Yet another grounding question

Options
Desert Rat
Desert Rat Solar Expert Posts: 138 ✭✭✭
in Off Grid Solar & Battery Systems #1
Hey y'all, new member here. I've been reading posts here with great interest and look forward to learning more, and perhaps helping out with some of my own experience. I've lived with solar electricity for 20 years and am in an ideal place for it (29° latitude and very rarely cloudy). In addition to designing and installing my own system, I've set up a handful of systems for others.
I was working on a system today (not one of my installations) that had several things not done properly, many of them ground-related. I usually feel like I have this stuff figured out, but sometimes when it comes down to making the connections, I have questions. Here's the question I had today:
This system has a Trace 2512 inverter. The installation instructions say to connect the chassis ground at the right end of the inverter to the DC grounding system. This system has a ground bus inside the Trace inverter disconnect where the grounding electrode conductor, battery negative, and equipment grounds meet. Then at the AC end, the ground wire which goes to the AC distribution panel connects to the chassis grounding lug on that end of the chassis. So the AC ground travels through the metal case of the inverter? Seems to me that the AC ground should be connected directly to the ground bus in the inverter disconnect for a more secure connection. Thoughts?
· Share on Twitter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #2
    Options
    Re: Yet another grounding question

    Ah grounding... One of the more "fun" conversations here...

    First, there are several reasons for grounding. One is safety with respect to your power circuits (12 VDC, 120 VAC, etc.).

    The second is for lightning/surge protection... So lets start with the first.

    Start with what you want call "ground". A 10' copper rod in the ground at the edge of the building next to the main panel... Cold water (metal) pipe, etc... for fixed installations.

    Now--we assume that the water piping grounds sinks, gas heaters, electrical boxes, water heater, etc... Any "large fixed metal objects".

    So, if there is a 120 VAC hot to metal ground somewhere, the return current will flow back to the source (main panel, inverter etc.) and pop a fuse or shut down the inverter (rather than make the metal sink "hot"--and electrocute you when you have a hand in the sink water and another on the faucet.

    Nominally, the ground rod (or cold water) pipe needs to be tide back to the common AC neutral bus bar (and if you have metal breaker boxes, conduit, green wire to outlets, etc... That neutral/earth ground bond bus bar is where all the AC power should return to... And a single ground wire (around 6-8 awg depending on your code) back to the earth ground rod. That way, if there is a short anywhere in the system, it will pop a breaker/fuse in the AC supply.

    NOTE: Most MSW Inverter cannot have their output "neutral bonded to ground"--It creates a dead short through the inverter from the battery bank and will usually let the magic smoke out of the inverter if you do this neutral/earth bonding (and ground the battery bank to the same earth bond). Most TSW Inverter have "floating" or "isolated" AC outputs and you can earth bond the Neutral to safety ground (just like any north American home). You need to check the inverter (and AC generator if you have one wired up) to see if it is "safe" to ground the Neutral/White wire with an AC Inverter (smaller generators generally have a floating H-H-G output and those over ~3.5 kWH actually have the H-N-G tied together (N-G) inside the genset).

    You do not want multiple Neutral/Earth ground bonds--It creates a parallel current path and can cause problems in some conditions (especially if you multi bond 12 VDC returns--Where the current is 10x larger than the 120 VAC current--you can over heat the "smaller" AC ground wires).

    For DC Battery, you want a similar setup... Take a ground wire from the common ground rod (or water pipe) connection to the Negative Terminal on the battery (or common negative bus). In that one point only. The wire should be at least 6 awg (can be heavier).

    Your "safety ground" becomes a bit of an issue... DC powered devices (like your AC inverter) can have a short between AC and chassis ground, or between DC and chassis ground. For a 17 amp AC output inverter (around 2kW), you will have ~170 Amps @ 12 VDC (or more) current available (fuse/breaker). Obviously, you would want the Inverter Chassis ground cable to be on the order of the same size of the DC cables supplying power to the inverter so that if there is a dead short to chassis, the ground cable is heavy enough to pop the DC breaker to the inverter:
    • 2,000 watt inverter * 1/10.5 volt cutoff * 1/0.85 inverter eff * 1.25 NEC derating for wire/breaker = 280 Amp rated fuse/branch wiring for rated power

    You would not want a 12 or 14 AWG wire to "safety ground" the Inverter's chassis ground. 14 AWG wire would "fuse" at ~167 amps.

    The place where you Bond the White Wire / Neutral to safety ground could take place inside the AC inverter (and some inverters to that from the factory). Or you can have the Neutral/Earth Bond in the AC Main panel (typical location for North America). Just choose one place.

