System Grounding

OffGridRoryOffGridRory Posts: 26Solar Expert
Hello again everyone!

I have a question about grounding. Should I ground every piece of hardware in my system that I can? I'd assume the big pieces of hardware that require grounding are the Charge controller and the Inverter, but is there anything else that I should ground in the system?

(I'm working with a 1500w inverter, 410ah battery bank at 12v, 45A charge controller, and 205w of panel at 24v)

Thanks guys...

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: System Grounding

    On February 2nd, the Groundinghog pokes his head out .... :p

    Seriously. The panel frames/mounts should be grounded. The charge controller. The inverter.
    Right there is where we start running into the "ifs".

    You should ground the inverter and the (-) battery terminal. Now, if it is an MSW type inverter avoid the neutral-ground bond on the AC side or bad things will happen. Ground only the AC ground; do not bond to neutral. If it is a True or Pure Sine Wave inverter there should be no trouble.

    Or the usual disclaimer; always follow manufacturer's instructions. :roll:

    And generators. They can pose some situations with grounding. most have 'floating neutral' - not bonded to ground. This is fine, as there should be only one neutral-ground bond on a system, if any.
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Is this a home, RV, on grid, off grid, lightning prone area, etc.?

    If you are powering 12 VDC gear, much of it is designed for automotive use with a + wire and a metal chassis connected to the framework of a car. The lack of isolation between battery "-" and the frame/safety ground can be an issue--especially if you have antenna (FM car radio, CB gear, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OffGridRoryOffGridRory Posts: 26Solar Expert
    Re: System Grounding

    The system is a "emergency power" system... AKA me having fun with solar because I can't afford the massive system that I would like right now... The idea is to have this system set up for the next few years in my rental house (completely separate from anything else in the house) and run a few laptops etc off it in the mean time... Living in rural Maine it will give me a few days of entertainment etc during the frequent power outages, but it is an off grid system...

    I'll ground the panels, inverter, and charge controller then... Thanks guys

    Oh... I guess I should ask advice on gague wiring... I have tons of extra 6 guage... Is this adequate for grounding? Its coated, but high temp "industrial" wiring, so I'd assume it can handle as much as any 6 guage wiring can...
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    Related to this, should the DC components (like a charge controller) in a system be grounded with a separate ground rod than AC components?

    I have the grounding system from our household grid system easily accessible right near my RE equipment. Is the ground rod for the grid AC acceptable, or do I need to drive a separate ground rod for grounding my DC stuff?

    Not sure if it's good to mix AC and DC on a single ground rod.

    Then there's stuff like small breaker boxes that house DC only breakers. My understanding is that grounding like this is primarily for shock prevention, so I'm not sure if it's really necessary to ground everything that houses only 48VDC.

    Edward
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Two general reasons for grounding...

    First would be to prevent metal from being energized and causing a shock/fire hazard. Ideally, all metal (plumbing, cases of electrically powered equipment, etc.) should be grounded. That way, if there is a short (for example inside a metal case) the short circuit will cause excessive current to flow and blow a fuse/circuit breaker.

    The second reason is to safely direct lightning strikes and static charges safely to ground instead of bringing energy into your home/work/etc.

    The recommendation is to only tie the AC and DC grounds together in one place to prevent multiple ground/return current paths...

    As an example... You tie the negative terminal of the battery to the AC ground rod (one place--per "rules"). And say you use a car 12 volt radio or CB unit. The meal case of the radio is -12 VDC (case is the power return in a car). And you have an outside whip antenna that has the ground braid tied to earth ground where it enters the home (for lightning/static charge build up suppression).

    And here is the problem... If the antenna ground is tied to AC ground--you now have two electrical paths for your return current... One is through the -12 vdc return lead, the second is through the antenna ground braid. Depending on 12 volt loads, wire gauge, and wire length, the return current may actually go through the antenna braid to AC/DC ground point to negative battery terminal. If there are high 12 VDC currents (say a 1,200 watt inverter, > 100 Amps DC), much of the 100 amps may return through the antenna braid/safety ground--And the antenna braid or safety ground may overheat with the >100 amps if the 12 vdc return has higher resistance or if the -12 VDC connection broke for some reason.

