Grounding Negative Battery Terminal

Options
lazza
lazza Solar Expert Posts: 336 ✭✭✭
Hi Forum, and a Happy New Year to all

I have a question. The issue of grounding PV system is somewhat of a puzzle to me, as no one here actually grounds anything!

But one thing i'd like to know is if one should ground the negative battery terminal? If so, why is it necessary?

thanks
Larry

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Options
    Re: Grounding Negative Battery Terminal

    Grounding it s fairly complex issue and has many reasons... A few are:
    • Grounding the return allows you to put fuses/breakers in only one side of a circuit. If you "short" the positive wire (assuming negative ground) to a piece of metal, the fuse in the fuse/breaker in the positive wire will blow. If you short the negative lead, nothing much will happen (no need to fuse negative/return lead).
    • Grounding all pieces of metal (plumbing, water pipes, conduit, metal electrical enclosures, etc.) will prevent the metal from becoming "hot" and being a shock hazard.
    • For Lightning, ground can help control static electrical charges (metal towers, elevated metal structures can become charged from earth's electrical field). Also allows you to control the path of lightning to safely be dissipated to earth.
    • Single point electrical grounding can prevent "ground loops" or heavy current traveling in both the return leads and earth/safety grounding paths (parallel wiring connections). For example, if you have both AC and DC grounded circuits (grounded AC neutral and grounded DC return), the heavy DC currents could flow in the AC wiring returns. DC currents can be 3-10x the AC currents and may overheat the lighter AC wiring.
    However, improper grounding can cause problems too:
    • multi point grounding creating ground loops
    • Grounding both the DC battery and the AC "neutral" for most MSW type inverters can cause the inverters to smoke
    • leakage current from exposed + DC wiring (say broken insulation in buried cable) can cause electrolysis in water/gas piping in negative grounded systems (example why telephone systems use positive grounding).
    • Lightning is a "high frequency" type current flow. It natural wants to flow on the outside/far apart conductors (lightning ground conductors should be on the outside of the building). Running a lightning ground down the middle of a building (say from solar array famework down middle of building) can cause arcs to other metal on inside of building.
    Grounding is frequently a requirement for fixed installations (homes, cabins, RV's connected to AC shore power, etc.). Small/portable PV/generator systems are not usually earthed (you don't drive a ground rod next to an RV when parked). However, even in metal trailers/vehicles, proper DC (and AC grounding) practices must be followed to properly protect against a short circuit from damaging equipment or causing a fire/injury.

    So, any grounding needs to be carefully understood and implemented per code/customary requirements. You need to understand your requirements because, sometimes, code/customary requirements are actually not safe.

    Floating electrical circuits is not a bad thing and can be actually a safety feature. You cannot get a shock from a "floating" circuit (no ground connection, no current flow to ground). However, they do have their issues (such as, normally, both of the power leads should have a fuse/breaker/ganged breaker to safely disconnect power if there is a short or somebody is working on the system. A short to earth with one power lead in a floating circuit turns the system into a ground referenced system and potentially causing an "unexpected" shock/fire hazard.

    And the larger a power system (such as a utility grid system/transformer drop to multiple homes), the less effective a floating power system becomes. There is always some leakage current somewhere or a wiring mistake/failure that eventually ground references the system (or even by design, such as capacitors in computers to reduce electrical interference which cause a small AC leakage current).

    In North American Code (NEC - National Electric Code), the idea is that any single fault anywhere in the system should be "safe". Either trip a fuse/breaker or nothing will happen (i.e., short to hot lead pops breaker. Short to neutral/return lead, nothing happens).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal

    HI Bill

    Thanks for the reply... another question then- surely putting a differential breaker on the AC side is the best protection against shocks to humans? I cant see how ground helps very much
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Options
    Re: Grounding Negative Battery Terminal

    Grounding is, more or less, a way of controlling high current flow (short circuits) and to reduce the chances of an electrical appliance from having an electrically "hot" metal case (i.e., you don't want the toaster to have a short to the metal case with 120 VAC, and you touch the toaster and the faucet at the same time). If you have a grounded case, then if there is a short in the toaster, then the grounded case will cause the breaker/fuse to pop.

    For electical appliances/computers/etc., there is the option of "double insulated". Basically, the physical design of the internal power supply has distance plus a couple layers of insulation to reduce the chances of a single fault causing a shock.

