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Old 03-12-2015, 01:02   #61
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Re: DC generator and inverter ground

Quote:
Originally Posted by ramblinrod View Post
At least you now acknowledge there is a voltage present with respect to ground. You're getting there.

However, I have to correct you again, as you have introduced yet another error.

AC Volts as read on an AC voltmeter are RMS volts, expressed without polarity. There is no such thing as - 60Vac (rms)

Ah another fan. No I have never said there was voltage present to ground with a transformer inverter. What I said was there was no ground path with a transformer type inverter. There is a ground path with a non-transformer inverter. like your xantrex.

You might want to check the wave form of typical AC power. AC 120v power cycles from +V to -V 60 times a second, with a 0v crossing in the middle. This is because the US uses center taped 240V transfomers for residential power.

One end of the secondary coil at 120V+ and and the other end is at -120V. Combined that is 240V. The center tab called neutral is at 0V. So you have a two 120V hot legs in the typical residence. Each is 180 degrees apart from the other at peak. One at plus 120V the other at -120v. It's how transformers work. Each leg will alternate between + and - with 60 hertz in the USA or 50 hertz almost everywhere else.

So typical USA power has a hot leg that alternates from +120 to -120V 60 times a second when referenced from the center tap held at 0V. End tap to end tap is 240V.

Now the power company will tie the center tap, called neutral as its at 0V to earth and it will be connected to earth at the house panel too. It's an actual 10' long copper rod that is earth, also called ground. The newer non-transformer inverters mimic that, hot leg to neutral center tap. And everything you say is correct for that.

But most of the world does not use a center tap, just the ends of the secondary for 220-230V, leg to leg. That does not mean that one leg is at 0V like USA house power. What it means is one leg is +110V and the other leg is -110V for a total V of 220V. That is how the cheap MSW inverters work too. Except they drop the end tap voltage to 60V+ and 60V-. Probably because that's what the Chinese EE's know. There isn't a center tap and one leg is not at a constant 0V. Both legs are hot but at opposite phase or pole if you like.

With switching power supplies and switching inverters you can rather easily, get voltage between neutral and ground. Even more so with 3 phase as 3 phase is 120 degree out of phase per phase and does not cancel the 120V harmonics as a 240V single phase would.

EE's have found lots O fun with 3rd order harmonics when using lots of 120V switching supplies on 3 phase power. To the point where ground wires over heat and burn up. I learned that somewhere around 1985. That is one reason ground is oversized in data centers. I believe the NSA has that little issue with their new data center just this year, or something very much like that.

In any event. AC power does cycle from positive to negative Volts, that's the sine wave we all know and love. Please check voltage with an oscilloscope. A RMS voltmeter merely gives correct peak to peak voltage readings. It does not tell you the polarity. That is +V to -V peak. It's how AC power alternates.
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Old 03-12-2015, 07:39   #62
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Re: DC generator and inverter ground

I wish there was a head spinning icon....

It seems the OP will have a similar system to ours with zero shorepower (none, nada, 0), but unlike theirs, our DC system is totally floating. I was planning on installing our Magnum inverter in a similar fashion as SC since the inverters installed outlet will be the only one used. Seemed simple enough.

So how is what the OP and I'm talking about different than a portable inverter 100 watt inverter plugged into a D/C outlet?

Matt
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Old 03-12-2015, 07:51   #63
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Re: DC generator and inverter ground

I really can't remember what the small inverters I have used personally said in their manuals as far as ground. But I don't remember connecting a ground so I don't think they did. However, for all the bigger inverters I have installed (lots) and on my own boat, the manufacturers require a large case ground (normally green) that is to be no smaller than one gauge below the DC power feeds to the inverter. I think ABYC has that requirement as well.

I don't think you would ever fuse a ground wire under any circumstances. The requirement for a large cable ground is obviously to ensure that it can carry as large a current as would be possible under any conditions from a faulty inverter, presumably it would be "fused" in reality by the fuse on the quick acting fuse on the DC positive if it came to full DC current going to the ground. Of course, DC voltage between 12v and 24v would not normally do much of any damage to a person, but 48v is much more dangerous. I have been tingled by 12v many times but burned by 24v (minor) but enough to be very uncomfortable.

