OFF GRID: Grounding and Bonding

Started by MountainDon, February 01, 2011, 08:10:48 PM

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MountainDon

No work today because of the snow, such as it is so far here. So I spent a little time on my continuing task of cleaning junk out the workshop and garage. I came across an old inverter, just a simple small 400 watt thing. I had experienced a mishap and let the magic smoke out of the case some time ago during the construction of our cabin. Now it doesn't work anymore.  >:(

I thought I might tell of what happened because I know there are a number of folks here using PV, batteries and inverters at their cabins.
The problem, a loud pop and a curl of smoke, occurred when I connected a wire from the inverter to the cabin service panel. This could happen to anyone. So maybe a heads up will save someone.

The cabin was wired 100% according to the NEC. This included the neutral to ground bond in the service panel. Many cheap inverters don't like this. I did not know that at the time. The inverter itself had a neutral to ground bond inside the unit. From what I've since read this is a much bigger problem with MSW, modified sine wave inverters, what I prefer to call square wave, than Pure/True sine wave inverters. It is also usually only a problem with inverters that are equipped with built in receptacles.

The NEC clearly states that a house AC system should only have one neutral to ground bond. This applies to any properly wired building; it matters not if the AC power is coming from the power grid, a generator or an inverter.

It may not be easy to determine if the unit in your hand has an internal bond. From reading a small assortment of inverter user manuals online I found that not all manuals reveal this. However, if the inverter has receptacles built into it and if it is a square wave inverter, then it very likely may have an internal bond.

Larger inverters that are designed and built to be hard wired to a dwelling do not have internal bonds. Inverters like my Outback VFX3500M have a ground lug as well as the 120 VAC hot and neutral connections. Those three wires are connected to the service panel. The service panel has the neutral to ground bond.

There must be many who find the subject of grounding and especially bonding a tad confusing. So together maybe we can sort it out. The simple rule to follow on the bonding subject, is there should only be one bond in the AC system. Knowing where all those bonds may be located is another matter as not all inverters or generators make it clear.

So if there are questions or good solid information to be added to this grounding and/or neutral bonding issue, let's have a discussion.

Just because something has been done and has not failed, doesn't mean it is good design.

williamt

Should the DC side of the system have its own neutral to ground bond too?


MountainDon

Note: I use the term "earth ground" in an effort to help relieve any possible confusion with an earth ground and a neutral-ground or a negative-ground bond.

In an AC system it is a neutral - ground bond. In a DC system it is normally going to be a negative - ground bond. But, yes, a negative - ground bond should be used, although a 12 VDC system is normally permitted to omit this. The NEC specifies that DC systems of 50 volts and higher must have the negative - ground bond, but learned experts in the PV system world recommend all systems have a negative - ground bond. As with AC neutral - ground bonding there should only be one negative - ground bond on the DC side of the system. There is an exception noted below. *

Most people would never think of mixing any AC hot or neutral wires with the DC positive and negative wires. For some reasons some folks will mix the grounds. The AC and DC grounds should be kept totally separate. Never mix them. That includes separate earth grounding rods.

Most DC equipment like the charge controller and inverter will have earth grounding terminals. These and any others are best individually connected to a buss bar. Then the buss bar would be connected to the earth ground rod with a single wire. This permits any one device to be removed from the system without disturbing the electrical path. It also prevents current loops from forming, something that can happen with multiple paths to the earth ground rod.

Using equipment from an automobile, a radio for example, can lead to problems if the case is internally bonded to the negative. I have no advice for the solution other than to "don't do that."



* Negative - ground DC bonding exception. If the PV array and the main controls and batteries of the system are widely separated the ground wire between array and the main system components may be omitted. In such a case there should be a negative - ground bond at the array and there should be a negative - ground bond at the main part of the system. Each of these should have their own earth grounding rods.

The NEC does not precisely define "widely separated", but 30 to 50 feet (or greater) is the general rule to qualify.  My own PV system is built this way.
Just because something has been done and has not failed, doesn't mean it is good design.