Electrical question - inverter 'chassis ground'?

[Helmsman]

Thanks steve,

i thing the question that some of us have is..

If the DC main negative bus and the grounding bus are connected then why it make a difference if the case grounding is attached to the dc bus or the grounding bus.

the windlass wires are a good example

Ofer, this write up by Jim Healey may be of interest to you (and to others reading this set of forum posts. He does some advisory work with the ABYC, writes for the MTOA, and I have found his articles extremely informative. He has many other articles that readers may find of interest.

https://gilwellbear.wordpress.com/c...opology/bonding-system-design-and-evaluation/

In this article he describes the bonding system and states that a B- cable should never be connected directly to the bonding system.

 

[diver dave]

And that link says this: The boat’s AC Safety Ground and the various branches of the DC bonding system are all connected together at one place, and at one place ONLY: the “DC Main Negative Buss

So, they DO connect. His point is the attachment should not routinely conduct appliance current. One point attach does that.

 

[SteveK]

Ofer, this write up by Jim Healey may be of interest to you (and to others reading this set of forum posts. He does some advisory work with the ABYC, writes for the MTOA, and I have found his articles extremely informative. He has many other articles that readers may find of interest.

https://gilwellbear.wordpress.com/c...opology/bonding-system-design-and-evaluation/

In this article he describes the bonding system and states that a B- cable should never be connected directly to the bonding system.

IMO where he says that it means connected in a way that the bonding becomes a path for current back to the battery. He also says ABYC has a recommendation for bonding, but ABYC does not mandate bonding.

The SDA diagram shows a single connection to two separate busses keeping normal current flow away from each other.

SDA. your diagram from ABYC has DC- go to engine block first and then to the DC- buss. IMO DC- should go to the distribution buss and from there to the engine block. Otherwise the connection failure at the block can affect all circuits.

 

[Helmsman]

IMO where he says that it means connected in a way that the bonding becomes a path for current back to the battery. He also says ABYC has a recommendation for bonding, but ABYC does not mandate bonding.

The SDA diagram shows a single connection to two separate busses keeping normal current flow away from each other.

SDA. your diagram from ABYC has DC- go to engine block first and then to the DC- buss. IMO DC- should go to the distribution buss and from there to the engine block. Otherwise the connection failure at the block can affect all circuits.

Yes, and that what is what I meant by directly. Unfortunately, probably a poor choice of words on my part without an attendant more lengthy explanation.

As Ofer stated above, and Jim also states in his article, the bonding system, the AC system, and the DC system all connect at one place, and one place ONLY. I have also personally found that to be the case in boats properly wired with a bonding system.

The exact wording from the referenced article provides a caveat against using the bonding system to carry B- current, which was the reason I used the term “directly”. Here is the article comment.

“Bonding circuits are intended to carry only galvanic and fault currents; never currents that power equipment or attachments. To avoid undesirable voltage drops in the bonding system, and problems with accelerated electrolytic corrosion, no B- connections should ever be made to any part of the bonding system. Such connections are analogous to a “code violation.””

I would not personally (nor would I advise anyone to) use a bonding circuit as a path for a B- circuit, or for a case ground coming from an inverter. The wires used for bonding are usually much smaller gauge than anything you might find between an inverter and a battery bank. It could create a potential for fire or melted wiring, not to mention the issues in the quoted comment above.

 

[SteveK]

“Bonding circuits are intended to carry only galvanic and fault currents;

That is where I cringe every time as bonding is designed to discharge AC current fault into the water if that is the only path for the fault.

 

[twistedtree]

That is where I cringe every time as bonding is designed to discharge AC current fault into the water if that is the only path for the fault.

Only on shore power, and only if there is a double fault. 1) you need an AC fault that energizes the AC protective ground. 2) you need a failure in the AC protective ground back to the shore power source.


If the power source is on the boat then fault current flow is back to the onboard power source.

 

[Helmsman]

Probably what happened to my boat. The AC to DC converter on my fridge shorted, sending current through the ground, through the bonding system, and then put through the rudder. Just a slight tingle in your arm, when checking the running gear when approaching the starboard rudder.

The “best electrician on the lake” connected a neutral to ground jumper in the AC panel and pronounced it fixed. I disconnected it, and began the search for the leak. After a few hours I narrowed it down to the fridge, figured out where it was, and replaced the power converter.

The leak was small enough to keep from popping the breaker, though the neutral to ground solution certainly tripped it.

 

[CharlieJ]

The “best electrician on the lake” connected a neutral to ground jumper in the AC panel and pronounced it fixed.

You are fortunate as his lack of knowledge could have easily led to a case of ESD.

The leak was small enough to keep from popping the breaker, though the neutral to ground solution certainly tripped it.

Absolutely textbook condition when AC stray current occurs.

 

[Helmsman]

You are fortunate as his lack of knowledge could have easily led to a case of ESD.


Absolutely textbook condition when AC stray current occurs.

It convinced me the only responsible solution was to invest the time in learning how electrical systems on a boat should operate, how they are set up when done so properly, and how to give myself the best chance of knowing. Not anywhere close to being an expert, but at least I know what to look for in most situations.

 

[CharlieJ]

Good on you!!.

 

[Gdavid]

It convinced me the only responsible solution was to invest the time in learning how electrical systems on a boat should operate, how they are set up when done so properly, and how to give myself the best chance of knowing. Not anywhere close to being an expert, but at least I know what to look for in most situations.

This is a very true statement and I would say could be said for any system on a boat.

 

[Steve DAntonio]

Thanks Steve. Every time you post I learn a little more. Can you comment on the ground wire between the two engines and what it’s purpose is?

