My wiring plans

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Postby TonyCooper » Sat Mar 19, 2005 12:05 am

I've not been following this thread recently but having read it I'm very impressed! You folks have been working hard! A couple of questions if I may...

1) 120VAC appears to use grounding from shore power. Is the consensus that ground should only be at the shore power plug and not trailer frame?
That gets my vote but I've seen references to grounding to the trailer.

2)I'm adding an inverter. The inverter has a chassis ground tab separate from the negative terminal. Should I simply ground this to the neg battery terminal (trailer chassis ground)? Somehow I don't think so... My instincts tell me this is for a true earth ground - a rod in the dirt! Any insights appreciated...
Last edited by TonyCooper on Mon Mar 21, 2005 12:16 am, edited 1 time in total.
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Postby TonyCooper » Sat Mar 19, 2005 10:57 pm

angib wrote:I came across this wiring diagram for the Scamp trailer and thought it might add to this thread:

Image

The source page is here.

Andrew


Andrew,

Interesting that the Scamp wiring doesn't show the 120VAC ground line grounded to the trailer. This is how I had planned to wire my 120VAC system but I saw references that the 120VAC should be grounded to the trailer...Some folks quoted code too... This picture reinforces my first thoughts on the subject. Thank you very much for posting it.
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Postby bdosborn » Sun Mar 20, 2005 1:08 am

TonyCooper wrote:1) 120VAC appears to use grounding from shore power. Is the consensus that ground should only be at the shore power plug and not trailer frame?
That gets my vote but I've seen references to grounding to the trailer(which to me seems dangerous).

2)I'm adding an inverter. The inverter has a chassis ground tab separate from the negative terminal. Should I simply ground this to the neg battery terminal (trailer chassis ground)? Somehow I don't think so... My instincts tell me this is for a true earth ground - a rod in the dirt! Any insights appreciated...


1) My opinion is that the trailer frame absolutely should be grounded to shore power. If for some reason the hot wire energizes the trailer frame, you want a path to conduct the current to ground (and pop the breaker). I don't want to be that path, which is what I would be if I touched it and the frame isn't grounded through the shore power ground. Since I'm a fairly high impedance path to ground, the breaker may not pop and the end result is barbeque Bruce. I may be tasty but I'm not that generous. So I'll ground the shore power plug(s) and the frame (as soon as I finish the stupid galley). If I were skinning with aluminum I'd ground that too.

2) I plan to ground the negative terminal of the battery to the chasis as well as the shore power ground. I don't want to have any voltage differences. And remember. the shore power ground does go to a rod in the dirt (or it's supposed to). The manufacturer of the inverter is counting on this, which is why they have a separate shore power equipment ground. Interestingly, a ground rod right at the trailer won't reduce the electrocution hazard. Here's an article about ground rods and touch voltage:
http://ecmweb.com/images/archive/0501btb.pdf

Here's some articles of what can happen with improper grounding:

http://ecmweb.com/mag/electric_electroc ... index.html

http://ecmweb.com/mag/electric_case_hot ... index.html

http://ecmweb.com/mag/electric_case_let ... index.html
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Postby TonyCooper » Mon Mar 21, 2005 12:15 am

bdosborn wrote:
1) My opinion is that the trailer frame absolutely should be grounded to shore power. If for some reason the hot wire energizes the trailer frame, you want a path to conduct the current to ground (and pop the breaker). I don't want to be that path, which is what I would be if I touched it and the frame isn't grounded through the shore power ground. Since I'm a fairly high impedance path to ground, the breaker may not pop and the end result is barbeque Bruce. I may be tasty but I'm not that generous. So I'll ground the shore power plug(s) and the frame (as soon as I finish the stupid galley). If I were skinning with aluminum I'd ground that too.


Hi Bruce,

You may be right... I'm open for discussion... Lets walk through the logic...
For discussions sake lets assume no trailer circuit breakers or GFCIs are installed (insanity!) and the only ground available is an 8' rod in the ground from shore power.

