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Old 10-04-2008, 08:34   #1
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ABYC requirement for DC & AC ground

Can someone explain the logic to me behind the ABYC requiring AC & DC shared ground?
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Old 10-04-2008, 10:07   #2
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Ewww....I don't like that idea at all and I don't do it. Unless somehow it is being done through my inverter which I am unaware of.

Voltage spikes will destroy 12VDC equipment so why increase the chances of that happening by tieing your 120 and 12 volt systems together in any way?

Just to get the terminology correct. DC has a positive and a negative and AC has a hot (or L1, L2), neutral and ground. Are the regs saying to tie the negative to the ground?

Where in the ABYC regs do you see this?
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Old 10-04-2008, 10:30   #3
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There's no reason for a 12v dc 'ground'. We attach the dc negative to our engines so that the starter and alternator have a dc negative path back to the battery. The AC system does require a ground for safety, so that current flows out to the water through the engine if your ac equipment faults and shorts to it's housing(or someother ground). That way there is less chance of electrocution. It just works out then that both the dc negative and the AC ground are electrically connected together, at the engine. The physical connection for the ac ground can be on say a dc negative bus, which does the same thing of tying the two systems to the engine. On an engineless boat (do they still exist?) I can't see why you would want to tie them together.
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Old 10-04-2008, 10:37   #4
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look at it this way, the ac ground is there as a saftey outlet, if for some reason the fan motor winding on your hvac system shorted to the case, if ungrounded, the 120 vac would be present on the case, waiting for you to ground it if you touched it. when grounded, it is sent straight back to shore to earth ground, thus triping the breaker and keeping you from getting the shock. and dc neg is at the same voltage as ac ground ( 0 volts) or earth. through bonding systems or ground plates. they sould never be any ac on your ac ground unless due to some equipment failure. it just waits to snatch it when something fails.
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Old 10-04-2008, 10:42   #5
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look at it this way, the ac ground is there as a saftey outlet, if for some reason the fan motor winding on your hvac system shorted to the case, if ungrounded, the 120 vac would be present on the case, waiting for you to ground it if you touched it. when grounded, it is sent straight back to shore to earth ground, thus triping the breaker and keeping you from getting the shock. and dc neg is at the same voltage as ac ground ( 0 volts) or earth. through bonding systems or ground plates. they sould never be any ac on your ac ground unless due to some equipment failure. it just waits to snatch it when something fails.
That explains why we have grounds as a safety but it does not explain why you would ever want to connect the AC and DC systems together in any way.

What happens if you have a big field collapse in your AC system at your shorepower isolation transformer for example causing a voltage spike and it crosses over to your DC system? Not good at all.

What happens if an hot AC wire hits a DC ground accidentally. It would have a path back to the AC neutral causing a direct short....not good either.
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Old 10-04-2008, 10:54   #6
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saftey! everything needs to get back to earth (o volt). you can get some pretty nasty stuff from dc also, so get it all to earth, and not through your fingers, arms and legs to earth.
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Old 10-04-2008, 11:05   #7
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the only way to " cross over" would be through your battery charger/inverter. so if it did then send it to ground.
and if ac hot came in concact with dc ground, it will not find its way back to ac neutral, but to ground. everything back to earth.
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Old 10-04-2008, 11:07   #8
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My engine is isolated from earth...how then does the AC through the engine get to earth without killing my 12V system?
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Old 10-04-2008, 11:16   #9
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My engine is isolated from earth
Your engine may be isolated from earth via shaft and prop, but how is it cooled?
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Old 10-04-2008, 11:18   #10
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ABYC E-11 says, in part:

"11.18.1. DC Grounding
If a DC grounding system is installed, the DC grounding conductor shall be used to connect metallic non-current-carrying parts of those direct current devices identified in E-11.17.2.3 to the engine negative terminal or its bus for the purpose of minimizing stray current corrosion..."

Emphasis on IF is mine.
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Old 10-04-2008, 11:19   #11
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are you swinging a bronze prop, with ss shaft? ac does not go through the dc system. ac ground should have 0 volts, 0 current. only carry current to earth when something fails in the ac system. it's just a saftey line waiting for something to fail, so if it does dump it back it earth, either through the prop shaft or ground wire in your shore connection.
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Old 10-04-2008, 11:24   #12
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I would hope everyone has an isolation plate between their engine and shaft...I do. As such, the engine is isolated from earth.
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Old 10-04-2008, 11:26   #13
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Many boats have AC and DC common ground. All European built boats do. We did, I broke the connections. If, you are plugged into shore power you must check it or your running gear can end up hot if shore power is not wired correctly.
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Old 10-04-2008, 11:45   #14
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Your engine may be isolated from earth via shaft and prop, but how is it cooled?
Hmmm...yes, but I would imagine that such a path would be discontinuous when the engine isn't pumping raw water. So sitting in the slip when it matters, it is likely not to have a path to ground via raw water cooling.
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Old 10-04-2008, 11:49   #15
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Non-isolated engines

Trim50; Unless you have a keel-cooler using deionized water for cooling (or have an air cooled engine) you are mistaken in thinking that your engine is electrically isolated.

Even fresh water cooled engines have electrical conductance to the outside water of the boat. One STILL should have a shaft brush to bond the shaft to the bonding system. The engine also should have a separate bonding wire via the starter negative connection to a bonding distribution point.

I have seen improperly bonded engines and shafts that had problems with corrosion especially with heat exchangers (engine and/or transmission coolers) making expensive repairs that would be prevented with proper bonding and anode protection. Visualize that the seawater that enters to your raw water strainer and heat exchanger as being an electrical conductor like a wire then you can understand how a battery can be created that can cause metal damage.
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