Ground necessary?

garynappi
garynappi Registered Users Posts: 105 ✭✭
edited April 2020 in Solar Beginners Corner #1
Sorry, but I found no discussion on the issue for a purely DC system.

I've finished my outdoor lighting and water feature installation, and added a second panel as recommended here to keep my 95Ah battery charged on days with less sun at "optimal tilt angle" and I'm very pleased with its operation. 

The panels (100w and 125w circuit breaker protected) are on a stucco house wall, 12' above a patio, below the roof line. 

My question is, given that I am not grid tied, and do not have an inverter for any AC purposes, do I need a ground wire between the panel brackets to the combiner, the charge controller case and to earth ground? 

If so, given a 35' run down and across a stucco wall, does the ground wire need to be in a conduit? Separate from the panel DC down leads, galvanized or schedule 40/80? Is 6 gauge adequate? 

To add to my confusion my combiner, DC down wire conduit and charge controller cabinet are all plastic or PVC. do these have to somehow be grounded? 

I know the subject of PV grounding has been discussed at length here and on the web, but my installation being DC only has given me reason to ask. 

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Are you in a lightning prone area? As short an answer as I can do:
    1. Grounding (frame, or battery negative bus) do not need grounding for functional reasons (everything will run fine if grounded or not).
    2. Grounding for Electrical Safety: Negative grounding the battery bus (to ground rod/cold water pipe) is "the right way" to let you use fuses/breakers in only the positive leads/wires from the battery bank. Grounding the solar panel frames/any metal panel mounts, again to keep a short circuit for "energizing metal" (at 12 volts, not usually a shock hazard).
    3. Lightning grounding: If you are prone to lighting in your area, running a 6 AWG minimum wire from panel frames/metal mounts down the roof, down the outside wall, to a ground rod close to the foundation (avoid sharp bends). You want lighting energy to not go into the house but follow 6 AWG cable to ground rod. Ground bonding the negative battery bus to a ground rod can be helpful for lightning control. Surge Supressors are nice, but may be overkill for small system). A 6 AWG cable from "panel frame lightning ground rod to the house/battery negative bus grounding system to "trip fuses/breakers in the event of a short circuit to battery bus).--
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappi
    garynappi Registered Users Posts: 105 ✭✭
    edited April 2020 #3
    Thanks Bill,

    Yes for sure we're lightning prone in south Florida!  They have lightning sensors all over my city with klaxon horns to warn that lightning has occurred somewhere in a 5 mile radius.

    Your point #1 is basically where I'm at for the last two years. 

    I've not been so worried about damage from lightning as there are a LOT of much taller structures nearby which will likely take the brunt of a hit though some level of protection won't hurt. I guess my main concern has been protection from a system fault like a panel, battery or cable failure. 

    Your points: "Negative grounding the battery bus (to ground rod/cold water pipe) is "the right way" to let you use fuses/breakers in only the positive leads/wires from the battery bank" and "A 6 AWG cable from "panel frame lightning ground rod to the house/battery negative bus grounding system to "trip fuses/breakers in the event of a short circuit to battery bus"
     
    Is not completely understood though. My system has basically a "floating" ground, but since it has a complete DC ground return, I am puzzled. Without an earth ground are you saying my panel breakers in the combiner, or the breaker on the lead to the positive terminal of the battery may not work at all? 

    PS, I have a supply of 6 AWG THHN which I "think" is best used in a conduit?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    A "floating" power system is the "safest" type. You cannot get a "shock" from touching anything. Just like a simple battery, you touch one terminal, nothing happens... Hit with wrench, gold ring, etc.... Nothing. The only "danger" is touching both + and - at the same time--A breaker/fuse in either line, and it will stop any short circuit issues.

    The problem--Is when you have a "fault" and any subsequent fault. Say you have an RV system. And it is "floating". You have the large 12 VDC connections (AC inverter, charge controller, etc.). And you have a small circuit for 12 VDC LED lighting. And you only have one circuit breaker in each positive lead (200 Amp + lead to inverter, 5 amp in + lead to LED lights/switches/etc.).

    Somehow, one of the LED "negative" leads gets shorted to sheetmetal (cut in insulation). All is still fine, because "one short' to ground, no extra current flow in ground.

    Two years  later, you have a fault from AC inverter to chassis ground (FET, inductor, something comes loose and shorts to case, which is tied to RV metal chassis ground).If there was "no other short" in the system, again nothing happens. But now, we have a short in the negative lead of the LED ceiling lamp to chassis ground. And we have a current path from + battery bus -> 200 Amp circuit breaker -> inverter -> chassis ground -> LED 18 awg negative wire -> negative battery bus (no 5 amp fuse/breaker in negative lead). And you now have "lit up" the LED lighting negative wire, insulation overheats, ruins the wiring and possibly starts a fire.

    That is why, a floating power system is supposed to have "two breakers/fuses" on each circuit.... If there was both a 5 amp fuse on the + and a second 5 amp fuse on the negative LED wiring, then the second negative fuse would prevent the overcurrent/fire.

    If you run a "negative/return" connection to chassis ground... The fault current will flow through the negative-ground bond--Sized to handle a 200 amp fault current and pop the 200 amp breaker (6 AWG should be good up to 200 amp ground connection for a quick >200 amp fault surge until breaker pops--6 AWG will "fuse" around 600 Amps.

    That is why most floating system that people make, even if fused on the + lead/branch circuits, can still be "unsafe". It is the "subsequent faults" that catches folks many times during a power system design/installation.

    NEC and such is not supposed to be concerned with two or more faults (one fault, safe shutdown). However, for floating power systems, the "first fault" is undetected and does not shut down the power distribution... Setting up for a second/subsequent fault situation.

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