Ground loops?

Hi all. What exactly is a ground loop? My drawings (for one of our installs) are being reviewed by a PE Electrical Engineer with his NABCEP cert. This guy works for the electrical contractor that we're hiring to do the work. He says that I need to change the system grounding because there is a ground loop.

I don't quite understand this concept and haven't encountered or heard of it before. I've googled some information and looked in the 2011 NEC but I'm not finding anything that doesn't reference audio-visual ground loop problems.

How does this ground loop problem pertain to solar PV installations?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Ground loops?

    Coincidentally I just explained this somewhere else today. :D

    Grounding is a safety function. Normally there is no current on it. If something goes wrong and the case or frame of some device gets energized the ground circuit is there to conduct that energy safely to Earth ground. To do this you need to control the path that energy takes. This means you want one point common to all ground connections: single point grounding.

    If you have a more than one Earth point or ground wires going this way and that between devices you get a ground loop. Literally it's an alternate path the current can take which would allow it to energize or leave energized something in the system. It is even possible for the current to enter the Earth ground through one rod and energize a nearby rod and thus bring the power back out of the ground and energize whatever is attached to the second rod. That's why when it is necessary to install multiple ground rods it's very important to do it right. This is often referred to as a "star pattern", where the connections from the system are made at one point and then connections to additional rods are attached at the same point radiating outwards. However you get enough contact with the Earth itself, it is important that the system connect at only one point.

    Too often people will run one ground wire down from the panels to a rod, then connect another wire from there (or another rod) to the AC ground, and perhaps even another line to the generator (if equipped). So imagine something shorting out in the generator, energizing the case. Some of the power goes in to the ground, but some of it could travel through wire or Earth and energize other parts of the system. How much and how severe the effects depends on what mistakes were made. The electricity will always take the path of least resistance. What that path is needs to be controlled and absolute.
  • arcturusk1arcturusk1 Solar Expert Posts: 26
    Re: Ground loops?

    Thanks for the response Cariboo! I'm sorry if I didn't see your explanation elsewhere. I did indeed search the forums for ground loops but mostly came up with just general grounding questions. Perhaps I overlooked your post.

    I sort of get what you're saying, although I'm confused when I look at Exhibit 690.6 on page 1107 of the NEC 2011. That's a simple grid-tied system. They have two grounding electrodes, one for DC and one for AC. They have the two connected. They also have an auxillary grounding electrode out at the array that the equipment grounds are connected to. Isn't this two paths? Or three? Isn't this a ground loop problem?

    I have attached a portion of my drawing to which I assume the guy is referring. Is there a problem here? The 1200A main must have an equipment ground running to the MDP. Both must be connected to the same grounding rod... oh wait, that's not necessarily true as the two panels are in different buildings on the campus and may have different rods.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Ground loops?

    Regarding that drawing, if "8" is AC Hot and "6" is AC Neutral and the solid green line is ground wire, then it's wrong because there are no fewer than 3 neutral-ground bonds in that depiction. This is one type of ground loop. Maybe the current goes to the Earth ground rod, or maybe it finds a different route to Earth, all depending on how much resistance there is, how much Voltage there is, which way the wind is blowing ... okay, maybe not that last one. The point is the circuit is unreliable as a safety ground because of the alternate paths available.

    The other type is with multiple grounding rods where current might go to one or the other or unevenly to both. With separate grounding rods there are NEC regs regarding how far apart they must be in order to be considered "separate". You often see this problem with off-grid applications where they've put one rod in for the AC panel, another for the PV frames, and another for the generator chassis. Then wired all the device grounds together.

    My remark about having explained it once already today is just to point out how often this comes up. There's nothing like discussing grounding to get a good argument going either. I fully expect someone to add a post here explaining why I'm wrong. Trouble is, neither one of us will be; the rules can be confusing and even contradictory at times. Throw in lightning protection or GFI issues and you can really get flummoxed! :roll:

    As a general rule, it's point-to-point chassis wire from one device to another on down the line terminating at a single ground rod with only one AC neutral-ground bond anywhere in the whole system.

    Then there's the problems with MSW inverters and grounding. That's a whole 'nother kettle of fish.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ground loops?
    Regarding that drawing, if "8" is AC Hot and "6" is AC Neutral...
    I'm pretty sure that 6 is the EGC. It would be bonded to neutral in the main panel, of course. I've seen many installations that passed inspections wired as he has drawn it minus the connection from Panel MDP to the electrode; is that the problem? I'm assuming that Panel MDP combines the output of other inverters.
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