Open Ground when on PSW Inverter -- Experts please advise

Thanks Salvo.. and you are correct. I should have thought of the RFI supressor cap's in the assorted devices. In addition to the inverter/generator, add and Microwave, add the TV, Radio and assorted other electronic devices. Most all have suppressor caps on the power leads.

I'm glad I came Closest... Validates all that money I spent on education :).

Salvo wrote:
It's more than just cable capacitive coupling. The converter has emi capacitors from neutral to ground and from hot to ground. Not sure, the microwave may also contain emi capacitors. Cable capacitive coupling is a minor component.

The 60V is created from a capacitor voltage divider from hot to ground and then from ground to neutral. If the capacitors are not equal in magnitude, you will see something other than 60V. Perhaps 65V on one leg and 55V on the other leg.

Placing an analog meter on one of the legs will reduce that leg's impedance, resulting in a lower measured voltage.

The point to all this is that the 60V is created by a capacitor voltage divider. It has little punch, just a little tingle when you touch it.

Sal

But that is only for non-bonded inverters, right? In my case, the inverter's neutral and ground WERE bonded, and I still got the tingle when I touched the case and the battery negative.

Huh, sometimes I look at these devices and shake my head in wonderment that they actually work - kinda tough to wrap my head around some of the theories.

Cheers!
Mike

Salvo wrote:
Sorry, wrong answer, but my fault. I didn't formulate question correctly. Why do you see 60Vac from ground to neutral, or from ground to hot when measured with a digital volt meter? And why does an analog meter measure perhaps only 6V? The inverter is not bonded.

wa8yam comes close to correct answer.

Sal

Hmm, good question then. Unless, the inverter is using a center-tapped transformer with the hot and neutral as the secondary's ends and the ground as the secondary's center-tap. But wouldn't make sense WRT safety. I would think then due to the lack of bonding you've got induction between the lines, like WA8YAM said.

Now, is this a PSW or MSW inverter? If PSW, then both meters should read correctly. IF MSW, most multi-meters, at least digital, don't read the correct voltage, UNLESS they can read RMS voltage. Not sure about the analog.

Cheers!
Mike

It's more than just cable capacitive coupling. The converter has emi capacitors from neutral to ground and from hot to ground. Not sure, the microwave may also contain emi capacitors. Cable capacitive coupling is a minor component.

The 60V is created from a capacitor voltage divider from hot to ground and then from ground to neutral. If the capacitors are not equal in magnitude, you will see something other than 60V. Perhaps 65V on one leg and 55V on the other leg.

Placing an analog meter on one of the legs will reduce that leg's impedance, resulting in a lower measured voltage.

The point to all this is that the 60V is created by a capacitor voltage divider. It has little punch, just a little tingle when you touch it.

Sal

wa8yxm wrote:

A1: Due to capacitive coupling between the wires forming a voltage divider.

A2: Due the the lower impedance of the analog meter "loading" the divider (Which can only handle a very small amount of current)

Salvo wrote:
A couple of test questions.

1. Why 60Vac?

2. Why does the analog voltmeter read about 6V (from gnd to neutral or gnd to hot)?

Sal

ve7prt wrote:

1) The hot and neutral wires will each carry about 60VAC. You will see 120VAC hot to neutral and hot to ground, though.

Cheers!
Mike

I recall answering both of those recently though the 2nd one by inference.

A1: Due to capacitive coupling between the wires forming a voltage divider.

A2: Due the the lower impedance of the analog meter "loading" the divider (Which can only handle a very small amount of current)

Sorry, wrong answer, but my fault. I didn't formulate question correctly. Why do you see 60Vac from ground to neutral, or from ground to hot when measured with a digital volt meter? And why does an analog meter measure perhaps only 6V? The inverter is not bonded.

wa8yam comes close to correct answer.

Sal

ve7prt wrote:

Salvo wrote:
A couple of test questions.

1. Why 60Vac?

2. Why does the analog voltmeter read about 6V (from gnd to neutral or gnd to hot)?

Sal

Sal, it all has to do with how inverters work, particularly the cheaper ones. A bit tough to explain, but in a nutshell, when producing the AC power is creates a positive and negative voltage (think sine wave) using different parts of the circuit. Instead of trying to invert the one side to increase voltage, it's easier to connect 1 output to the hot and the other output to the neutral. Run each at 60VAC, and you get 120VAC across the outlet. Most devices won't care either, so long as what they get is 120VAC. Clear as mud? Anyway, read Inverters By Wikipedia; that could probably explain it far better than me.


At least, that's how I see it.

Cheers!
Mike

wa8yxm wrote:
As an aside: Yes, I know that.

Now to the point. What I'm trying to say is this.

If you are holding a metal cased device (Did I mention a Skill hand grinder.. I actually held it) that shorts hot to case (It did) on mains power, which is earth grounded YOU become a "Return path".

In a standard system, yes. And you definitely DON'T want to be that return path!

But since the inverter has no ground.. Even if you grabbed the hot wire with your bare hand (Assuming nobody put in a ground when you are not looking) THERE IS NO PATH it is an open circuit, SWITCH OFF, no flow. (Still I do not recommend it)

Neither do I. I've felt the upside of 120VAC up my arm a couple times. IT definitely don't tingle! It hurts like h3ll!

