Battery Bank as Ground?

Tulumtam
Tulumtam Solar Expert Posts: 37 ✭✭
in Off Grid Solar & Battery Systems #1
I wanted to share an interesting grounding situation that I came across the other day for comment. Someone told me it is called a floating ground and that it is acceptable practice. I have my doubts, but am open to learning about it!

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This is how the equipment grounding conductor is "bonded" to the modules and mounting structure. These go to a busbar in the combiner box.
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(Also, check out the knife switches! I didn't even know what these were when I first saw them and had to look it up!) From here, it goes to the grounding lug in the charge controller (with little crimped on ring terminals):

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And from here it goes to a negative terminal on the battery bank:

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And that is the end of the journey for the equipment grounding conductor on the DC side. The only physical connection to the grounding rod is from the AC side of the inverter.

Any and all comments appreciated!
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Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: Battery Bank as Ground?

    The DC ground and the AC ground in the inverter are almost certainly connected internally.
    For lightning protection, there should be an uninterrupted GEC connecting the panel frames and racking to a local ground rod or other ground electrode. That electrode then is required by NEC to be bonded to the rest of your grounding system (there are arguments about the wisdom of that and the best methods to use if you do it, but that is current code.)

    From there, there are any number of ways of handling the bonding of the DC + or DC - or neither to the grounding system. This is too detailed and contentious an area, with too many changes from one NEC cycle to another for me to comment on here. What is done will depend on the array system voltage, the inverter type and a host of other issues.
    SMA SB 3000, old BP panels.
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  • Tulumtam
    Tulumtam Solar Expert Posts: 37 ✭✭
    #3
    Re: Battery Bank as Ground?

    Thanks for your comments! I do understand how the panel frames and racking should be connected to a ground rod. In this installation, the array and racking are connected to the battery bank and not to a ground rod. I had never seen this before and it does not seem like a good practice to me. Could the battery bank absorb the surge from lightening?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Battery Bank as Ground?

    "Floating ground" is an oxymoron. If the ground wiring isn't actually connected to Earth it isn't grounded, nor is it safe.

    It is common on off-grid systems to have the ground connected to DC negative as well, and both connected to the single ground point which is the same on the AC side.

    No, your battery bank can not absorb the energy from a lightning strike nor even a near-miss. As it is, if any of the wiring gets energized it will be high Voltage through the circuit and things will fry.

    Connect the negative battery post to Earth ground.

    Note that new NEC regs require the (-) to not be connected to ground, but instead utilize DCGFI circuitry. In this case there is still wiring to Earth ground, but it is isolated from negative by the circuitry.
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  • Tulumtam
    Tulumtam Solar Expert Posts: 37 ✭✭
    #5
    Re: Battery Bank as Ground?

    Thanks Cariboocoot! I saw the system and my initial reaction was "that is crazy dangerous"! But I wanted to get confirmation from someone else before suggesting a re-wire. I talked to a couple of local electricians (I am in Mexico) and they seemed to think that this was nothing unusual or uncommon.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Battery Bank as Ground?

    Let's put it this way: without that Earth connection all the wiring is doing is making it possible to energize every metal case of every component in the even of a short. Since the DC is typically low Voltage, most of the time even if this happened you wouldn't even notice. This is why people might think it's safe. But most of the time isn't all of the time. It would actually be safer to have no ground connections between components.

    This is the problem with all types of safety grounding; it does nothing when everything is working correctly. It's there for when something goes wrong. When that happens it can be difficult to sort through the ashes of the house and figure out if it was wired properly or not.
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #7
    Re: Battery Bank as Ground?

    I think that those wires that are spirally wound are a serious error, because all mass and ground connections, especially the connections of the surge protectors, must be as straight and short as possible ...

    in my humble opinion, I would put it straight and as short as possible, to maintain connections impedances as low as possible, so that the earth and surge protectors will be effective ...
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #8
    Re: Battery Bank as Ground?
    unicornio wrote: »
    in my humble opinion, I would put it straight and as short as possible, to maintain connections impedances as low as possible, so that the earth and surge protectors will be effective ...

