Marine Grounding

[Rusty123]

Question 1: Does the grounding approach in this West Marine tutorial represent the current collective wisdom of this forum? I ask because my current grounding setup more closely resembles the "ground everything" approach. But the wiring is installed poorly. If I follow the recommendations of the article, I'll certainly have less rework to do (mostly rip out).

https://www.westmarine.com/webapp/wc...s#.UljzaaS9KK0

Question 2: Since the AC and DC connected together (in my case, at the engine block), where is the best location to physically connect the chassis ground of my battery charger? Also at the engine block? Or does it matter?

[smac999]

bond everything. it's the safest. you can't give off ac stray current if everything is bonded (you'd just trip the breaker). where as an unbounded boat with a problem can kill swimers. (though only really in fresh water)

idealy nothing is joined at engine block. you should have a neg distb bar. then cables from bar to battery, engine, ac ground, bonding system, dc panel, chassis grounds.

[transmitterdan]

Originally Posted by Rusty123

Question 2: Since the AC and DC connected together (in my case, at the engine block), where is the best location to physically connect the chassis ground of my battery charger? Also at the engine block? Or does it matter?

In the US the AC and DC grounds ought not be connected together at the engine. Unless you have a generator or inverter they should never be connected together. When on shore power the AC ground should only connect to the galvanic isolator (assuming you don't have an isolation transformer). There was another thread a few months back that debated this issue pretty much to death.

The WM recommendations are pretty close to consensus. There are some who will argue finer points but these arguments revolve around trying to avoid the lesser of two evils and the issue is which evil is lesser.

[transmitterdan]

Originally Posted by transmitterdan

In the US the AC and DC grounds ought not be connected together at the engine. Unless you have a generator or inverter they should never be connected together. When on shore power the AC ground should only connect to the galvanic isolator (assuming you don't have an isolation transformer). There was another thread a few months back that debated this issue pretty much to death.

The WM recommendations are pretty close to consensus. There are some who will argue finer points but these arguments revolve around trying to avoid the lesser of two evils and the issue is which evil is lesser.

Correction: I was dyslexic this morning for some reason. The inside boat AC ground should go to the engine block. It's the dock side of the galvanic isolator that should not connect to the engine. Sorry for the confusion.

[Rusty123]

As I read more about grounding systems, three things have me stumped. Maybe someone can educate me. I'm not saying anyone is wrong (I'm not smart enough to say that), but these things just don't make sense to me.

1. Why does the DC system need to be grounded at all? If all loads have their own return path to the battery, I don't see what advantage grounding offers. And doing so seems to open the door for stray current corrosion.

2. Why does the AC system need to be grounded to the engine block? Grounded via shore power ground, sure, but what advantage is grounding the engine block? Is this just a precaution against the block becoming energized by an AC short? If so, is this the same concern that makes some people advocate grounding all metal objects (perhaps also answering question number 1)?

3. Why can't the shore power ground be adequately isolated with an appropriately sized capacitor(s)? As I recall my basic electrical engineering classes (I'm a civil), capacitors pass AC and block DC. Seems like just what we need. So why the need for diodes or transformers?

Russ

[transmitterdan]

1) Without grounding the engine to battery minus it will not start.

2) This is the subject of religious debates. Search previous threads for long discussions.

3) Google "galvanic isolator".

[Rusty123]

1. Only if the starter return path is via the block, which is common, but not mandatory, I believe.

2. The safety aspect seems to be somewhat dependent upon how extensive the on board AC system is. For example, on my boat, the AC powers three receptacles and the battery charger (the case for which is grounded). There is virtually zero chance that my engine block, fuel tank, etc. would become energized, since there are no AC wires anywhere nearby. But I could see a bigger concern on a boat with lots of AC loads.

3. Google hasn't helped yet. Lots of information on isolators design features, and a few indications that capacitors can provide isolation in some circumstances, but nothing about why boats aren't one of them. 60 hz is pretty low frequency - perhaps a capacitor would need to be really big to work I that circumstance.

[transmitterdan]

1) The starter return is not isolated from the engine block. Thus the engine is "grounded" whether we like it or not.

2) ABYC was worried that DC negative wires used to be black. AC hot is also black. Someone might mistakenly criss-cross these thinking they were the same. That puts AC on the engine block and without the block AC grounded the breaker won't trip. Newer ground fault interrupters can detect this condition but ABYC has found them to be prone to nuisance tripping. In Europe ground fault interrupters are mandated but with a higher trip current to prevent nuisance trips. So they don't connect AC ground to engine or DC ground. US will no doubt adopt this in the near future. But there are thousands of boats with the AC/DC grounds connected.

3) The capacitor has to be really big and there is no easy way to know if it has failed open. Diodes do the same job and generally fail shorted which is best for human safety. This too is a religious subject for some.

[Rusty123]

1. No dispute, but does the starter circuit use the engine block because it is convenient/cheap, or to explicitly place the block at DC ground potential? I guess I'm splitting hairs here, but I don't see a technical requirement for the block to form part of the DC circuit, and doing so connects the DC system with the ocean, which would seem to introduce unnecessary galvanic and/ or stray current issues.

2. Thanks for that nugget of info about black wire confusion - gives context to much of what I've read elsewhere.

3. As I was suspecting.

Finally, I can get some sleep!

[transmitterdan]

Originally Posted by Rusty123

1. No dispute, but does the starter circuit use the engine block because it is convenient/cheap, or to explicitly place the block at DC ground potential?

It would cost money to isolate the starter negative from the starter casing thus it is extremely unusual bordering on nonexistent.

[Tantalus]

I fon't know much about deisel engines but gas engine park plugs could not fire if the block was not in the circuit.

[transmitterdan]

Originally Posted by Tantalus

I fon't know much about deisel engines but gas engine park plugs could not fire if the block was not in the circuit.

Not precisely true. The spark plug circuit does not have to be connected to the battery. The spark plug is fired from a transformer (aka coil).

But your point is right on. Diesels don't have spark plugs but many have glow plugs. These don't work unless the block is tied to battery negative. Both gas (petrol) and diesels use controls and sensors that rely on the block being tied to battery negative. It's all about saving money on the extra wires and fewer connections.

[Rusty123]

This really throws a wrench into my plans to develop a Fiberglas engine block.

[transmitterdan]

Originally Posted by Rusty123

This really throws a wrench into my plans to develop a Fiberglas engine block.

Just make it out of carbon fiber and no worries mate.

[Rusty123]

Just came across this post from Mark Johnson in another thread:

"In my case, I isolated the engine from the AC and DC system (except when its running), by installing an on off switch in the black - wire, as well as the red + one. I shut off both during the 99.9% of the time that its not running. This minimizes any stray current, that eats zincs."

Would need some pretty big switches, but that's one way around it.