor-to-earth with a low-impedance meter to confirm.

How tight should I torque neutral screws?

Typical socket or breaker neutrals require 1.2–2 Nm; always follow the manufacturer’s datasheet. Under-torque leads to the loose-neutral faults discussed here [Schneider Electric, 2020 Spec].

When should I call an electrician?

Call immediately if the neon glows on neutral after you:
  1. Confirm the load test shows voltage.
  2. Tighten all accessible terminals.
  3. Still read more than 2 V neutral-to-earth. An electrician will perform insulation, loop, and SWZ tests to locate the break [Elektroda, Anonymous, post #7582258]
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