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Bad A/C DC wiring

Emerald

Moderator
Hi All,

First, I did post the same question over at the Sailnet list. Please forgive the double post, but not sure who is looking where at this point.


this is more of a survey question than I need help, but I am also looking for feedback from those with more experience when it comes to wiring up our lovely boats. I have discovered, and undone, what I believe was a modification to my wiring that I feel was potentially very very dangerous. Curious if anyone else has found what I am about to describe, and if perhaps there is some reason I can't fathom why it was done this way.


Here's what I found. Stage one was installing a polarity indicator on my original A/C shore power panel - please don't tell me to go buy a nice Blue Sea panel - I don't have the boat bucks currently allocated and all I do with A/C is keep my batteries topped and occasionally run a light bulb or a little fan - the Blue Sea solution is on the horizon, but I have to focus on the "keep her sailing" stuff, not the big upgrade stuff. OK, so I open up my A/C panel, and discover that the bus bar that has the green wire from the A/C shore power - what I think of as the safety ground has a wire to the block of my engine, effectively tying the DC ground of the system to the A/C safety ground. My first thought is that if there is an A/C fault, it's going to go to the "green" wire, and thereby electrify everything else attached to it e.g. my engine block, and my whole DC system with 120 A/C. The wiring diagram inside my A/C panel shows no such wire grounding the "green" bus to the the boat - it shows all of it as a self contained system with the green running back to the green provided by the shore power cable. The diagram as show in the A/C panel makes sense to me, so I remove this wire (from the "green" bus bar to the engine block) that appears to have been added. Roll forward several days, I am back on Emerald, and discover that my 12 volt DC side is not working properly. I start digging around and found a negative bus bar behind the 12 volt DC panel, and it's got a wire that snakes down to the "green" bus bar in my A/C panel! I look at this and think odd thoughts I can't post, and then take the wire I had disconnected from the A/C green bus bar to the engine block, and connect it back to the 'green" A/C bus bar, and voila, my DC is working again. Still not feeling good about this, for the time being I have left the A/C panel just as it is shown in the wiring diagram inside the A/C box, which is to be a self contained system with no ties to any part of the boats structure, and all wiring feeding back to the shore power cable itself. In conjunction with this, I have restored a DC ground from the DC bus bar directly to the engine block, and have not restored the wiring connecting the DC bus bar with the A/C green bus bar in the A/C panel. I have left the only path on the "green" bus bar in the A/C panel to be the shore power cable itself, and the only path from the DC ground bus bar is to the engine block, and the two systems are currently separate.

Anyone care to comment on all this? And, to make things a little more complex, I found this link on the Blue Sea website, but I don't think it applies to me because it has an DC to A/C inverter involved, which I don't, but, it makes an interesting comment about tying the two systems together! Argh! Which end is up? Here's the link:


http://www.bluesea.com/Article_detail.asp?Section_ID=145&id=103



-David
Independence 31
Emerald
 

gjersvik

Member II
For what It's Worth...

For what it is worth you may want to make a wiring diagram so you can see both circuits (use colors). As you work through making the schematic it may help you understand the relationship of the two circuits to each other.
 

Steve

Member III
Me too

For what it's worth, I also found the same issue on my 35-3, looked like factory applied. We noticed our zincs boiling off following our first season in a slip with shorepower 24/7, Prior we were on a mooring with out noticable problems. After tracking down grounding with a meter, it was clear the AC and the DC shared a common ground to the engine block. Yikes, now I was the A/C ground for the whole freaken marina. We seperated the A/C from the D/C ground and isolated the A/C from anything else, using the marina ground. Left the D/C to the block only. We installed a simple A/C ground fault beween the shorepower entry and the A/C panel. And guess what, now my zincs last the whole season and likely could go a second. I'm not a marine electrical expert and unsure of the "codes", but that one didn't make sense to me? Only thing I can rationalize is the wiring design was done before the common use of ground-fault breakers (20 years), thus they needed a ground to direct fault to ground... a thought.

Steve
 

Joe Benedict

Member II
"but it all grounds to earth"

I spent some time in the Navy and they saw fit to train me as an electrician - when it comes to elecrical power people can be extremely creative - that is why so many end up receiving Darwin Awards. As a complete guess, is it possible someone tried to build a lightening arrestor and got "confused?" See the attached file which comes from the Coast Guard website. While you are investigating make sure the innards of boatside of the shore power connector are tight. I just finished chasing an off again on again polarity problem that turned out to be loose screws inside the connector.
 

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dave_g

Member II
ABYC and NFPA connect the two grounds

The ABYC and NFPA both connect the AC "safety" ground to the Engine block and DC ground. (Blue Seas has an excerpt of the ABYC code http://www.bluesea.com/Article_Detail.asp?ID=119&Page=1&section_Id=139)

As Steve states, this can promote galvanic action on the thru hull fittings. Hence the purpose of the galvanic isolator between the AC and DC grounds. The isolator blocks low voltage DC current (which is causing the corrosion).

I think the purpose is to provide an alternate low resistance path, to the AC ground back on land, for any stray AC appearing on the DC ground. Otherwise the stray AC would go into the water via the prop shaft and cause a dangerous condition for anyone in the water. (stray AC could be from a miswire, a chafed AC wire touching a DC grounded object, leakage through a charger or inverter )

Dave
 

Steve

Member III
Yikes, the electrical police are coming

Wow, this subject can get interesting, just read all the posts on sailnet. Well then what the heck was going on back in 84 when they OEM wired our boats. Since isolation transformers were not OEM from Ericson, did they figure all boats would share the same AC ground in the marina with each other and then throw a bit of DC around to make things interesting. Maybe the guys who sold zincs and shafts were in on this?

Safety is every sailor's goal, but unless you install an $$ isolation $$ tranformer, you can't meet the code (this is what I'm concluding), and risk burning up zincs and more. I thought my GFI would satisfy, but doesn't seem to enter into the conversation? Common sense says physically isolate the two systems, but this does not seem like a prudent idea to the sailnet experts. I'm off to the big marine stores to see how much these isolation transformers cost, gasp! And to hear more ideas.
 

Howard Keiper

Moderator
The green ( AC grounding) wire should connect to the engine block as should the black (or more likely yellow these days) DC ground. Never, ever connect the green to AC neutral in the boat. In any case, it's a good idea to invest in an isolator (not necessarily a transformer), and one should be installed in every boat as an OEM product. It adds maybe $200 to the cost.
Howard Keiper
Sea Quest
Berkeley
 
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