Fuse on + or -?

[Richard Selman]

Just opened up my wiring to fit a standard horizon FF525 blackbox fishfinder to the plotter and found the fuse to the chartplotter on the negative side. Thought I had made a mistake when I fitted the plotter but I checked and thats what the manual says to do. The fishfinder manual shows a shared fuse for the fishfinder and plotter on the positive side as you would normally expect.

Any idea why the change?

Richard

[bella]

Richard,

Seems unusual, but fuse will function equally well since any fault current will flow through circuit.

[chewy]

I personally would fit the fuse on the live side, that way it can be used as a point of isolation.

[BogMonster]

It's less of an issue on a boat where a lot of it is non conductive with a GRP hull and consoles, but conventional wisdom in most applications is to put the fuse as close as possible to the battery and on the live feed wire.

This is because a significant proportion of problems in typical automotive applications are down to chafing of the live feed rather than a fault with the actual device. As an example we once had a vehicle in our workshops which had a small fire in the dash where somebody had installed a VHF radio, the radio had 25A fuses on both wires in its power lead but they were just behind the radio, so when the positive wire chafed through on the sharp edge it was resting on, the rest of the fairly chunky wire was a direct unfused link to an 800A battery... not ideal! Fortunately it just did a bit of welding for a couple of minutes and then burned out so there was not too much damage but things like that are just as likely to burn something out completely.

You can put a fuse on the earth lead and it doesn't hurt but ...

a) it doesn't help in the above scenario so you'll still want one on the live side somewhere

b) there is no benefit in it protecting against a chafe to earth as the metal chassis of most substantial machines is earthed and therefore even if a wire does chafe through on the earth side no current would flow, and

c) there's no benefit to the device over putting in on the live feed

[jwalker]

Originally Posted by BogMonster

It's less of an issue on a boat where a lot of it is non conductive with a GRP hull and consoles, but conventional wisdom in most applications is to put the fuse as close as possible to the battery and on the live feed wire.

What he said. The purpose of a fuse is normally to protect the wiring not the device. Unless there's something seriously quirky with that unit. I dunno.

[jyasaki]

Originally Posted by BogMonster

b) there is no benefit in it protecting against a chafe to earth as the metal chassis of most substantial machines is earthed and therefore even if a wire does chafe through on the earth side no current would flow

Doesn't really apply to a small boat, where power is supplied by discrete wiring on both pos and neg sides. An abraded wire on either supply feed most likely won't be noticed operationally, until the conductor actually corrodes or abrades through. Just not enough energized stuff around to short to (unless you're really unlucky and you short to say, a power distribution panel or something.

A fuse on either side should do, but the "normal" method is to fuse the positive. The advantage to that would be that a troubleshooter would likely start looking for fuses on the positive lead.


jky

[mitchc]

Read the manual. I think it says for SH plotters and ff525 the fuse protects the nmea -ve.
I'd put a fuse on both the feed (before the plotter isolation switch) to stop wiring faults melting wires and the 1A fuse on the - to protect the nmea circuit

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[Richard Selman]

Did read the manual. or manuals! They conflict.

Richard

[mitchc]

Originally Posted by Richard Selman

Did read the manual. or manuals! They conflict.

Richard

Sorry didn't mean to funny about reading the manuals...

I'm driving down the M6 at the moment so don't have them to hand but If you're not sure as others have said protect the +ve in case of short circuit.

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[jezza2011]

Fuses should be as close to the battery as possible on the + side
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.
The steering and control cables are metal in most cases so are good returns to ground (-)

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!

-______________
+______________\_____________UNIT
-___x________________________/

In this diagram there is a short to - from the + and although the fuse may blow the grounding is still there.
so possible fire.

Idealy you need to fuse both + and - Sides but practically always fuse the + closeted to the source. This is why a demestic supply fuses in the plug as close the the supply as possible..

[jyasaki]

Originally Posted by jezza2011

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!

-______________
+______________\_____________UNIT
-___x________________________/

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.

jky

[jezza2011]

Well you example wont work if the stray + is also fused. as it should be :-)

[Justribing]

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.