electrical bonding...

I am in a project someone else has started. The question I have is the bonding system and the 12volt negative are bonded together in the engine room.. seems the previous mechanic is using the bonding system as a negative on the 12volt side for trim tabs, engine meters, ect.. I was under the impression the 12 volt system should be totally isolated from the bonding and 110 volt grounds.... am I incorrect?

 

The bonding system and the 12VDC will be tied together, most likely in the engine room, however that does not mean that it should be a current carrying wire. The neg bus and the bonding system is the correct junction. The neg bus has been installed to carry the full current of the 12VDC system, the bonding system is only connected without it being a conductor of any part of the 12VDC system.

If there are two engines, they should be connected with a battery cable capable of carrying the full start current of either engine, usually 0000 cable on diesel motors. This is connected from the neg connection of the engines at the starter motors of each motor. Make sure that the neg of the emgine is actually attached onto or very close to the starter motors also, not just attached to some convenient part of the block.

The AC earth wire will also connect to the DC neg and the bonding cable at a convenient terminal post position. If this does not exist, then create it.

 

i agree with what landlubber has said and would like to add to it.
by ABYC and USCG bonding is to carry less than an amp (ABYC sec 11.5)
this can be measured will a carbon and an amp meter. this will help find any presents of problems
please be careful about what you call grounded (return) and what you call bonded (these are two differant this)

 

I'll second...uh third, that motion. It is good to keep the idea of bonding and ground as separate systems, but they do have a common point. Usually this is the engine block. But as was said, the bonding circuit should never carry current. It is there to put all metal fittings in contact with the seawater at ground potential which in theory is zero volts.

There's a section on bonding on this page: http://newboatbuilders.com/pages/electricity6.html

 

Errrr just a minute.!!! the engine is is both under these circumstances . They may be 2 different systems for 2 different jobs but in actual fact overlap their duties. The engine is grounded to your shaft anodes and is connected to the neg of the battery. Unless as on my boat I have pos and neg switches.

Its not just the engine start its every thing else from temp guage to tachc, oil pressure, unless your using true 2 wire marine senders.

 

Frosty is right. I left a whole lot of stuff out. If wired correctly a 12V DC system should have either a two wire (positive and negative wire) system, or a negative ground buss that runs through the boat. The ground buss is connected to the engine block. Then all negative (ground) wires from all dc equipment would go to the negative ground buss. This saves a lot of wire and stringing two wires all through the boat.

You would ideally have the positive from the battery go to a positive buss in a fuse or circuit breaker panel, and then separate positive circuits to the equipment. That way each circuit is fused at the power source. Start here.
http://newboatbuilders.com/pages/elect.html

You would have a separate bonding buss connected to the engine block and all metal fittings in contact with the water would be connected to the bonding buss. NOT all Negatives. Negative wires go to the ground buss.