# Electrical Question - Voltage Even when Switch is Off **URL:** \ **Category:** Factual Questions\ **Created:** [September 28, 2008, 1:31am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521 "2008-09-28T01:31:39Z")\ **Posts on this page:** 19\ **Page:** 2
**Author:** ![jkleohart22193](https://avatars.discourse-cdn.com/v4/letter/j/5f9b8f/32.png) [@jkleohart22193](https://boards.straightdope.com/u/jkleohart22193)\ **Post date:** [May 24, 2014, 2:14am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/21 "2014-05-24T02:14:01Z")
So what was the answer? I have a similar issue where a switched outlet reads 16 VAC between neutral and hot when switched off and 118VAC when switched on. The outlet heats up too. I replaced the outlet. Something crazy us going on. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [May 29, 2014, 4:13am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/22 "2014-05-29T04:13:59Z")
Don’t know about this particular problem, but it’s likely an open or loose neutral in the circuit. It could also be that there is not a proper ground for the system in conjunction with an unbalanced load. I’d guess the former, but have seen the latter. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [May 29, 2014, 12:19pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/23 "2014-05-29T12:19:13Z")
**Una Persson** has had 6 years to work on it. Hope there is a solution by now. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [May 30, 2014, 2:52am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/24 "2014-05-30T02:52:56Z")
No, I ended up just replacing everything. ---
**Author:** ![NB4U](https://avatars.discourse-cdn.com/v4/letter/n/258eb7/32.png) [@NB4U](https://boards.straightdope.com/u/NB4U)\ **Post date:** [December 23, 2015, 7:40pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/25 "2015-12-23T19:40:40Z")
ok, here is what I have, same as above, 42 Volts when off measured with a DVM, all switched outlets in the hose read the same when off. House is 8 years old, romex wiring. I noticed this when I plugged in my LED Xmas lights and when I turned off the switch the LED’s continued to glow slightly… When I plug in a circuit tester with the three indicator lights on it, and turn on the switch, the circuit analyzer indicates correct wiring. [http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg](http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg) ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [December 23, 2015, 10:36pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/26 "2015-12-23T22:36:21Z")
> [@NB4U](#): > > ok, here is what I have, same as above, 42 Volts when off measured with a DVM, all switched outlets in the hose read the same when off. House is 8 years old, romex wiring. > > I noticed this when I plugged in my LED Xmas lights and when I turned off the switch the LED’s continued to glow slightly… > > When I plug in a circuit tester with the three indicator lights on it, and turn on the switch, the circuit analyzer indicates correct wiring. > > [http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg](http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg) Sounds like the place is ungrounded or inadequately grounded, to me. ---
**Author:** ![boytyperanma](https://avatars.discourse-cdn.com/v4/letter/b/4491bb/32.png) [@boytyperanma](https://boards.straightdope.com/u/boytyperanma)\ **Post date:** [December 23, 2015, 11:24pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/27 "2015-12-23T23:24:17Z")
NM ---
**Author:** ![eulalia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eulalia/32/457_2.png) [@eulalia](https://boards.straightdope.com/u/eulalia)\ **Post date:** [December 24, 2015, 4:45am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/28 "2015-12-24T04:45:21Z")
> [@Chefguy](#): > > Sounds like the place is ungrounded or inadequately grounded, to me. Yes. Neutral should be bonded to ground at some point; it probably isn’t. If you switch your meter to ohms and measure from the neutral side of the plug to earth ground it should be near zero ohms. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [December 24, 2015, 4:03pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/29 "2015-12-24T16:03:25Z")
> [@eulalia](#): > > Yes. Neutral should be bonded to ground at some point; it probably isn’t. If you switch your meter to ohms and measure from the neutral side of the plug to earth ground it should be near zero ohms. I’d suggest killing the power first, to avoid frying the meter. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [December 24, 2015, 11:23pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/30 "2015-12-24T23:23:44Z")
Or check neutral to ground for voltage first. Then Ohms. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [December 25, 2015, 4:10pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/31 "2015-12-25T16:10:24Z")
