earth connections

HI Forum

I have a question regarding ground connections

Is it ok to connect the same circuit in two places but to the SAME ground point? In this particular case it would consist of the connection of the PE wire to the single ground point after the protection panel but before the inverter (as this connnection already exists), and then again through the PE wire/connection of the inverter which itself will be connected via the inverter chassis to the same single ground point.

Have added a picture which might clarify what i'm banging on about :)

Comments

  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: earth connections

    Hi Lazza
    My opinion (others may disagree) is that as long as the two ground wires running from your panel to ground are very close together, or perhaps even twisted together, there shouldn't be a problem. If however they are separated and especially if they take two different routes to the common ground point, they could appear as a loop (coil) of wire to the magnetic pulse from nearby lightening, which could induce large voltage differences between the two wires where they enter your panel. Would such high energy pulses and possible arcing cause problems in your panel? Something to think about. Of course it would also depend on how long those ground wires are.
  • BB.BB. Super Moderators, Administrators Posts: 32,596 admin
    Re: earth connections

    Is this a Grid Tied or Battery Based System?

    I am trying to figure out if you are talking about DC grounding the Solar "-" bus and the Inverter "-" bus or what...

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

    It should not be a problem as long as there is only one point of connection to Earth and no "loop back" from the chassis ground to the PE wire. The idea is to take any power that might connect to cases, et cetera and sink it to Earth rendering it harmless. As such you want to avoid making any alternate paths it can take that would avoid the Earth grounding (or allow conducting through Earth and back out at another ground rod point).
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: earth connections

    The chassis PE ground is what is called an "equipment ground". In a UL listed inverter it will not have the chassis bonded to the inverter's (-) DC current carrying conductor (battery cable). Some older, non-listed or small portable inverters will have the negative battery connection directly connected to the chassis and should not be connected to the permenant wiring of a structure.

    If your inverter is not installed yet you can easily check with a continuity or an ohm meter by putting one lead of the meter on the battery connection (-) and the other on the chassis ground. If there is no continuity when the inverter is not connected to any wiring there will not be two "common grounding points" (neutral bonding points).

    Chassis grounds (equipment grounds) can have many earth connections and in fact should be solidly bonded to earth so there is no voltage potential when someone touches a piece of equipment. In a properly operating system there should be no current flowing through any chassis, frames, racks or "equipment ground" wires.
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    It's an off-grid system.

    I'm not talking about either grounding of the -ve side, nor of the neutral wire (blue wire in European systems). Purely I am discussing the ground wiring in the system, the PE wire. Maybe inside the inverter the chassis earth and the PE wire slot are not even connected. I will perform some continuity tests as suggested
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: earth connections

    be careful as any ground loops that may be present could induce a damaging voltage to a meter trying to read ohms.

    i'm still a bit confused as to envisioning this even with the diagram. what is this protection panel you are referring to? a fuse or cb box? if so what is the other wire that isn't going to the chassis connecting to in the "protection panel"?
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    The PE wire is going to the normal panel with breakers etc (although it doesnt have to be there). In the panel simply all the PE wires from circuits will be joined to this main PE wire running to the common ground point
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: earth connections

    is this the pe wire the pv - or is it the 3rd wire ground that each pv or string of pvs must have? i'm assuming this to be the 3rd wire for grounding and if so the chassis does not need the extra wire going independently back to the main ground point. you can double check if you disconnect the independent chassis ground and put your ohm meter from chassis ground to the internal ground bus (that pe wire you illustrated). 0 or near 0 ohms indicates the chassis is already grounded. note here that if that wire has voltages present that it could blow out the meter so check for voltages first.
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    Its the 3rd wire in a normal (european) house system... ignore the pv system for now. The 3rd wire, that is to say, the PE wire, must be connected to a grounding point within or near the house (this is an off-grid system).

    And yes, the chassis is connected within the inverter to the PE wire connection point which I checked with a continuity test. Hence without connecting the chassis to ground, the inverter chassis is in effect, already grounded.

