Why Grounding

Henrique T
Henrique T Registered Users Posts: 1
Hello ,

I'm learning about PV systems , but I have a question...

What is the purpose of grounding the negative pole ( or sometimes the positive ) ?

Can somebody help me?

Thanks

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Why Grounding

    Welcome to the forum.

    Grounding the negative side of DC gives it zero Voltage potential, making it safe to touch. Then if a short between positive and any other point (such as a metal casing of an inverter) should develop the current flow should instantly go high and trip the circuit protection. Shuts everything down safely without risking fires from arcs or high current and without possibly shocking anyone touching anything.

    This is the same principal as with AC grounding, but most of the time the DC Voltage is low enough that the hazard is minimal anyway. For instance your car has a 12 VDC system but obviously is not Earth grounded: it's almost impossible to get a shock from that low a Voltage and the system runs below the sustainable arc threshold. The higher the DC Voltage, however, the greater the risk. A PV array may be well above system Voltage (70+ for a 48 Volt system for example) and quite capable of sustaining an arc (hence the new inclusion of arc fault protection on MidNite equipment) or delivering a nasty shock. Grounding minimizes but does not eliminate these risks.

    Some of us have argument with the NEC regulations for DC ground fault because the negative is now required to not be grounded. This opens up a new set of potential problems in efforts to eliminate one.

    And since this is a thread about grounding, I will now sit back and let the disagreements begin. :p
  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    Re: Why Grounding

    I always had this question on the mind lol sorry if i sound sillly



    So when we have a PV system .. lets say 48v .

    Scenario 1: No grounding on anything:

    What happens when i touch the Positive pole or Negative pole.. seperately? I believe it shouldnt result in a shock right? or would it still do it?

    what happens when i short them with hands? this is sort of obvious , it would provide a shock definitely! i m not sure too lol.. Not sure what a human skin resistance will do at 48V.


    Scenario 2: same cases but with grounding. ?

    Does adding ground help with the lighting protection or something?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Why Grounding

    If you touch either positive or negative of a battery you won't feel anything because you are not completing a circuit. The problems arise when something goes wrong and something that is not supposed to be connected to power is; then you can touch both sides accidentally and get zapped.

    Normally conductive casing is tied to ground so that if any positive wiring comes in contact with it the circuit protection will trip. If the connection is made by you personally you will feel it. Low Voltage is a tingle, but the higher it goes the more dangerous it becomes.

    This is where the NEC DC GFCI comes in to play; by judging that equal current is flowing through both negative and positive lines it presumes there is no "incorrect parallel path" as in through your body. This works fine on the GT arrays it seems to have been intended for. Through batteries in and things are different (two power sources, one current based the other Voltage based, need to interrupt both lines of both sources simultaneously to shut down all power - complicated).

    Touch positive to negative at any Voltage and you get a spark. You can start a fire with a couple of 'D' cells and some steel wool. How much spark/arc, current heating, or shock potential depends on how much power is available from the source. A 12 Volt car battery won't shock you, but you can weld a wrench across its terminals. A 400 Volt PV array can stop your heart even at very low current.

    Human skin resistance is variable.

    Add grounding and what you get is a lot of debate about how best to do it. :p Most DC grounding practices have been centered on sinking current to trip circuit protection in the event of a short. Now there are efforts being made to protect against electrocution (ground fault interrupt) and fire hazard from arcing.

    Two other aspects of grounding: RFI control and lightning protection. Both of these topics are really complex. There is a very long and involved thread about RFI here: http://forum.solar-electric.com/showthread.php?5887-Radio-Noise-from-PV-System Note that it is not as simple as "add ground = noise goes away". Lightning has been and no doubt will be discussed numerous times. The short version is you can't do anything about a direct strike, you can only minimize the damage potential from stray Voltage caused by nearby strikes.

    Oh and someone is bound to mention EMP's and grounding's relation to them too. Another of those complicated "do-I-really-have-to-know-this?" subjects. ;)

    Electricity is dangerous. You can not make it 100% safe. You can only minimize the risks.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Why Grounding

    Grounding is mostly a safety feature. All the exposed parts, such as electric boxes, cases, pipes, is supposed to be grounded. One of the AC power electrodes is also grounded. This makes it a "neutral" electrode, while the other becomes "hot". As a result, if "hot" elecrode touches something, which is grounded, it creates a short circuit. It is therefore safe to touch everything grounded. It cannot be "hot". This wouldn't be the case if "neutral" wasn't grounded. The same with DC. You make one of the electrodes "neutral", most often negative.

    Now, if you're standing on the ground and touch something grounded, you're safe.

    On the other hand, if you accidentally touch something "hot" while standing on the ground, you get zapped (killed in the extreme case). If neutral wasn't grounded, you wouldn't get zapped (survived).

    So, the grounding does make some things safer, but at the same time may create a danger elsewhere.

    Lightning protection devices, GFCI and other things may need grounding. In your case, charge controllers and inverters may assume that a negative terminal is grounded. If you don't take this into consideration, the equipment may get destroyed very quickly.

    IMHO, the best way to deal with that is to do whatever codes require - you do not have any other choice anyway, and there is no good reason to violate the codes.
  • KnowledgeSponge
    KnowledgeSponge Solar Expert Posts: 173 ✭✭✭
    Re: Why Grounding

    Excellent thread. VERY useful. I was hoping someone would touch on this topic.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Why Grounding

    NorthGuy mentioned in passing what is known as "floating Neutral" which is often found on portable generators. In that instance the center of a 240 VAC (or one leg of a 120 VAC) gen is not tied to the (metal) generator frame as there is no guarantee that there will be an Earth ground when in use.

    If it were and no ground was supplied then the frame would be energized in respect to any hot lead; very dangerous. When Neutral is left floating the circuit should be protected by a double breaker per leg in case of over current: this will shut off both "hot" and "neutral" leaving all wires completely disconnected from the power source.

    This is why there are often issues with connecting a generator to an inverter or household power system: some may have an N-G bond, others will not. You need to check this always. Likewise some inverters have a built-in N-G bond and others don't. This gets in to the troubles with multiple N-G bonds creating loops: alternate current paths that can actually defeat the purpose of the grounding by allowing "grounded" conductors to become energized due to resistance difference in the two paths.

    Did I mention this stuff isn't simple and even engineers disagree about how to do it right? ;)
    Did I mention you usually only find out its wrong when something fails (at which point it's probably too late)?