    And, there is the Ground Fault Interrupter (ground fault breaker). There, if you have more than (approximately) 0.005 amps (5 milliamps) of unbalanced current flowing out the L1/Neutral/L2 power leads, then the fault current will trip the breaker and cut all power to the outlet/device. Typically required where there are wet areas (sinks, kitchens, bathrooms, outside, near pool, etc.). Above ~5 mAmps is considered to be hazardous current (around 10 mAmps applied "correctly" to the heart can stop it from beating).

    Safety agencies assume any voltage under around 25-60 volts (depending on where/who is exposed) is thought to be incapable of causing >5 mAmps of current flow--so there is no GFI requirement/screens/cases/insulation required (i.e., you can touch your 12 volt car battery and not get shocked).

    Of course, a 12 volt car battery can put hundreds of amps into a dead short--so you don't want to get your wedding band between + and the car chassis. So, there are other safety limits (like any "touchable" electrical circuit has to have a maximum of 8 amps--or whatever the exact number is--of current available even if it is low voltage).

    And there are ELV and SELV (Safety Extra Low Voltage) that go back to the definitions of where the power comes from and its voltage potentials (i.e., a SELV circuit has to have less than ~25 volts and 8 amps, and must have a double insulated or other method between the 120/240 VAC input and the SELV output.

    The problems with putting GFI everywhere--As the equipment/power needs get larger, you can have over 5 mAmps of leakage current (capacitive coupling, capacitors for EMI control) during normal operation. So for larger power systems, you will not typically see GFI Breakers, but hardwired or green wire grounding required.

    Also, you would not want a single GFI breaker on both sink outlets and also powering your home's lights. Otherwise, if you get a shock/trip at the sink (good thing), you will be left in the dark at night (not a good thing) from the single breaker/circuit GFI trip.

    Back to the DC GFI and 1 amp fuse... It was intended to reduce the chance of Arc Faults with Solar arrays. Normally, if you short a power circuit, the inline fuse/breaker would trip and everything would be "safe".

    With solar panels, because they are (more or less) constant current sources--If you short a 10 amp solar array, you may get ~10 amps of short circuit current, which is not enough to trip a fuse/breaker. And that short circuit (rat chewed through insulation, piece of metal fell on wiring/conduit) will tend to have a sustained arc (current limited source + DC current with >12 volts--A very good arc welder).

    The 1 amp fuse was intended to catch shorts between "hot" and earth ground and interrupt the current flow to a possible Arc Fault situation.

    Sort of works, but causes a whole bunch of further safety and DC circuit design rule safety (fire and electrocution) issues.

    But, in any case, DC GFI was not intended to prevent dangerous shocks (i.e., > 5 mAmp). It was intended to reduce some types of Arc Faults (Hot to earth ground specifically).

    AC GFI was intended to protect people against electrocution.

    Anyway, I have written a paper and a couple threads on the subject if you want to read more about the issues of DC GFI and my concerns:

    System Grounding

    Ground Fault Protection


    There are a whole bunch of safety issues rolled into the problem.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal

    HI Bill

    Thanks again for your very complete reply. I will study it later with a bit of time

    As for differential breakers.... (aside from grounding etc) ... are these however the best protection against electrocution?

    or maybe when you refer to Ground Fault Interrupter (ground fault breaker)... is this a differential breaker (as we call them here)?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Options
    Re: Grounding Negative Battery Terminal

    Lazza,

    I am not quite sure I understand your questions completely...

    In the US we have two major type of AC GFI devices. One is a outlet that replaces a "normal" AC outlet that can detect ground faults and will turn off the power to whatever is plugged in if there is a ground fault.

    The second is a GFI/Circuit Breaker that plugs into the main house AC distribution panel. The GFI type breakers are installed on circuits that go to kitchen outlets/outside outlets/etc. and will "protect" the attached branch circuit.

    Is this the best way to protect against electrocution? It is pretty good... However, it does not protect against somebody getting shocked by somebody who touches the two power wires/connections. GFI's only protects against power to earth/green wire current flow.

    Another way to protect against short circuits is to use a Isolation Transformer. That way you cannot ever get a shock from power to earth fault because there is no way for current to flow ground ground back through the floating output of a transformer.

    A better question--What is it that you are looking to do/protect against? Perhaps I/we can give you other suggestions on how accomplish what you are trying to do.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal
    BB. wrote: »
    However, it does not protect against somebody getting shocked by somebody who touches the two power wires/connections. GFI's only protects against power to earth/green wire current flow.

