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Old 09-20-2020, 11:00 AM   #1
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How to identify 12v positive and negative

Hello Helpful Stream’ers
Can someone please help identify what is positive and negative on the 12v side.
I am wiring a 12v dc Caframo fan in the bedroom of our 2017 international.
I am pulling 12 V constant power from one of the light switches I have a black wire white wire a red wire and two purple wires . I’m also wanting to wire one above the TV in the kitchen area and poor constant power from the smoke detector but want to identify what is positive and what is negative
A second question is while camping last weekend in the rain in the upstate I noticed getting shocked when entering the trailer and grabbing a hold of the handle door (:is there a grounding problem ? I’ve never had that experience before . The ground was wet and I was standing outside in the rain Entering the trailer
Thanks to anyone who can offer help on these two issues since I’m not an electrician
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Old 09-20-2020, 11:07 AM   #2
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Dangerous situation!

This is above my pay grade, but you should immediately unplug the trailer from shore power, and do not plug it back in until a licensed electrician has found the source of you being shocked.

Hopefully more expert opinions will be forthcoming soon.

You might also want to check the following forum and sub-forum for relevant threads:

https://www.airforums.com/forums/f37/
https://www.airforums.com/forums/f447/

FYI -- "electric shock touching trailer" search results: https://www.google.com/search?q=elec...=airforums.com

Plenty of helpful reading in those search results IMO.

Good luck,
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Old 09-20-2020, 12:06 PM   #3
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YouTube search for Mike Sokol no shock zone hot skin presentation.
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Old 09-20-2020, 12:41 PM   #4
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I carry an outlet tester ($5 or so at Lowes) and 30A to 15A adapter. I check the 30A receptacle at the campground by plugging the 30A to 15A adapter at the pedestal and plugging the outlet tester into the adapter. It will identify if there are any wiring problems in the campground.


Once the trailer is plugged in to the pedestal, check your internal outlets. There may be a wiring problem in the trailer. If you find a problem with these tests or if you don't and still get shocked, a qualified electrician may be required to find it.


That said, in a campground with a large distributed electrical system, the ground reference may be some distance away from where your trailer is parked and it is possible for the voltage potential of the ground at the trailer to be different from that at the ground reference (typically at the meter). In that case a shock is possible because the trailer shell is connected by the green wire ground to the ground reference at the meter. The only way to avoid that is to ensure that the trailer is grounded at your site. Putting the jack and stabilizers down without insulating material between them and the ground can help but the best way is to drive a ground rod.
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Old 09-21-2020, 09:16 AM   #5
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With regards to your other question about finding positive, the RV industry makes me crazy. Somewhere along the line some genus decided to follow the home standard (also nuts) of black being “hot” and white being neutral.

So in RV’s black is POSITIVE and white is negative. HOWEVER that’s only true sometimes because other devices will use the vastly more universal red/black pair in which case black is NEGATIVE and red is positive.

Typically if you see a black and white wire together as a pair you can be fairly confident the black is positive and the white is negative in an RV.

If you see red and black wires in a pair the black is negative and the red is positive.

Using a multimeter will tell you for sure.

There is no real standard for what the other colors mean.
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Old 09-21-2020, 10:14 AM   #6
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Dc +-

You should have a DC voltage test meter with a red probe on positive side of meter and black probe on negative side of meter. It will read the DC voltage.
If you have the red probe on the positive wire, meter should read voltage with no + or - on screen. If you have red probe on negative wire, meter will have a minus (-) sign on the screen beside voltage reading.
You may first check with an AC meter to be sure you do not have wires with AC power.
Or if you know you have DC wires and you know the direction motor should turn, you can hook motor to wires and see if it turns in the right direction if not right direction swap wires to right direction.
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Old 09-21-2020, 10:40 AM   #7
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Daleycum is right on the mark.
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Old 09-21-2020, 10:46 AM   #8
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Al and Missy are correct about the possible difference in ground potential. That’s the intrinsic problem with parallel ground paths. But, more importantly you must have voltage bleeding to ground somewhere (trailer shell). Enough to give a shock but not enough to trip a breaker, or blow a fuse?
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Old 09-21-2020, 10:56 AM   #9
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Originally Posted by daleyocum View Post
Typically if you see a black and white wire together as a pair you can be fairly confident the black is positive and the white is negative in an RV.