As to the 60 volts on the ground lead (I do see that and it does tingle a bit)

This is because the ground wire is sandwiched between the black and white in standard ROMEX and in other types they also lay side by side.

Two conductors sepererated by a non-conductor (Dielectric) form what is called a capacitor or condenser (Two names, same device). Given time to do the research and a bit of math I could tell you the capatience per foot of say 14ga.

Well in this case you have two capacitors in the same can forming an AC voltage divider.. In most RV's there is not enough wire for this to pass enough current to do much in the way of damage (Why it only tingles) but that is where it comes from....

Basically a very tiny current flows right through the insulation wire to wire.. A sensitive voltmeter will show 60 volts (1/2 the voltage) a el-cheapo might show less and if you put a load on it (Say a 10 watt lamp) It goes by by fast.

You're talking inductance, right? The 2 current carrying conductors are INDUCING a voltage in the ground wire. And since, in some systems, it's not connected to anything, you measure a voltage, but its with respect to the hot or neutral.

Now, I have an older Xantex MSW unit I recently retired, but just after I installed it many years ago I was working around it with it on (it was providing power for one of my tools). I happened to lean my arm on the case of the inverter (metal casing) and my hand came to rest on the 12VDC negative terminal. There was most definitely a tingle there! I read the manual and it stated the chassis was connected to the ground, and the ground was bonded to the neutral, as per code for an installed power source (which it was). I verified this with my multimeter. I also saw the 12V negative was bonded to the chassis.

Having said that, I don't know if the replacement ProWatt SW unit would do the same. I'd have to meter it.

Cheers!
Mike

Salvo wrote:
A couple of test questions.

1. Why 60Vac?

2. Why does the analog voltmeter read about 6V (from gnd to neutral or gnd to hot)?

Sal

Sal, it all has to do with how inverters work, particularly the cheaper ones. A bit tough to explain, but in a nutshell, when producing the AC power is creates a positive and negative voltage (think sine wave) using different parts of the circuit. Instead of trying to invert the one side to increase voltage, it's easier to connect 1 output to the hot and the other output to the neutral. Run each at 60VAC, and you get 120VAC across the outlet. Most devices won't care either, so long as what they get is 120VAC. Clear as mud? Anyway, read Inverters By Wikipedia; that could probably explain it far better than me.

For your second question, is the inverter loaded or idling? If loaded, then the meter might have an issue. If idling, the inverter may have gone into search mode (or power saving mode). This is where it puts out a small voltage so that loads can operate, yet it draws almost no power (or very little). Once a load turns on, the inverter "wakes up" and provides full voltage to the load. Your analog meter might not produce enough load to wake the inverter up.

At least, that's how I see it.

Cheers!
Mike

ewarnerusa wrote:
My yamaha generator and PSW inverter (which I plug my shore power cable into) both show open ground with the little tester. Any interior outlet will show the same open ground result when the TT is plugged into the gen or inverter. When the TT is plugged into true shore power, tester shows all systems good

Sounds about right. Your genset probably has a floating neutral. It should say right on it. I know my Honda does. I'd have to meter the outlets to see if the ground is even connected to anything. It might not be. As for the inverter, if it's a portable one it might be using the same connections.

Cheers!
Mike

ve7prt wrote:

You have some good points there. However, even with an isolated power supply (which a portable inverter or generator would be) a ground wire can and will still do its job if you are using a metal cased device. Also, a GFCI outlet (which many mid-size inverters now have) can be used to enhance your protection.

BTW, as an aside, did you know that if you are in a house that has an older 2-wire electrical system (And not a 3-wire grounded system), you can put GFCI outlets in as a substitute? I read somewhere that if you have a house with older wiring and it is not practical to replace/upgrade that wiring, use of GFCI outlets is permitted instead.

Cheers!
Mike

As an aside: Yes, I know that.

Now to the point. What I'm trying to say is this.

If you are holding a metal cased device (Did I mention a Skill hand grinder.. I actually held it) that shorts hot to case (It did) on mains power, which is earth grounded YOU become a "Return path".

But since the inverter has no ground.. Even if you grabbed the hot wire with your bare hand (Assuming nobody put in a ground when you are not looking) THERE IS NO PATH it is an open circuit, SWITCH OFF, no flow. (Still I do not recommend it)


As to the 60 volts on the ground lead (I do see that and it does tingle a bit)

This is because the ground wire is sandwiched between the black and white in standard ROMEX and in other types they also lay side by side.

Two conductors sepererated by a non-conductor (Dielectric) form what is called a capacitor or condenser (Two names, same device). Given time to do the research and a bit of math I could tell you the capatience per foot of say 14ga.

Well in this case you have two capacitors in the same can forming an AC voltage divider.. In most RV's there is not enough wire for this to pass enough current to do much in the way of damage (Why it only tingles) but that is where it comes from....

Basically a very tiny current flows right through the insulation wire to wire.. A sensitive voltmeter will show 60 volts (1/2 the voltage) a el-cheapo might show less and if you put a load on it (Say a 10 watt lamp) It goes by by fast.