There are two conductors, DC negative and bonding. The DC negative is to ensure proper ampacity when paralleling starting batteries. The bonding cable between the two is a rare case of sanctioned parallel grounds, the notion being the blocks should be, if non-current carrying, bonded and if so that wire must be able to carry full fault current, and cranking current if the DC negative parallel conductor is disconnected or compromised. All engiens and gens that have parallel start capability should use this approach.


Here is the ABYC citation...

11.5.2.4 MULTIPLE ENGINE INSTALLATION - If a boat has more than one engine with a grounded cranking motor, which includes auxiliary generator engine(s), the engines shall be connected by a common conductor that can carry the cranking motor current of each of the grounded cranking motor circuits (see FIGURE 5).
NOTE: Outboard engines may be connected at the battery negative terminals or DC main negative bus.

11.5.2.5 CROSSOVER (PARALLEL) CRANKING MOTOR CIRCUITS - In multiple inboard engine installations, which include an auxiliary generator(s) with crossover (parallel) grounded cranking motor systems, the engines shall be connected with a large enough conductor to carry the cranking motor current. This cable and its terminations shall be in addition to, and independent of, any other electrical connections to the engines including those required in E-11.5.2.4 (see FIGURE 5).

 

[Steve DAntonio]

Probably what happened to my boat. The AC to DC converter on my fridge shorted, sending current through the ground, through the bonding system, and then put through the rudder. Just a slight tingle in your arm, when checking the running gear when approaching the starboard rudder.

The “best electrician on the lake” connected a neutral to ground jumper in the AC panel and pronounced it fixed. I disconnected it, and began the search for the leak. After a few hours I narrowed it down to the fridge, figured out where it was, and replaced the power converter.

The leak was small enough to keep from popping the breaker, though the neutral to ground solution certainly tripped it.

Perhaps the most common error committed by "dirt" electricians working on boats. I learned that the best way to enlighten them is to tell them to treat a boat like an out building rather than a house, houses have N to G bonds, out buildings and their sub panels do not. Then I tell them, "You guys like to say, 'wiring is no hobby, call a pro' (I worked for a commercial electrician while in HS and college, all their trucks had that bumper sticker) , so unless you learn the peculiarities of marine electrical systems, and the ABYC standards, you aren't a 'pro'"

More on N to G here https://stevedmarineconsulting.com/neutral-to-ground-demystified/

 

[Cheechako]

Great advice from Circumnavigator and CharlieJ. Invertors = RTFM thoroughly and follow. Quote from Steve Zimmermans very helpful article on shore power pedestals posted on another TF thread:

https://www-passagemaker-com.cdn.ampproject.org/v/s/www.passagemaker.com/.amp/technical/troubleshooter-shorepower-danger?amp_js_v=a3&amp_gsa=1#referrer=https%3A%2F%2Fwww.google.com&amp_tf=From%20%251%24s&ampshare=https%3A%2F%2Fwww.passagemaker.com%2Ftechnical%2Ftroubleshooter-shorepower-danger

If the industry presented an award for the device most likely to be improperly installed, inverters would win and there wouldn’t be a second place.

 

[catalinajack]

There are two conductors, DC negative and bonding. The DC negative is to ensure proper ampacity when paralleling starting batteries. The bonding cable between the two is a rare case of sanctioned parallel grounds, the notion being the blocks should be, if non-current carrying, bonded and if so that wire must be able to carry full fault current, and cranking current if the DC negative parallel conductor is disconnected or compromised. All engiens and gens that have parallel start capability should use this approach.





Here is the ABYC citation...



11.5.2.4 MULTIPLE ENGINE INSTALLATION - If a boat has more than one engine with a grounded cranking motor, which includes auxiliary generator engine(s), the engines shall be connected by a common conductor that can carry the cranking motor current of each of the grounded cranking motor circuits (see FIGURE 5).

NOTE: Outboard engines may be connected at the battery negative terminals or DC main negative bus.



11.5.2.5 CROSSOVER (PARALLEL) CRANKING MOTOR CIRCUITS - In multiple inboard engine installations, which include an auxiliary generator(s) with crossover (parallel) grounded cranking motor systems, the engines shall be connected with a large enough conductor to carry the cranking motor current. This cable and its terminations shall be in addition to, and independent of, any other electrical connections to the engines including those required in E-11.5.2.4 (see FIGURE 5).

Steve, thank you for this explanation. I always wondered why my boat has that big black cable going from one engine to the other. The only fault I have found since I purchased our DeFever 44 eights years ago was a jumper someone installed between the neutral and green busbars. I went looking for it after reading on this forum five years ago when those pesky new pedastals were being tripped mysteriously.

 

[Steve DAntonio]

Great advice from Circumnavigator and CharlieJ. Invertors = RTFM thoroughly and follow. Quote from Steve Zimmermans very helpful article on shore power pedestals posted on another TF thread:

https://www-passagemaker-com.cdn.ampproject.org/v/s/www.passagemaker.com/.amp/technical/troubleshooter-shorepower-danger?amp_js_v=a3&amp_gsa=1#referrer=https%3A%2F%2Fwww.google.com&amp_tf=From%20%251%24s&ampshare=https%3A%2F%2Fwww.passagemaker.com%2Ftechnical%2Ftroubleshooter-shorepower-danger

Interestingly some inverter manual even get this wrong, and they provide undersized lugs for the necessarily large wire used for the chassis ground. This is likely because they are used primarily for off grid solar applications, with standards different from ABYC. Therefore, I say, RTFM and comply with ABYC Standards, those are usually bit not always the same.