Just came over here from Fine Woodworking. I was about to add two more eight foot lights to my shop and opened up a swithced light box that was at the end of a run. That’s ok cause I want the new lights to be on the switch that controls the light I’m tapping into. Since I just can’t figure out which circuit it is on I used my cheap little circuit tester to be sure the switch is turned off before I connect the new wires. The cheap tester shows a bright light when touching the black and the white screws. With the switch in the off position it shows a faint light, like the one that is supposed to show a poor ground when used for that purpose. My more expensive circuit tester only shows the bright light with the switch on, and no light at all with the switch off. Does this mean anything?
OK I lied when I said this was a quick question. But it should be a quick easy answer for anyone who knows even a little bit more than I do about electricity. Thanks.
Replies
There is something called "phantom voltage" (and a few other terms) that you encounter when metering power circuits with a "high impedance" device. Basically, when a wire is not connected to anything on either end, and it runs close to a wire that is "hot", it picks up some of the voltage of the other wire via capacitance. The amount of current that it can sustain is minimal -- far below the amount needed to shock you (but of course you kinda want to be sure before you touch it).
Your bigger problem is likely to be that there's about a fair chance that that switch is on a "switch leg", and "tapping" it at the switch would be impossible.
Thanks for the reply. I'm not tapping in at the switch, I'm tapping into the ceiling light that is at the end of the run, This light is controlled by two three way switches at the top and bottom of the basement stairs to my shop, with unfinished ceiling and walls. The box has only one white and one black and will be a snap to tap. I currently have ten eight foot fluorescent fixtures with twin tubes. They have been great but two more fixtures will kill some shadows and help the 71 year old pair of eyes I'm currently using. I know I used the word currently twice... us electric guys like to stick in terms like that every time we can. Thanks again.
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Since you are currently concerned you should kill the circuit at the breaker panel. Turn on the lights, go to the panel, flip off a breaker at a time then back on until you find the one that controls the lights (you will be concurrently watching the lights as you flip the breaker). Label it. Leave it off. Do the work. Turn the breaker back on. (You labeled it right?) Done, and you didn't get a current overload into your currently 71 year old body.
Exactly.
That is just what I did when I started. Then, as I flipped the second breaker, a great voice from above yelled"You just turned off my computer!" I labeled that one "don’t touch, circuit for the computer of She Who Must Be Obeyed" Then I tried all the lighting circuits and everything that wasn't labeled dishwasher or disposal or hot tub etc. The light never went out. Getting tired of that I decided to just turn off the switch and be careful. There are only two wires and a ground to contend with as the light box is at the end of a run. But when I double checked with a simple 2 lead bulb tester I was getting a faint light with the switch turned off and was curious about it, that is all. There is nothing more shocking than SWMBO yelling down two flights of stairs. Thanks to all for the advice.