Fuses - Battery side of component or earth/return?

[NPMR]

Do you put the fuse before the unit or after the unit - (I have a mental picture of the electricity going from the positive side of the battery to the unit and then back to the negative side).

Anyway - where does the fuse go?

 

[David2452]

In the feed i.e. the positive line.

 

[pvb]

The purpose of the fuse is to prevent the wire overheating in the event of a fault or short. Therefore the fuse needs to go at the supply end of the wire, ie the end nearest the battery. Convention is to put the fuse in the positive wire.

 

[Aurai]

Can we suppose

That there is a battery feed to a Distrbution Board or a selection of Circuit Breakers. Then there are a few instruments and perhaps some delicate or sensitive ones.

The we might fuse the outgoing positive battery cable to the Distribution Board, as close to the battery as practicable. The Circuit Breakers protect the wiring from various fault conditions including overcurrent.

While you might fuse on the positive cable, any feeds to sensitive equipment e.g. VHF as close to the instrument as practicable.

So we fuse protect at different levels and for different purposes in different ways! I took all my cabling and protection / Fusing advice from Merlin Marine electrical guidance and they do some useful wiring guides on their website and that does not mean it is the only way.
Cheers

 

[nimbusgb]

Since current is the same all the way around a single circuit it makes not the blindest bit of difference.

Convention has it in the positive line and close to the appliance.

 

[pvb]

Since current is the same all the way around a single circuit it makes not the blindest bit of difference.

Convention has it in the positive line and close to the appliance.

Convention's incorrect - the wire needs to be fused close to the battery to give proper protection.

 

[David2452]

Can we suppose

That there is a battery feed to a Distrbution Board or a selection of Circuit Breakers. Then there are a few instruments and perhaps some delicate or sensitive ones.

The we might fuse the outgoing positive battery cable to the Distribution Board, as close to the battery as practicable. The Circuit Breakers protect the wiring from various fault conditions including overcurrent.

While you might fuse on the positive cable, any feeds to sensitive equipment e.g. VHF as close to the instrument as practicable.

So we fuse protect at different levels and for different purposes in different ways! I took all my cabling and protection / Fusing advice from Merlin Marine electrical guidance and they do some useful wiring guides on their website and that does not mean it is the only way.
Cheers

Exactly the way to do it, this mantra of a fuse being there simply to protect the cable is just plain wrong, a fuse is there to protect the circuit, now in the case of a cable from battery to a fused distribution panel the cable is the circuit but in other instances this is not the case and that is exactly why I dislike the simplicity of a fuse is there to protect the cable.

 

[nimbusgb]

It makes no difference!

 

[pvb]

It makes no difference!

It does. Suppose you have a device supplied by a 2 metre long cable. With the fuse at the device end of the cable, if there's a short anywhere along the cable, the cable will potentially heat up and may cause a fire. With the fuse at the battery end of the cable, a short anywhere along the cable will simply blow the fuse.

Most small circuits, lights, etc, tend to be fed from a switch panel on which each circuit has a fuse or breaker. The wire from the battery to the fuse panel should also be fused close to the battery.

 

[halcyon]

Exactly the way to do it, this mantra of a fuse being there simply to protect the cable is just plain wrong, a fuse is there to protect the circuit, now in the case of a cable from battery to a fused distribution panel the cable is the circuit but in other instances this is not the case and that is exactly why I dislike the simplicity of a fuse is there to protect the cable.

Which ever way you look at it, the cable must have a larger current rating than the circuit breaker, intern the cable must have a higher rating than the unit it is supplying, so yes the circuit breaker protects the cable.

If you attach multiple circuits to one circuit breaker, each cable must be capable of individually carrying circuit breaker trip current.

Brian

 

[single]

It does. Suppose you have a device supplied by a 2 metre long cable. With the fuse at the device end of the cable, if there's a short anywhere along the cable, the cable will potentially heat up and may cause a fire. With the fuse at the battery end of the cable, a short anywhere along the cable will simply blow the fuse.

Most small circuits, lights, etc, tend to be fed from a switch panel on which each circuit has a fuse or breaker. The wire from the battery to the fuse panel should also be fused close to the battery.

