Boat Mains Switch

[malliadan]

when fitting a mains switch, which in your opinion is best to isolate , the +ve or the -ve

i know i'm supposed to know the answer since i'm a technician but i ve heard a lot of opinions and got confused

Daniel

[bigboy]

my boats -ve mains are isolated

[skip]

The battery switch is usually always connected to the positive circuit as it isolates the feed from the batteries to the engine/electronics.

I like to come out from the mains switch on to bus bars one strip for +ve and one strip for -ve although you can get a dual bus bar from the likes of BlueSeas etc.

Then from there you can have fused protected etc, have a look at the Gaffer's re-wiring diagram from his winter projects on the gaffer thread.

[The_Gaffer]

In DC theory, you always isolate the +ve side.  Allow me to explain why. 
DC operates on low voltage but high currents.  Currents are carried via the +ve circuit and neutralised to ground, -Ve.  If for any reason you isolate the -ve side of the current, and you have a short circuit, the current will still flow through your boat wiring circuit until it encounters a -ve element, be it the engine, boat grounding, or the -ve circuitside.  Once contact is made, the circuit is complete, and because of the high currents involved, you are severely risking (a) DC wiring burnout (b) fire on board (c) damage to electronics

[Destination Sea]

isolate positive

[busumark]

if you dont have a main switch which wire you disconnect from the terminals the  + ve or - ve ? i disconnect the - ve because the outboard manual says to connect the motor wires first connect the + ve than the - ve and to disconnect the wires first disconnect the - ve than the + ve. so i leave the + ve always connected and disconnect and connect the - ve. will the battery be disconnected if it has only the + ve connected ?

[Destination Sea]

Disconnect negative first and connect last. Connecting the postive last can induce voltage sparks to 100V and if a ecu is present it would not be happy.a circuit needs to be complete for current to flow.

[malliadan]

that's the same opinion i had

thanks guys

Daniel

cu out there on the gypsy spirit