120VAC Frame Grounded
You ground the 120VAC to the trailer frame. You then have a short from the 120VAC hot line directly to frame ground. What circuit completes the path from frame ground to shore power ground? Hopefully your extension cord and the camp site wiring circuit.

As noted above, if you are tying the trailer frame to the shore power ground, you are counting on the shore power ground integrity to be intact. And even if it is intact you can fry (this possibility based on the first article you referenced). (hopefully you pop the shore power CB before doing terminal damage)

Now lets go the other way...

120VAC with no frame ground
You run 120VAC utilizing only the shore power ground. You then have a short from the 120VAC hot line directly to frame ground. What circuit completes the path from frame ground to shore power ground?

Answer, The trailer frame is not grounded to anything so a possibility of up to 120VAC difference of potential exists from the trailer frame to shore power ground. You touch the trailer frame and you complete the circuit and fry.

In both cases you are counting on the shore power ground integrity to be intact. If it fails you fry. (again, hopefully you pop the shore power CB)

Remember electricity seeks the path of least resistance which <should> always be shore power ground but based on your first article, resistance can grow enough to cause a difference of potential by as much as 100VAC in as little as 3-5 ft from the ground rod. Even at 1 ft the picture illustrates the possibility of an 80VAC difference of potential! How many tears are located 1ft from the ground rod? I'm showing just the image from the article you referenced in Mike Holtz Enterprizes site to illustrate the point I'm trying to make.

Image

Now add in circuit breakers and GFCIs to either scenario...
If one installs a circuit breaker and a GFCI in ones trailer, you protect both the wiring(CB), and the individual(GFCI) with or without grounding to the frame.

A quote from How Stuff Works regarding GFCIs...
"One of the newer circuit breaker devices is the ground fault circuit interrupter, or GFCI. These sophisticated breakers are designed to protect people from electrical shock, rather than prevent damage to a building's wiring. The GFCI constantly monitors the current in a circuit's neutral wire and hot wire. When everything is working correctly, the current in both wires should be exactly the same. As soon as the hot wire connects directly to ground (if somebody accidentally touches the hot wire, for example), the current level surges in the hot wire, but not in the neutral wire. The GFCI breaks the circuit as soon as this happens, preventing electrocution. Since it doesn't have to wait for current to climb to unsafe levels, the GFCI reacts much more quickly than a conventional breaker."

I'm counting on good basic wiring, and GFCIs and CBs to protect me. If I have this wrong I need folks brighter then me to tell me so...

2) I plan to ground the negative terminal of the battery to the chasis as well as the shore power ground. I don't want to have any voltage differences. And remember. the shore power ground does go to a rod in the dirt (or it's supposed to). The manufacturer of the inverter is counting on this, which is why they have a separate shore power equipment ground. Interestingly, a ground rod right at the trailer won't reduce the electrocution hazard. Here's an article about ground rods and touch voltage:
http://ecmweb.com/images/archive/0501btb.pdf

Here's some articles of what can happen with improper grounding:

http://ecmweb.com/mag/electric_electroc ... index.html

http://ecmweb.com/mag/electric_case_hot ... index.html

http://ecmweb.com/mag/electric_case_let ... index.html
Bruce


The first article clearly shows that counting on shore ground is not viable and supports my (and many others) suggested use of GFCIs. The ground rod will most likely be at the nearest utility pole which could be many feet away from our teardrops.

The other articles are sad but my tear isn't a houseboat sitting in a fresh water lake with rigged electrical wiring. I'm not sure the pool house had GFCIs installed. The article did say all electrical connections in the building were up to code. They fixed the problem by placing rubber mats under the machines large enough to isolate the users while using the machine and repairing the defective power cord.

Regarding my Inverter
I am not real clear about the inverter tab ground.... shore power ground seems unlikely as it is used to provide 120VAC from a completely disconnected system running on 12VDC.

