Marine AC - Why Tie Neutral and Earth?

[boatpoker]

Originally Posted by smac999

so if you take the hot feed wire at the AC panel and short it to the green ground wire what happens?...

If your DC negative and AC ground are bonded as per ABYC, you will send AC into your DC system. This is what causes ESD in fresh water.

[smac999]

Originally Posted by boatpoker

If your DC negative and AC ground are bonded as per ABYC, you will send AC into your DC system. This is what causes ESD in fresh water.

Yes like this... 15a leaking through (salt) water was not enough to blow the 30a main. Elci would have though. But I’m curious what would happen on his boat with a floating ground.

[Thumbs Up]

Originally Posted by s/v Jedi

...
So the ground wire (green/yellow, earth, bare copper) on the outlets all go to the ground busbar as shown in my diagram. This is where the ground wires from transformers, inverter/chargers, boat-side of galvanic isolator and isolation transformer etc. go. It is not connected to DC negative and not to AC Neutral. It’s only job is to connect all external metal surfaces of appliances together.

So you are connecting your outlet grounds back to ship/shore ground? You are still connected to shore AC neutral! (if you are plugged into the 50 amp, the 30 amp depends on how you have your isolation transformer configured) Shore ground and shore AC neutral are connected (at the source). You have an isolated supply at your outlets but the outlet ground connection is connected to the water, the shore ground, and the shore neutral. The power at the outlets will not recognize these as a ground. A fault from the outlet power to the ground wire would not cause anything to trip. A second fault from the opposite phased outlet power would cause a trip (of an AC panel breaker), but it would do that only by virtue of the fact that your outlet grounds are wired together (no need to connect them to ship/shore ground). I don't understand why you have the third outlet wire (safety ground) connected back to ship/ shore ground. Why is it not connected to the autotransformer nuetral? Also you are losing out on the safety and corrosion advantage of having a completely isolated system.

[boatpoker]

Originally Posted by Thumbs Up

Also you are losing out on the safety and corrosion advantage of having a completely isolated system.

AC can cause corrosion on a boat, but at such a slow rate it is unlikely you'll notice it for years or more likely, decades.

[Thumbs Up]

Originally Posted by boatpoker

AC can cause corrosion on a boat, but at such a slow rate it is unlikely you'll notice it for years or more likely, decades.

He is also grounded to the neighbor boat's DC. Although the galvanic isolator is there, is the isolation transformer passing it's earth connection? (depends on how it is configured)

[s/v Jedi]

Hahahaha... guys, when the ground becomes active... you may get a vulcanic eruption! The ground in my diagram are just a bunch of wires. You may get a fault in a wind instrument which may make your ground active.. the horror!

About ground to neutral relays: these exist in inverters as well as auto transformers. It is correct that these must not be used. In the Multiplus this is just a configuration setting.

About the genset connected wrong: that’s hilarious it’s actually the only way to get full genset output because it enforces perfect balance over the output windings, which you don’t get when you connect it’s neutral. In fact, only connecting L1 and L2 from genset into an auto transformer which creates a new neutral (like I do) is the recommended way for full genset output.

If you don’t know how a floating power distribution system works then go read up and study it instead of posting that it’s wrong. It’s not wrong, it’s in common use even when your house doesn’t have it!

About my 50A shore power: it should have an isolation transformer but I don’t want a 12kW transformer aboard. I get away with this because my auto transformers don’t ground neutral (I tried what happens: 1.3kW went into shore ground!) and all the regular grounding is in place. Also, I only use this in my own slip where I’m sure the pedestal is in good order (I own the slip).

[Thumbs Up]

Your 50A shore power earth is connecting all of your green wires to shore neutral. Your isolated outlets have their grounds connected to shore power. They should connect to the autotransformer neutrals.

[s/v Jedi]

For reading up on this, start with section 7.5 on page 60 here: https://www.victronenergy.com/upload...limited-EN.pdf

The section is about mobile installations, where no grounding rods are hammered into the earth.

