Ready to make a UPS, just need some clarification

Hehehe. Good question. For the average homeowner the answer is likely to be faith-based.

It's kind of an elaborate process using specialized equipment, some interesting math, and more labor than you would expect. Here's one nice overview article about it, with some diagrams. You can google "fall of potential method" for more.

They may have the equipment to test it; I don't know if they would send someone to do it because you were curious, or only if there was reason to suspect a problem. I've never asked.

As the ground path really only affects what happens during a fault or surge event, I would not expect its resistance to work much to the utility's monetary favor or disfavor.

All of Article 250 in the National Electrical Code is about grounding. It covers a lot of things I couldn't here (like what you can do if your ground rod hits rock above the specified depth). Basically you can have made electrodes (like a driven rod) and "found" ones (like a metal water service pipe, or rebar in a poured foundation). Whatever things are there that meet the definitions of electrodes have to be wired together, and become your grounding electrode system. If it doesn't have enough non-made electrodes you have to add some made ones. If you add a ground rod but it turns out above 25 ohms you have to add another one. There's no requirement to keep testing and adding more beyond the second, so some installers will just drive two without any testing and call it good. Electrodes within 6 feet of each other are less effective than electrodes farther apart. And so on. A whole little universe of things to know that most people probably haven't imagined ... I know I hadn't!

-Chap

 

Andy,
There are commercial outlet testers available for about $6_US (or so). do a Google search for "outlet tester", the first link (Eclipse 400-029) looks identical to mine.

It has a 3 prong plug and 3 lights, 2 yellow & 1 red. The center light is wired between the hot (H) & neutral (N) prongs of the plug, the right (Y) between hot (H) & ground (G), and the left (R) between neutral & ground.

There is a chart on the top showing the 6 possible combinations of lighted and dark lamps.

If the right two (Y&Y) are lighted, everything is good, there is 120V between H & N & 120V between H & G. Since both G & N are tied together in the CB box, that is good.

If only the center is lighted, the G wire is open.

If only the right (Y) is lighted, the N wire is open.

If center (Y) & left (R) are lighted, the hot & neutral wires are swapped. (the N wire is 120V from G & the H wire is at the same potential as G)

If the left (R) & right (Y) are lighted, the H & G wires are swapped.

Note this outlet tester is good for quick, but not all inclusive tests of outlets. Since the tester draws very little current, it may show 'good' if there is a high resistance connection in the circuit.

You asked about a megger. A megger is used for verifying that high resistances are truly high. For instance, after pulling a number of wires through a conduit, you could verify that the insulation on each wire is good by meggering each wire to the other wires and ground (the conduit). All but one wire would be tied together, and to the conduit, and the megger connected between the remaining wire & the conduit. It is basically an Ohmmeter that operates at about 1000V. Needless to say, you want to megger the wires before connecting them to any electronic equipment.

Norm

 

As Norm points out, the outlet testers can be handy for checking the condition of the wiring from an outlet back to the point where G and N are bonded together at the box. They won't tell you anything at all about the resistance from that point, through the grounding electrodes, to the earth itself - that's where those more involved ground testing protocols come in.

-Chap

 

I just searched that and I think I'll pick one up. Maybe Home Depot sells them. It's strange that in my formative years (before 30} I looked at a multimeter as a foreign object. After 30 I could not understand how I got along without one. Thanks. Andy

 

Forget all the other hyperbole, Tom's post #28 answers your question. If your connecting to the house, the green wire (safety ground) must always be there. Yes it is usually the round, third post. If your using the output of the inverter for camping then the safety ground (green wire) has no effect. For the 12VDC Aux battery, chassis, ground, negative are all one and the same. As you have no doubt observed, the Neg lead on the 12VDC battery connects to the chassis. The really important point in all this IS: Watch out for wires getting hot, if they are, disconnect immediately and figure out why.

 

TO: Mike Baker, I just looked up the Aims 1250Watt inverter you installed. You said you used a 80Amp fuse. Did you notice that site calls for 150 Amp fuse? In fact they sell the fuse and holder.

 

For the AIMS to have full capacity it would need the larger fuse, not to mention very heavy inpput wiring. However when connecting to the Prius 12 volt system, a more important consideration is to protect the main system fuse from blowing. That would render the car inoperable until replaced. I suggest that 80 amp is a good starting point, and smaller ones might be more appropriate.

