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Old 08-05-2014, 02:33 PM   #1
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ground question

At the risk of starting a controversy.....
a simple question that should have a simple answer...

if a boat has an isolation type transformer (i.e. 120 to 240 VAC transformer that "separates" the shore power from the boat.) will it cause the boat to seem as if it has an open ground on the AC outlets?
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Old 08-05-2014, 03:16 PM   #2
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At the risk of starting a controversy.....
a simple question that should have a simple answer...

if a boat has an isolation type transformer (i.e. 120 to 240 VAC transformer that "separates" the shore power from the boat.) will it cause the boat to seem as if it has an open ground on the AC outlets?
I would think the neutral and ground wire on the boat would be connected together at the transformer (point of origin) so it shouldn't seem like it has an open ground.

It would help if you posted or quoted the installation instructions for the transformer.

Understand though that if the boat's wiring is isolated from shorepower with a transformer, the boat's wiring will have no relationship to shorepower.
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Old 08-05-2014, 03:23 PM   #3
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No. The electrical code requires the neutral and the ground to be tied at the power source- main panel in most cases but also at any isolation transformer. So the neutral and the ground are tied inside the case of the isolation transformer.

FWIW, Charles Industries does not do this automatically at the factory. They expect the installer to do it. And on my new Mainship 34 the ground and the neutral were not tied. I contacted Charles and got their installation instructions which said to tie them. I did and the open ground indication went away.

And of course the foregoing is only applicable to US 120/240V wiring. European will be different, which is probably why Charles doesn't do it at the factory.

David
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Old 08-05-2014, 06:29 PM   #4
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We have a Isolation transformer, the ground and neutral are tied at the panel.
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Old 08-05-2014, 07:14 PM   #5
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here's one of many diagrams available...
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Old 08-05-2014, 08:25 PM   #6
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Electrical is beyond my comprehension. I leave it to the experts.
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Old 08-05-2014, 08:34 PM   #7
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No. The electrical code requires the neutral and the ground to be tied at the power source- main panel in most cases but also at any isolation transformer. So the neutral and the ground are tied inside the case of the isolation transformer.

FWIW, Charles Industries does not do this automatically at the factory. They expect the installer to do it. And on my new Mainship 34 the ground and the neutral were not tied. I contacted Charles and got their installation instructions which said to tie them. I did and the open ground indication went away.

And of course the foregoing is only applicable to US 120/240V wiring. European will be different, which is probably why Charles doesn't do it at the factory.

David
The panel is not a power source and the neutral/ground should never be joined there or anywhere else on the boat except at the isolation transformer, inverter (automatically switched) or generator also automatically switched.

If you join them at the panel you run the risk of feeding AC into your DC system if that is properly bonded as required.
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Old 08-06-2014, 12:27 PM   #8
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so any electrical experts here live in Portland, OR and be available some day to help trace wires... I can probably afford a couple of Coronas
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Old 08-07-2014, 09:42 AM   #9
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In general ABYC calls for safety ground and neutral to be bonded "At the Source", and with regards to isolation transformers 11.5.5.2.2 calls specifically for this.

However, there is an exception as follows: "For systems using an isolation transformer or polarization transformer, both the generator or inverter neutral and the transformer secondary neutrals may be grounded at the AC main grounding bus instead of at the generator, inverter, or transformer secondaries."
ref: Diagram #5 - E-11.17.3


So, it is fully acceptable to provide the bonding point inside the AC panel, but one needs to make sure to read all rest of the spec to make sure all is done consistently and correctly. Example: Depending on how an inverter is wired into the boat, it may be necessary to defeat the auto bonding capability inside inverters.



Back to the OPs question: With an on-board isolation transformer there should be somewhere a bonding between the safety ground and the neutral wire. (North America standard here). If you are not able to measure that, something is wrong.



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Old 08-15-2014, 11:02 AM   #10
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OH, if it helps any.. this is the diagram sent to me by the manufacturer... am I wrong in thinking this is an isolation transformer?

http://i143.photobucket.com/albums/r...16/winding.jpg
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Old 08-15-2014, 03:24 PM   #11
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OH, if it helps any.. this is the diagram sent to me by the manufacturer... am I wrong in thinking this is an isolation transformer?

http://i143.photobucket.com/albums/r...16/winding.jpg
It's a transformer. It can be used to double or halve the voltage and would isolate one side from the other. We don't know from the diagram if yours is installed correctly and actually isolating the boat from shore power.
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Old 08-15-2014, 04:20 PM   #12
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OH, if it helps any.. this is the diagram sent to me by the manufacturer... am I wrong in thinking this is an isolation transformer?

http://i143.photobucket.com/albums/r...16/winding.jpg
As rwidman pointed out, it is a transformer - and it is indeed an isolation transformer. Key here are the separate windings which are not connected. Another type of transformer, not as often seen but out there, is an "Auto-transformer" which can also change voltages, but shares part of the one winding between both the 'input' and 'output' sides. Though an Auto transformer will allow voltage changes, it provides no isolation.

The photo you posted is a proper isolation transformer, but again as Rwidman pointed out much depends on how it is wired into your boat.

Should also note: ABYC has some specific requirements for a 'marine' isolation transformer. Specifically with regards to the construction of the internal isolation shield and labeling requirements around same. Industrial transformers often (never?) will meet this unique spec.

Not to get into a ABYC vs. rest of the world discussion, but just to point out a knowledgeable surveyor may flag any transformer that meets industrial / ship specifications but not 'marine' when used in recreational boats.

-al-
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