    Note, if you have a larger genset, you will need to research its neutral bonding... You may lift the bond at the generator (use the main panel bond). Or you may get an AC transfer switch which will switch all power wires (hots and neutrals) so that on the inverter you have one bond point in the inverter or main panel, and a second ground bond at the generator (depending on which circuit is active). This is very common with RV's where the RV Park supplies earth bonded neutral, but you need your own White/Green wire bond on the road/dry camping.

    If you have lightning issues, we can discuss that next... Or here are some threads that talk about grounding too:
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Options
    Re: Yet another grounding question

    I think the NEC specifically forbids the use of component cases or conduit as conductors. My advice would be to run wires and not rely on a metal housing to make the connection from A to B.
    · Share on Twitter
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Options
    Re: Yet another grounding question
    Cariboocoot wrote: »
    I think the NEC specifically forbids the use of component cases or conduit as conductors. My advice would be to run wires and not rely on a metal housing to make the connection from A to B.

    You cannot use cases or conduit (either rigid or electrical metallic tubing (EMT) which most people think of as conduit) as a current carrying conductor (e.g. the neutral)
    But you can use it as (part of) the Equipment Grounding Conductor, and under some limited circumstances as part of the Grounding Electrode Conductor.

    It is common and code compliant, although disliked by many electricians, to connect the neutral bus and the ground bus in a service panel entirely by means of the little green bonding screw which optionally connects through the neutral bus into the panel case.

    In other situations, the conduit which is acting as an EGC on one side of a junction or breaker box might have to be connected (bonded) to the ground bar from each entry point rather than just using a bushing which is specifically designed to bond the conduit to the box.

    Within a single UL listed assembly, whatever the manufacturer got approval for is de facto acceptable.
    SMA SB 3000, old BP panels.
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Options
    Re: Yet another grounding question
    inetdog wrote: »
    Within a single UL listed assembly, whatever the manufacturer got approval for is de facto acceptable.

    And yet not necessarily a good idea. :p

    Thanks for clearing that up!
    · Share on Twitter
  • Desert Rat
    Desert Rat Solar Expert Posts: 138 ✭✭✭
    #6
    Options
    Re: Yet another grounding question

    Thanks for your answers. I love reading about grounding, even though it gives me a headache.
    :confused:


    BB. wrote: »
    If you have lighting issues, we can discuss that next...

    No, my lights are working just fine.
    ;)
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #7
    Options
    Re: Yet another grounding question

    Lightning... :p

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Options
    Re: Yet another grounding question

    desert rat,
    i love your avatar. i bet that doesn't bode too well with your power production, but i'm sure the cat is nice and warm.

    ftr, grounding gives us a headache too.
    · Share on Twitter
  • Desert Rat
    Desert Rat Solar Expert Posts: 138 ✭✭✭
    #9
    Options
    Re: Yet another grounding question

    Way off topic, but since you mentioned the cat:

    The cat in my avatar pic is Moses, who showed up at my house back in the summer of 2005. I have no idea where he came from. I had other cats at the time, and he and the other male cats were trying to kill each other as male cats do. I trapped him and took him to a neighbor ( 8 miles away) who was willing to take him. Six days later he was back at my house! I trapped him a second time and returned him to the same neighbor. Another six days passed and he showed up again! At that point I realized he was going to live here no matter what I did. Bear in mind that this was June in the Chihuahuan Desert, 110 degrees, no water anywhere, and coyotes and hawks all over the place. How in the h*ll did he find his way, let alone survive all the perils? I named him Moses in honor of his trek across the desert. All my other cats died off after a few years, but Mo remained until this past Thanksgiving week when he died of kidney failure. He will be missed. Here is a pic of my house just to give you an idea of the terrain here.

    Attachment not found.
    · Share on Twitter
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
    Options
    Re: Yet another grounding question

    thanks for that story. look for a pm so the thread isn't kept off topic.
    · Share on Twitter
  • Desert Rat
    Desert Rat Solar Expert Posts: 138 ✭✭✭
    #11
    Options
    Re: Yet another grounding question

    Niel,
    I got your PM and replied, but it's not showing up in my Sent folder. Did you get it? Seems like I've seen some forums where users can't use the PM feature until completing a minimum number of posts. ??
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #12
    Options
    Re: Yet another grounding question

    You should be OK... In this software, you have to check the "save sent messages" box save the sent messages.

    Try clicking on this link and about 1/2 way down is the option to save sent pm's.

    http://forum.solar-electric.com/profile.php?do=editoptions
    Sent Private Messages:
    • Save a copy of sent messages in my Sent Items folder by default

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • Desert Rat
    Desert Rat Solar Expert Posts: 138 ✭✭✭
    #13
    Options
    Re: Yet another grounding question

    Got it. Thanks, Bill.
    · Share on Twitter

Categories