small electric indication when switch is off

[Bob Glenn]

I was changing a light fixture and wanted to make sure the power to the light was off before removing the fixture. I checked it with one of the small pocket probes with a little light. It showed bright with the switch on, however showed dimly light with the switch off.

Any thoughts on whats going on here?

[ray hampton]

are the switch a three-way switch by any change

[Tom Cornish]

I was changing a light fixture and wanted to make sure the power to the light was off before removing the fixture. I checked it with one of the small pocket probes with a little light. It showed bright with the switch on, however showed dimly light with the switch off.

Any thoughts on whats going on here?

Is this a dimmer switch?

If not, probably induced voltage from other nearby wires.

[Doug Palmer]

I agree with Tom that it sounds like induced voltage. Induced voltages are of no danger.

[Arvid Podsim]

I suggest that you measure between each of the wires connected to the light fixture and a grounding conductor (not neutral) using a direct contact multi-meter. Induced voltage bellow 300 volts is very unlikely. The fact that you pocket probe was indicating the presence of voltage is an indication that you many not have a good connection between the neutral white wire and ground back to the distribution panel. Take care as YOU do not want to be the current path between Neutral and Ground. Be aware that CURRENT can stop your hearth a very low levels.

[Angie Orfanedes]

I seem to recall that the "silent" light switches actually leak a little voltage (probably doesn't make sense electrically). You really should kill power at the circuit breaker. If some jackleg switched the white wire on a light switch (instead of black), you can get a large charge out of an "off" light switch.

[Tom Cornish]

Induced voltage bellow 300 volts is very unlikely.

The percentage of voltage induced in a nearby wire depends on the length in common and proximity of the wires in question. The source voltage affects things in an absolute sense - you'll get a higher induced voltage if you're next to a 5,000 volt wire than if you're next to a 120v wire when all else is equal, but there's no magic about 300 volts. In my home I have personally observed induced voltages in shared electrical boxes of over 40 volts, which is definitely enough to light up some non-contact testers.


Edit: This is a bit academic, but if you're really concerned about this, you can get a "wiggy" - an electromechanical volt meter that has a lower impedance than the usual digital volt meter. This draws enough current to hold down the floating voltages and actually tell you what's going on.

Several companies make them - Ideal is one:
http://www.idealindustries.com/produ...age_tester.jsp

[Bob Glenn]

Thanks for the info. I think it probably is induced voltage, since every thing else checks out okay. I measured it at less than 10 volts. As I recall, I've seen this before in another house.

[Bob Glenn]

Just out of curiosity, would the induced voltage be DC or AC?

[Ken Fitzgerald]

Bob....it would be AC. Induced voltage is a result of induction or transformer action between two wires. Two wires running parallel to each other with AC voltage passing through them have the potential to couple/induce voltage into each other.

[Joe Angrisani]

I seem to recall that the "silent" light switches actually leak a little voltage (probably doesn't make sense electrically). You really should kill power at the circuit breaker.....

They most certainly do. The kind of switch in this picture still passes enough power to give a shock even in the off position. As the instructions that come with these switches say, you need to pull out the little "kill switch" tab at the bottom of the big switch to shut off all power in the light circuit. I've circle the kill switch in red. Doesn't matter if it's "one-way", three-way, four-way, whatever.

Light Switch.jpg

[Jim Rimmer]

Just out of curiosity, would the induced voltage be DC or AC?

It's the same principal as a transformer. A conductor with a current passing through it will develop a magnetic field and conversely, a magnetic field passing through a conductor will induce a voltage in it. So when the AC current flows through a nearby conductor, the magnetic field passes through a nearby conductor and induces the current in it. It will only work with AC because it needs the movement of the magnetic field caused by the 60 Hz reversal of current. Amount of current induced is dependent on lots of variables; number of turns (conductors), strength of magnetic field, dielectric penetration, etc. Voltage and current are inversely related and that's how transformers work. Reduce the voltage and increase the current; increase the voltage and decrease the current. That's why transmission have very high voltage. Drop it down to 220 or 110 and the current goes way up. This is very simplified but the basics of induced current.

[Steven DeMars]

Use the breaker ! ! !

Use the breaker luke ! ! !

[Tom Willoughby]

Even after you are sure that the power is off, I always make a habit of holding the hot and neutral wires by the insulated part of the wire and touch the bare wires together (making a circuit). If there is still voltage, it should blow the breaker since there is no resistance.

Stay safe.

Tom

[ray hampton]

Even after you are sure that the power is off, I always make a habit of holding the hot and neutral wires by the insulated part of the wire and touch the bare wires together (making a circuit). If there is still voltage, it should blow the breaker since there is no resistance.

Stay safe.

Tom

if there is any current in the wires and you touch them together, DO YOU KNOW WHAT FLASH BURN MEAN ?