My instructions are not clear and that is why I asked. It clearly is a inverter chassis ground tab.

Any and all insight on anything here would be very much appreciated.
Tony

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Postby bdosborn » Mon Mar 21, 2005 10:27 am

TonyCooper wrote:Hi Bruce,

You may be right... I'm open for discussion... Lets walk through the logic...
For discussions sake lets assume no trailer circuit breakers or GFCIs are installed (insanity!) and the only ground available is an 8' rod in the ground from shore power.

<snip>

Regarding my Inverter
I am not real clear about the inverter tab ground.... shore power ground seems unlikely as it is used to provide 120VAC from a completely disconnected system running on 12VDC.

My instructions are not clear and that is why I asked. It clearly is a inverter chassis ground tab.

Any and all insight on anything here would be very much appreciated.


Tony,
If there's no circuit breaker, you're right it pretty much doesn't matter what you do. I assume that you will have a circuit breaker from the the shore power circuit because its always required by code to be there. The difference between grounding the trailer frame and not (with a circuit breaker) is how fast the circuit breaker will clear the fault. The circuit breaker will open within a second or two when the hot wire touches the frame if its grounded. The circuit breaker won't open on an ungrounded frame untill something completes the path to ground. This leaves the trailer frame energized untill someone touches it. Also, people are a high resistance path to ground so the circuit breaker may not see enough current to open, leaving the frame energized untill the next person touches it.

Yeah, some of those articles I posted aren't specifically in reference to trailers, they were just meant to show that proper grounding is important. The pool house article does relate though. The ground pin had been cut off the vending machine plug so that when the chasis of the vending machine was energized, the circuit breaker never opened.

Circuit breakers and GFCI protection are important and I'm putting them in my trailer now. But, good grounding is important too in case you have a problem upstream of the GFCI.

I reviewed the NEC on trailers and it requires that the case of the inverter be grounded. I still think that's what the tab is for but I'd check with the manufacturer to make sure.

Thanks for the good discussion and I hope I helped,
Bruce
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Postby TonyCooper » Mon Mar 21, 2005 12:46 pm

bdosborn wrote:
Tony,
If there's no circuit breaker, you're right it pretty much doesn't matter what you do. I assume that you will have a circuit breaker from the the shore power circuit because its always required by code to be there. The difference between grounding the trailer frame and not (with a circuit breaker) is how fast the circuit breaker will clear the fault. The circuit breaker will open within a second or two when the hot wire touches the frame if its grounded. The circuit breaker won't open on an ungrounded frame untill something completes the path to ground. This leaves the trailer frame energized untill someone touches it. Also, people are a high resistance path to ground so the circuit breaker may not see enough current to open, leaving the frame energized untill the next person touches it.

Circuit breakers and GFCI protection are important and I'm putting them in my trailer now. But, good grounding is important too in case you have a problem upstream of the GFCI.

I reviewed the NEC on trailers and it requires that the case of the inverter be grounded. I still think that's what the tab is for but I'd check with the manufacturer to make sure.

Thanks for the good discussion and I hope I helped,
Bruce


Thanks for the clarification Bruce,
Couple more questions... in the case of a 120VAC short to trailer wouldn't the GCFI see the difference in voltage between neutral and hot, and trip?

With regards to grounding the inverter, not to show my stupidity here but ground it to what??? If I'm not plugged into shore power I can't ground it to shore power ground... Grounding it to the frame does nothing unless the battery neg is grounded to frame as well. And it that is the case why not just run a line directly to the battery neg terminal...

I stay in a perpetual state of confusion...

Tony
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Postby bdosborn » Mon Mar 21, 2005 10:43 pm

TonyCooper wrote:Thanks for the clarification Bruce,
Couple more questions... in the case of a 120VAC short to trailer wouldn't the GCFI see the difference in voltage between neutral and hot, and trip?