    The other issue is lightning. Say you tied the antenna braid to a separate ground rod at the edge of the home (separate DC ground--should be OK?--No)... Now you have a two point DC ground system. One point is at the AC ground rod, and because of the common -12 VDC connection, you have a second ground at the antenna earth ground.

    You get a nearby lightning strike and it causes a high voltage "gradient" from the lightning strike through the ground out to your home. You have two ground rods XX feet apart. Once may be thousands of volts above the other because of the lightning strike--and that voltage/current is now injected into your DC ground/return system from the antenna ground rod, through your 12 VDC return, to the battery bank, to the AC/DC ground rod.

    The above is one example of a "ground loop" that should be avoided--and can be difficult to avoid with "car electrical equipment" which assumes that 12 volt return and 12 volt ground are the same connection.

    There are other possible DC ground loops (several DC powered equipment and audio cables with common shield grounds connecting the equipment together would be another example).

    These sort of issues are another reason why I tend to like "AC inverters" better than many direct 12 VDC equipment. AC equipment has a separate AC safety ground separate from the AC electrical power connections (and a single point AC ground to AC neutral connection typically in the main electrical panel).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    Thank you Bill for the great explanation. I read it through a few times, and I'm still chewing on and digesting the good information.

    I have the new Midnite Classic controller with built in ground fault protection. In the Classic instructions, it says that with this safety feature built in, then the negative side of the battery bank should not be tied to ground. So, what I was planning to do was change my system so that I have a grounding busbar where I will tie together the following grounds: Classic controller, 2 DC breaker boxes, case ground from inverter. Then I plan to run a 6 gauge ground wire several feet over to my AC ground and tie it into that one place.

    Fortunately, everything is very closely situated. My RE equipment and battery bank are in my utility room. A few feet away is my water heater which has the big braided ground wire connected to the copper pipes. Right next to that (on the other side of the wall in the garage) is my electrical panel. So, the ground wire from my DC equipment can tie in right at the water heater. I like that fact that the run is short. I have plenty of 6 gauge wire and I could even use 4 gauge if that would be better.

    1) Does my plan of changing my ground system as described above sound sensible?

    My main concern is for safety, but I'm not too concerned about getting shocked from the metal cases on the DC breaker boxes. It's 48VDC (nominal) and when I've touched across 48VDC before, I've only felt a tingling if I had some perspiration on my hands. So, I don't think that shock is the biggest issue with the lower voltage DC stuff. Probably sparking and fire is a bigger concern there.

    As far as lightning protection, I did drive a ground rod in at the base of my steel wind turbine tower. I used a 4 gauge wire to tie the tower pipe to the ground rod. That ground rod for the wind turbine is located far away from the AC grid ground rod (maybe 200 feet). That ground is independent of any of the turbine's 3 phase electrical output. For the 3 phase output of the turbine, I have a Delta 3 phase lightning arrestor. I have had the ground wire tied from the Delta arrestor to my AC ground at the water heater.

    Lightning is a very low risk here. I live in a canyon that runs along the base of a small mountain range. Lightning almost always strikes up above. The top of my wind turbine tower is still very LOW relative to most of the surrounding terrain. Lightning in general is very rare here, but when I've seen it, I've never even seen lightning strike down in the canyon. I've always seen it strike up on the surrounding higher elevations. Even so, I know it's possible, so I tried to take some precautions with my wind turbine.

    2) I wonder if somehow that 3 phase lightning arrestor being connected to my single point AC/DC ground tie will cause any potential problems?

    Thanks again for the help!

    Edward
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Part 1:

    You are very welcome Edward... I try to give the reasons behind the rules since it is almost impossible to have a rule that addresses every situation.
    I have the new Midnite Classic controller with built in ground fault protection. In the Classic instructions, it says that with this safety feature built in, then the negative side of the battery bank should not be tied to ground. So, what I was planning to do was change my system so that I have a grounding busbar where I will tie together the following grounds: Classic controller, 2 DC breaker boxes, case ground from inverter. Then I plan to run a 6 gauge ground wire several feet over to my AC ground and tie it into that one place.
    Ok, I am going into deep water here... And take what I say with a grain of salt. boB or Ryan would probably a much better source of information regarding this than I.

    As I understand people's solutions to the NEC requirement for ground fault protection for DC solar arrays, it really does violate the very definition of safety ground.

