Ground strap

Stodge

I was in the bilge the other day and glanced at the ground strap/bar at the back of the bilge.  It has 2 large posts, one at either end, then smaller posts in the middle.  Lots of black wires going to it.  One large black wire went to 1 large post then to (as far as I could tell) the port engine.  But the other large post did not have anything.  It made me wonder, should there be a similar large black wire from the starboard engine to the strap/bar?

rasbury

I have seen that...what exactly does that do?

Stodge

I imagine it's to provide a common ground connection.  It just seemed odd to me that only 1 engine appears to be connected to it.

LaRea

[Edited with corrections suggested by @raybo3!]

The bus bar in the engine room just provides a convenient way to connect multiple small DC negative wires to a common return / negative point.  The second large post on the bus bar is used to connect an extra bus bar if you run out of small posts.  

raybo3

RANT TIME.................. 12 VDC does not have "grounds". It has 2 wires. 12vdc+(red) and 12vdc- (black). The word "grounds" on DC is getting used a little too much for my liking. We have a lot of guys/women on the forum that do not understand the difference between 12- (negative) and AC "ground". If these 2 get mixed up and people start putting their 12- with AC ground they are going to have a big massive problem. I do not want to see anyone get hurt or hurt their boat.......... Just say'n.............. Rant over! 

Stodge

I get the confusion with AC netrual and AC ground.  But I was under the impression that the negative DC connections go to a common location (engine block) considered to be "ground".  AC ground in a house goes to a ground rod or other thing in the earth.  But on a boat I'd expect AC ground to go to the ground pin on the shore power connection.  I have assumed that it was also tied to the DC negative "ground" on the boat.  I'll take that to mean they are not meant to be connected.

raybo3

@Stodge see that's the problem I was trying to point out. Under no circumstances should 12- and AC ground (bare or green) be connected. We had a problem down out club a few years ago with everyone's zinc's being "eating up" in a matter of a couple of weeks. I found on a boat that someone put the AC shore power ground to the engine block along with the 12-. When I questioned him he said "ground is ground" so I thought I would put them together. WRONG!!!!!!............. Using the term ground on 12 DC system is just a wiring method and not a true ground. It gets confusing for people not really used to wiring things. I just dont want anyone getting confused. 12- should never be connected to AC ground.....

Stodge

Good explanation.  I don't think I have that condition in my 342.  And I learned something.

Dream_Inn

I completely agree with what's been stated (as an EE, I better know the differences).  Note, there are actual green "ground" wires that connect all the metal objects that are touching water (ie, stern drives & seacocks).  This is for corrosion

I know cars use the actual chassis as a 12-, note there is no 120VAC on the vehicle.  Note that boats do not tie the 12- to the engines or any part of the "chassis" on a boat.  Not to confuse things even more, but there can also two more grounds on a boat.  Lightning ground which is usually tied to mast on sailboats and go to a large plate on the keel.  The other ground is an RF ground which is mainly used for keeping noise free radios.

RVs on the other hand, would have the chassis at 12-.  Anyone know how the green AC ground is done on RVs?  Is there a rod that should be put in the ground when you campout?

reneechris14

Ok I do understand electricity and I must agree. 12 volt is a circle(loop) in the boat and 120 volt comes in and goes out to a ground to work(ie a house). But I do believe the though hull and drives are 12-. Green ground for lack of a better word. I with @Stodge the gound block should tie together both motors.          I do understand the danger of 120 v ground entering the water that will make a positive charge.

Ian

A boat floats and so should the 12vdc - don't tie it down to and "ground". Unlike your house which doesn't float so it is tied to ground.

In theory the AC ground in an RV should track back through the source cable. You can put a stake in but then you'll introduce a ground loop and potentially trip a GFI if it's hih enough.

I wonder though for protection if they tie the chassis to the ground wire but somehow isolate the 12vdc -ve which is as you point out, for vehicles is normally the chassis also. Our RV buddy @Handymans342 should be able to tell us this.

reneechris14

If you run a house on a Generator they suggest you run a ground to it. ?? But in the boat its grounded to the block??