Join Date: Jul 2016
Location: British Columbia
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12V ground on the boat
I am beginning the electrical system refurb on my boat today, starting at the batteries, main switch battery multiplexer, and motor electrics. (Perhaps I will do a write up of what I find a little later as I get into it a bit more.) My question for today concerns the ground leg.
First off, there are two grounds on the boat. The ground we are all familiar with is the battery negative terminal. This ground is tied together throughout the boat: batteries, instruments, lighting, etc. all on one bus. The positive side of the electrical system is +12.6V or thereabouts under normal conditions with a charged battery and no charging current from either the alternator, solar charge controller, or mains charger. (It may useful to note that the voltage of the system represents charge potential, and that electrons flow from the negative side of a power supply to the positive leg, which some find counterintuitive. Be that as it may, my understanding is that the +/- nomenclature is a historical artifact of early electrical engineering.) At any rate, the second ground is "earth ground", and on my boat is accessible by way of a connection to the keel-bolts --> iron keel --> saltwater.
On this boat there is a 2" wide strip of copper affixed to the keel-bolts and runs to a 4AWG or so cable which runs to the engine block. The aluminum mast is electrically connected to the keel as well to provide protection from lightning strikes. Incidentally, the VHF antenna is at the top of the mast with the cable running down the backstay and forward into the cabin. I'm sure a lightning strike will cause the radio to explode spectacularly. At any rate, I question the wisdom of grounding the keel and battery ground through the engine casing. I speak some Electron, but not enough to fully understand the subtleties, and it is my suspicion that the battery ground may in fact float around but not quite at the earth ground. If this is true, my lack of EE expertise is not sufficient to know whether there is an advantage to tying the battery ground to the keel, or whether it is a bad idea on general principles, lightning strikes notwithstanding.
Any sparkies out there with opinions on the wisdom of the existing arrangement here?
Disclaimer: Not an EE.
Last edited by Frogc; 07-18-2016 at 07:58 PM.