Re: [electricboats] Common DC ground with multiple banks

Hi Jack,

You ask some good questions.  There's a lot of info here, but you asked, so I'll tell you.

1) Pick a single point on the boat and declare it "ground zero." 

2) Now connect each subsystem through one and only one path to that point. Like a tree branches from a single trunk. Use separate 'grounding' conductors.  _Not_ the current carrying conductors used for power.

Basically that's it.

Green wires are convention.  Bare copper is ok too.  The _negative_ side of battery banks should be bonded there according to abyc. I don't mind floating power sources, but see below on over current protection and theory. Any user accessible conductive surfaces (panels, conduit,etc.) should go to ground too through, again, one and only one path.

If you've got a mast (an antenna mast even.) You might want to get that one grounded to the water _not_ through your motor too.  If you're out, that's where lightning is going to strike.  Bet on it.

Be careful of control signals that have ground for a return conductor too.  Ethernet is usually transformer isolated, but simple potentiometer types are suspect.  Check it on an ohmmeter.  One and only one path to earth from any point so there's no loops.

Ideally, you would not ever wire a conductive 'ground loop,' because changing magnetic fields can induce large currents in conductive loops. You mentioned having two independent 'electrodes' via two separate motors.  Technically, there's a loop formed through them and the water....if you connect them.  It's a geometry problem.  Maybe keep those two particular grounding wires hugging the hull. Pick a point between the two motors?  IDK. Minimize the loop area if you can't avoid it, but avoiding is better.  You should be good to go with a little thought.  

It's not a big deal usually.  But floating systems can accumulate charge so you have to give any stray charge a path to return to earth by grounding all systems to at least one electrode.  Two electrodes is usually considered superior by inspectors for house wiring...FYI...but they're usually right next to each other too so ...

Moreover, grounding is primarily intended for fire safety, to trip the over current protection devices, which you should have on all your circuits' positive (ungrounded for ac systems) conductors, if something shorts.

So you, you know, don't start an electrical fire.  

You probably don't want an electrical fire on a boat, so ground it all and use fuses or breakers on ungrounded conductors...ok?

And don't sweat it.

Unless that is, lightning strikes nearby, in which case you will probably want to hope like hell that everything is grounded _and_ your loop area is minimized :-)

On Thursday, March 16, 2023 at 08:37:59 AM CDT, Jack Agarwal <yacht.alcazar@gmail.com> wrote:

In my planned design I have two 48v propulsion Lifepo4 banks (one in each hull) and a 24v Lifepo House bank.

The propulsion banks will be grounded by their respective pod motor which will be fixed in the water.

How should I handle the negative cabling? Is it okay or advisable to have a single common negative bus which all banks negatives are connected to? 

The 24v house bank is quite far from the propulsion banks, and I plan to charge it with a simple isolated DCDC converter from each propulsion bank. In this case is it necessary to have a common ground between the two banks or can I essentially operate them as two separate systems?

any advice appreciated!
Jack