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post #9 of Old 09-27-2012
Maine Sail
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Re: Grounding Batteries to Eng. Block

Originally Posted by L124C View Post
Started this thread regarding Shunts:
How do Shunts work?
A issue I solved there prompted me to research the Battery/Ground relationship. The book; The 12 Volt Bible For Boats, by Miner Brotherton states:

"Ground is considered to be either an infinite "sink" or an infinite "source" for charge. That means if we have an excess of charge, ground will take all we can give it. If we have a deficiency of charge, ground will supply us with all we need."

I think I understand the "sink", meaning ground gives excessive charge some where to dissipate. However, I do not understand how ground supplies us with all the charge we need. If my alternator fails (for example), ground ain't going to help me, right?
Can someone explain what Botherton means?
There is a lot at play here:

Wiring batteries to the engine block is required with case grounded starters and alternators or there is an incomplete circuit. On some boats, such as metal hulls, the starter can be only momentarily connected via solenoid then disconnected and the alternator is isolated ground.

There are four main things at play:

Lightning grounding (earth potential non current carrying)
Dissimilar metals bonding (earth potential non current carrying)
Negative DC current carrying conductors
AC grounding

There is a difference between "grounded" and "grounding".

DC Negative Conductor (grounded conductor) = Black OR Yellow (Yellow is ABYC preferred but black still acceptable)

DC Grounding Conductor = Green or Green W/Yellow stripe. This is for AC and DC GROUNDING or EARTHING and is not for negative return conductors. DC grounding is optional on the DC side unless you are wiring an inverter or charger and then the case needs to be earthed with green or green/yellow tracer. Bonding wires and lightning grounding also use green for earthing...

Your boat is attached to "Earth" for safety reasons or to help minimize corrosion. Your inboard engine is connected to the batteries to complete the starter, fuel pump or alternator circuits unless other means of connecting the neg conductors to the battery exists. With a DC only system you can have a system totally isolated from Earth, as many small OB powered sailboats do, but Earthing the boat somewhere is normally preferred and this spot is most often the engine on inboard sailboats..

-Maine Sail / CS-36T

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Last edited by Maine Sail; 09-27-2012 at 12:21 PM.
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