Fuse in both +ve and -ve wires?

[Plum]

As originally fitted by Beneteau, there is a 40A fuse in the positive wire, close to the master switch, that goes to the switch/fuse panel for all the ancillary circuits. However, next to this fuse is an identical 40A one in the negative return wire. Why did they fuse both +ve and -ve wires?

Www.solocoastalsailing.co.uk

 

[lw395]

To con-fuse you obviously!

proper answer, because there are unfused, or high-amp-fused +ve lines on the boat, e.g. the starter circuit.

 

[Plum]

To con-fuse you obviously!

proper answer, because there are unfused, or high-amp-fused +ve lines on the boat, e.g. the starter circuit.

Ha! Thanks, I had not thought of that but now I see the logic.

Www.solocoastalsailing.co.uk

 

[JumbleDuck]

proper answer, because there are unfused, or high-amp-fused +ve lines on the boat, e.g. the starter circuit.

How would the current return from those without going through the 40A fuse in the negative? And what sort of weedy started draws a measly 40A?

 

[Alex_Blackwood]

As originally fitted by Beneteau, there is a 40A fuse in the positive wire, close to the master switch, that goes to the switch/fuse panel for all the ancillary circuits. However, next to this fuse is an identical 40A one in the negative return wire. Why did they fuse both +ve and -ve wires?

Www.solocoastalsailing.co.uk

You don't say hold the boat or wiring is. However used to be common practice with DC to fuse both +ve & -ve. At least on big stuff 110 & 220volt

 

[VicS]

How would the current return from those without going through the 40A fuse in the negative? And what sort of weedy started draws a measly 40A?

I think ( at least what my two remaining grey cells have figured out) is that the 40 amp fuse in the negative supply to the domestic panel would offer protection from an ( unlikely ) short between the unfused starter circuit and the negative connection of any a equipment powered from the domestic panel.

 

[KompetentKrew]

proper answer, because there are unfused, or high-amp-fused +ve lines on the boat, e.g. the starter circuit.

That seems to suggest all our boats should be double-fused like this, doesn't it?

 

[greeny]

That seems to suggest all our boats should be double-fused like this, doesn't it?

Yes.

 

[lw395]

That seems to suggest all our boats should be double-fused like this, doesn't it?

How many boats had damage due to unfused negatives last year?
How does that compare with the risk of damage due to return currents flowing through unintended paths (e.g. the aerial coax outer)?
It's probably an indifferent solution to a non-problem.

 

[Plum]

How many boats had damage due to unfused negatives last year?
How does that compare with the risk of damage due to return currents flowing through unintended paths (e.g. the aerial coax outer)?
It's probably an indifferent solution to a non-problem.

I am tempted to replace the negative wire in one continuous length without the fuse just to eliminate a few more opportunities for a poor connection particularly as the "unfused, or high-amp-fused +ve lines" you referred to in post 2 do not run anywhere near the negative wire from the fuse/switch panel.

Www.solocoastalsailing.co.uk

 

[Plum]

You don't say hold the boat or wiring is. However used to be common practice with DC to fuse both +ve & -ve. At least on big stuff 110 & 220volt

She is 19 years old and the fuses in question are original.

 

[Heckler]

She is 19 years old and the fuses in question are original.

My 1999 Bene 381 has two 100amp fuses as you describe.

 

[lw395]

She is 19 years old and the fuses in question are original.

If the original fuses haven't blown in 19 years, you possibly didn't need them?

 

[Zing]

To con-fuse you obviously!

proper answer, because there are unfused, or high-amp-fused +ve lines on the boat, e.g. the starter circuit.

In that case why not fuse the starter wire with 40 A at the + end as normal as it would blow the 40A neg wire fuse anyway if there was a short in its circuit?

 

[VicS]

In that case why not fuse the starter wire with 40 A at the + end as normal as it would blow the 40A neg wire fuse anyway if there was a short in its circuit?

Because 40A would not be large enough to carry the starter motor current.

 

[Zing]

Because 40A would not be large enough to carry the starter motor current.

Then why is the neg fuse not blowing? All the current on starting flowing through the - wire will be equal to that in the + wire.

 

[VicS]

Then why is the neg fuse not blowing? All the current on starting flowing through the - wire will be equal to that in the + wire.

The negative fuse under discussion is one in the negative fed to the domestic panel. The return current from the starter motor does not flow through this wire

 

[Zing]

The negative fuse under discussion is one in the negative fed to the domestic panel. The return current from the starter motor does not flow through this wire

Ok. It still seems daft. The risk from an infused starter circuit, say from a stalled starter motor must be far higher a risk, yet is unprotected. I realise that is normal.

 

[VicS]

Ok. It still seems daft. The risk from an infused starter circuit, say from a stalled starter motor must be far higher a risk, yet is unprotected. I realise that is normal.

The starter circuit can be protected by a "mega-fuse", (although often/ usually is not)

Even if the starter circuit has a mega fuse the argument for the fuse in the negative feed to the domestic panel is still valid.
If there is a risk of a short occurring between the un-fused or mega-fused starter circuit wiring and any of the negative "domestic " wiring then the fuse in question will protect the negative domestic wiring which will not safely carry the current allowed by a mega fuse.

However , IMHO if there is risk of a short between these two the wiring needs a very serious "looking at"

 

[JumbleDuck]

In that case why not fuse the starter wire with 40 A at the + end as normal as it would blow the 40A neg wire fuse anyway if there was a short in its circuit?

I think the idea (thanks, VicS) is that there is a completely unfused starter circuit and a domestic circuit, and that the domestic circuit has been given two fuses in case either the engine live shorts to the domestic return or the domestic live shorts to the engine return. Which doesn't seem unreasonable.

Edit: Actually, it does sound unreasonable, because it leaves the starter circuit completely unprotected, and those wires will almost certainly be closer to each other than either of them is to the domestic wiring.