It amazes me why this is so hard to understand... let’s make it simple: you have a car or motorhome with an inverter: what is your ground? You drive a rod into the soil when you switch on the inverter or start the generator? No you don’t! Yet all ground wiring is in place... it’s called chassis ground instead of earth ground.

[smac999]

so answer this question...

Originally Posted by smac999

so if you take the hot feed wire at the AC panel and short it to the green ground wire what happens?... under inverter, gen , 240v, and 120v shore

[s/v Jedi]

Originally Posted by Thumbs Up

Your 50A shore power earth is connecting all of your green wires to shore neutral. Your isolated outlets have their grounds connected to shore power. They should connect to the autotransformer neutrals.

Well... no, they connect to the galvanic isolator. And with an ungrounded neutral... guess what... you don’t connect ground to neutral

Haha. I know the 50A is different. What is supposed to be the neutral in the outlet gets a potential to ground, which isn’t normal. But with an ungrounded neutral, that’s how it is.

Let’s take a different approach: you show me a “dangerous” situation with ungrounded neutral and I’ll explain why it isn’t dangerous. Nobody ever came up with something except for the two simultaneous appliance isolation failures that are touched simultaneously... which highly unlikely situation is protected by gfci outlets.

[Thumbs Up]

Originally Posted by s/v Jedi

For reading up on this, start with section 7.5 on page 60 here: https://www.victronenergy.com/upload...limited-EN.pdf

The section is about mobile installations, where no grounding rods are hammered into the earth.

It amazes me why this is so hard to understand... let’s make it simple: you have a car or motorhome with an inverter: what is your ground? You drive a rod into the soil when you switch on the inverter or start the generator? No you don’t! Yet all ground wiring is in place... it’s called chassis ground instead of earth ground.

A mobile system typically connects to a variety of power sources it is sometimes not clear which of the leads in
the shore power supply is connected to earth or if earth is connected at all. Also phase and neutral may have
not been wired correctly. Connecting a supply like this to a mobile system can potentially create a short circuit
to earth. Or earth is missing completely

As per your diagram, if you have your 50A supply plugged in, you are connected to shore ground and therefore shore neutral. You don't want your isolated outlets to safety ground to that.

[Thumbs Up]

Originally Posted by s/v Jedi

Well... no, they connect to the galvanic isolator. And with an ungrounded neutral... guess what... you don’t connect ground to neutral

A galvanic isolator passes the ground connection but filters out any DC current. You had stated in an earlier post that all of your outlet grounds are connected to these wires. They should not be. Not only are you introducing shore ground/neutral potential to your appliance cases, this connection is also not providing any safety benefit. Your outlet grounds would be better off left open.

[Thumbs Up]

Originally Posted by s/v Jedi

Let’s take a different approach: you show me a “dangerous” situation with ungrounded neutral and I’ll explain why it isn’t dangerous. Nobody ever came up with something except for the two simultaneous appliance isolation failures that are touched simultaneously... which highly unlikely situation is protected by gfci outlets.

I am not arguing against the merits of a floating AC supply, It is just that you have your outlet grounds tied to the wrong place.

[s/v Jedi]

Originally Posted by smac999

so answer this question...

Okay. Remember, both wires are to be considered “hot” because the neutral isn’t grounded. When you create a short between a hot and boat ground, nothing happens. I know it’s dull but absolutely nothing happens. You can freely touch the ground wire, metal appliances, even when touching earth with your other hand. In fact, like someone mentioned, in some cases they do this on purpose. The simple reason is that nor ground, nor earth is a return path.

So there is one exception and that is when the 50A shore power is active. In that case a breaker trips. This is one of the reasons why a true floating power distribution is better: you need at least two isolation faults before a breaker trips. If you install a detector, you can most likely prevent that by installing an isolation fault monitor which warns when the first fault develops so it can be fixed before another fault develops. This is why they use floating power distribution in demanding high availability situations like operating rooms etc.

[Thumbs Up]

If you connect your outlet grounds to your transformer neutral, it is still a floating system, because those grounds are not earthed, and your neutral is not really neutral. And if you have RCD breakers in the panel, that outlet ground wire becomes a sense wire that will trip the RCD at 30ma as a safety backup to your GFCI outlets.