    My question is whether there is a problem in connecting the chassis to ground again... just in case, to have it connected TWICE but to the SAME COMMON ground point.?

    My thinking is that it's not a problem, unnecessary maybe... but i could be wrong
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: earth connections

    Grounding is one of the most difficult things to get a handle on. Even experts disagree.

    But I'll take another stab at it. ;)

    Your PV frame ground is not connected to either the (-) or (+) PV wires.
    The frame ground goes to the single Earth grounding point.
    Your charge controller and/or inverter frame ground may or may not be internally connected to (-) wire.
    The frame ground for same goes to the single Earth grounding point.

    Where you get in trouble (one of the ways anyhow) is if the PV (-) is connected to the PV frame ground.
    You then would have the potential for current flow through the grounding wire if something were to happen to the Earth ground point and/or negative wire (high resistance or disconnection).

    You also do not want a separate ground wire running from the PV frame ground to the controller/inverter ground that bypasses the Earth grounding point.

    Not sure yet that's enough to explain it.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: earth connections

    another ground wire can set up ground loops. these aren't desirable. is there any other reason that you might think your pe wire is insufficient?
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    The PE wire will probably only be 2,5mm2 from the inverter (one of the 3 wires in a typical domestic installation cable).. I was going to put the chassis ground with a 6mm2 or 10mm2 cable as the DC battery disconnect size will be 63A or 80A. Hence reducing the resistence to ground in case of high current leakage.

    I dont really consider it as a ground loop as both wires are connected to the same common grounding point... or is this still a ground loop?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: earth connections
    lazza wrote: »
    I dont really consider it as a ground loop as both wires are connected to the same common grounding point... or is this still a ground loop?

    It depends on whether or not the two wires are connected to common points at each of their ends.
    You can run separate grounding wires from each device to one common Earth ground (often a good idea), but if you do you can't run ground wires from device to device. That's the sort of thing that creates the dreaded ground loop with its potential for allowing one faulty device to energize the case of another rather than sinking the Voltage to Earth.

    You also do not have to make ground wires as large as the Voltage conductors. Under normal operation they carry no current at all. They're there in case something goes wrong. Then they only have to carry current momentarily - just long enough to cause the circuit protection to trip and de-energize the system. I'm sure NEC probably has a table rating for ground wire sizing vis-a-vis conductor sizing. Or else one of the engineers here knows how to determine the minimum size needed.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: earth connections

    to put it simply a ground loop can be created when there are 2 or more paths to ground from the same starting point.
  • SUNUPSUNUP Registered Users Posts: 24 ✭✭
    Re: earth connections

    Hi
    why would anything have to be tied to earth if the system is off the grid?
  • BB.BB. Super Moderators, Administrators Posts: 32,596 admin
    Re: earth connections

    Grounding is a huge issue... It takes into account metal piping in the home, shock/fire hazards, lightning, static buildup, fusing/breakers, connection of other power sources (generators), etc... There are even specialized issues for solar--One (very good) brand of solar panels need to be positive ground referenced for optimum performance. Another issue is that (most) MSW Inverter cannot have their output setup with a "ground referenced neutral" plus ground referenced DC battery bank--It will destroy the MSW inverter (most TSW type inverters can have ground referenced neutrals--You need to read the manuals to be sure which supports what grounding techniques).

    In the end, it depends on the size/needs of your system. A solar PV system does not need to be grounded to operate. Just like the flashlight/car battery does not need to be grounded to be "earth referenced" to operate correctly.

    However, if you have a Cabin with DC/AC wiring, metal water pipes, and on occasion run an extension cord outside to work on the well system, etc. in an area subject to lighting... Then you may need to have a ground referenced system for safety.

    Here are some links to detailed grounding discussions:
    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

    If you are working with NEC Code (local building permits/inspectors), then usually grounding will be required for Cabin/Homes... And, for the most part, proper grounding is the 'safer' thing to do for fixed installations--especially as they get larger in physical size.

