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Old 12-26-2009, 08:14 PM   #15
Ronald D
 
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Rich the Viking and John,

I have read 551-20 of NEC. It states that the converter enclosure shall be bonded to the chassis with at least a #8 copper conductor. It also states that the converter be provided with a separate chassis bonding conductor that shall not be used as a current carrying conductor. John, does that sound right?
Rich, If the 110 volt system is isolated from the chassis and a short (fault) or mis-wiring occurs and the chassis becomes energized there is no place for it to go until ground is made, possibly by someone on the ground touching the chassis.

It seems to me that the chassis MUST be grounded. I could be wrong, I just don't understand Airstream wiring.

Retired Ron
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Old 12-26-2009, 08:45 PM   #16
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Rich the Viking,

For your edification the ground wire does NOT carry current back to its source, the white wire does. The ground wire carries fault (short circuit) current back to the distribution panel, eventually through the main breaker to the faulted (short circuited) breaker and trips it. At the speed of light.

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Old 12-26-2009, 11:06 PM   #17
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Ron,

You ask good questions.

Quote:
Originally Posted by Ronald D View Post
Andy,
Rich the Viking stated NOT to ground the 110 volt to the chassis but DO ground the 12 volt to the chassis. If the chassis is not grounded to the shore power and there is a fault, that is; hot wire in contact with the chassis, wouldn't the chassis be live? I believe that in a three wire 110 volt cord there is a black wire (hot) white wire (neutral) and green wire (ground). Unless the chassis was grounded (to the shore power) the fault would have no place to go until someone or something on the ground touched the trailer. Or am I wrong?
It is my understanding that the green grounding wire from the shore power cord is supposed to be bonded to the trailer frame. However, the service to the trailer is not treated as a service entrance, and therefore the neutral is not bonded to the ground. Matters become more complex in trailers that have a separately derived power source, such as a generator or inverter, because when shore power is not connected, there is no neutral-to-ground bonding point. A result of this is that some transfer switches have the additional components necessary to bond the neutral to ground only when shore power is not present.

Quote:
My other question is IF the shore power cord was grounded to the chassis AND the battery negative was also grounded to the chassis AND there was a fault, wouldn't that put 110 volts on the negative side of the battery, just for and instant? Could that mess up the 12 volt electronics? I'm confused, I just want to do it right.
The main problem is that not the brief presence of 110 volts on the negative side of the battery (which is inconsequential since the battery has a low series resistance and will maintain a 12 volt nominal DC differential between the +12 volt supply and ground), but rather the fact that there are high impedance components in the frame. As with any ground fault, the high current flow can cause a voltage gradient along the frame components between the fault and the bonding point. So, for example, their might be some AC voltage between the negative side of a 12 volt outlet and the - post on the battery. Yes, it can be enough to damage 12 volt electronics. That is a consequence of using the frame as a 12 volt return, and is lessened by using a wired return to negative bus. Supplying the trailer through a GFCI can limit the fault duration, which also helps.

The best thing to do, from a wiring quality standpoint, is to wire your 12 volt returns the same way as your supplies, rather than running anything to the trailer ground. This is much the same as you would run a wired neutral in a 120 volt system.
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Old 12-27-2009, 05:28 AM   #18
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That's my understanding

Ronald D-

"Is that right?"

I'm not an electrician. My son is an apprentice and is 3/4 of the way through his training. Our Airstream has come up in his class and the answers from his teacher -- a Master Electrician -- sound exactly like Jammer's post.

Here are my takeaways on our discussion of grounding:

(1) the ground on the Airstream has one purpose -- to save your life. Get it right.
(2) Ground the 110v system with #8 or (preferably) #6 wire to the steel frame, not to the aluminum.
(3) Neutral carries as much current as supply. Never connect neutral to ground inside the trailer. It is the responsibility of the supplying system to bond neutral and ground outside of the trailer. They are intended to be separate paths.
(4) Both 12v and 110v systems can be grounded to the chassis.

The more I read about grounding issues, the more concerned I am about some of the trailers out there. I hope that anyone reading this thread makes sure that their neutral and ground are not connected AND they check the polarity of the power supply at the campground. Think about the opposite -- connecting live power to what is assumed to be neutral, then bonding it to the aluminum shell. Proper wiring and polarity testers are cheap insurance.

