Ground loops?

Re: Ground loops?

Coincidentally I just explained this somewhere else today.

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.

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.