> [@NB4U](#): > > ok, here is what I have, same as above, 42 Volts when off measured with a DVM, all switched outlets in the hose read the same when off. House is 8 years old, romex wiring. > > I noticed this when I plugged in my LED Xmas lights and when I turned off the switch the LED’s continued to glow slightly… > > When I plug in a circuit tester with the three indicator lights on it, and turn on the switch, the circuit analyzer indicates correct wiring. > > [http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg](http://ecx.images-amazon.com/images/I/41k0DfHOl6L.jpg) You need to take a closer look at the switch. - Is it an illuminated switch (i.e. does the switch have a built-in light that illuminates when off)? - Is it a dimmer switch? - Is it a three-way switch? - How old is the switch? - Does it switch anything else other than the receptacle? - Is it switching the hot or neutral side of the receptacle? ---
**Author:** ![Me\_Billy](https://avatars.discourse-cdn.com/v4/letter/m/e480ec/32.png) [@Me\_Billy](https://boards.straightdope.com/u/Me_Billy)\ **Post date:** [December 25, 2015, 5:10pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/32 "2015-12-25T17:10:58Z")
Phantom voltage when using a digital multimeter… [http://support.fluke.com/find-sales/Download/Asset/2105317\_A\_w.pdf](http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf) Analog meters do not do this… [http://www.amazon.com/s/ref=nb\_sb\_noss?url=search-alias%3Dtools&field-keywords=Analog+Multimeter](http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dtools&field-keywords=Analog+Multimeter) ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [December 25, 2015, 5:48pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/33 "2015-12-25T17:48:30Z")
> [@Me\_Billy](#): > > Phantom voltage when using a digital multimeter… > [http://support.fluke.com/find-sales/Download/Asset/2105317\_A\_w.pdf](http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf) > > Analog meters do not do this… > [Amazon.com : Analog Multimeter](http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dtools&field-keywords=Analog+Multimeter) (Pssttt - post #11 from 2008, though it’s probably capacitance, not inductance.) ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [December 25, 2015, 7:45pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/34 "2015-12-25T19:45:12Z")
> [@Me\_Billy](#): > > Phantom voltage when using a digital multimeter… > [http://support.fluke.com/find-sales/Download/Asset/2105317\_A\_w.pdf](http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf) > > Analog meters do not do this… > [Amazon.com : Analog Multimeter](http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dtools&field-keywords=Analog+Multimeter) A “phantom voltage” is very real. What it _really_ means is that the voltage source is modeled as an ideal AC voltage source (120 VAC[sub]RMS[/sub] in the U.S.) in series with a high impedance, _and_ you’re measuring the voltage with a meter that has high input impedance. To explain further, let’s say you wire a switch in series with a receptacle. When the switch is on, there will be 120 VAC[sub]RMS[/sub] at the receptacle. But what is the voltage at the receptacle when the switch is off? You might think it’s 0 V. But in reality the voltage at the outlet (with the switch off) must be modeled as an ideal AC voltage source in series with a high but non-infinite impedance. Where does this impedance come from? Some of it comes from the switch; when the switch is off, there is still a finite resistance and non-zero capacitance between the terminals. Some of the impedance may also come from capacitive coupling in the wiring. At any rate, when you measure the voltage at the receptacle with the switch off, your meter is really measuring the voltage of a [voltage divider](https://en.wikipedia.org/wiki/Voltage_divider). Using [this](https://en.wikipedia.org/wiki/Voltage_divider#/media/File:Impedance_voltage_divider.svg) diagram, V[sub]in[/sub] = 120 VAC V[sub]out[/sub] = voltage reported by your meter _Z_[sub]1[/sub] = impedance of the switch when it is off _Z_[sub]2[/sub] = input impedance of your meter when measuring AC Let’s say the switch has an impedance of 100 MΩ when it’s off. If you use an ideal voltmeter, i.e. one that has infinite input impedance, then the meter will report 120 VAC at the receptacle. If you use an old Simpson 260 meter, it will have an input impedance of around 1.25 MΩ, and the meter will report 1.5 VAC. If you use a modern DVM (e.g. Fluke 87), it will have an input impedance of around 10 MΩ, and the meter will report 11 VAC. (Note that I would expect a good switch to have a much higher resistance than 100 MΩ when it’s off.) Now if you measure 42 VAC at the receptacle with the switch off using a Fluke 87 DVM, this implied the resistance of the switch is 18.6 MΩ. Why would the switch have a resistance of 18.6 MΩ when it’s off? A few possibilities come to mind: - The inside surfaces of the switch are contaminated with something semi-conductive (carbon?). - It’s a illuminated switch. - The switch has an RC snubber circuit across the contacts to help reduce arcing. - Outside surface of switch is very dirty. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [December 26, 2015, 1:21pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/35 "2015-12-26T13:21:08Z")