    Pretty much all DC GF systems put a 1-5 amp fuse/breaker/ or in the case of the Classic a 1 amp PTC Resistor (basically a resistor that when it passes too much current gets hot and dramatically increases its resistance) between DC ground and safety ground (add missing text).

    At this point, the popping of a fuse/opening a breaker will open the connection between the + solar array and the DC charge controller (or GT inverter) (the 5 amp breaker trips, then this trips the 80 amp breaker too with the handle bar/rod).

    In the case of the Classic and a few others, the "high voltage" across the open fuse or hot PTC signals the charge controller (or GT inverter) to stop conversion.

    Sort of sounds OK--but, from my humble point of view, all they have done is stopped the power transfer into/through the DC input of the device.

    The have not limited to less than 5 milliamps (~limit that can cause heart failure), and they have now potentially energized the DC grounded section of the circuitry by "floating" the ground (open fuse, open breaker, hot PTC)...

    In the "olden days", when something was ground referenced, it was done with at least a 6 gauge piece of wire that could handle >600 amps of fusing current and would guarantee to trip a circuit breaker/fuse in the + or hot leads from the energy source (DC or AC systems).

    Now, your DC "ground" is a 1-5 amp fuse, that when popped, it has now un-referenced your ground system. And, depending on what happens, it has de-referenced your otherwise operating battery bank--while just stopping the solar panels from charging the batteries (hope you notice in time the warning LED, beeper, etc.).

    This is typically considered very bad form (again from my humble opinion) to have the ground automatically lift... Heck, when I designed/installed AC equipment we had to use a separate ground stud so that when a person was servicing the device they could not unhook the green wire by accident (or double nut the safety ground/other ground connections).

    And you don't know where the ground loop problems are going to develop... For example, a very common ground loop problem in large systems and high current systems was for people to burn out the RS-232 ground wire between a computer and a modem/printer/terminal/etc. because of mismatched ground references (RS-232 is a terrible interface because of the common ground signaling system).

    Anyway... Either connect the Ground Fault system as defined in the manual and as required by the NEC...

    OR, hard ground your DC battery negative bus to your common AC/DC ground rod (one point). And disable the GF detection circuitry on the DC Charge controller.

    Again, this is not a hit against the Classic or other brands/models of charge controllers... It is just my personal belief that the NEC does not know what it is doing with DC.

    If they want the safety of a floating system (in AC, this is done with an isolation transformer, and can be done with DC switching power supplies with slighting higher costs)--They should mandate that (and the testing/monitoring system required to self test/determine when the isolation has failed)... But, nnnnoooo, they will not do that because the could do the Rube Goldberg Machine with a ganged 5 amp / 80 amp (or whatever) dual circuit breaker setup to protect against one sort of failure (positive lead of array to safety ground short)... Does not protect against any other types of failures and creates a whole new set of problems.

    OK--Off my soap box now. And I completely understand if others have a different opinion than I... Always open to new ideas/interpretations.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Part 2:
    Fortunately, everything is very closely situated. My RE equipment and battery bank are in my utility room. A few feet away is my water heater which has the big braided ground wire connected to the copper pipes. Right next to that (on the other side of the wall in the garage) is my electrical panel. So, the ground wire from my DC equipment can tie in right at the water heater. I like that fact that the run is short. I have plenty of 6 gauge wire and I could even use 4 gauge if that would be better.
    Again, in the "olden days", the ground wire needs to be heavy enough to carry the maximum expected short circuit current AND trip the upstream Fuse/Breaker (also the green wire run has to be low enough resistance that it will guarantee to trip the protective device--Which can actually be a big problem with long cable runs at lower voltages we typically see in DC off-grid battery bank systems--I.e, will a dead short on a 100' 14 gauge cable trip a 15 amp breaker, or will the voltage drop be so high, that the cable will just "cook" with 15 amps in the event of a short circuit at the far end).

    Which gets into yet another issue with DC power systems... With Boats and Cars, it was always assumed that the 4 awg (or whatever) battery cable was heavy enough that it would short out the battery and drain it before the cable caught something on fire...

    Now, with people installing large DC battery banks, you can have enough current in a short circuit to actually fuse your 4 awg battery cable (fusing current ~1,000 amps; a 100 AH 12 volt battery is probably cable of around 1,000 amps into dead short).