    Quite interesting, many surge protecting breaker-like devices sold here are sold with spirally wound ground connecting electrode. Why? The instruction says to straiten it out and make it as short as possible, but who reads instructions ....
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #9
    Re: Battery Bank as Ground?
    NorthGuy wrote: »
    Quite interesting, many surge protecting breaker-like devices sold here are sold with spirally wound ground connecting electrode. Why? The instruction says to straiten it out and make it as short as possible, but who reads instructions ....

    is a philosophy of pure aesthetics, there are several schools in this, but I have always liked the american philosophy of wiring: cables and connections for the shortest path even though it is ugly, it is both, technically much more advantageous, and too, so much cheap! .... hehehehe
    inetdog wrote: »
    From there, there are any number of ways of handling the bonding of the DC + or DC - or neither to the grounding system.

    starting to say that I have my battery negative grounded (via a GFDI from outback), I'm saying it's a silly thing to connect the negative to ground in DC systems, for which it is best to connect the positive to ground ... in 95% of industry (telecoms/military/gov) that run in 24v and 48v DC, connects the positive to ground ...

    the main reason for what is better the connection of the positive (which may seem whatever) is just for the corrosion and durability of the ground rod, but this seems to me an important thing, because really, technically, there are no one difference to put the positive or negative to ground ...

    a ground rod with positive to ground last a lifetime! ...;-)

    but take care! ... equipment sometimes (usually) is designed for positive ground or negative ground! ... very few equipment allows choose it, so foolishly, someone connect our negative to ground...so even know that is beter the positive to ground, we have no way to do it because of the equipment we use ...

    silly thing! ...negative to ground!...;-))))
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Battery Bank as Ground?

    For what it's worth, some components may come with their chassis already connected to negative making that the only grounding option. It's one of those annoying things you have to check up on when you look into equipment.

    I've been alive forever it seems, and I remember positive ground for electronic circuits and automotive applications (used to be standard). I know they started switching to negative ground in the fifties, but can't for the life of me remember why. Some confusion over which way the electrons actually did flow, perhaps. Or maybe it was just to make sure cars rusted faster so you'd have to buy a new one. :roll:
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #11
    Re: Battery Bank as Ground?
    Cariboocoot wrote: »
    For what it's worth, some components may come with their chassis already connected to negative making that the only grounding option. It's one of those annoying things you have to check up on when you look into equipment.

    I've been alive forever it seems, and I remember positive ground for electronic circuits and automotive applications (used to be standard). I know they started switching to negative ground in the fifties, but can't for the life of me remember why. Some confusion over which way the electrons actually did flow, perhaps. Or maybe it was just to make sure cars rusted faster so you'd have to buy a new one. :roll:


    yeah!...that is fun! ... in antiquity (hehehehehe) was believed that the way of the electrons was from positive to negative, by the way, hence the name of the positive and negative, but later was found that are the contrary!... ;-)

    now, in this time, current travels from negative to positive, but people were ashamed to admit it and change everything! ... or might please all is based, as you say! ... on the duration of the frame of the cars! .... hehehehe
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #12
    Re: Battery Bank as Ground?
    unicornio wrote: »
    ... or might please all is based, as you say! ... on the duration of the frame of the cars! .... hehehehe
    The biggest reason still remaining for choosing a positive or negative ground system is corrosion of the hot wire. If copper wire is positive with respect to earth and there is a little bit of insulation leakage, etc. the current will carry away the copper until it is gone.
    If the wire is negative with respect to the earth contact, other crap will plate out onto the surface of the wire making it look bad, but the integrity of the conductor will not be compromised.
    That is why copper loop phone systems still use positive ground for their DC.
    In the automotive case, I suspect that the positive ground was chosen to protect the wiring when the insulation materials were not as good for wet conditions, rather than to protect the frame of the car.
    SMA SB 3000, old BP panels.
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