What constitutes a “sufficient” ground?

Question

Electricity novice here. Thanks for reading!

I have a 3000 watt DC to AC inverter attached to a 12V battery bank. The inverter has "DC ground lug" on the exterior of the inverter's case which the manufacturer says should be "tied into the DC ground of the system".

My "system", however, is a small box on wheels that I take around with me. There is no vehicle chassis or house ground that I can tie into.

As I understand it, the purpose of the ground connection is to bleed off current in the event that the inverter's case should ever become energized, which in turn would trip a breaker.

My questions are as follows:

1. How much metal does the ground wire need to be connected to in order for the ground connection to work as intended? Is it enough for me to attach a strip of metal inside of my "mobile power box" and attach the ground wire to that? A car's chassis is massive and I see that it might conduct a lot of energy, but I'm not sure how much mass is enough.

2. Can the ground in a system like this be tied into the negative side of the battery bank? The battery in this case is not tied into a vehicle chassis, so I'm not sure that would even make sense.

Thanks in advance!

Answer 1

@rdtsc provided a link to the manual which covers what to do with the grounding. You are going to connect DC negative to chassis ground with heavy gauge wire (see manual), and you are going to have to decide what "chassis" means in your case. Maybe if this box of yours is metal, you can just bond the chassis ground to the metal box. But this is not really the important part. More importantly, make sure you use a GFI somewhere between the inverter and your load. Inside the inverter, since it is a UL 458 inverter, there is a relay which connects green wire to neutral only when the inverter is supplying power. When it is in standby or pass-through mode (when you supply AC to the inverter so it can recharge its batteries) the connection from neutral to GND is open. In either case, there is exactly one place where GND is bonded to neutral. Your 12 battery is not a shock hazard. It stores an impressive amount of energy, and can be a fire hazard if there is a short, so make sure you have proper fuses. But if you screw up the AC grounding, you can introduce a shock hazard. Luckily it is not that complicated. Just read the manual again.

Answer 2

In some jurisdictions a system that size would require being pushed to location and driving an earth peg (1.6m into the dirt) for the ground and connecting it to the lug you mentioned before it could legally be operated. Same rules apply to generators.

Noting they you say:

"tied into the DC ground of the system"

This may refer to the solar panel earth as if the inverter was designed for a solar battery system. Again in some jurisdictions, the solar array is required to be earthed with a not less than 4mm² earthing conductor which is taken into the main earthing system of the premises.

These arrangements prevent the inverter neutral floating above ground voltage and ensure that there is sufficient current return to operate RCD type protective devices in the event of a fault or electrocution.

The whole purpose of doing it properly is to avoid risk of electrocution.

Note: In the manual linked in another comment it shows both the chassis ground and the battery negative connected to earth on pages 23 & 24. If you do not have a suitable earthing system available that you can connect to legally then you need to drive an independent earth stake and connect both to that.

If you are unsure of electrical work use a licenced and qualified electrician.

Answer 3

A ground is for a return current. A return current should be able to flow through the ground without it affecting the design. This can happen if:

1) There is too much resistance creating a voltage in the current return path which can create a common mode problem. For example, if you have 12A flowing through a 0.1 ohm ground cable, this will create a 1.2V voltage at the point of the current input (at the device) $/ V = I*R $/.

2) The other problem is inductance, if you have a fast signal (or ground EMI) you need to have a sufficient reduction of inductance to ground out the high frequency signal.

So short story is measure the current through or voltage across the cable, and keep it lower than what your design needs.

Answer 4

All that these guys said is great, to dumb it down, especially for me, attach your ground to a screw in the bottom of your mobile box, attach the wire to it. That screw should go all the way through the box and be able to touch the ground, therefore creating your ground situation.

If this mobile battery box is used inside the house, it creates more problems. I am doing the same thing, but I will be connecting the ground to the baseboard wall as my house is grounded already. Your mobility is good as long as its outside or you don't mind burnt floors in case a discharge happens, rare, but it does.

If you don't provide an adequate ground, anything 3-prong might have difficulty working if there is a sensor reading that third prong. Xbox One, Electric cars and the smart equipment are the example. On the other hand "dumb" equipment like weed eaters, and toaster ovens don't care.