Is a chassis grounded needed in a mobile application?

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TucsonAZ
TucsonAZ Solar Expert Posts: 139 ✭✭
in Off Grid Solar & Battery Systems #1
I'm just wondering if I can keep the entire solar system isolated in an RV or if I should ground it to the vehicle chassis? I would much rather keep the system independent of the vehicle in every possible way but if it's somehow better or safer I would of course choose whatever option that way.

Thanks a lot!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Is a chassis grounded needed in a mobile application?

    The only reason automobiles have negative tied to the chassis is because it's cheaper than wire.
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  • TucsonAZ
    TucsonAZ Solar Expert Posts: 139 ✭✭
    #3
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    Re: Is a chassis grounded needed in a mobile application?
    Cariboocoot wrote: »
    The only reason automobiles have negative tied to the chassis is because it's cheaper than wire.

    Thank you, now that is some sense I can sink my teeth into!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: Is a chassis grounded needed in a mobile application?

    Another reason to ground your battery bank... If you get a short circuit somewhere between + and chassis somewhere (cable is cut/worn and conatacts chassis), the short circuit will pop a fuse or circuit breaker.

    If you have a floating battery power system--Then the "first short" sets the system grounding (i.e., a - short to ground makes negative ground; a + to ground short makes a positive ground)--Then the second short causes the current to flow. And at that point, you don't know where and how much current will flow. Say that your 2 awg AC inverter + wire makes the first short. And the second short is a - 14 AWG wire from a DC lighting fixture--In a "normal" system, you only have fuses in the + leads--So you have your 200 amp fuse in the plus lead and no fuse in the 14 AWG return lead--And your 14 AWG wire gets red hot and causes a fire.

    So--For floating power systems, to be safe, you need a fuse/breaker in both the + and - wire runs--2x more protective devices.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Plowman
    Plowman Solar Expert Posts: 203 ✭✭✭✭✭
    #5
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    Re: Is a chassis grounded needed in a mobile application?
    BB. wrote: »
    Another reason to ground your battery bank... If you get a short circuit somewhere between + and chassis somewhere (cable is cut/worn and conatacts chassis), the short circuit will pop a fuse or circuit breaker.

    If you have a floating battery power system--Then the "first short" sets the system grounding (i.e., a - short to ground makes negative ground; a + to ground short makes a positive ground)--Then the second short causes the current to flow. And at that point, you don't know where and how much current will flow. Say that your 2 awg AC inverter + wire makes the first short. And the second short is a - 14 AWG wire from a DC lighting fixture--In a "normal" system, you only have fuses in the + leads--So you have your 200 amp fuse in the plus lead and no fuse in the 14 AWG return lead--And your 14 AWG wire gets red hot and causes a fire.

    So--For floating power systems, to be safe, you need a fuse/breaker in both the + and - wire runs--2x more protective devices.

    -Bill

    I understood about half that, and probably misunderstood that half.

    But based on my dim understanding of your post, in a system like mine---small, ungrounded solar electric system set up in a cargo trailer---I should have circuit protection on both positive and negative wires coming from the battery bank. I currently only have a breaker on the positive cable leading to the positive bus bar.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Is a chassis grounded needed in a mobile application?
    Plowman wrote: »
    I understood about half that, and probably misunderstood that half.

    But based on my dim understanding of your post, in a system like mine---small, ungrounded solar electric system set up in a cargo trailer---I should have circuit protection on both positive and negative wires coming from the battery bank. I currently only have a breaker on the positive cable leading to the positive bus bar.

    Don't worry about it.
    You could look long and hard before you found any automobile with positive and negative fusing. In practical terms it simply isn't necessary.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: Is a chassis grounded needed in a mobile application?
    Plowman wrote: »
    I understood about half that, and probably misunderstood that half.

    But based on my dim understanding of your post, in a system like mine---small, ungrounded solar electric system set up in a cargo trailer---I should have circuit protection on both positive and negative wires coming from the battery bank. I currently only have a breaker on the positive cable leading to the positive bus bar.

    Yep--You got it right. That will give you 100% over current protection on all DC wiring. Is that overkill? What is the chances of any + to - short in the system (to sheet metal, crossing of high and low current connections, etc.). If the chances are low of that ever happening--Then double fusing is probably overkill (from a safety point of view).

    Ideally, if you have a negative grounded battery bank... And there is a + to sheet metal short anywhere in the system--It will pop the appropriate fuse (sized to the cable).

    If there is a short between any of the negative leads to sheet metal--Then nothing will happen. And you do not need any fuses on the negative bus.

    You need to look at the system... If there is really no chance for positive or negative shorts to the trailer chassis/sheet metal (plastic/wood trailer around the electrical station), then negative grounding/bonding to the trailer frame is "over kill".

    However--If I remember correctly, your in an area where you can have some lightning and the equipment trailer is (more or less) powering your permanent residence.

    Earth grounding of the trailer frame and DC return (negative) is probably a good thing for you (as well as surge protection, etc.)--Helps reduce the chances that lightning will propagate into your home.

    Just remember that automobiles use the frame/sheet metal as the return/negative lead--There is almost no way to over current that path (even then, there are "jumpers/ground bonds" across the motor mounts and across the rubber vibration isolators to the cabin, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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