> [@Sparky812](#): > > Can I suggest that anyone with older homes (or even new) pick one of these up next time you are at the hardware store! I’ve used to countless times at work and home and it may help you here. > > [http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100062242](http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100062242) > > I also alway have one of these in my pocket : > > [http://global.flukebiomedical.com/usen/products/Fluke+VoltAlert.htm?catalog\_name=FlukeUnitedStates&category=ELT(FlukeProducts)](http://global.flukebiomedical.com/usen/products/Fluke+VoltAlert.htm?catalog_name=FlukeUnitedStates&category=ELT(FlukeProducts)) I just saw the date-a zombie thread. Oops. ---
**Author:** ![engineer\_some](https://avatars.discourse-cdn.com/v4/letter/e/b5e925/32.png) [@engineer\_some](https://boards.straightdope.com/u/engineer_some)\ **Post date:** [March 6, 2016, 12:22am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/36 "2016-03-06T00:22:20Z")
I found that I had about 40 +/-Volts between neutral and hot when I had the switch turned off. I checked everything and finally resorted to looking on the internet and found these posts. I read several posts and was surprise to see that someone suggested that using a digital volt meter may give an incorrect reading showing a voltage where none existed. So, I tried two other volt meters, and the other two meters that I used showed no voltage where the first one I used showed 40 volts!!! Boy was I surprised. My 30 year old analog meter showed no voltage and my free harbor freight one also showed no voltage, while my $40 home depot digital one was the one that showed 40 Volts. So, be cautious when using a newer digital volt meter. ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [March 6, 2016, 12:47am UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/37 "2016-03-06T00:47:42Z")
> [@engineer\_some](#): > > I found that I had about 40 +/-Volts between neutral and hot when I had the switch turned off. I checked everything and finally resorted to looking on the internet and found these posts. I read several posts and was surprise to see that someone suggested that using a digital volt meter may give an incorrect reading showing a voltage where none existed. So, I tried two other volt meters, and the other two meters that I used showed no voltage where the first one I used showed 40 volts!!! Boy was I surprised. My 30 year old analog meter showed no voltage and my free harbor freight one also showed no voltage, while my $40 home depot digital one was the one that showed 40 Volts. So, be cautious when using a newer digital volt meter. Or be careful when using a cheap DMM. Also, make sure it isn’t swinging between V and mV. 40 millivolts is nothing but when you’re trying to check voltages and you don’t realize that the units changed on their own from v to mv because it’s dark and the letters are so tiny, it can make things difficult. I usually lock mine to V. I had an old one that would do this and I threw it out because of that, it made it more or less useless. It was like trying to get a reading from an analog meter with someone shaking it for the first few seconds. At that point, it’s easier to just use my cheap neon tester when I just need to know if something is hot or not. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 6, 2016, 2:03pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/38 "2016-03-06T14:03:54Z")
> [@engineer\_some](#): > > I found that I had about 40 +/-Volts between neutral and hot when I had the switch turned off. I checked everything and finally resorted to looking on the internet and found these posts. I read several posts and was surprise to see that someone suggested that using a digital volt meter may give an incorrect reading showing a voltage where none existed. So, I tried two other volt meters, and the other two meters that I used showed no voltage where the first one I used showed 40 volts!!! Boy was I surprised. My 30 year old analog meter showed no voltage and my free harbor freight one also showed no voltage, while my $40 home depot digital one was the one that showed 40 Volts. So, be cautious when using a newer digital volt meter. Each meter is reporting the correct voltage from the meter’s perspective. (See post #34 for more info.) Though the reading from the $40 meter is more useful. If the $40 meter has an input impedance of 10 MΩ, then it means the switch has an impedance of 20 MΩ when it is “off”. When the switch is off, the max current is 6 mA, and this occurs when the hot and neutral are shorted together. If the hot and neutral are connected to anything other than a short circuit, the current will be less than 6 mA. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [March 6, 2016, 3:03pm UTC](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521/39 "2016-03-06T15:03:53Z")
> [@Una\_Persson](#): > > No, I ended up just replacing everything. Yup … sounds like you were a bit overdue for this. [Previous page](https://boards.straightdope.com/t/electrical-question-voltage-even-when-switch-is-off/465521.md?page=1)