Using a multimeter will tell you for sure.
If you are on the DC setting for your multimeter and you are reading 60 volts, that means that the line is actually 120 VAC, right? You're measuring only half the voltage. I always check with both settings, AC and DC.

If I was checking for DC only voltages I would disconnect from shore power, just to be safe.
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Old 09-21-2020, 11:04 AM   #10
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YouTube search for Mike Sokol no shock zone hot skin presentation.

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Old 09-21-2020, 11:34 AM   #11
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Hot Skin RV

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Old 09-21-2020, 11:52 AM   #12
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Hot Skin RV
I've seen that one a few times. I have the Progressive Industries surge protector which lets me know if there's an open ground. Is that what the problem is with getting shocked on the AS skin? I prefer using that tool as opposed to the hand held, but I guess the hand held is handy if you're visiting another AS.
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Old 09-21-2020, 11:53 AM   #13
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Originally Posted by mikeoxlong View Post
Al and Missy are correct about the possible difference in ground potential. That’s the intrinsic problem with parallel ground paths. But, more importantly you must have voltage bleeding to ground somewhere (trailer shell). Enough to give a shock but not enough to trip a breaker, or blow a fuse?

Even if the trailer is perfect, it is referenced to ground at the facility entrance panel. Ground potential at the trailer could be different and it could be enough to feel a shock. One cause could be a leak in the park power system. In cases such as these one could disconnect the trailer and measure between the ground pin on the pedestal and earth ground and see a voltage.
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Old 09-21-2020, 12:00 PM   #14
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Originally Posted by daleyocum View Post
With regards to your other question about finding positive, the RV industry makes me crazy. Somewhere along the line some genus decided to follow the home standard (also nuts) of black being “hot” and white being neutral.

So in RV’s black is POSITIVE and white is negative. HOWEVER that’s only true sometimes because other devices will use the vastly more universal red/black pair in which case black is NEGATIVE and red is positive.

Typically if you see a black and white wire together as a pair you can be fairly confident the black is positive and the white is negative in an RV.

If you see red and black wires in a pair the black is negative and the red is positive.

Using a multimeter will tell you for sure.

There is no real standard for what the other colors mean.

IF the black white pair is Romex cable or heavy gauge solid wire (12 or 14) it is probably AC. If it is smaller gauge stranded wire it is probably DC.


A multimeter will not necessarily tell you what it is because they operate differently depending on design. What I have seen is an analog meter (with a needle movement) will typically show 0 volts for AC on the DC scale and will show some level of voltage for DC on the AC scale. Digital meters, all bets are off. They repeatedly sample the voltage and give you a number based on the voltage measured at the time of the sample.


IF you know the characteristics of your meter, then you can tell the difference. The moral of the story is check the voltage of known sources on both scales to learn how your meter works.


Al
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Old 09-21-2020, 12:18 PM   #15
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In all the airstreams I have worked the white is negative (I am referring to the single strand wire, not the jacketed cable, that would have 3 single wires inside)
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Old 09-21-2020, 12:33 PM   #16
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If you have to ask this question .
Then you do not know enough to do any work on any electrical .
You need to find someone qualified in both low voltage & household AC electrical .
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Old 09-21-2020, 02:59 PM   #17
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Quote:
Originally Posted by JohnTF View Post
If you have to ask this question .
Then you do not know enough to do any work on any electrical .
You need to find someone qualified in both low voltage & household AC electrical .
Bingo!

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Old 09-21-2020, 03:29 PM   #18
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If you have to ask this question .
Then you do not know enough to do any work on any electrical .
You need to find someone qualified in both low voltage & household AC electrical .
Well said.
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Old 09-21-2020, 04:42 PM   #19
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pop the cover on your Converter panel. there is a chart that tells you what color wire supplies power to what appliances. The range is brown, orange, purple, blue, etc. All these are positive wires. All white wires you find in the trailer are negative.
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Old 09-21-2020, 11:47 PM   #20
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If you are on the DC setting for your multimeter and you are reading 60 volts, that means that the line is actually 120 VAC, right? You're measuring only half the voltage. I always check with both settings, AC and DC.
It doesn’t work that way. The DC reading could be all over the place if you stuck it on an AC line. Try not to do that! On some meters you’ll damage them putting that much voltage on a meter expecting DC. On a modern autoranging meter it might not hurt anything but the reading you get will be meaningless.

Typically, as was said, an AC line will be inside a heavy sheath of some sort.
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