    What is the size/configuration of your system? Do you have lightning in the area? Dry/rocky soil? etc...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SUNUPSUNUP Registered Users Posts: 24 ✭✭
    Re: earth connections

    Ok now could you explain how it would be a shock/fire hazards by not being referenced to earth?
  • SUNUPSUNUP Registered Users Posts: 24 ✭✭
    Re: earth connections

    Another issue is that (most) MSW Inverter cannot have their output setup with a "ground referenced neutral" If off grid no reason to tie the neutral to ground.
  • SUNUPSUNUP Registered Users Posts: 24 ✭✭
    Re: earth connections

    What is the size/configuration of your system? Do you have lightning in the area? Dry/rocky soil? etc...

    We have 4 130 watt panels,, 8 6 volt batteries, 1 outback charge controller flex 60, 1 xantrex 3000 watt psw inverter, 1 1500 watt msw inverter, and 1 600 watt psw inverter, and 1 trimetric monitor. The panels are series connected supplying approx 80 volts to the controller on a good sunny day. yes we have lightning in our area, and all kinds of soil.
    Charlie
  • BB.BB. Super Moderators, Administrators Posts: 32,596 admin
    Re: earth connections

    Hi Charlie,
    Ok now could you explain how it would be a shock/fire hazards by not being referenced to earth?

    By itself, an isolated supply is not a shock/fire hazard in relation to "earth referenced" ground. Take a car battery and put it on a table and watch. And touch one terminal or the other--And take your other hand (or even a length of wire) and touch your kitchen faucet. Nothing happens. ;) In general, and isolated supply is very safe.
    Another issue is that (most) MSW Inverter cannot have their output setup with a "ground referenced neutral" If off grid no reason to tie the neutral to ground.

    Here is where it "gets complicated"... We are now talking about "systems". (note, the following is all generalizations--There are exceptions to virtually everything I type below--You need to refer to the specific device documentation to be sure).

    In the case of a MSW inverter, its output is not isolated from the DC battery bus. For example, if you have a 12 volt automotive radio (or ham set) connected to the 12 volt battery bank. And use a MSW inverter for lighting, soldering iron, drill, etc... And there is a short between one lead of the MSW AC output and the radio chassis (for example), there will be an excessive flow of current. Which may blow a breaker/fuse, pop the wire, or even cause internal damage to the inverter.

    If this was a "floating" TSW inverter, you can (in general) touch one of the AC output wires to any metal object (wired to battery or not) and not get a current flow.

    Other when referring to Lightning and Atmospheric Static Electric Field--There is nothing "magical" about earth ground. It generally has fairly high resistance and takes a fair amount of metal and good soil conditions to be "conductive enough" to matter regarding shock hazards (galvanic corrosion is another issue).

    When Earth Ground becomes more interesting is when you have, for example, a buried metal water pipe going to your sink/faucet. Now--That creates a "conductive terminal" that has interest too us.

    If you permanently wired your home/cabin -- there is always the possibility that insulation fails, a wire connection "falls apart", etc... And it can energize you or something metal.

    Say you have a mixer that has a shorted winding that energized the metal base. And you grounded the DC battery bank (and possibly ground referenced the inverter AC White wire and made it a grounded neutral). It is now possible for you to get a shock if you have the mixer in one hand and the other turning one the water to rise the beaters. That is why we now have GFI (Ground Fault Interrupting outlets) around sinks/outside near pools, etc. to reduce the chance of electrical shock.

    Another reason for Ground Referencing a Neutral circuit. Look at your AC home wiring (North America). We have Line1, Line2, and Neutral. We only place a breaker/fuse on the L1/L2 "hot" lines and not on the neutral.

    Because the neutral is ground referenced (really connected to all major metal objects in the home--Water Lines, Gas Lines, third wire in outlets, etc.), it can never become other than Zero Volts respect to ground--So there is never any reason to put a breaker/fuse on the "white" lead.

