grounding

wastwast Registered Users Posts: 41 ✭✭
is it okay to ground everything to the one grounding rod.. 

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,186 admin
    I am not sure what your system is (AC, DC, multiple buildings, etc.)... The basic requirement is:

    https://www.ecmweb.com/content/who-cares-about-25-ohms-or-less

    The "good ground" rod has 25 Ohms resistance to earth (using special meter). In some areas, that can be a very difficult value to obtain (rocky, sandy, dry soil, permafrost, etc.)....

    But, the basic answer is that you want all of your AC (neutral to earth bond), and DC systems (negative battery bus to ground), and other stuff (j-boxes, green wire ground, plumbing hot and cold water pipes, if metal to ground, now we are even seeing gas pipes tied to ground--This gets complex, more discussion may be worthwhile).

    Besides ground rods, sometimes using a buried "ground plate" is OK.

    Mostly, ground rods are there for directing lightning strike energy to the ground. So, you want a short run (from main breaker panel, solar panel frames on roof, mounting frames, etc.) with a short run of 6 AWG (typical, check your code) in a short straight run to the ground rod.

    If you have two or more ground rods because of your home's configuration (AC main panel on one side, DC battery bus + solar panels on the other side), drive a ground rod next to each (outside of the foundation)... And connect both rods with 6 AWG together (this allows a short circuit to find the return current path and trip a breaker).

    For lightning grounds, you do not want to run them down the middle of the house (from roof top solar), and you do not want long horizontal runs (more than 10-15 feet, the lightning will probably find another path), or sharp bends (lightning may not follow sharp bends).

    That is the quick overview... There is a lot more information if you need it (let us know). Note that NEC historically did not address lightning grounding correctly/optimally.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    edited July 25 #3
    Great Advice Above - Thanks again Bil

    It is also my understanding thru much research that for a whole house AC ground, a GES (grounding electrode system) can also achieve 25 Ohms or less by having 2 grounding electrodes driven at least 6' apart and connected by a 6 AWG conductor.   In many soil conditions this will give you 25 Ohms or less without testing and in some jurisdictions (in the USA) is all that is needed to pass an inspection for grounding (rather than testing).  But as noted in the article,  still may not be good enough to achieve the under 25 Ohms mark.

    But you do not want to have 2 grounding electrodes in an electrical system  to the same house,  when the 2 electrodes are NOT bonded.  Again my understanding is this will give lighting a path into and thru your house electrical system.   This would include sub-panels, arrays, etc.  The grounds should be bonded.

    My only question mark in my head is the battery negative itself.   MidNite Solar's Classic Charge controller has a build in Ground Fault Protection (GFP).  But I don't think it works properly if the battery negative is connected to earth ground. From the classic manual p21-22

    The built in DC-GFP eliminates the need to purchase and install an external DC-GFP. If the internal grounding jumper is installed in a Classic, the battery negative and DC source negative must not be connected to the system grounding conductor anywhere in the system. Grounding of these circuits will defeat the GFP function.

    Somebody else comment here about this?   Not sure how this would effect Any/All charge controllers.

    - Mike


    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • BB.BB. Super Moderators, Administrators Posts: 28,186 admin
    edited July 25 #4
    Note that a standard Midnite classic controller has an Arc-Fault detection system which is a pretty good way of protecting the array from "arc faults" (actual arcing between bad connections or hot to ground).

    There are other models of Midnite classic controllers which do not have "arc fault" detection but instead use a fuse between battery bus ground and safety ground (blown fuse turns controller off). Many other solar controllers have this setup. And you can get a stacked pair of circuit breakers, one is a ~1 amp breaker that goes between the two grounds, and a second (or pair) of ganged breakers that go in the solar panel +/- output which is turned off is that 1 amp breaker trips.

    This is an NEC option for DC Ground Fault protection which puts a fuse between Battery Ground (typically negative terminal) and the rest of the system green wire grounding system. Personally, I believe that this is dangerous and should never be done:

    http://forum.solar-electric.com/discussion/9345/system-grounding
    http://midniteftp.com/forum/index.php?topic=142.0

    Not that not everyone agrees with me...