+1

 

[Hydrozoan]

It does. Suppose you have a device supplied by a 2 metre long cable. With the fuse at the device end of the cable, if there's a short anywhere along the cable, the cable will potentially heat up and may cause a fire. With the fuse at the battery end of the cable, a short anywhere along the cable will simply blow the fuse.

Most small circuits, lights, etc, tend to be fed from a switch panel on which each circuit has a fuse or breaker [my emphasis]. The wire from the battery to the fuse panel should also be fused close to the battery.

Quite. Is some confusion being caused by an assumption, in some other replies, that all sub-circuits will be protected by CB's, such that the only fuses being considered by some here are either main battery fuses or appliance fuses?

 

[KellysEye]

Our switch/fuse panel was about 6 metres from the battery with no problems. Any short trips the fuse immediately so the cable is unlikely to heat up provided you have the right size fuses for all the equipment. Obviously a fuse too big that doesn't trip can potentially start a fire. The great majority of shorts are in the equipment it's rare to have a short in a cable unless it is cut and the wires touch. We saw a boat in Antigua Sailing Week and one on a buoy in Bonaire on fire and both were electrical fires, the first in the cockpit locker and the second in the fuse panel.

 

[pvb]

Quite. Is some confusion being caused by an assumption, in some other replies, that all sub-circuits will be protected by CB's, such that the only fuses being considered by some here are either main battery fuses or appliance fuses?

Maybe, I don't know. The OP's question wasn't very clear either. But a lot of people tag in extra equipment, wired direct to the battery or to a distribution busbar, often with only the device's own inline fuse.

 

[Hydrozoan]

Maybe, I don't know. The OP's question wasn't very clear either. But a lot of people tag in extra equipment, wired direct to the battery or to a distribution busbar, often with only the device's own inline fuse.

Nor me for certain, though Aurai's case description (for example) seemed to be based on a distribution board with, or a selection of, CB's for circuit protection.

Some of us have, and are happy (even perhaps happier?) to have DC distribution boards with fuses to protect sub-circuits.

But of course I also take the point in your last sentence as a good reason to emphasize fuse location, as you do.

 

[Aurai]

Small Addition

To return to the original question and various responses suggesting it does not matter which side of an accessory you place any fuse. All this discussion has to be in the context of DC and not AC power supplies.

DC is unidirectional unlike AC and so it is a bit more than convention to fuse on the positive side of anything, it is logical.

Thank you

 

[pvb]

DC is unidirectional unlike AC and so it is a bit more than convention to fuse on the positive side of anything, it is logical.

May I ask why it's "logical"?

 

[VicS]

Small Addition

To return to the original question and various responses suggesting it does not matter which side of an accessory you place any fuse. All this discussion has to be in the context of DC and not AC power supplies.

DC is unidirectional unlike AC and so it is a bit more than convention to fuse on the positive side of anything, it is logical.

Thank you

Its not logical at all!

In fact the actual flow of electricity ... the direction the electrons move ... is from negative to positive. The positive to negative flow is merely a convention,

 

[William_H]

Fuse negative or positive? Well in a simple circuit of a battery and a light bulb then it does make no difference. For best protection the fuse should be near the battery. If the fuse were near the light then you have 2 wires from the battery which if they touch one another could give a high current overheating the wire. The current path not being via the fuse.
However in the case of a car. We have a metal body connected to the negative of the battery. This connection is also via the engine block or starter motor body with heavy wire.
Now there is a risk of chafing of any positive wire to the car body so any fuse in the negative might be bypassed by the negative to body. Hence you fuse the positive supply.
Very old cars were positive earth but same applies.
Now on a boat we do not unless it is a steel boat have the body connected to negative. However we usually except in some cases connect the engine body to negative with a heavy cable. This means also that the water is connected to negative. The radio will probably have a definite connection negative to the mast for a groundplane.
So while not always the case often the negative of your service device will be connected to negative of the battery via structure or other earthing wires.
This means that fusing and switching the positive supply has become the norm and is perhaps the easist way to go. Certainly makes it easier for any person coming along later because that is what he will expect. So yes fuse in the positive line if only to be like every one else good luck olewill