12v PV power distribution problem

sidsid Registered Users Posts: 4
A couple of days ago, after years of problem-free operation, my power distribution circuits went crazy. Here is the layout:

I ran a vacuum cleaner off the inverter, something I've done many times before without issues, and after a minute or two it stopped. The charge controller displayed 13.8v battery voltage but the voltage across the breaker bus went down to 6v. The Low Voltage Disconnect didn't activate. I set all of the breakers to OFF and the voltage there returned to 13.8v. If I turned any of them ON, with nothing active on the circuit, eg, no lights switched on, the breaker bus voltage dropped 1 - 2 volts. I measure 1.25v from negative to earth.

Does this mean I have a ground fault? How do you locate ground faults?

Thanks for help with this.


  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Is the LVD part of a Li Ion BMS system?

    Normally, "switches" should be in the "positive" wiring of a 12 VDC negative ground system. Your LVD in the negative/return lead to the inverter would mean that +12 VDC lead stays at +12 to ground, and the negative lead to the inverter would go upwards to +12 VDC (depending on DC loading).

    You also have a 250 Amp fuse where it really does not "protect" the 12 VDC wiring anywhere and (in theory according to your drawing) the 30 amp breaker is the "operative" over current protection device...

    The 30 Amp breaker is in the correct area (positive lead) and should be "near the source of high current" which is the battery bank for almost all solar power/DC power systems. This breaker is to protect against short circuits anywhere "down stream" of the breaker/fuse. So keeping the breaker near the battery bank is ideal (shortest length of "unprotected" wiring).

    Do you have a physical Negative to "Ground" (cold water pipe, ground rod, AC/DC System "green wire grounding") somewhere?

    Otherwise, check the wiring in the system. Look for loose/corroded/browned/blacken wiring and connections. "Bad connections" tend to overheat and show visible signs (insulation changing color, etc.).

    Ground faults can occur--But need to better understand your system... Where is the grounding done now. Measure voltage with respect to "ground" (i.e., DMM black lead on "system/home ground", and measure the voltage on all the V+ wiring, and the V- wiring.

    The "voltage" between V+ should be "battery voltage" and between V- and "ground" should be less than 1.0 volts typically.

    If the battery bank is getting old (>3-5-7 years for flooded cell batteries)--It may be possible your bank is having issues. Which you can only measure when you have loads (like your vacuum running) on the inverter and/or other heavy DC loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mvasmvas Registered Users Posts: 395 ✭✭✭
    Start by putting your volt meter directly on the battery bank terminals.
    Do you read 13.8 volts?
    Next move the volt meter neg black lead...
    to the disconnect switch.
    Then move after the disconnect switch
    Then move it to lvd switch
    Then move it after the lvd switch
    Then move it to the 250A fuse
    Then move it after the 250A fuse
    Then more it to the shunt
    Then move it after the shunt

    Then move the volt meter pos red lead ...
    To the 30A breaker
    Then move it after the 30amp breaker

    By "walking" the meter down the circuit
    at some point, you will see the voltage drop.
    This will identify the bad component.
    Done ...

  • sidsid Registered Users Posts: 4
    edited May 8 #4
    mvas, thank you for your helpful reply which has put me on the right track to find the fault(s), I think.  I will try out the process at the site.

    Bill, thanks for these ideas.  I can see that the system needs improvement.  
    - the LVD is stand-alone: https://www.amazon.ca/gp/product/B06VWLFTJ8/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
    - the ammeter I bought said to put it on the low side so I put the fuse, LVD and switch there too, all in the same box, for convenience.  I will rearrange them as you suggest.
    - I'll move the 250a fuse and inverter to connect to the batteries directly, not through the 30a breaker, LVD and shunt.
    - I don't have negative connected to earth.  Only the inverter enclosure and breaker box are grounded.  I drilled a 6" hole in bedrock, put the ground wire in and filled it with molten lead.

    Should I ground the negative side?
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    Grounding is a complex subject....

    Do you have a chance of lighting strikes in your area? Then grounding can be a good thing. Lots of things to consider... And rocky ground is very difficult to get a "good earth ground".

    Otherwise, if this is a vehicle (RV, trailer, etc.)--Then grounding to the metal frame is done for electrical safety (avoid shock/electrocution)--And if short to ground, then will trip breaker(s) and/or fuse(s).

    If a home or cabin, then grounding cold water pipe, gas pipe, etc. to avoid a short from 12 volts (really 120 VAC) to something "big and metal" has a return path to the source of power (again to trip fuse/breaker rather than "energizing" a sink and getting electrocuted to a faucet, etc.).

    Things like antennas, telephone, cable TV, etc. can "bring" lightning into the home... Usually more complicated than just "grounding 12 Volt negative to earth" as an answer.

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