    The other issues with putting two ground rods that are not bonded together in a system are:
    • If there is a DC or AC short to earth ground, the 25 Ohms minimum resistance is not enough to trip the AC or DC breaker/fuse--And just leaves the systems feeding the short circuit through the ground rod (unsafe and can waste energy). There have been issues where homes were mis-wired and it was only found when somebody notices the snow melting around the ground rod. If you have two unconnected power systems (home and shed), the two ground rod are fine if unconnected with 6 AWG between grounds. If the two locations share the same power source(s) (such as AC power from home to shed, DC/AC Solar backup power from shed to home, etc.), then the two ground rods should be connected together. That way if there is a short in the "other location", the AC/DC current can go back to the source and trip the circuit breaker/fuse/etc....
    • The other issue with two ground rods separated is that when you have a lightning strike, you get a "voltage gradient" in the earth... And you can have hundreds or thousands of voltage difference between two ground rods. In an industrial (water pumping) setting, I have actually received a 60+ VAC shock between two ground rods driven ~100 feet apart because of wiring/grounding issues.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    edited July 25 #5
    Bill ... Poetic.  Taking the very complex and helping the likes of me understand it.   I can see this in my head,  but would have taken many more words and done a much poorer job explaining it.
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    But when the OP says, "one grounding rod",  would it be better to have 2 ground conductors bonded together in most cases, creating a GES (grounding electrode system) for just about any whole house grounding?
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • BB.BB. Super Moderators, Administrators Posts: 28,186 admin
    Most people (in the US) have metal cold water pipes. As long as you ground to them too... You probably have much better ground than any humber of ground rods can make.

    If you are worried about lightning, having multiple ground rods that make the run from the "grounded object" directly to ground is probably a "better thing".

    The issue is that lightning is not DC or low frequency AC... It is still, relatively, low frequency, but really behaves more as a radio wave and complex impedance is now coming into play.

    With lightning, braided wire, flat braid/conductors/ground plates, and other shapes (shallow bends vs sharp bends) all affect the impedance of the condutor. And when you start taking about feet and 10's of feet, these differences/accumulations of impedances can inhibit lightning current flow, and the current can/will find other paths.

    25 Ohms DC/60 Hz is one thing for a ground rod... More or less, bends, diameter/shape of conductors does not matter too much (although, because AC current tends to migrate to the "skin" of the conductors, for example, utilities will use hollow copper pipe as busbars in power stations (I believe/remember from past lives).

    https://en.wikipedia.org/wiki/Skin_effect

    If you need to control for lightning (ground strikes relatively common in your area), here are some starting reads/links:
    Re: Working Thread for Solar Beginner Post/FAQ

    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 FAQ:

    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
  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    edited July 25 #8
    BB. said:
    Most people (in the US) have metal cold water pipes. As long as you ground to them too... You probably have much better ground than any humber of ground rods can make.
    Yeah ... no metal pipe here.  while I am in the US, everything from my pump (25 ft from the house) to and thru the house is PVC.  funny story though,  When I went to check the whole house ground,  I discovered that a water faucet was 2 ft from the only grounding electrode.  The previous owner / DIY builder of this house ran a braided copper conductor from the grounding electrode to the brass water faucet.  I assume to pass inspection.  Funny thing was,  when the facets pipe went thru the wall,  there under the house it was connected to PVC ... LoL.  No ground there.   I cut it off from the faucet and sunk another grounding electrode 7 ft from my original solo electrode and bonded the 2 together. 

    BB. said:
    If you are worried about lightning, having multiple ground rods that make the run from the "grounded object" directly to ground is probably a "better thing".
    I actually have my array and DC equipment and inverter grounded to a grounding electrode closer to them directly,  and then have that electrode bonded to the rest of my GES (grounding electrode system) for the whole house.

    Still waiting for the OP to respond.  Not trying to hi-jack the thread.  But perhaps the OP will find this discussion of value

    Mike
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • wastwast Registered Users Posts: 41 ✭✭
    thank you for the response.. water lines are pvc so will have to use grounding rods...don't see much lightning in our area but will use a couple of ground rods to ground system
Sign In or Register to comment.