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I’m in the middle of replacing a bunch of fluorescent Tubes with LED And bypassing the ballast. Everything was going great, but in one room with the power off (at the switch, not the breaker), after I’ve made all the connections, I started putting the tubes in, and they were turnings on (full brightness) with one end in the Keystone, and the other end touching the metal box. The switch is still in the off position.

In another box, after connecting the black and red wires, I was pigtailing the blue wires to white, and had some arcing as well as all the lights (including the fluorescents I hadn’t touched yet) came on. I realize that this fixture is “downstream from the two I had already direct connected.

The switch has a tiny light inside that glows when the switch is turned off. Is this the problem? Or is there something worse/more dangerous?

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    This is why you always turn off the breaker and never trust the switch alone. If the switch is in a switch loop, there is always power in the ceiling box with the switch off or on.
    – crip659
    Commented May 7 at 12:32
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    Not turning off the breaker is a great way to end up at the hospital or the morgue...
    – Ecnerwal
    Commented May 7 at 12:34
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    Clearly the switch is not turning (all) the power off. One explanation, if a commercial building, is that some of the fixtures are not switched for safety.
    – Jon Custer
    Commented May 7 at 13:19
  • Thank you all for your feedback. Laying personal safety aside for a second. (I’ve learned my lesson.) In the first instance listed above, I was installing the tube. Do you turn off the breaker just to change tubes? Have you ever known T8 tubes to turn on when one end is in the keystone and the other is touching the metal housing? This is my biggest concern. And it doesn’t matter which keystone I use (positive or neutral).
    – Jason M.
    Commented May 7 at 21:52

1 Answer 1

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Turn Off the Breaker

Any time you are doing any electrical work on a circuit beyond simple "replace light bulb", you MUST turn off the breaker. After turning off the breaker, verify that power is off, ideally with a Non-Contact Voltage Tester (NCVT), such as this Klein from Amazon:

NCVT

Testing is important because sometimes you have either multiple circuits connected to one appliance or light (they should always be handle-tied if that is the case, but sometimes things are not done correctly) or multiple circuits running through a box - e.g., the junction box above a light might have wires for a totally unrelated circuit running through it, so you want to make sure all power is off in any box before you start pulling wires apart.

Reverse Hot/Neutral

In the US, all 120V circuits are supposed to be switched on the hot wire. But sometimes things are done incorrectly and the neutral is switched instead. In that situation, you may find live wires where you don't expect them when the switch is off.

240V Circuit

While most residential lighting is on 120V (hot/neutral, hot switched) circuits, commercial lighting is often on 240V circuits and a lot of lights can be powered by 120V or 240V. If a light is on a 240V circuit and only one hot wire is switched then the other hot wire will still be live when the switch is off. I don't know if code requires both hot wires to be switched, but even if it does, things are not always done properly so turning off the breaker is the safe thing to do.

Lighted Switch

A lighted switch requires power for the light. There are a few ways to do that:

  • Hot/Neutral - This is the modern way to do things. The catch is that many older homes have switch loops with no neutral in the switch box, so many older lighted switches would assume that neutral is not available.

  • Hot/Ground - This is marginal. Under certain circumstances, returning a small amount of current through ground to power an indicator light or other things is allowed. But that is very limited and since older homes often do not have ground at switch boxes, many older lighted switches would assume that ground is not available.

  • Hot/Switched Hot - This used to be extremely common. The lighted switch (or timer or other device that needs a little bit of power for itself) "leaks" a little bit of power through the switched hot wire to complete its own circuit. This is not enough to light up an incandescent or a typical fluorescent light. But it is enough to:

    • Cause LEDs to light up (dim, but noticeable) or blink
    • Present dangerous power at wires in the light fixture
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    While contactless detectors are nice, a suitable voltmeter is the gold standard for zero energy verification.
    – Jon Custer
    Commented May 7 at 16:09
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    NFPA 70E 120.5(7) is quite clear that establishing and verifying an Electrically Safe Work Condition requires testing of each phase conductor both phase-to-phase and phase-to-ground. I like my NCVT, but it is not a zero energy verification test instrument.
    – Jon Custer
    Commented May 7 at 16:22
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    Indeed, that is why I like my NCVT (just like the one in your picture). It can provide useful information (like some other circuit running through the box) before you start physically interacting with anything. But I'm not touching anything until I measure directly that it is dead.
    – Jon Custer
    Commented May 7 at 16:32
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    One issue with NCVT is that they give too many false positives. They provide no indication if a line is energized, or just floating and picking up a voltage through capacitance to a parallel wire. There are many questions at the DIY site about users not understanding how a high-impedance device works.
    – DrMoishe Pippik
    Commented May 7 at 16:52
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    An advantage of the NCVT is that it doesn't rely on the integrity of the ground (or neutral) to indicate voltage.
    – MadMonty
    Commented May 10 at 3:18

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