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Old 09-01-2004, 04:45 PM   #1
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Polarity light function (diode?)

How does the polarity light on my 64 Safari function? Is it simply a light with a diode?

My polarity light is on constant. I removed all power out sources from univolt, same thing. I removed the light unit and wiring and plugged it into house outlet (2 different houses). It lights with leads either way. Perhaps the diode is burnt?



Is there a source for these polarity lights? What value diode would I need to put inline with a new indicator light?

Thanks, Walter
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Old 09-01-2004, 05:14 PM   #2
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Short answer: No diodes required.

Longer answer: Univolt is not involved.

I am willing to give a dissertation if necessary/requested involving hot & neutral wires, but the short answer is your black & white wires are reversed, most likely at your power source or you have constantly encountered an inappropriate ground.

Tom
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Old 09-01-2004, 09:57 PM   #3
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Not sure I understand your question. The polarity indicator lights when you have reversed polarity. Meaning that the neutral (white) wire in the 120 volt system is "hot" (carrying the load the black wire should have). However, IMHO, the Univolt is involved because the polarity indicator light is grounded to the body of the trailer and only the 12 volt system is supposed to be grounded to the body or shell. The 120 volt system is supposed to be isolated, meaning the romex used has its own ground wire. The 120 volt system grounds to the utility shore power box, not the trailer body. With the Univolt, if the 120 is wired with polarity reversed then that polarity is carried over to the 12 volt system via the Univolt causing the indicator to light. More modern converters are designed to prevent that, but not the old Univolts. That is why there is a indicator light. If yours lights then it is working - why do you want to replace it? Track down the source of the reversed polarity so that the indicator light does not light.

Forrest
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Old 09-02-2004, 04:06 AM   #4
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Quote:
Originally Posted by Forrest
...The 120 volt system grounds to the utility shore power box, not the trailer body...
That is not the case in my '67 Overlander. While the shore power ground wire does ground to the Overlander's 120 vac breaker box, the box itself is attached to a metal plate which in turn is attached to the trailer body effectively grounding the entire trailer body.

Tom
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Old 09-02-2004, 07:10 AM   #5
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Ground conductors

The Ground should be isolated from chassis.

Polarity indicator will only light when there is 120 volts nominal between neutral and ground conductor.
Seeing that you have problem a two different sources, (houses), the problem is in your connection at trailer. Is you polarity light in a junction box or protrude from the skin of the trailer?

Check the connections at Polarity light and check connections on shore power cable. I bet the wires are reversed in the Plug of the shore power cable. Is it molded or is a serviceable connector? (plug has screws in it). If it does have screws, take it apart and insure that white wire is on light colored screw, dark wire is on dark screw and green wire on green screw. Also check that "Pins or contacts" of plug are in correct position

Another drawback to this reverse polarity is that you will have no power when plugged into generator. Most generators are polarity sensitive.

Smily
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Old 09-02-2004, 07:17 AM   #6
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Quote:
Originally Posted by smily
The Ground should be isolated from chassis.
And your reasoning is?
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Old 09-02-2004, 08:28 AM   #7
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Potential

Quote:
Originally Posted by tcwilliams
And your reasoning is?
I believe I was off base, I meant to say isolated from neutral as in Neutral should not be conncted or bonded to ground and or chassis. Some in the electrical field consider the neutral a ground conductor also.

Any ground connected to chassis without being grounded to source ground, could provide potential for voltage to chassis.

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Old 09-02-2004, 12:33 PM   #8
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Well, I think my previously stated thoughts were not thought out well. So much for writing late at night. I defer to Ken's last post. Obviously, the 120 is grounded to the shell of the trailer, I just checked that by inserting one probe of my voltage tester into an outlet hot slot and touched the tip of the other probe to the shell and the lamp lit. I was also off base on the neon polarity indicator light. I pulled mine out and it is rated to 240 volts, and has one ground wire connected to the shell and another I presume to 120. So, obviously, the indicator lamp is not 12 volt. My apologies.

Still, I find the whole thing somewhat contradictory. My circuit breaker box is mounted to the fiberglass shell in the bath and doesn't appear to be grounded to the aluminum shell at that location. Then the 120 outlets are mounted in bakelite boxes with no discernible ground to shell at those locations. At first glance then it would seem that there was some design to keep 120 completely isolated from the shell, but then the outside "porch" 120 outlet is mounted in a way that grounds it to the shell. So, what was the point of using bakelite boxes and mounting the circuit breaker box to fiberglass? Just convenience, or evolution?

