When it comes to electrics I'm a real Bozo. The wires behind the control panel are a dogs breakfast. I don't have
much in the way of electrical components but what I do have are not working except for one weak LED cabin light.
So I put the multimteter to work and got 13.8 v across the bus bars. The negative bus bar does not seem to be the
common earth for the components, only for the switches ( which all work ). The live bus bar feeds the switches from
which a wire goes to the component, but where is the earth wire. The boats wiring is cast into the fibreglass
moulding of the boat so it's not exposed and I can't follow where it goes. Is each component earthed separately
or is there a common earth somewhere and if so where do I find it ??.
Maybe they're all just led back to your battery negative terminal? Hard to say without looking.
I would run a new 10 AWG wire (black) from the Neg battery terminal to the Neg Bus & then use this as your earth for everything.
Select to expand quoteLazzz said..
I would run a new 10 AWG wire (black) from the Neg battery terminal to the Neg Bus & then use this as your earth for everything.
OK, I can do that, so you're saying the earth wires are coming back into the control panel and I've just got to find them
I think Planeray 's idea is more likely. Start at the battery and work backwards from the negative, and see if they go to each component directly.
And remember we are talking DC, so no separate earth for most components.
When I say it's a dogs breakfast I really mean it, it's just a jumble of wires in there. I did take a pic on
my phone but I don't know how to put it on the computer.
The components have to be earthed so is it logical for the return wire to come back to the control box
and hook on to something.....it certainly isn't the earth busbar. Or would each wire join up to each other
on a common wire that goes to earth ?. I can't say I've seen any evidence of that.
Lets not get confused by the terminology. By earth do you mean connected to the negative terminal? Have you looked at the wiring out of the negative terminal at the battery?
All switches and fuses are typically on positive connection.
You don't need earth/ minus there.
Can be easily chain connection similar, the most powerpoints
in the house.
It is possible all the earth wires are going to a small bus mounted out of the way. Sometimes they have a grey cover on them (as seen in the top right). The cables wont necessarily be black either, as you can see most of mine are white. One cable will take it to your earth strap or possibly the motor.
Thanks everybody. Trying to think logically, when the wiring was cast into the fibreglass mould it should have been twin core with
the earth wire returning to the control box and connecting to the bus bar, which it does not do. So I think, as EC31 said, all the earth
wires must join up to an out of the way bus bar. I'll have search around for it.
G'day Sam,
That looks pretty normal mate, so don't feel bad.
In the perfect world, every device's earth would tie back to a negative bus bar, like the top picture. 
In the real world, it's the bottom picture. People will often add a device and attach the earth to any other earth wire they can find nearby. It may be scotch locked on, soldered, bare wires tied together with insulation tape.
Sadly, this will work fine as long as the earth cable your 'piggybacking' off connects cleanly back to the battery earth. It's crappy, and not good practice, but it will work.
Why it is a crappy idea...If it doesn't have a connection/clean connection back to the battery, all devices piggybacking off that single earth cable fail.
To test...with your multimeter connected to battery power on DCV, measure at the negative terminal of each device that is not working. For the + input to the multimeter, I've shown it from the + side of the device, which is fine if you know if you have good power there.
If unsure, run a spare cable directly to the + on the battery with a 20A inline fuse. If you do this, take care not to connect your - probe to the + side of your instruments by mistake (might blow a fuse for that instrument)
0 volts means no connection to the battery earth = Bad.
A positive (or negative ) voltage means you have an earth circuit. The voltage level shown depends upon the device and position in the circuit etc etc, just assume any reading other than 0 means a connection from your - test probe position to the battery earth is present.
Write down all the widgets that fail (0 V) in one column.
Write down all the widgets that work (+ /- V) in another column.
Once you've checked them all, with the multimeter connect the + to the battery + . Starting from your neg bus bar, and using your - neg probe go looking for a logical earth cable heading in the general direction of the closest failed widget/s. Look for where yu can stick your - neg probe. Measure. A 0V is what you are looking for , this means no connection to battery earth.
You already know your busbar connection to the earth is good, you've measured that point already.
So now keep working outwards towards the widget/s until it drops to 0V. That's where you're break/s in the circuit is.
Hope this helps mate.
Thanks Shaggy. I've got the gist of all that and I'm sure I'll sort it. If i can, I'll earth everything to the bus bar
as per your diagram. I'll let you know how I go. Thanks everybody
I'm starting a movement in support of using battery positive as common, switching the negative. We're gonna fight the corrosive negative common'rs... Are you with me?
These things are handy for finding where hidden wiring goes, and only $30 on Ebay. Emitter puts a signal onto the end of the wire you can find and the receiver is like a sniffer to find the other end 
I had dogs breakfasts in wiring many times and found sometimes its better to chop out the whole mess then one by one put the wiring back neatly and correctly. That way you look at each wire one by one and know what it is. Your dogs breakfast would take about a day to fix Sam.
That looks like a great bit of kit Trek. Looking at the pics would you agree that there is no apparent common
earth connection so should i follow through and connect the wires to the earth busbar of the control panel ??.
Select to expand quotesamsturdy said..
That looks like a great bit of kit Trek. Looking at the pics would you agree that there is no apparent common
earth connection so should i follow through and connect the wires to the earth busbar of the control panel ??.
Its really hard to tell where the wires go from the pics Sam. Its possible there isn't an "official" earth bus bar at all, if all the loads (radios, lights etc) have their ground wire leading straight back to the battery negative terminal. If you find a stack of wires on the battery negative terminal that's whats been done. I would go to there and trace everywhere the negative terminal of the battery is wired to. (With the gadget above).
Select to expand quotesirgallivant said..
Gee! You'r lucky your hull is not aluminium! ??
Trust SirG to come up with that!!. Though aluminum would stop an electric field, the magnetic field would still get through. In theory. It might still work. 
Managed to get to the boat yesterday determined to fix the electrics. I found the 'common earth', it's just a bunch of wires
soldered together in a clump hiding behind the alleged 'loom'. Despite being ugly it looked intact but I couldn't pick the earth
lead, so i ran a new one, direct from the battery terminal to the 'clump'. Hey presto...everything works. So it seems it was
just the earth wire breaking down somewhere.
you probably realise this already but a bunch of wires soldered together in a clump is not ideal. electrical solder isn't designed to support mechanical stresses, and it can sometimes be hard to tell visually if a solder joint has got internal stress fractures in it that can result in a flaky connection...