One of the areas I have been interested in but do not have much knowledge on is AC transformers. I understand the basics, and how the center tap (in the US) is grounded to neutral, but that is about it. So I'm not qualifies to comment on the discussion you guys are having about the actual voltages. I do know that AC voltage does vary between positive and negative at 50 or 60Hz, so it is pretty fast switching. My understanding of RMS (root mean square) is that it is a measure of the true "power" available in an AC voltage/current circuit so you can compare output between MSW and non-MSW inverters.
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Old 03-12-2015, 07:58   #64
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Re: DC generator and inverter ground

Quote:
Originally Posted by funjohnson View Post
I wish there was a head spinning icon....

It seems the OP will have a similar system to ours with zero shorepower (none, nada, 0), but unlike theirs, our DC system is totally floating. I was planning on installing our Magnum inverter in a similar fashion as SC since the inverters installed outlet will be the only one used. Seemed simple enough.

So how is what the OP and I'm talking about different than a portable inverter 100 watt inverter plugged into a D/C outlet?

Matt
I think if you read your installation manual for the Magnum it requires a safety case ground. It is not one of the small inverters that SC is describing. Larger MSW marine inverters require a case ground as well (e.g. the Xantrex one RR notes). But you're allowed to do what you want. A good surveyor would call it out if they found it.
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Old 03-12-2015, 08:18   #65
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Re: DC generator and inverter ground

Quote:
Originally Posted by exMaggieDrum View Post
I think if you read your installation manual for the Magnum it requires a safety case ground. It is not one of the small inverters that SC is describing. Larger MSW marine inverters require a case ground as well (e.g. the Xantrex one RR notes). But you're allowed to do what you want. A good surveyor would call it out if they found it.
Ah yeah.... read the manual. Sure enough, it states that it needs a "chassis ground" connected to the DC grounding system. Heck, it even says so right on the lug in the back of the inverter.

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Old 03-12-2015, 08:59   #66
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Re: DC generator and inverter ground

Rather than spinning the roulette wheel, Vegas style, with your inverter or inverter-charger it is often easiest to simply follow the standards and source units that already meet the minimum requirements...

"31.5 REQUIREMENTS
31.5.1 GENERAL
31.5.1.1 Battery Chargers, inverters and inverter/chargers shall be tested by an independent laboratory to establish compliance with this standard."

31.5.3.2 All marine power inverters shall meet the applicable requirements of UL 458, Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts, and Supplement SA, Marine Power Converters/Inverters and Power Converter/Inverter Systems.

31.5.5.7 Battery chargers, inverters or inverter/chargers installed in spaces requiring ignition protection shall meet the ignition protection requirements of ABYC C-1500, Ignition Protection for Marine Products or SAE J1171, External Ignition Protection of Marine Electrical Devices (see A-31.8 for labeling requirements)."
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Old 03-12-2015, 09:31   #67
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Re: DC generator and inverter ground

I'm sorry, but you keep introducing new errors faster than I can correct them.

Please stop spewing nonsense. It is not serving you well by anyone who does understand basic electrical principles and your advice could kill someone who doesn't and follows your lead.


Quote:
Originally Posted by sailorchic34 View Post
No I have never said there was voltage present to ground with a transformer inverter.
I wish you had of. That would mean you had learned something in this thread that could save your life.

Quote:
What I said was there was no ground path with a transformer type inverter.
And what I said is there is a ground path, as soon as one part of your body is touching anything grounded while another part is touching the inverter output. Should you do so, current will flow from your inverter output, through your body, while it electrocutes you. You denied that you could even get a shock.

[/QUOTE] There is a ground path with a non-transformer inverter. like your xantrex. [/QUOTE]

Regardless of transformer design (transformer or non-transformer in your terminology), if it is ABYC compliant, on out put leg is grounded ("Neutral") under all conditions. With any inverter design, there is always a ground path from the output, as soon as anything touches the output that is grounded, LIKE A PERSON!

Quote:
You might want to check the wave form of typical AC power
Deary, I have been an electronic engineering technician for over 35 years. I assure you I have looked at a 120Vac waveform with an oscilloscope, 1000's of times. I know exactly what they look like and obviously you don't.

Quote:
AC 120v power cycles from +V to -V 60 times a second, with a 0v crossing in the middle. This is because the US uses center taped 240V transfomers for residential power.

One end of the secondary coil at 120V+ and and the other end is at -120V.
Incorrect, "AC" stands for alternating current. Your residential supply is constantly switching back and forth at 60 Hz (NA).