    Make that a 1,000 AH battery bank--and you are now in the realm of needing a large fuse/breaker at the (typically positive end) of every battery string (4x strings, four fuses/breakers--one for each string--to limit maximum short circuit current to prevent cable or battery failures in the event of a dead short).
    1) Does my plan of changing my ground system as described above sound sensible?
    I will defer and let people tear apart my rant above and see if there is a consensus on how best to ground a system with NEC GF protection (if that is even possible to do "safely").
    My main concern is for safety, but I'm not too concerned about getting shocked from the metal cases on the DC breaker boxes. It's 48VDC (nominal) and when I've touched across 48VDC before, I've only felt a tingling if I had some perspiration on my hands. So, I don't think that shock is the biggest issue with the lower voltage DC stuff. Probably sparking and fire is a bigger concern there.
    I have been bit (a little) on 24 volt systems (sweaty hands and clothing, to grounded metal frame rail).

    Shock can be an issue--But protecting against short circuits (grounded metal, upstream fuse/breaker/etc.) is generally the recommended plan.
    As far as lightning protection, I did drive a ground rod in at the base of my steel wind turbine tower. I used a 4 gauge wire to tie the tower pipe to the ground rod. That ground rod for the wind turbine is located far away from the AC grid ground rod (maybe 200 feet). That ground is independent of any of the turbine's 3 phase electrical output. For the 3 phase output of the turbine, I have a Delta 3 phase lightning arrestor. I have had the ground wire tied from the Delta arrestor to my AC ground at the water heater.
    Sounds fine... I am no expert in proper lightning grounds--But it may not hurt to drive multiple ground rods (star pattern) and "home run" each extra ground back to the "common/central" ground rod, then that to your tower.

    I know the number/amount of ground rods do depend on your local earth/soil conditions.
    Lightning is a very low risk here. I live in a canyon that runs along the base of a small mountain range. Lightning almost always strikes up above. The top of my wind turbine tower is still very LOW relative to most of the surrounding terrain. Lightning in general is very rare here, but when I've seen it, I've never even seen lightning strike down in the canyon. I've always seen it strike up on the surrounding higher elevations. Even so, I know it's possible, so I tried to take some precautions with my wind turbine.
    Yea, a wind turbine on a tower is the same as a "kick me" sign to lightning.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Part 3:

    We have a few threads that talk about lightning protection. Sometimes the worst is mixing different grounding theories together in one installation.
    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.
    2) I wonder if somehow that 3 phase lightning arrestor being connected to my single point AC/DC ground tie will cause any potential problems?
    Not by itself... It should not make things worse... Personally, unless somebody has a better idea, I would put the 3 phase arrester fairly close to your main AC/DC ground rod and run a nice heavy "multi-strand" wire from the arrestor ground to the AC/DC ground rod (solid wire does not carry lightning as well as stranded/braided cable--You can read about skin effect).

    Again, I am certainly no lightning expert--but the above is how I understand the basics of the problem.

    If others here (or you have a good local expert)--certainly listen to them and see who has a good record regarding lightning suppression.

    At times, after reading other "stories"--I am not sure if it is very good lightning/grounding practices that make for a rugged/safe system--Or it was simply very poorly done (or none) lightning control systems that are problematic.
    westom wrote: »
    A professional application note demonstrates single point earthing. Two structures. Each must have a single point ground. Any wire entering but structures must connect to that structures single point ground before entering:
    http://www.erico.com/public/library/fep/technotes/tncr002.pdf

    Earthing requirement applies to every wire in every cable. Even if that cable is underground.

    If two structures violate the concept, then a direct lightning strike to one building may become a lightning rod connecting that surge into appliances inside the second building.

    Earthing wires to some 3 meter ground rods means a massive increase in surge protection. But better informed builders start protection when footings are poured. Ufer grounds were pioneered in munitions dumps so that direct lightning does not cause explosions. Technology is that routine when surge damage is not acceptable. Effective because concrete is a good electrical conductor.

