Burned ground wire on inverter

[transmitterdan]

Originally Posted by a64pilot

Unless I’m mistaken my AC ground bus and DC ground bus are connected? Have to open the panel to be sure.

That should only be true if you have no shore power. When you have shore power the AC and DC grounds must not be connected inside the boat anywhere. Only when using generator or inverter should they be connected and then only directly at the generator or inverter. No where else on board should the AC and DC grounds be connected. For sure not inside your electrical panel.

[boatpoker]

Originally Posted by transmitterdan

That should only be true if you have no shore power. When you have shore power the AC and DC grounds must not be connected inside the boat anywhere. Only when using generator or inverter should they be connected and then only directly at the generator or inverter. No where else on board should the AC and DC grounds be connected. For sure not inside your electrical panel.

The DC negative and AC ground should always be bonded on the vessel
(as per ABYC).

I believe you may be thinking of the AC ground and AC neutral which should only be bonded at the power source whether generator, inverter or shore power.

[a64pilot]

I believe your talking about the neutral and ground.
I just looked boat was wired 32 yrs ago with both grounds bonded to each other.

[boatpoker]

Originally Posted by sailingunity

No it is not a marine inverter, it is a "motomaster" 1000W pure sine wave. It says "mobile inverter" on the casing. If the cheapo inverter was the issue, it won't be for much longer as it is going to be replaced with a victron inverter / charger soon.

The Canadian Tire "Motomaster" inverter is not a marine inverter and should never be used on a boat. Like automotive type battery chargers the AC and DC sides are not isolated. AC and DC leaks are common and can cause significant corrosion through the DC side and are potentially lethal through the AC side particularly in fresh water. (read up on ESD-Electric Shock drowing).

[transmitterdan]

Originally Posted by boatpoker

The DC negative and AC ground should always be bonded on the vessel

(as per ABYC).



I believe you may be thinking of the AC ground and AC neutral which should only be bonded at the power source whether generator, inverter or shore power.

Ok, ABYC says connect boat AC shore ground to DC negative. Personally I think that is bad advice but I agree it is the ABYC recommendation. However, it should be done at one and only one point else there is risk of DC current in the AC ground wire. And when shore power is switched off so too should the shore ground be disconnected. Just connecting all the green wires together inside the boat is a poor practice IMO.

[transmitterdan]

Originally Posted by a64pilot

I believe your talking about the neutral and ground.
I just looked boat was wired 32 yrs ago with both grounds bonded to each other.

It’s a pet peeve I have. When the boat is on internal power it should not be connected to shore at all.

[Frankly]

In earlier days the connection between the AC ground (green conductor) and the 12 Volt negative (black or yellow conductor) was where the galvanic isolator was inserted. Recent thinking has inserted the galvanic isolator in the shore power AC ground circuit and connected the AC ground to the 12 Volt negative with a conductor with the same ampacity as that feeding the 12 VDC to the inverter. Apparently with some inverter designs/ topology there are failures that can put large currents into this ground / return conductor.

Frankly

[sailingunity]

Originally Posted by transmitterdan

Ok, ABYC says connect boat AC shore ground to DC negative. Personally I think that is bad advice but I agree it is the ABYC recommendation. However, it should be done at one and only one point else there is risk of DC current in the AC ground wire. And when shore power is switched off so too should the shore ground be disconnected. Just connecting all the green wires together inside the boat is a poor practice IMO.

Ok so I just went and traced all my green wires and it looks like one of them does bond my DC ground to the AC. So I guess I'm good there.

I'm sure the cheapo inverter had something to do with the issue.

I don't have a galvonic isolator, are you saying I need one?

[Neptune's Gear]

Yes. It's a safety device that should, IMO be mandatory. Either a galvanic isolator or an isolating transformer.

And be careful! if there was sufficient current to burn the ground wire, it could be dangerous - we don't know if it was AC or DC....

If you are not certain what you are doing, seek qualified assistance.

[transmitterdan]

Originally Posted by sailingunity

Ok so I just went and traced all my green wires and it looks like one of them does bond my DC ground to the AC. So I guess I'm good there.



I'm sure the cheapo inverter had something to do with the issue.



I don't have a galvonic isolator, are you saying I need one?

Definitely I think these are a good thing. They are not a safety device in the sense they protect people. They protect the boat from corrosion caused when DC current flows from the boat to shore ground via the green safety ground wire. But I don’t think a galvanic isolator would help for the problem you had. You are doing the right thing by replacing with a proper marine rated inverter.

These truck stop inverters are a menace to boating.

[GordMay]

Originally Posted by boatpoker

The DC negative and AC ground should always be bonded on the vessel
(as per ABYC).

I believe you may be thinking of the AC ground and AC neutral which should only be bonded at the power source whether generator, inverter or shore power.

Indeed.
Which the transfer switch (should) accomplish.

[Brewgyver]

Shouldn't the inverter AC ground wire also be sized to carry the fault current without burning?
Or is the concensus (i.e. group guess) that it was DC current, which shouldn't have been present on that conductor at all, that caused the charring?

[sailingunity]

Originally Posted by Brewgyver

Shouldn't the inverter AC ground wire also be sized to carry the fault current without burning?
Or is the concensus (i.e. group guess) that it was DC current, which shouldn't have been present on that conductor at all, that caused the charring?

Yep I think the consensus is that the inverter was not isolated internally, thus allowing this fault to happen.

[transmitterdan]

Originally Posted by Brewgyver

Shouldn't the inverter AC ground wire also be sized to carry the fault current without burning?

Or is the concensus (i.e. group guess) that it was DC current, which shouldn't have been present on that conductor at all, that caused the charring?

In a proper installation there should be virtually zero current in the AC ground wire.

My guess is DC or a fault in the inverter. Since there are usually no fuses in the AC ground wire it’s important that the system design and installation follow all the standards.

But the root cause of this issue, in my opinion, was using a “trucker” type inverter. These have no place on a boat. Inverters are marine rated for a reason.

[sailorchic34]

Back in the early 1990's, Some offices where many personal computers were installed, were seeing ground wires burning up. The cause was the 208v/120v 3 phase service and 3rd order harmonic's in the PC's switching power supplies.

In layman terms the grounds were carrying several volts due to phase imbalance, where different groups of PC's were on a different 120 degree phase circuits in the 3 phase system. The fix at that time was to use a 240/120v transformer for switching power supply circuits as the 180 degree phase of a 240/120v transformer canceled out the 3rd order harmonics

On a boat, you don't have that, at least in theory. But many cheap inverters (I use one also) may have the ground connected to one leg of the inverter output. This to comply with the NEC. A residential 120v circuit uses a hot leg and neutral (center tap). The neutral always is at zero volt potential.

The typical single phase inverter does not have a neutral. That is there isn't a center tap. So while a inverter output looks like 120V 60hz, each leg alternates hot. Inverters on most boats don't have a neutral leg as shore power does. OK some inverters might have a neutral leg but I'm not aware of them.

When you connect the ground to the boats AC ground and your still plugged into shore power, it is quite possible for a phase imbalance to cause voltage potential on the ground to the inverter. This due to the switching nature of the inverter. This is exasperate as most marines are using 208v/120v transformers where leg to leg phase is 120 degrees apart.

One fix is to install a much thicker ground wire #6 or larger from inverter ground to AC ground. That to comply with ABYC. Assuming a 1000/2000 watt inverter.

I will note that I agree with TransmitterDan and I feel that connecting the AC ground to the boats DC ground can create problems. ABYC is a good code, but it assumes everything will always be perfect. On a boat that is almost never the case. So I do not have AC and DC ground connected on my boat. But that's just me.