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Old 05-28-2010, 10:18 AM   #15
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I guess I'm still not making myself clear. The word "Bond" to me means they (the ground and neutral) are tied together. What I am saying is that they should NOT be tied together in the trailer. What is the point of having a ground wire to the trailer if it is not connected to the trailer skin? All metal boxes are to be grounded just as VENTPORT says. To me the trailer is just another metal box. If any hot wire comes in contact with the frame or skin of the trailer and it is not grounded, the skin of the trailer would be at 120 volts. You may not even be connected to the power with your shorepower plug. You could have an extension cord running thru the door of the trailer for example: (when you are doing a make over). The extension cord is running over metal door jamb, if that extensiopn cord has a bad spot in it or gets damaged because the door closed on it, the hot wire could come in contact with the metal part of the trailer. If the trailer is not grounded, you may get shocked when you are standing on the ground and you reach up to grab the metal handle to step into or out of the trailer. One might say OH Well! a little shock never hurt anyone. If you grab that metal handle on that trailer and you can't let go because you can't open your hand to let go. It could kill you or maybe one of your kids!
I am one of those crusty old farts with relatively dry skin and don't get shocked easily. But there are a lot of people out there that have very moist skin, especially children and they will feel the effects.
That's my story and I'm sticking to it.
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Old 05-29-2010, 09:41 AM   #16
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re: "I don't understand why the generator-alternator thing is such a touchy issue." -- me, neither, which is why I considered it indicative of something other than productive discussion and worthy of note in that regard. It was particularly noteworthy in the context of its two adjacent comments in the same vein.

re: "I guess I'm still not making myself clear." -- you are not alone! The topic is one of those that is simple yet complicated at the same time. That, with changes over time (learned via experience) in deciding how things should be done create a lot of confusion and misunderstanding.

re: "If any hot wire comes in contact with the frame or skin of the trailer and it is not grounded, the skin of the trailer would be at 120 volts." -- any voltage has to have a reference. That 120 volts only exists between the power leads. If one power lead is connected to the chassis ground, the 120v is only between that chassis ground and the other power lead. (the caveats involve induced voltages and split phase systems which can complicate the simple picture somewhat)

Unless you complete a circuit you will not get any current to flow and with no current, no shock.

In other words, in an isolated system a single wiring fault is not a shock hazard.

You can think of the neutral to chassis ground (and also to earth ground) bonding as a fault like this intentionally done to make one side of the power circuit at the same potential as the 'environment.' The reasons for this in modern, on grid, practice were thoroughly explored by the 'Standford Professor' and 'Grizzy' over on rv.net a while back. (interesting stuff).

What this gets into is the GFCI and how it implemented in small, 120v, plug type gensets. Most of the discussion (like the link) assume NEC compliant grid attached wiring. For RV's and gensets, the context is a bit different (see separately derived systems - ( ECM1 or EMC2 but take care about the proper definition of neutral in the code to get the context, though)).

Note that the GFCI has essentially replaced the purpose of the polarity indicating light on Airstreams.

Again, my suggestion is to trust the manufacturer to be following code (he has significant financial and legal reasons to get it right) and make sure any modifications you make are also in accord with current code described practice for the circumstances of your application (RV, separately derived, on grid, etc). It is not generally a good thing to think you know more than codified experience or those who make a living at it.
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Old 05-29-2010, 01:03 PM   #17
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Here is a web site that gives all the details of the AC wiring on travel trailers. Just came across it today. Please note the paragraphs regarding grounding and wiring the neutral. www.dasplace.net Once there just click on the RV wiring article. And I do make a living at it, have been for 35 years.
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Old 05-30-2010, 12:41 AM   #18
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interesting link. thanks. Do note that it does not address what the code calls "separately derived systems" but only shore power or grid connected power.

I also didn't see much on wiring except for plugs. I did note a few common myths in his list of mods, though, but nothing to worry about IMHO. Some good stuff, too. It certainly isn't a website that should be treated as an authority but rather its ideas should be checked carefully against more authoritative resources.

Generators used for RV power are generally not connected to 'shore power' or the electric power grid in any way. That is why they are called "separately derived" power systems.

Do check the link I provided earlier for references to OSHA documents as well as some code tutorial sites that address these distinctions directly. Their citations back to sources (such as the NEC) as well as explanations of reasons and circumstances tend to make them a bit more trustworthy than most handyman sites, I think.

The polarity light can be a useful indicator if there is a current path between neutral and chassis ground, which would indicate a wiring fault in the system. Modern systems use GFCI to not only take note of such faults but to shut down the circuit with such wiring faults.

Also note that you can have wiring faults that create a current path between chassis ground and earth ground. The polarity light might not tell you about this but the GFCI should still catch it.

Take care.
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