    Buried ground loop is another solution. Or how one solve earthing atop a rocky mountain:
    http://scott-inc.com/html/ufer.htm

    A case study of how all surge damage was eliminated in a Nebraska radio station by not wasting money on protectors. By fixing defects - bad earthing. They even fixed the utility pole transformer earth ground. Each layer of protection is only defined by that earthing electrode - each single point ground; not by any protector:
    http://www.copper.org/applications/electrical/pq/casestudy/nebraska.html

    Welcome to what advertising and salesmen in a big box TV / computer store will not discuss to enrich themselves at your expense.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: System Grounding

    i am deterring away from grounding questions myself as i too feel the nec is botching things up and it puts my opinions outside the law where the nec is adopted. i don't feel comfortable recommending the ways of the nec, but i can't advise against it either for the reason in my 1st sentence.:cry:
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    Thanks again for sharing your wealth of information, Bill. I've read through what you wrote a couple times, and I'm now leaning towards the older method of grounding the negative buss of the battery bank. Like Bill wrote, if anyone else has an opinion about the new GFP systems, then let us know. Otherwise, I think I'll go with the tried and true older method.

    Edward
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Edward,

    Given this is a Midnite Solar "Classic" question... You may wish to take the same question (you are welcome to copy my posts too) over to their forum and ask.

    boB and Ryan still come by here--but they are trying to provide support at/through their forum so I am not sure they will see/respond here (Obviously, I am not speaking for them).

    If you get a different/better reply from them, post a link to their thread and/or post the results here too. It would be nice to hear other's opinions too.

    -Bill

    PS: I did make a few text additions to the first post to clarify my first draft.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    Good idea Bill. I just copied and pasted a chunk from your first post and started a new topic on the Midnite forum. Here's the new thread.

    http://midnitesolar.com/smf_forum/index.php?action=post;board=2.0

    Also, thanks Neil for your comments regarding grounding.

    Edward
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    I don't know why, but it appears that my post on the Midnite forum was removed.

    Guess we might have accidently stepped on some toes or something???

    Maybe they don't like any discussion that is contrary to the NEC. Not sure. All I know is that I did post a new topic and now it's gone.

    Edward
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    I am not sure... It may not have gone through. The link you posted did not work (appeared to be just a "create new post" link or similar--it was not to a specific thread/post).

    And I checked pretty quickly after you posted the link, and did not find any new posts at all (other than an earlier post to an existing thread by you on another subject).

    Just send a PM to the admin (Ryan?) and see what he says.

    I think they are pretty open and he probably would have notified you somehow if the post was disallowed for some reason.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    I sent a PM to Ryan earlier, and I'm waiting to hear back from him before reposting anything. Perhaps it was just a technical glitch. Just in case, I figure it was better to wait and see how Ryan responds.

    Edward
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: System Grounding

    edward,
    try it again being careful to be sure you are doing all correctly. sometimes ryan gets busy with business and can't get back to you right away.
  • keyturbocarskeyturbocars Posts: 375Solar Expert ✭✭✭✭✭✭
    Re: System Grounding

    All is well. Ryan got back to me, and it looks like it was just a technical glitch with my post being lost. I'll repost again.

    Edward

    EDIT: Here it is: http://midnitesolar.com/smf_forum/index.php?topic=142.0
  • halfcrazyhalfcrazy Posts: 717Solar Expert ✭✭✭
    Re: System Grounding

    Robin asked me to add this for him he can not get into the forum.

    I read through the different scenarios and it made my head hurt. I believe I
    did catch references to some issues that would be caused by having more than
    one bat/PV negative to earth ground connection. If there is more than one
    connection, then the DC-GFP may not trip. It is not fair to come up with
    scenarios that are not hooked up per code to begin with. I don't plan on
    figuring out current paths on non compliant installations. You must connect
    the DC-GFP as the only PV minus to earth bond.

    In a charge controller setting the DC-GFP breaker or fuse is never more than
    1 amp. MidNite uses ½ amps in the breaker versions. The Classic trips at
    about 7/10 amp. I for one do not recommend a DC-GFP in off-grid systems that
    won't be inspected. I do not believe they are unsafe, I just don't think
    they do much to protect anything. The voltages are too low. Arc fault
    protection does do some good though and the Classic has this feature
    designed in.

    The scenario described may indeed be fixed by breaking the + and - circuits.
    I never thought about it much due to my lack of faith in a DC-GFP system in
    the first place. We have designed systems that meet the NEC. That is our
    job. To second guess the NEC and design something that takes into account an
    incorrectly wired system would put us at a competitive disadvantage. I would
    suggest that given the new requirement for arc fault protection, the older
    DC-GFP requirement should go away. It really causes issues in telecom sites
    that have positive ground! John Wiles wrote the DC-GFP requirement into the
    NEC. He also wrote the arc fault protection into the NEC. May I suggest
    talking to him to ask why the DC-GFP is first of all safe, and second still
    required?

