Battery Replacement/Rewire

[PeteW]

Dale,
I'm not sure you got my understanding correctly. My last post was an example of how not to do it. It was how an "expert" told me I should do it. I go on to describe the sequence of disasters that could result.

I run separate ground busses so I know what all those wire are, Bonding,  DC ground,  AC ground. I believe it came that way from Pearson. But I absolutely do tie them all together with one heavy gauge wire. That's in agreement with the ABYC standards.

The reason I tie them together with one wire is so that I have a single point failure in event of a huge fault current.  I want this connection to be deliberate and to be ale to point out the exact wire that makes it a certainty. An inadvertent connection between ground systems caused by  some other appliance which is redundant is OK,  but it goes away if you take that appliance out of service. Now you introduced a danger.

I would never recommend that you intentionally try to keep these grounds isolated from each other. Its not safe.

Pete

[Della and Dave]

Wow, that was Pandora's box, sorry about cracking the lid:-)  Actually, thanks a lot, I just learned a lot.  I think I see why it's a controversial topic, depending on what you are trying to protect against, it might result in a different "correct" way to wire it.  It took me a while to read the article, it's pretty well written, but dense, then I went to Don Casey's book, and read much of it again.  Casey says "To reduce the risk to swimmers, the green wire should be connected to the ground terminal on the engine." Since, as Pete mentioned, the DC neutral is connected to the engine, the AC ground and the DC neutral would be at the same potential, i.e. connected.  Casey's book clearly tells you how to check. 

Captain Rifkin in the Article says "The ABYC standards and the National Electric Code are consistent when they require that the neutral and ground only be connected at a “newly derived source.” In our case, this means that the connection is made at the Marina's electrical service entrance (which is a transformer ashore) " So it's connected, but not on the boat.  Then he goes on to explain for inverters and gensets you need to connect them. Because they are "newly derived sources". 

Casey essentially says connect at the engine, and Rifkin says on the dock, but which changes with inverters or generators. I agree with Pete, the best plan is to at least try to follow ABYC. 

So applying this to my situation, on my battery charger, I have the AC input cable with three wires.  I'm figuring they just go to the AC bus feed marked charger, assuming the breaker has the capacity, if not, upgrade.  The AC charger ground goes to AC ground, if I can find where that is.  I also have what I think is the DC ground, on the DC output.  I can either connect that to the engine block, or a grounding buss then to the block, or connect it to the AC ground for the shore power ground.  If I understand Pete correctly, the best thing, and ABYC thing, to do is collect this to the grounding buss, then connect that to the engine.   Did I understand that right? 

The other thing are our green bonding wires.  I don't think we have a galvanic isolator, but I am not sure I would recognize one if it bit me on the bum. I assume the galvanic isolator connects in series to the ground wire on the back of the shore power plug.   The bonding wires from the through hulls, chain plates etc, should go to a buss, then to the engine block grounding stud.

I have read the about whether or not to ground the through hulls.  One opinion is that grounding promotes lightning currents, which could blow the fitting out, leaving you with a hole in the bottom of the boat.  The counter argument is that corrosion can be countered by grounding to the prop shaft zinc, so you should bond them. For Polaris, I am inclined to bond them because lightning is pretty rare up here, so corrosion is the bigger threat.  If we sail south, at some latitude that would change, and the bigger risk might be lightning.   Beth Leonard's article on the other string seems to support that, as Alaska didn't even make the map. 

We do occasionally get thundersnow, but it's very rare and I have never heard of a boat getting hit. 

Did I understand this correctly? 

[Sailorlou]

The statement in a previous post that, "The best advice I can give for electrolysis is get a galvanic isolator.", was not completely accurate.  The best thing you can do to isolate the shore power is to use an isolation transformer.  An isolation transformer breaks the shore connection completely by transferring the power through a magnetic field.  The so called galvanic isolators sold for marine use don't actually "isolate" you from anything.  They do block DC stray current through the diode used in the circuit, but they do not block AC current.
Galvanic isolators are prone to failure and most of them on the market have no indicator to tell you they have failed.  You may have one installed thinking you are blocking DC current and in reality it may have failed months ago.  The only ones approved by ABYC are the ones with an indicator that shows it has failed, these typically run around $300.00 or more.  Galvanic isolators also do nothing to protect from electrical shock.  AC stray currents in the water around boats at docks kill people every year.  Galvanic isolators do nothing to prevent this.  Only isolation transformers block both DC and AC currents and protects from shock hazards.  If you truly want to have the safest system on your boat and extend the life of your zincs, then install an isolation transformer not a galvanic isolator.

[PeteW]

Lou,

I have an isolation transformer and the only reason I have not dragged it down to the boat and wired it in  is that it weights 50 lbs. I will agree with you 100% that an isolation transformer goes a long way in preventing electrocution in and around your boat because it allows you to permanently establish the bond between isolated  neutral and the AC ground buss once and for all at the secondary winding of the transformer (the newly derived source ) located on your boat. With some of the screwed up wiring that I have seen in marinas its might be too much to expect that those connection are too be trusted to be made at with the transformer on the dock. The isolation transformer will protect you from that.

But based on some of the schematics that I've seen for marine isolation transformers I would not go so far to say that all of them isolate you from the shore power AC ground. There are ground connections to the transformer core primary and secondary. Without checking it with an ohm meter there is a good chance that because of IEC requirements you will remain connected to AC shore ground, in which case you will still need a galvanic isolator.

A truly isolated high voltage AC system that is not bonded to a solid earth connection is dangerous.  At a minimum it needs to be tied to the boats underwater bonding system to establish a safe reference between you and the other boats around you that don't have an isolation transformer and are also connected to shore ground.

Pete