Galvanic Isolator/Inverter Wiring

When it comes to electrical systems aboard boats, Arild Jensen knows his stuff. Here's where to ask the simplest or most complex questions about marine electrics.

Galvanic Isolator/Inverter Wiring

Postby TheAussies » Sun Feb 14, 2010 1:18 pm

Hello.

I have an electrical question please. As a Mechanical Engineer with limited electrical Engineering knowledge this issue has me stumped.

Our boat is fitted with a Guest Model 2450P Galvanic Isolator. It's the wiring associated with that, and our Freedom 3000 Inverter that concerns me. It seems the Galvanic Isolator separates AC input grounding from the boats AC and DC grounding, only allowing connection between the two under limited (fault) circumstances.

Our AC panel has a small bus to which the Shore Power Grounding Wire is attached. From that small bus one heavy wire runs to the Galvanic Isolator's Shore Ground Connection Terminal, also three other wires run from that small bus as detailed below. From the Galvanic Isolator's Boat Ground Connection Terminal one heavy wire runs to the boat's DC Ground Bus and another heavy wire runs to the boat's AC Ground Bus (that's the one connected to all the AC wires that run to the various AC using equipment on the boat).

So I can see those wires provide no direct connection between the Shore AC Grounding and the Boat's Grounding, other than through the Galvanic Isolator, that seems correct to me. What concerns me is the other three wires that are connected to the small Shore Ground bus, as follows:

The Inverter's output (single output) ground wire is connected to small Shore Ground bus. I am concerned that's an incorrect connection because I suspect that provides a direct connection to the boat's grounding, bypassing the Galvanic isolator. Am I right about that? Where should the Inverter's output grounding wire be correctly connected?

A second small wire runs from the small Shore Power Grounding bus to one side of the reverse Polarity LED on the AC Panel. Is that correct wiring? Where should that wire be connected.

A third small wire runs from the small Shore Power Grounding bus to one side of the ampmeter on our AC Panel. Is that correct wiring? Where should that wire be connected?

Another thing that surprises me is that there is no separate AC Grounding wire running from our Onan 17KW generator. Only three wires run from the generator's AC connection point, L1/L2/N (we have a regular 50amp AC system). I presume therefore that the generator's AC output is grounded via the generator's DC grounding wire. Is that normal? Is that correct?

Thank you, from Rod and Pauline
2003 Nova Scotia 47 Pilothouse Trawler
TheAussies
 
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Re: Galvanic Isolator/Inverter Wiring

Postby TheAussies » Sun Feb 14, 2010 2:07 pm

A clarification:

Our boat does not use the Inverter's Transfer Switch to tranfer external power to our AC panel. Each of our four power sources, Shore 1 (Bow), Shore 2 (Stern), Generator, Inverter, has it's own breaker, all set up so only one of those four breakers can be engaged at any one time.

We read that when external power is supplied to our Inverter (ie. from shore power or generator) and we are using the Inverter to charge our batteries, then the Inverter's internal connection between neutral and ground is opened. However that may not mean the inverter's output ground wire is no longer connected (internally in the inverter) to the boat's grounding system. That we don't understand.

Also we have a backup 12V battery charger, a Sentry 40A unit, if we use that to charge our batteries we turn off external power to the Inverter before engaging the Sentry Charger. In that case, we read, the Inverter's internal connection from neutral to ground would be closed.

Thanks from Pauline and Rod
2003 Nova Scotia 47 Pilothouse Trawler
TheAussies
 
Posts: 9
Joined: Wed Jul 01, 2009 8:27 am

Re: Galvanic Isolator/Inverter Wiring

Postby elnav » Sun Feb 14, 2010 7:34 pm

Your description sounds like someone misunderstood the ABYC specifications. One of the ongoing issues with the ABYC standards is how they portray schematics. All of them deal only with one specific aspect being covered by one paragraph and there are clear notices saying the schematic does not represent a complete circuit. So where does someone go to see a complete schematic?
All your boat grounds regardless of wire size should go to a common ground bus. One wire goes to the DC ground and also to your boats external bonding system or external connection to the water. Builders take a short cut and use the thru hull fitings plus shaft prop and rudder as the ground connection to the water. the Galvanic isolator only has two connections one to shore ground and one to vessel ground bus.
The reverse polarity light should also wire to ship side of the galvanic isolator because the isolation diode only blocks galvanic voltages below 1.4 volts above that the diodes conduct. This would cause the reverse p0olarity light up if a 120V AC voltage was present between neutral and green ground.

Your Onan should have a green ground wire sized to have the same ampacity as the supply wires going to the main panel.
a questionable short cut is to ground the white neutral to the genset frame. This saves the builder from having to pay for the wire. Instead it relies on the DC negative wire for connection to AC ground bus. But what happens if you use a double pole DC disconnect switch? I am inclined to fix this wiring omission.
Without a schematic of your boats wiring its hard to say if it is wired correctly or not. Several things you mention raises concerns in my mind. i have found grounding and bonding to be among the least undersatood issues in marine wiring and one of the most troublesome to find a problem in.
Arild Jensen
Custom electrical design for cruising boats
Telephone: 250 998 4474
Site: http://www.electronicnavigator.com
Email: mailto:2elnav@netbistro.com
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Re: Galvanic Isolator/Inverter Wiring

