I just moved into a new apartment and I would like to install simple ceiling light. As shown in the picture, there are two wires outside the ceiling, a blue one and a red one.

The light switch button is on the wall. When I install light in other rooms, if I switch off by pressing the power button on the wall, the electricity tester does not beep at all. Only when I switch on the power, the electricity tester makes an audible beep. But for the wires in this room, no matter the light switch is on or not, the my test makes a beep all the time, as shown in the picture.

Now I have the following questions:

  1. What could be the potential problems? Some wires are connected improperly?

  2. When I install a bulb to the light fixture (the black thing in the picture), the bulb doesn't light up? Why? As I said, there is always voltage detected, but why doesn't the bulb light up? I've checked the bulb is in good condition.

enter image description here

  • Does the bulb never light up, even when the switch is on? Is it a known good bulb?
    – Tester101
    Dec 9, 2015 at 15:09

4 Answers 4


Switched Neutral

It could be a symptom of a "switched neutral", where the switch interrupts the grounded (neutral) conductor instead of the ungrounded (hot) conductor. In this case the bulb would turn on and off with the switch, but the socket would always show as hot.

To test:

WARNING: This procedure requires working on energized circuits. If you don't feel comfortable, or lack the knowledge or tools to do so, please contact a local licensed Electrician.

  1. Use a voltmeter/multimeter to measure AC line voltage.
  2. Take a measurement between the contact at the bottom of the socket, and a known good ground (possibly grounding conductors in the box).
  3. Take a measurement between the threaded contact within the socket, and a known good ground.
  4. Flip the switch, and repeat steps 2 and 3.

With the switch OFF, both measurements should be 0 VAC. If you're measuring line voltage at the socket when the switch is off, it's likely you have a switched neutral situation.

With the switch ON, the measurement from step 2 should be line voltage. If you get 0 volts in step 2, and line voltage in step 3. That means the socket leads are reversed.

Phantom Voltage

It could be the meter is picking up phantom (induced) voltage. This type of voltage could be picked up by a non-contact voltage tested, or a high impedance voltmeter (common for digital multimeters). The voltage could be induced by other electrical lines, which are in close proximity to the switched conductor.

To test:

WARNING: This procedure requires working on energized circuits. If you don't feel comfortable, or lack the knowledge or tools to do so, please contact a local licensed Electrician.

  1. Use a low impedance volt/multimeter to measure the voltage across the socket.

If there's phantom voltage, the low impedance meter will not pick it up.


I had a similar issue with a switched light. Whether on or off the socket showed voltage but would not illuminate a light bulb. Turned out to a disconnected neutral (white) wire in a basement junction box.


Sounds like the hot and neutral (typically black and white, but it seems that standard romex wasn't used for your wiring) wires are swapped. Continuity test to confirm. Also, is your switch a 2 or 3 way switch?


  • 2
    On this site, whenever wire colors don't match your expectation, its pretty safe to assume they are in a country other than the USA and don't have the same color conventions. We get questions from many different countries.
    – JPhi1618
    Dec 9, 2015 at 17:06

If the socket is controlled by 3 way switches this can happen when a home owner wires in the switches and gets the hot and neutral reversed. (2 switches) to have power but no return. If that is the case one of the switches is wired wrong. The second possibility the neutral line is open so you have power to the fixture but no return path. this is where I would look sense you said you checked the bulb. A third idea did you check the bulb in the fixture it is possible the center contact is not connecting to the bulb. The last thing if the switch is illuminated it could be the power for the neon lamp in the switch causing the tester to light and the switch is bad

  • A 3-way switch would not energize the socket, except when both switches were toggled properly. This would not be normal with 3-way switches.
    – Tester101
    Dec 9, 2015 at 16:44
  • there are 2 single pole double throw switches the line can have power but no return and this is a common issue as a professional electrician I have seen this many times
    – Ed Beal
    Dec 9, 2015 at 19:54
  • An open neutral would never be described as "normal".
    – Tester101
    Dec 9, 2015 at 22:25
  • I am not saying that is normal but some folks dont know how to wire 3 ways and get the hot and neutral swapped so there is always power on the fixture, I have run into this at least 20 maybe 30 times. it works but have gotten the tingle because I did not tick test it first
    – Ed Beal
    Dec 9, 2015 at 22:30
  • 1
    "If the socket is controlled by 3 way switches this is normal". You did say it was normal. You're talking about a switched neutral, and it can happen with single pole switches as well. It's a wiring error, and far from " normal".
    – Tester101
    Dec 9, 2015 at 22:33

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