    There was a reference to breaking the neutral to earth bond in this thread.
    You NEVER do that!! There is no requirement in any code that would suggest
    this. Also, make sure that the requirement for equipment grounding is not
    getting confused with negative DC bonding. Every piece of electronics and
    electrical equipment must have the case bonded to earth ground. There are no
    switches or GFP devices in this circuit. The "equipment ground" connections
    must be made uninterrupted. I saw mention of "safety ground" and have to
    really work hard to determine what this means?
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Safety and earth ground are the same (at least as I talk about it here).

    I am comparing the dc gfp set up to the equivalent of a 1-5 amp fuse between a 120/240 vac neutral and earth/safety ground... Which as you say, nobody would ever do. But that is exactly what the dc gfp does between dc return and earth ground.

    And when this happens (say +V panel/array shorts to pv panel frame, we now have -Voc offset between earth ground and -V panel. That could be over -100 vdc between -V battery and the earth case ground. I am not sure any low voltage dc system designer hipots their system for V input max of 150 to 250+ vdc between the frame ground/negative earth bonding and +/- input power leads.

    And if hipot tested, it would be 3x rated maximum rated input voltage. With a 250 vdc rated max Voc, that would be a highpot of 750 vdc minimum.

    And to be clear, midnite has implemented dc gfp as written by the nec. My gripe is with the nec and their (I believe unsafe, in my opinion) hacked together dc gfp requirement that has little in the way of additional safety and if implemented as written is actually very unsafe.

    I too do not believe that miswired systems (multiple dc to earth grounds) areanything to worry about in this discussion... That was just answering a question asked earlier.

    Midnite implementing arc fault protection is very cool!

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    I have just posted this also on the Midnite forum:

    After a PM to boB--I will post this statement. This is my personal opinion and has not be blessed by Midnite Solar, this forum, or any others. This is just my opinion/interpretation of the various regulatory codes as they exist today (as I understand them). (boB is not making add this--It is my honest opinion :)).

    The NEC DC GFP system has little to no safety improvement and in fact violates several NRTL Recognition/Listing requirements for multiple countries.

    And because of the fundamental flaws in the NEC DC GFP requirements/system, it should immediately be disabled and permanent DC Return/Earth Grounds be used in place of the sensing fuse in DC / Off Grid Systems as a safety/fire hazard.

    This includes Hybrid inverter systems that use DC Solar Charge Controllers as part of their DC power design.


    I am not too keen on having NEC DC GFP on Grid Tied (Utility Interactive) Inverter systems (I don't think it adds very much in the way of safety) and it does create a nightmare in terms of a ambiguously grounded solar array. And it creates potential confusion of what is or is not grounded in the DC PV Array side of the circuitry--which is always a potentially dangerous situation. Also, I do not know how the NEC DC GFP system affects the new non-galvanically isolated GT inverters. I don't like NEC DC GFP and believe it makes things less safe for a service person--but I don't know any method of bypassing the NEC DC GFP circuitry--and in any case, it may be different by vendor/model number anyway.

    Now the details why:

    Per Kent0's nice drawing--Yes, that is one of the two (as far as I can tell) failure modes, that DC GFP, is designed to detect/protect against, it actually makes the system very unsafe and an actual safety/fire hazard if it operates as designed (per NEC requirements--Again, not picking on Midnite--They are giving their customers the option of NEC DC GFP grounding or the traditional hard ground of the return bus/earth ground that does not "meet" current NEC).

    If there is a fault between +Vpanel (assuming negative ground system) and frame/earth ground, this causes the whole DC battery wiring to no longer meet ELV ratings (more or less, no longer "touch safe") because of the opening of the Ground Fault Sense Fuse/Breaker. Now all down stream 12-48 VDC components/wiring/devices/etc. have to rated for 150+ VDC and Highpot tested to a minimum of 750 VDC. (It has been decades since I have been at this level of design and test for safety/code issues--so I have probably messed some of the details up--but the basics, I believe, are still correct).