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Old 09-02-2004, 12:46 PM   #9
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Quote:
Originally Posted by Forrest
... My circuit breaker box is mounted to the fiberglass shell in the bath and doesn't appear to be grounded to the aluminum shell at that location... So, what was the point of using bakelite boxes and mounting the circuit breaker box to fiberglass?
Forrest,

In another post, InlandRV pointed out the existence of a ground wire between the breaker box & the frame somewhere around the black tank drain. I do not recall what year Airstream he was talking about. Yours may have something similar.

National Electrical Code, and hence RV electrical code specifies the use of electrical boxes when 120 vac is used.

Tom
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Old 09-02-2004, 12:56 PM   #10
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I'll find out more about where and how it's all wired fairly soon because I'm in the process of disassembling my '66 Overlander. I'll be taking the inner skin off so that I can replace the aluminum wiring with copper (also to re-insulate, seal seams, etc.) Oddly, I think, my '66 Globe Trotter has copper wire, not aluminum, even though both trailers were manufactured the same year at Jackson Center!

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Old 09-03-2004, 12:58 PM   #11
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I’ve been away. Thanks for the help. This explains a lot.



Now I understand the idea of the 120vAC system being isolated from the trailer shell while the 12vDC system uses the frame as ground. This could be my problem area.



From here on I refer to Black is hot, White is neutral, uninsulated Copper is ground.



I was wrong about the univolt. A former owner replaced the original univolt with a modern 1998 mfg 120vAC/12vDC converter/charger. Same basic function, 120vAC shore power in but only puts out 12vDC @ 15A or 30A (charge line) instead of 12vDC and 19vDC as univolt. All lights and outlets work properly, 120vAC shore and 12vDC battery power.



So, perhaps my answers to polarity light stays lit lay within Forrest’s response of “The 120 volt system grounds to the utility shore power box, not the trailer body. With the Univolt, if the 120 is wired with polarity reversed then that polarity is carried over to the 12 volt system via the Univolt causing the indicator to light. More modern converters are designed to prevent that, but not the old Univolts.”. It might be that the polarity light wiring itself is providing the link to body/frame ground. Perhaps the modern converter wiring does not require the polarity light in system and allows it to remain lit.



Shore power plug is a 50A molded plug, with 30A molded plug adapter, wired to replaced/modern main circuit breaker box. Black to Main breaker, White to neutral buss bar, copper ground to breaker box case. The breaker box is isolated from body shell, mounted to bathroom wood cabinet.



The converter is wired directly to “converter” breaker, black hot to black, white neutral to white, copper ground to converter internal case ground lug. The converter case is screwed to wood floor and has an additional external ground lug with wire running back to breaker box case.



The polarity light is installed in city/battery switch panel with one wire direct to black converter input power line connection and other wire lug screwed to city/battery switch panel face. The switch panel is sheet metal screwed to inside skin in rear service area. Looks like polarity light is in fact wired 120vAC power hot through light to body shell ground (ground for 12vDC system).



Maybe I have no wiring problem? Perhaps all I need to do is disconnect the polarity light as no longer needed with new converter, as opposed to its not working or polarity is actually reversed?



There is also a “new” exterior waterproof power outlet, I’ll have to check it’s internal wiring/grounding.



I know this is a lot, but don’t want to get zapped in rain touching trailer skin or worse burn it all down.



Thank for all the help, Walter
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Old 09-03-2004, 02:05 PM   #12
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I would highly recommend getting and using a plug in tester. The one I have also tests GFI outlets. It has three diodes with a little chart on it, and depending on how the diodes light up the chart diagnoses any problems. It might confirm that you have a hot to neutral situation, which is what the polarity indicator seems to be saying. Take what we have to say on this thread with a grain of salt because we are just hobbyists (at least I am), not professional electricians. Please, don't assume that you can disregard the polarity light without using other diagnostic tools and/or getting a professional to look at the problem.

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Old 09-03-2004, 03:59 PM   #13
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Thanks Forrest,

I am taking all this advise VERY seriously. You have taught me much about A/S already. I'm a wildlife biologist not an electrician. I know bats and birds, now learning electricy flows. I am assuming nothing about this 40 year old / modified electrical system and I really want it fixed right.
I used a DVM to test voltages, 12vDC shows +/_ polarity but 120VAC does not. I also tried a plug in 3 diode light tester like you mentioned at several outlets. All outlets indicated properly wired. This just adds to my confusion leading to the polarity light used with newer converter in system. I'll test every last outlet tonight including shore power source.


I can also slave back in the original univolt unit to see if polarity light works with it in place. That may answer questions. I really appreciate the time you guys have given and don't want an undiagnosed polarity problem to become a real problem. I’m thinking I may have a link to ground that shouldn’t be there, somewhere.

Walter
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