Had a lead for low voltage 60 amp breakers at a good price but now can't find it.

 

Hello tochatihu: So the only time a larger fuse would be needed is with a much larger 12VDC battery? If the Prius is rated at (about) 30AHr then what is the criteria for determining the 80Ampere fuse? I suppose the final use in Watts would determine the current draw on the 12V battery. Since the company designed the inverter they are familier wirh (normal) current draws. Since 150Amp is only twice an 80amp then it's not out of line. Or is the inverter company normally thinking in terms of a larger DC supply? Anyway one can always overfuse but never underfuse. Not seeing the schematic I'm guessing the fuse links the plus on the bat to the plus on the inverter. Andy.

 

A larger fuse is needed to draw more power. A 1250W inverter will draw over 100A when it is fully loaded. The exact amount depends on efficiency and input voltage, but we don't need to be exact for this discussion.

Note that I said fully loaded. Just idling, that same 1250W inverter might draw only a couple of amps. The amount of current goes up more or less proportionally with the load.

When you use a smaller fuse, the inverter will work fine up to the point that the fuse blows. In other words, by using a smaller fuse, you effectively have a smaller inverter. Instead of 1250W, your inverter might be good only to 800W (just as an example).

As one of the earlier poster said, there is a safe limit to how much current you can draw from the Prius 12V system. The amount varies depending on whether you are taking it from the Prius 12V bus, or just drawing it from the little 12V battery. I wouldn't want to run an inverter from the Prius 12V battery alone, as it is a very small battery, and it is not designed for deep cycles. The smaller fuse size is suggested to protect the Prius 12V system and battery from large current loads.

Tom

 

Hello Tom. OK that all makes sense, especially the last sentence. I was thinking about having a inverter at hand and the car wired up for the adaptation But now I wonder about the total logic of it and would such a emergency (likely) occur? Thanks for the reply. Andy.:cheer2:

 

I'm going to offer one suggestion: single ground point.

It doesn't matter if the ground is vehicle-to-ground or ground-lead-to-ground but having both the vehicle and ground-lead tied to SEPARATE grounds is a bad thing!

The problem is multiple grounds can sometimes lead to a ground loop. They are pernicious and though seldom fatal, hell to pay for the grief they can cause.

IMHO, the best answer is to ground the vehicle to earth and the problem is likely to be solved.

Bob Wilson

 

It has previously been suggested that a good way to get max power (including some big current peaks) from the Prius 12 v system is by adding a larger 12 v battery near the outboard inverter. Using short, heavy gauge cables. I don't remember the exact size now, but the Prius' cabling from the front is about 4 gauge, and it's several meters long. Thus it would appear to make sense that the demand peaks of the outboard inverter would largely be met 'more locally' from the added battery and its big pipe. I would still like to see conservative current protection/limiting for the Prius main fuse, but the other positive connections could be 'wide open'. Then a 1250 w (or even larger) outboard inverter could deliver power peaks without bring the whole system down.

 

Correct me if I'm wrong, the input to the inverter is 12VDC, one plus and one minus with the minus at Vehicle chassis ground. The output(s) are 3 or 4 ordinary 3 prong AC outlets, just like your house right??? Should you make a nice heavy cable, the type an RV has, with a male on each end , wiring up the plug-ins, with the safety ground, then......you could plug it straight into the house , at a good secure female outlet. The green safety is automatically protecting you, the house and the chassis of the inverter. In fact the green safety is actually attached to the inverter chassis. Should you actually be using the inverter in this manner, Make sure you pull the main circuit breaker on your house FIRST! And if you run an extension from the inverter, make sure it's heavy duty and as short as possible. If you do not have a multimeter, by all means buy one and learn to use it, it's an education. Never run inverter current and house current together within the house circuit. There are many reasons for this, but I will not go into it. One more point whenever we speak of chassis ground on the vehicle (any vehicle) any metal lic point thruout the car is electrically one and the same and they are the same as the negative lead at the battery, HOWEVER there are preferred points of connection as some random chassis point could be floating or of a slightly higher resistance thereby lowering current flow. Paint, rust etc could affect the ground connection.