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Old 17 April 2011, 21:06   #11
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,623
Originally Posted by jezza2011 View Post
In the event of a short in the cable to ground along the + rail the fuse will blow and isolate the + supply. If its on the - then the + line will stay engergised and if a ground point touches it , it will just burn and spark untill the cables melt.
Which ws my point. This applies on a car, as the metal chassis and everything bolted to it is part of the negative return of the electrical system. This is not true on most boats. On a boat, the electrical system is discretely wired, rather than using the boat's hull as a return, so there aren't a whole lot of ground points that could touch.

Close to the source is better in any case, as it protects the most amount of wiring. But the positive side is better doesn't apply to a system without a chassis based return.

The steering and control cables are metal in most cases so are good returns to ground (-)
Not unless you electrically connect your sheath to some ground point somewhere. The cable end may make electrical contact to the motor block, but I'd hardly call that a good return.

BATTERY+ __X______________________UNIT
BATTERY- ___________________________/

X= fuse
if the + shorts anywhere along its length to - the fuse will blow and isolate the cable

if its on the + side you could potentially get a short from another - thats not fused and BOOM!


In this diagram there is a short to - from the + and although the fuse may blow the grounding is still there.
so possible fire.
To illustrate, this problem duplicates your second example:

stray wire (+)___
BATTERY+ __X____\__________________UNIT
BATTERY- ___________________________/

Same result, same likelihood.


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Old 18 April 2011, 16:45   #12
Country: UK - England
Town: Northampton
Make: RibTec
Length: 5m +
Engine: Outbaord mariner 75
Join Date: Apr 2011
Posts: 506
Well you example wont work if the stray + is also fused. as it should be :-)

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Old 27 April 2011, 15:29   #13
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Country: UK - England
Town: Devon
Boat name: Sweet Chariot
Make: Ribcraft 5.85m
Length: 5m +
Engine: Suzuki DF140
MMSI: 235065385
Join Date: Jun 2008
Posts: 45
Fuses protect the cables feeding the equipment to stop heat and then fire, so as long as the neutral is tied down to ground zero volts then positive side for fuse. If the neutral is not tied down then current can flow between earth and neutral.

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