    As far as I know, modern GFCI detects an imbalance to ground or neutral, so you are protected either way.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Options
    Re: Grounding Negative Battery Terminal

    No, GFI's (again, as far as I know), can only measure unbalanced current flow on Line+Neutral, not Line or Neutral.

    Basically, it is a small transformer wrapped around the Line+Neutral. This measures current in Line+Neutral. Any "shock" that is Line to Neutral will look just like a normal load (Line+Neutral equals a "NET" zero mAmps if the current flow is between L and N).

    If there is any Net/Unbalanced current flow on L or N (or both) to "elsewhere" (usually earth ground), when it exceeds ~5 mAmps, then the breaker/outlet/GFI will trip.

    Techntrek, I think that you are saying the same thing as I am (i.e., unbalanced current flow on line or neutral or line+neutral is detected).

    What I am trying to say is if a person touches Line and Neutral, they will get a shock and the GFI will not trip (and an isolation transformer will not protect). The GFI only protects against net current flow that uses another path (typically green wire/water/plumbing/etc.) through the human body.

    A person touching Line and/or neutral is only "protected" if they are also touching water/plumbing/etc. If they just touch line/neutral they will look like a normal load and they will be shocked.

    Here is a nice PDF document of a GFI outlet with drawings and pictures:

    http://personal.cha.bellsouth.net/j/o/johngd/files/rv/gfi.pdf

    -Bill

    To be very clear, when I type about Power connections, I am referring to both the LineA/LineB and the Neutral wire too. Any connections/touching between LineA/B/Neutral will not trip a GFI. One touching any/all of A/B/Neutral AND other conductive/grounded object will cause the GFI to trip.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal

    HI thanks again for the replies

    Yes, that's what it must be a Ground Fault Interruptor, called a differential here, for example: http://www.electrodescuento.com.ar/Interruptor-diferencial/141-diferencial-2-x-63-a-30-ma-siemens.html

    the reason we are asking, is that we are looking at a small installation for a pumping (irrigation) system in the countryside. Putting in a system with ground will imply more costs and i expect the client wont wish to pay.

    So to ensure a minimum amount of safety, i'd like to at least protect anyone turning the pump on from an electric shock- so I was wondering if putting in a GFI on the AC side would be sufficient-

    the truth is, ideally i'd like to ground everything, but the reality is always a different story

    Thanks
    Larry
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Options
    Re: Grounding Negative Battery Terminal
    lazza wrote: »
    Putting in a system with ground will imply more costs and i expect the client wont wish to pay.

    Putting in a grounding system is cheap, depending on the ground type you may just need a ground spike (pica de tierra) hammered in and then cable connecting this to the rest of the equipment - this is what's used for most rural properties in spain http://es.wikipedia.org/wiki/Toma_de_tierra, I have this same setup at my pump, because it was too expensive to lay down 300m of additional grounding cable, I hammered a spike in and used this as the ground. Which is effectively a TT grounding system http://en.wikipedia.org/wiki/Earthing_system#TT_network

    When hammering in the spike, choose soft preferably moist ground, e.g. a ditch.

    A residual current device (= diferencial = ground fault interruptor) should then ALSO be used, but not instead of proper ground.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Options
    Re: Grounding Negative Battery Terminal

    ...also, check the manual of the pump for RCD requirements, some equipment won't work with a standard 30mA RCD.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Options
    Re: Grounding Negative Battery Terminal

    Yes a GFI (at least a North American model) is designed to work without a green wire/ground/defined neutral (one power wire grounded at the main panel).

    However, larger equipment may have enough leakage current to actually trip a GFI (~5 mAmps is the trip point for our GFI).

    Generally, what you want is to ground motor housing and other boxes that contain electrical equipment/wiring. You don't want a wire with bad insulation to make a metal box "hot" which will shock the user. The green wire is there to short the hot wire and trip its breaker/fuse. No green wire ground, then metal box/conduit/etc. can become hot/energized and shock the end user.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal

    Bill, yes, we were saying the same thing.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    Options
    Re: Grounding Negative Battery Terminal

    Thanks all that's great. Good to know it's cheap too. I think my colleague was looking at pure copper ground spikes, but it appears iron copper plated ones are ok... which, of course is much less expensive.

    We want to make sure our systems have the best protection possible, so thanks for all your assistance

    Larry