Understanding DC grounding

eeqmc2
eeqmc2 Registered Users Posts: 3
in PV Installers Forum - NEC, Wiring, Installation #1
Hello All,

I've found a lot of very useful information on this forum and have since completed my first small solar installation. I have a question however on the actual purpose and hence wire size for a DC grounding system.

As I understand it (probably incomplete at best) the ground bus, which is connected to the DC Neg bus is to provide a place for current to go in the event of a short circuit, or lightning strikes. Are there any other reasons a ground line exists?

In the case of a lightning strike, is any wire size going to actually be enough to not melt and risk fire?

In the case of a short circuit, since the battery is connected to the DC Neg bus, isn't the maximum current potential to flow thru the ground wire the maximum AMPs that can be pulled from the battery(ies)?

I'm a bit confused because all the inverters or charge controllers I've seen carry smaller ground terminals (usually a 1/4 bolt or #8 raw wire screw lug) than the Pos or Neg terminals (usually a 3/8 bolt or #1 or #2 capable raw wire screw lug).

Basically I just don't want to undersize my ground wire, but at the same time I don't want to overpay for 4/0 cable if I don't have to. My earth ground rod is about a 10ft run from my batteries and about 40ft from my panels.

Any insight would be enormously helpful.

Many thanks,
Geoff
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Understanding DC grounding

    Welcome to the forum Geoff.

    The purpose of the grounding system is mainly to supply a path that will force current high enough to trip circuit protection in the event something goes wrong. It does not carry any continuous current the way normal conductors do. As such the wire size does not need to be as large because it will only carry current momentarily - until the CP breaks power. Much of a conductor's current capacity depends on its ability to dissipate heat, you see. Usually only 6 AWG wire is used for grounding conductors on both AC and DC systems. It's actually capable of taking 100 Amps for a minute or so and even more for less time. As always, there will be exceptions.

    As far as lightning goes there is nothing that will protect against a direct hit: enough electrical energy to jump miles and create temperatures hotter than the sun's surface is not going to be controlled by a bit of copper or steel. It is more a case of protecting equipment against the stray Voltage that builds up in an area where the strikes occur: the further from the strike the lower the Voltage, but there can be plenty available to fry components meant to operate at <600 or < 60. Thousands of high frequency Volts can still do damage even if there are lightning arrestors connected. It's a bit of a crap shoot.

    Grounding in general and lightning protection in particular are two of the "liveliest" subjects there are in electrical engineering. :D
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  • eeqmc2
    eeqmc2 Registered Users Posts: 3
    #3
    Re: Understanding DC grounding

    Fantastic. Thanks. That was the other area of confusion. The fuses/breakers. So basically before the fuse blows or the breaker trips the excess current is funneled (for a very short time) thru the ground wire. Then the circuit becomes open and no more current passes thru the loop so the cables / equipment survives.

    What about battery short circuits? Is that a concern? I like to completely isolate my terminals as much as possible, but never say never. Is the battery ground thru the DC Neg bus connection to the ground bus enough to handle that?

    Thanks again for your prompt lucid response.
    Geoff
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Understanding DC grounding

    The best thing you can do to protect against battery-drive problems is to use post terminal fuses (Blue Sea) and be very, very careful. Not so much Voltage there, but enough current potential to weld steel. We are talking hundreds of Amps in an instant.

    If the battery itself shorts (it happens - usually only one cell) it will drop approximately 2 Volts from its potential but the current is pretty much all still there. They're dangerous things; treat with care.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
    Re: Understanding DC grounding

    Batteries have very low internal resistance (on the order of 0.001 to 0.010 Ohms for larger flooded cell banks--AGM probably have less).

    So, it is possible to get hundreds of amps into a dead short from even a couple golf cart batteries (220 AH capacity).

    Each battery should have a fuse that is 1.25x the maximum nominal current you expect that bank (or string) to feed.

    So, if you have a 1,200 watt 12 VDC inverter and want to run it near 1,200 watts for a period of minutes or longer, then:

    1,200 watts * 1/0.85 inverter eff * 1/10.5 volts batt cutoff * 1.25 NEC derating = 169 amp minimum branch circuit

    If you have one or two batteries strings in parallel, I would suggest that each be fused for 175-200 amps minimum (using above example).

    If you have 3 or more strings, I would suggest fusing to at least 1/2 of the max continous loads (i.e, 1/2 * 169a = 85 amps)--So ~100 amps minimum (and wired appropriately).

    Next is sizing the DC ground wire... A simple rule is the ground wire should be the same AWG as the +/- cables to the load.

    In practice, that is expensive and probably over kill in most situations. We usually suggest a minimum of 6 awg for grounding (partially, that is because the NEC uses bare 6 awg wire for grounding and buried ground wire as a minimum--as I recall).

    Looking at the generic fusing current for a 6 awg is ~ 600 amps. There for a maximum of 200 amp fuse should be tripped if the output is shorted to a 6 awg cable...

    If you have >200 amp fusing/breakers to your loads--I would suggest that you look at increasing the AWG by +3 gauge for every 2x 200 amp current rating you use (i.e., 400 amp mains, 6awg-3awg doubling of copper=3awg -- So 3 or 2 AWG would be appropriate.

    For ligthning--that is a whole 'nother kettle of fish... Basically, we suggest bringing grounds from the solar panel frames directly down the outside of the building to the ground rod(s) driven next the foundation. Shortest way to ground, straight runs with wide turns (like 18" radius) if needed.

    The lightning ground rod may be alone, or can be the same as the main AC panel ground rod--If they are in the same location.

    The DC ground(s) should run to the AC Main Ground Rod... And I would recommend that you tie your multiple ground rods with a 6 awg wire from the remote rod each as a "home run" to the master AC/DC ground rod.

    If there is a short circuit in the solar array, you want the current to return down to the lightning ground rod, then back through the 6 awg ground wire to the master ground rod, then to the DC ground.

    What you don't want is for a DC to metal short (i.e., inverter frame ground) to take a 14 AWG AC safety ground wire back to the AC Main Panel ground to AC ground rod. A 200 Amp fuse/breaker may supply enough current to fuse the 14 awg AC Safety green ground wire. (as one example).

    Grounding can get more complex... We talk about "single point grounding", but when you run 100' to an out building (or well head pump)--what do you do. Compromise is usually the answer. Lightning grounds need to be short and close to what is being protected. AC/DC grounds can be longer (lighting is actually a ~7,500 Hz maximum AC wave form--Basically a low frequency radio wave--And has to be wired as if it was RF energy.

    Some more reading:
    BB. wrote: »
    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidated FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    -Bill

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • eeqmc2
    eeqmc2 Registered Users Posts: 3
    #6
    Re: Understanding DC grounding

    Thanks guys, that's very helpful.
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #7
    Re: Understanding DC grounding

    For more further reading, see the debate we had in the second half of this thread.

    www.wind-sun.com/ForumVB/showthread.php?17880-Ungrounded-PV

    Down the rabbit hole, for sure, but I thought it was a good discussion.
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