With regards to grounding the inverter, not to show my stupidity here but ground it to what??? If I'm not plugged into shore power I can't ground it to shore power ground... Grounding it to the frame does nothing unless the battery neg is grounded to frame as well. And it that is the case why not just run a line directly to the battery neg terminal...

I stay in a perpetual state of confusion...

Tony


Yup, the GFCI would trip and really fast. It only takes a .005 amp difference between the hot and neutral to trip the GFCI. But, if the short is on the line side of the GFCI, then the GFCI won't see the imbalance. There's something to be said for GFCI circuit breakers...

If you're not using shore power then I would ground the inverter case to the negative terminal of the battery. That way if your 120V shorts to anything metal it would go to the battery. I think I would tie all the grounds (and the frame ) to the negative of the battery to keep everything at the same potential and to have a return path to the negative of the battery for faults.
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Postby TonyCooper » Tue Mar 22, 2005 12:29 am

Thanks! I appreciate the time taken for the explanations... I've read and reread our dialog a few times and it's finally sinking in... Again, thanks for the help!
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Postby bdosborn » Wed Mar 23, 2005 2:40 pm

TonyCooper wrote:Thanks! I appreciate the time taken for the explanations... I've read and reread our dialog a few times and it's finally sinking in... Again, thanks for the help!


My pleasure. I actually like all this electrical stuff. :D
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Postby toypusher » Thu Mar 24, 2005 2:57 pm

Bruce,

I know I ask this before, but it seems to fit in here and I hope everyone would want to know the answer.

Is there a CB for the AC side that can be used without a breaker box? I don't want to have to spend $70 or more for a breaker box to hold only one 20A CB. You gave me a link to Granger (I think it was), but I don't understand the breaker that it led me to. Or how to use it for that matter. I was hoping that there is some kind of 120VAC Circuit Breaker that can be used stand alone or in-line (like the 12VDC ones).

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Postby Nitetimes » Thu Mar 24, 2005 3:08 pm

toypusher wrote:
Is there a CB for the AC side that can be used without a breaker box? I don't want to have to spend $70 or more for a breaker box to hold only one 20A CB. You gave me a link to Granger (I think it was), but I don't understand the breaker that it led me to. Or how to use it for that matter. I was hoping that there is some kind of 120VAC Circuit Breaker that can be used stand alone or in-line (like the 12VDC ones).

Kerry


You can pick up small breaker boxes at Lowes, HD for under 20 bucks that would work fine. Look at the ones for spa's and such. They are usually under 10" high and 6" wide.
I don't know of any stand alone AC breakers off-hand and if there are I would think they would be a little pricey.
If I come across any I will certainly post it here tho.
Rich


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Postby toypusher » Thu Mar 24, 2005 3:20 pm

Nitetimes

Thanks, :thumbsup: I don't remember seeing any of them :? , but will definitely go back and look again.

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Postby Nitetimes » Thu Mar 24, 2005 3:42 pm

I have no experience with these, but either of these two might be what you are looking for. I'm not sure how they mount but you should be able to get that info from the manufacturer. I think you would still want to put it in some kind of small box tho.

http://www.allbreakers.com/Circuit-Brea ... mnsf0.html
http://www.allbreakers.com/store.asp/pg ... jmqnnomnp4
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Postby Nitetimes » Thu Mar 24, 2005 4:13 pm

Found these ones on Lowes website, I think the first one would be the best for TD applications as there is no main, just main lugs connected to either 2- 1" breakers or 4- 1/2" breakers. But the others are possibilties too.

http://www.lowes.com/lkn?action=product ... lpage=none

http://www.lowes.com/lkn?action=product ... lpage=none
http://www.lowes.com/lkn?action=product ... lpage=none
http://www.lowes.com/lkn?action=product ... lpage=none
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Postby toypusher » Thu Mar 24, 2005 4:22 pm

Nitetimes,

Thanks again :thumbsup: I can't get out until tomorrow, but you bet I will be checking them out.

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