Postby TheAussies » Sun Feb 14, 2010 8:18 pm

Thank you very very much for your help Arild, we very much appreciate it.
We suspect our Galvanic Isolator may have been an add-on, and that might explain some of the incorrect wiring.
You did not particularly mention the Inverter ground wire (we don't mean the chassis ground).
You say "ALL boat grounds should go to a common ground bus".
Are we correct to assume that applies to EVERYTHING EXCEPT the Shore Power grounding wire?
Would that therefore mean the Inverter grounding wire, and the generator grounding wire would go to the the common ground bus?
Would that therefore mean the Galvanic Isolator is only active on Shore Power, and not on Inverter or Generator Power?
Would that effectively mean the ONLY thing that's connected to the Galvanic Isolator's Shore Ground is the Shore Ground wire?
Would that therefore mean that EVERY other ground is effectively connected to the Galvanic Isolator's Boat Ground.
We apologize for these questions being rather simplistic but we're trying to make sure we correctly understand your advice.
Regards from Pauline and Rod
2003 Nova Scotia 47 Pilothouse Trawler
TheAussies
 
Posts: 9
Joined: Wed Jul 01, 2009 8:27 am

Re: Galvanic Isolator/Inverter Wiring

Postby elnav » Mon Feb 15, 2010 10:54 am

The Inverter green wire ground is wired to the same green wire bus as the rest of the boat's AC system uses.
the Chassis case ground is tied to the vessels bonding ground which is a totally separate ground bus . At one and one point only does the bonding bus gets connected to the green wire ground.
The inverter should contain a relay which closes to tie the white neutral and the green wire ground together when the inverter is working as a power generator but is open when it functions as a charger.
From your description of the inverter it sounds as if it might be a TRACE model by Xantrex. Back in 2003 this model did not have an internal grounding relay. It required an external module to be compliant to ABYC standards. Check to see if your model does in fact ground the green wire and neutral together when inverting.
Arild Jensen
Custom electrical design for cruising boats
Telephone: 250 998 4474
Site: http://www.electronicnavigator.com
Email: mailto:2elnav@netbistro.com
elnav
 
Posts: 65
Joined: Fri Apr 10, 2009 4:54 pm

Re: Galvanic Isolator/Inverter Wiring

Postby elnav » Mon Feb 15, 2010 10:58 am

Here is another way to answer your questions.

You say "ALL boat grounds should go to a common ground bus".
Are we correct to assume that applies to EVERYTHING EXCEPT the Shore Power grounding wire?

Yes

Would that therefore mean the Inverter grounding wire, and the generator grounding wire would go to the the common ground bus?

Yes


Would that therefore mean the Galvanic Isolator is only active on Shore Power, and not on Inverter or Generator Power?

Yes.

Would that effectively mean the ONLY thing that's connected to the Galvanic Isolator's Shore Ground is the Shore Ground wire?

Yes

Would that therefore mean that EVERY other ground is effectively connected to the Galvanic Isolator's Boat Ground.

Yes.

We apologize for these questions being rather simplistic but we're trying to make sure we correctly understand your advice.
Regards from Pauline and Rod
2003 Nova Scotia 47 Pilothouse Trawler
Arild Jensen
Custom electrical design for cruising boats
Telephone: 250 998 4474
Site: http://www.electronicnavigator.com
Email: mailto:2elnav@netbistro.com
elnav
 
Posts: 65
Joined: Fri Apr 10, 2009 4:54 pm

Re: Galvanic Isolator/Inverter Wiring

Postby TheAussies » Mon Feb 15, 2010 11:27 am

Thanks again Arild.

What you state is beginning to make sense to me.
Our inverter is a new one (2009), with the grounding/neutral relay you mentioned.
the original owner, or his ABYC Electrician destroyed the original inverter.

The previous owner of our boat had an ABYC Electrician involved in continual boat maintenance and upgrading.
I met that guy and was NOT impressed, I asked him what was involved in becoming an ABYC Electrician.
My thinking was it might be something a bit less than my seven years advanced study to become a Mechanical Engineer.
His response staggered me, and explained my lack of confidence in him and what he had been doing.
His answer was 'about 7 days study and test'.

Now some more questions if you don't mind please.

So there should be TWO separate buses, an AC ground bus and a boat (DC) ground bus, joined at one point only. I think I can see how I might easily arrange things to meet this requirement, so the connection point between the AC Ground bus and the boat's (DC) bus would be the connection to the Galvanic Isolator.
Is it acceptable to have more than one boat (DC) ground bus, say one in the engineroom another in the DC panel, joined with a heavy wire?
Where should the bonding wires connect, presumably to the boat's (DC) ground bus?

I'm sure our boat's AC ampmeter should connect to the boat's neutral not to the incoming AC ground as it's now connected, but I'm unsure about the reverse polarity LED lights (one for Shore Bow and one for Shore Stern). presently one side of those LEDs is connected to each incoming neutral (in a reverse polarity situation one of those would be hot at connect but all boat breakers would be closed so no damage could occur) the other side of those LEDs is connected to the boat's incoming ground. That seems to me to provide a pathway around the Galvanic Isolator.
Would those LEDs work OK if connected to the boat's AC Ground, that is (effectively) to the boat's ground side of the galvanic isolator?

Thanks Arild your help is VERY much appreciated.

Regards from Pauline and Rod
2003 Nova Scotia 47 Pilothouse Trawler.
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