    Things get more complex with grounding when you have multiple power sources (AC mains, generator, battery bank, inverters, etc.). Some are required to ground bond the neutral (NEC code in the main panel). Generators over ~5,000 watt output are required to ground the Neutral (if I recall correctly). And if you tie the Generator to the Home wiring--you can have parallel ground/neutral connections--Aka a "Ground Loop" where you can have parallel paths for current flow (which can have safety issues).

    To see how complex that can become, here is a discussion I was involved in a while back discussing "Hard Grounding" vs ground a system through a 1 amp fuse/breaker as done by NEC/UL:


    My paper is a bit "over the top" -- But it does talk about the issues of "soft grounding/floating" power circuits and its implications. Short answer, solid ground referencing "larger" power systems increases the safety (fire, electrical) over that of "floating" a system at a significantly lower cost (you can make floating systems "safer"--but it costs a lot more).
    What is the size/configuration of your system? Do you have lightning in the area? Dry/rocky soil? etc...

    We have 4 130 watt panels,, 8 6 volt batteries, 1 outback charge controller flex 60, 1 xantrex 3000 watt psw inverter, 1 1500 watt msw inverter, and 1 600 watt psw inverter, and 1 trimetric monitor. The panels are series connected supplying approx 80 volts to the controller on a good sunny day. yes we have lightning in our area, and all kinds of soil.

    Regarding Lightning--You can refer to my post #17 above... There is a lot of reading to understand what happens with lighting as it is not "simple" DC but a ~1-10kCH radio frequency effect (and even static charge buildup in clear weather--~100-300 volts per meter as you go up in altitude--An insulated metal tower/solar array can be statically "charged" if not grounded) and give a very large static shock to somebody touching the structure--let allow the 1,000's of volts per meter under a thunder storm).

    So, grounding the metal frames of the array (8-10 foot rod driven into the ground with a 6 awg wire back to the frames is usually the starting point). Because of static build up, it is not a bad idea to negative ground the DC battery terminal. Also, this can help direct lightning energy from the bank to the earth (but is more complex--read the links).

    If you do not have metal water pipes, gas lines, etc... There is not really a reason to ground the battery bank or AC output (note, MSW inverters are "battery referenced" because they are not isolated).

    If you have water pipes, significant metal structures (antenna tower, metal sinks/plumbing, work bench, well+pump, etc.), then connecting all of those to your common ground rod with ground bonds may be a good idea.

    In your case, you have three inverters on this system... Having all of those sources of power (Pure/True SW and MSW inverters) and "isolated" loads can make things confusing. If they are all in the same workspace--It is possible that if you get a short from "hot to ground" on once circuit will energize a "piece of metal" with respect to your MSW inverter output--Creating a shock or fire hazard.

    I assume you need the large 3kW inverter for Well pump and/or power tools... And use the smaller inverters to run your smaller loads? 520 Watts of solar panels and a few hundred AH of 12 or 24 volts batteries with ~5kW worth of inverters is not usual... I guess you have an AC generator too?

    Guessing your existing bank would be hard pressed to supply more than ~1,000-1,500 watts continuously (and maybe 2-3kW maximum surge) at best (purely guessing here). And with 520 Watts of solar panel, you can only get ~1-2kWHour per day worth of solar power (1-2 hours of "full power" use per day from pure solar).

    Is the system meeting your needs so far?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    HI Bill, is there a link to your paper somewhere? or is it confidential?

    thanks
    Larry
  • BB.BB. Super Moderators, Administrators Posts: 32,596 admin
    Re: earth connections

    Good Evening Larry,

    It was buried in that 3 page thread on Midnite Solar's forum I linked too. They are kindly hosting my paper here:

    http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Solar Expert Posts: 336 ✭✭✭
    Re: earth connections

    Thanks Bill, I look forward to having the time to read this.. cheers
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