    And this causes a whole set of nightmares for device manufacturers. For example, the fuse/breaker/PTC needs now to be rated to 150 VDC (or higher for high voltage array product). And if there are any capacitors from -DC to Frame ground, those now need >150 VDC rating. Also, need to check barrier requirements between -DC (and +DC) and Frame ground now that you can have >150 VDC between DC power and frame ground.

    Now this means that the DC side of a PV system cannot be considered to be SELV (Safety Extra Low Voltage).

    Assuming Wiki is more current than my knowledge:

    http://en.wikipedia.org/wiki/Extra-low_voltage
    Separated or safety extra-low voltage (SELV)

    IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".
    This outright affects and violates UL/NRTL registration/listings...

    The only way that a NEC DC GFP system would not violate the SELV clause is if Voc-cold is guaranteed to never exceed 120 VDC. I am not sure how solar panels are rated to ensure that this requirement could be satisfied, especially now that Listed Solar panels are rated to 600-1,000 volt (600 VAC / 1,000 VDC?).

    And even if the the SELV rating of 120 VDC is not exceeded, I still believe the inversion of the safety ground reference is unsafe because the normal single pole DC breaker/fuses are now on the return lines when the ground reference is reversed (and the Ground Fault detection fuse/breaker is opened). This is also a violation of multiple safety requirements and create both a serious shock hazard and fire hazard.

    Personally, I would recommend that people never install (or configure) a NEC type DC GFP system. It is not safe.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    Just an FYI to everyone...

    With the help from the folks at Midnite solar, I have just submitted a 26 page document to John Wiles asking for him (and NEC) to review the entire NEC DC Ground Fault Protection requirement in face of the serious issues of safety that affect Solar PV Power Systems when installed per NEC Code Requirements.

    If anything further happens, I will let you guys know.

    Thank you all for the ongoing discussions that we have had about safety. I have documented many of those in the submission.

    Cross posted to:

    http://midnitesolar.com/smf_forum/index.php?topic=142.0

    Sincerely,
    -Bill B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    For anyone that is keeping track, a member from the Midnite forum contacted Mr. Wiles recently, and this was the answer:

    http://midnitesolar.com/smf_forum/index.php?topic=142.msg2467#msg2467
    author=laszlo « Reply #34 on: December 21, 2011, 11:33:41 AM »
    I emailed Wiles a while back asking for his comments about Bill's paper.

    He said no changes are planned in the NEC regarding GFP, and the standard the Bill was referring to is a European standard and does not apply here in the US.

    All I know. And thank you Laszlo.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,207Super Moderators admin
    Re: System Grounding

    To add to the options out there--A MorningStar applications engineer just emailed me a link to their new(?) DC GFPD system:

    http://www.morningstarcorp.com/en/support/library/GFPD-ds-Eng.pdf

    A very nice system that actually does DC GFI safely. It does not quite provide the level of GFI protection that AC GFI requires (300 mamp dc vs 5 mamp AC for human protection)--However, it is a huge improvement over the NEC DC GFI protection system.

    We may not see a discussion in the forum with MorningStar folks--I believe their corporate polices do not allow discussions of their employes in other public forums (I have no problem with that--It can be difficult to manage from a corporate point of view).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: System Grounding

    Bill;

    To my mind it looks like the Wiles/NEC DC GFCI implementation was a quick adaptation of the AC principle.

    Frankly I do not see DC GFCI as a necessary safety feature. Arc fault on the array, however, would be.

    Even in its AC application ground-fault protection tends to be problematic in application. Although the recent implementation of arc-fault on AC here in Canada has generated some problems too, which manifest the same way: most people don't like having their power shut down for no good reason. :p
  • westbranchwestbranch Posts: 4,993Solar Expert ✭✭✭✭
    Re: System Grounding
    Bill;

    Although the recent implementation of arc-fault on AC here in Canada has generated some problems too, which manifest the same way: most people don't like having their power shut down for no good reason. :p

    Marc, tell us more please...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
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    Cotek ST1500W 24V Inverter,OmniCharge 3024,
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    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: System Grounding
    westbranch wrote: »
    Marc, tell us more please...

    You know arc-fault breakers are now required on bedroom circuits? Seems they don't always trip for actual arc-faults. A big vacuum cleaner will do it sometimes. And as with the occasional false trips of GFCI it will not happen consistently, so the poor homeowner is left wondering what happened.

    In some ways I'm glad my power service is an old 100 Amp unit that no longer meets code.
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