One leg is 120Vac (rms) and the other leg is 120Vac (rms). These legs are 180 degrees out of phase. There is no such thing as +120 Vac (rms) and -120 Vac (rms) as they are constantly alternating. It is just 120 Vac.


Quote:
Combined that is 240V. The center tab called neutral is at 0V.
A residential supply has two conductors each at 120Vac (rms) and 180 degrees out of phase with each other. Please do not combine them or you will see a brilliant flash, a puff of smoke, and a woof of ozone, and perhaps burnt hair, if you are still breathing.

However, if you measure the voltage across them, you will read 240 Vac (rms).


Quote:
So you have a two 120V hot legs in the typical residence. Each is 180 degrees apart from the other at peak.
Annnghhgh! Wrong! Correct until the last 2 words, you are confusing rms (what you read on an AC voltmeter, that is converted to display the value equivalent to DC power) and peak to peak volts.

Quote:
One at plus 120V the other at -120v. It's how transformers work. Each leg will alternate between + and - with 60 hertz in the USA or 50 hertz almost everywhere else.
Well, wrong again, as there is no such thing as plus or minus 120Vac (rms).

So typical USA power has a hot leg that alternates from +120 to -120V 60 times a second when referenced from the center tap held at 0V.

Wrong again for same reason as above. This is not DC (which does have a steady polarity.) The hot legs are both 120 Vac (rms) but 180 degrees out of phase.

[/QUOTE]End tap to end tap is 240V.[/QUOTE]

First correct statement in this post.

In any event. AC power does cycle from positive to negative Volts, that's the sine wave we all know and love.

Yes, alternating current "alternates", but unfortunately you do not know your sine waves as well as you believe you do. A 120Vac (rms) sine wave is actually 340 V peak to peak. It is these two sine waves, 180 degrees out of phase that gives 240 Vac rms in a residential supply in NA.

http://www.samlexamerica.com/support...aveACPower.pdf



Quote:
Please check voltage with an oscilloscope. .
Please, I have looked at more sine waves with an oscilloscope than you can count.


Quote:
A RMS voltmeter merely gives correct peak to peak voltage readings. .
WRONG! An AC voltmeter displays the RMS (Root Mean Square) value. When it reads 120Vac RMS, it is actually measuring a 340 Vac Peak to Peak sinusoidal wave form. The reason it does so, is that this sinusoidal AC waveform has the equivalent power of 120Vdc. This was a standard developed well over 100 years ago.

OK, so this was a not so pretty stroll down "AC Electrical Fundamentals".

All that said, your 120Vac modified sine wave inverter output is quite different from a sinusoidal waveform, the peak to peak voltages will be different, and the waveform itself will be stepped rather than smooth curved, but the power will be equivalent to 120Vac rms. Appliances designed to run on a sinusoidal waveform won't work quite as well on modified sine, but most (not all) will work to some extent.

Now back to the real discussion.

An inverter with floating output of ANY DESIGN, is not safe on a boat (or anywhere really), for the reasons I have now stated numerous times. If you touch one leg and ground you will definitely receive a shock and could be electrocuted. Because it is so easy to be accidentally grounded in a marine environment, they are particularly dangerous there. A "marine" inverter that is ABYC compliant and has a grounded "Neutral" is much safer.

If, against all this advice, you choose to use a non-marine inverter, choose one that has integral GFCI outputs. In this case, as long as the GFCIs function correctly (and that's a big and very important if) you should not be electrocuted if something bad happens. (The cheap floating output inverters won't think twice, they'll kill ya without a care.)

NEVER connect a floating output inverter to the vessels on board AC system. When shore power is disconnected, your AC breakers won't trip if a faulty appliance is connected and the ground plug on the appliance, designed to protect you from electrocution, can't.

Ramblin Rod
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Old 03-12-2015, 10:00   #68
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Re: DC generator and inverter ground

ISTM a lot of the confusion on this thread is the varied and mixed usage of "ground" and "earth ground". Where "earth ground" on a vessel-based electric supply means connecting the boat ground to the water. A safety "ground", or the 3rd wire in an AC electric system, works the same regardless if the electric supply is connected to "earth ground" or not. I don't believe there is any argument over the value of the 3rd wire "safety ground" connecting to one side of the supply.

There is continual debate about the value of connecting a vessel-based AC supply (genset, isolation transformer, inverter) to "earth ground" (water). If there is no path back to the source, there is no danger of shock.
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