Here's what I understand about generators. If the trailer is bonded to the ground wire on the supply, you're counting on the generator to be properly grounded or ground fault protected. You may see or hear discussion of driving a rod into the ground and bonding it to the generator -- a little impractical. We're going to count on the Ground-Fault-Interruptor on our new generator and will probably bond ground to neutral on the generator itself. (We will never connect it to the house circuitry.)

Finally -- to repeat -- I am not an electrician. I hope it's helpful to share my conclusions from discussions about my son's training.

John
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Old 12-27-2009, 11:06 AM   #19
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Jammer, I believe that you have it just right. The only reason I got in on this thread was because of Rich the Viking post #4 when he said to NOT GROUND THE CHASSIS.

John, you are also right, your son is paying attention in his classes. The Airstream is treated just like a stove except it is 110 instead of 220, three wires instead of four. The frame is grounded and that ground is NOT connected to the neutral inside the stove. An electrical panel has two buss bars inside, one for the neutral (isolated from the enclosure) and one for the ground which is bonded to the enclosure) The grounding buss is then bonded to the chassis. You say that you're concerned about some of the trailers out there, after seeing post #4, so was I. I hadn't read 551-20 for 40 years, thanks for that information as I knew nothing about 12 volt wiring. All my questions have been answered, especially by Jammer when he says the 12 volt wiring is a 2 wire system, unlike a car that uses the chassis for a return.
Thanks to all, this is my last post ever on this site, I asked what I thought was a simple question and I was met with derision in some quarters. John and Jammer are tops, even Rich the Viking is ok, he was just repeating what someone told him. When he realized his information might be incorrect he asked for clarification. I am now going back into retirement to work on my Overlander, never to look at this site again.

Retired Ron
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Old 12-27-2009, 11:47 AM   #20
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Quote:
Originally Posted by Ronald D View Post
Jammer, I believe that you have it just right. The only reason I got in on this thread was because of Rich the Viking post #4 when he said to NOT GROUND THE CHASSIS.

John, you are also right, your son is paying attention in his classes. The Airstream is treated just like a stove except it is 110 instead of 220, three wires instead of four. The frame is grounded and that ground is NOT connected to the neutral inside the stove. An electrical panel has two buss bars inside, one for the neutral (isolated from the enclosure) and one for the ground which is bonded to the enclosure) The grounding buss is then bonded to the chassis. You say that you're concerned about some of the trailers out there, after seeing post #4, so was I. I hadn't read 551-20 for 40 years, thanks for that information as I knew nothing about 12 volt wiring. All my questions have been answered, especially by Jammer when he says the 12 volt wiring is a 2 wire system, unlike a car that uses the chassis for a return.
Thanks to all, this is my last post ever on this site, I asked what I thought was a simple question and I was met with derision in some quarters. John and Jammer are tops, even Rich the Viking is ok, he was just repeating what someone told him. When he realized his information might be incorrect he asked for clarification. I am now going back into retirement to work on my Overlander, never to look at this site again.

Retired Ron
Ron.

All Airstream and Argosy products use a "one wire 12 volt system" and use the chassis and/or shell as the ground return.

They have used that system for over 70 years.

However, in the case of the fiberglass sided Argosy and Land Yachts, then the 2 wire 12 volt system is used, in most places.

The 12 volt DC systems are far removed from the 120 volts AC systems.

Andy
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Old 12-27-2009, 11:58 AM   #21
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The only comment I have is that the shell may be bonded to the ground for the 120V service, rather than the steel rame.

There is a specific exception for this in NEC 551.56 (B), where unitized metal body RV (Airstreams, Avions, etc) use the body rather than the steel frame.
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Old 12-27-2009, 02:40 PM   #22
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Quote:
Originally Posted by Ronald D View Post
Jammer, I believe that you have it just right. The only reason I got in on this thread was because of Rich the Viking post #4 when he said to NOT GROUND THE CHASSIS.

John, you are also right, your son is paying attention in his classes. The Airstream is treated just like a stove except it is 110 instead of 220, three wires instead of four. The frame is grounded and that ground is NOT connected to the neutral inside the stove. An electrical panel has two buss bars inside, one for the neutral (isolated from the enclosure) and one for the ground which is bonded to the enclosure) The grounding buss is then bonded to the chassis. You say that you're concerned about some of the trailers out there, after seeing post #4, so was I. I hadn't read 551-20 for 40 years, thanks for that information as I knew nothing about 12 volt wiring. All my questions have been answered, especially by Jammer when he says the 12 volt wiring is a 2 wire system, unlike a car that uses the chassis for a return.
Thanks to all, this is my last post ever on this site, I asked what I thought was a simple question and I was met with derision in some quarters. John and Jammer are tops, even Rich the Viking is ok, he was just repeating what someone told him. When he realized his information might be incorrect he asked for clarification. I am now going back into retirement to work on my Overlander, never to look at this site again.

Retired Ron
Sorry to hear you are not coming back Ronald D. I find that you have to be rather thick-skinned to put up with some of things put forth here, as you have experienced. I try to keep my experience positive by reading a lot more than I post. The old information on this site is way more valuable than the new in most cases, exceptions being for things that are actually changing and improving, such as hitching, lighting, heating and a few others.
Since you state that the above was your last post I will send this as a pm also, in the hope that you will continue to use this resource to your benefit. As you stated, there are some good folks here, and your interaction with us is appreciated. Also, keep in mind that there is a private message system where you can contact any individual and ask for, and give help or guidance without having to expose yourself to the whole community.

After re-reading my post #4 I find that I was so incomplete as to be incorrect. Sorry for the gibberish, I know better than what I wrote there.
To explain my situation: I had no ground wire going to the chassis in my safari at all. the grounds all tied together at the buss bar and the enclosure was attached to a wood post rather than the metal chassis. I'm not stating that this is a correct set-up, just what I actually found in my safari. I also opened up every electrical connection in my rig and nothing anywhere on the 110 circuits was grounded to the chassis. I purchased a progressive dynamics power control center and in the installation instructions it doesn't say to ground the control center to the chassis and it doesn't show a ground indication other than the ground wire coming in with the shore power. In a previous thread I read that some convertors are grounded and some are not, and that, along with the instructions, lead me to believe I had one that did not require it. Then, in an earlier reply to this thread Inland Andy stated that you do need to ground the convertor to the chassis. After reading through some old threads on this I see that there is way more confusion with regards to this than I was experiencing myself, and the fact that master electricians are questioning the correct set-up make me feel like I'm in the right company, anyway. Now I'm in contact with Progressive Dynamics to see what they have to say. As stated before, this is too important to not get it right.

Rich the Viking
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Old 12-27-2009, 02:49 PM   #23
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Quote:
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Then, in an earlier reply to this thread Inland Andy stated that you do need to ground the convertor to the chassis.

Rich the Viking
It is not necessary to ground the converter to a ground, but you can if you wish.

You "MUST" however, ground the negative wire from the converter, or nothing will work, since the negative wire, is the return circuit for the all the 12 volt DC circuits.

Think of the trailer 12 volt system as being the same as a car or truck, as far as how things work on DC.

Andy
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Old 12-27-2009, 03:20 PM   #24
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It is not necessary to ground the converter to a ground, but you can if you wish.
.......
Andy
It's not necessary, but it is required by NFPA/NEC.

Don't know why this keeps coming up.
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Old 12-27-2009, 03:44 PM   #25
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Quote:
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It's not necessary, but it is required by NFPA/NEC.

Don't know why this keeps coming up.

Thanks, Mark. As I stated above, The actual instructions with the unit I purchased do not indicate a grounding of the unit to the chassis. If the manufacturer doesn't even address the issue of grounding it obviously raises some questions, and I am no electrician, as you hopefully have discerned by my lame attempts at trying to help someone do something that I don't know well enough myself to advise on. I read through several threads on this but there is varying information and conclusions in them. See the first paragraph in the first post of this thread http://www.airforums.com/forums/f449...ter-56389.html
misinformation still abounds, hence the need for clarification.
Thanks again,
Rich the Viking
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Old 12-27-2009, 10:11 PM   #26
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It's not necessary, but it is required by NFPA/NEC.

Don't know why this keeps coming up.
Mark

Could you please post that info, or e-mail it to me.

Thanks.

Andy
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Old 12-27-2009, 10:34 PM   #27
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Andy,

Ronald D in post #15 has the correct section. It's 551.20 (C).

The 2008 edition of NEC is available in open format, you need to go to NFPA and register or sign in to view it. Doesn't cost anything. However you can't copy, save, or print it.

It's on page 480.
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Old 12-28-2009, 06:52 AM   #28
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Quote:
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Ron.

All Airstream and Argosy products use a "one wire 12 volt system" and use the chassis and/or shell as the ground return.
Andy,

Is this true for the higher current connections as well? One might hope that the converter output, at least, would have the negative side wired straight to the battery bank. Same for an inverter if present. I'm not an expert on Airstreams but I've seen other RVs wired this way --- the chassis return is only used for small, distributed loads.
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