VHF fused on pos & neg

[scurr]

My VHF has an inline fuse on negative as well as positive. I understand the positive, but am I missing something with the negative? If not I’m going to move it to the AIS which seems to be missing one. I’ll check the fuse ratings.

The VHF is a Icom IC-M411

 

[VicS]

The manual talks of one fuse only (10 amp) section 8 page 45

 

[justanothersailboat]

That's definitely not normal practice. A fuse on both out and return paths is redundant. If I were you I would happily cannibalise one for something else that needs it.

 

[Boathook]

Years ago a fuse on both cables was a recognised way of doing things. Some of my older circuits are like that. Whether it was done as fuses weren't as good then I don't know.
I wouldn't necessarily do it today, but I'm not going to alter wiring because 2 fuses are involved.

 

[pmagowan]

Are you sure it is a fuse. Lightning protection, interference suppression etc can look like similar

 

[MADRIGAL]

My VHF has an inline fuse on negative as well as positive. I understand the positive, but am I missing something with the negative? If not I’m going to move it to the AIS which seems to be missing one. I’ll check the fuse ratings.

The VHF is a Icom IC-M411

Fuses protect the leads as well as the device. I had a fuse on the positive VHF lead when I noticed smoke escaping from the lazarette. A quick check revealed that the unfused negative lead from the battery to the VHF was smoking along its length and the insulation was boiling. Now the negative lead is also fused and an on/off battery switch has been installed.

 

[pmagowan]

How was it failing?

 

[justanothersailboat]

That's puzzling. Current reaches the device going down one wire and back via the other. If it's not the same in both, something is pretty wrong.

It's probably possible to have a hot wire and a cold wire the same size as the two sides of one circuit if one is corroded and the other isn't, plus a fuse big enough that the corroded wire can draw enough current to get hot without blowing it. This would be best avoided with a smaller fuse rather than an extra one.

 

[Alex_Blackwood]

That's puzzling. Current reaches the device going down one wire and back via the other. If it's not the same in both, something is pretty wrong.

It's probably possible to have a hot wire and a cold wire the same size as the two sides of one circuit if one is corroded and the other isn't, plus a fuse big enough that the corroded wire can draw enough current to get hot without blowing it. This would be best avoided with a smaller fuse rather than an extra one.

That is why it was smoking
As previously stated it was at one time common practice, in D.C. Circuits, to fuse both +ve. &-ve. As far as I know still is in places where you may have 220 Volt D.C. circuits. Not fully up to speed on the regs. for that so not 100% sure.

 

[justanothersailboat]

Where did the energy come from that's making the bad wire smoke? ...through the other wire, and the fuse. (I guess if your zero is not ground there could be a case for it, but in boat electrics?)

 

[B27]

It's not normal, but there can be some sense in it if there's possibility of other current flow, such as via the aerial or data connections which would not blow the fuse in the +ve supply.
Also when -ve is not connected to earth anywhere.

 

[lustyd]

That's puzzling. Current reaches the device going down one wire and back via the other. If it's not the same in both, something is pretty wrong.

Not puzzling at all. You’re assuming the cables are matched size. If one is thinner and the fuse is sized to the other larger wire it would be easy to have this happen.
This would also be the case if one wire was corroded, effectively making it thinner.

 

[justanothersailboat]

lustyd - no assumption the wires are the same. the current is the same, the heating is different with wires of different sizes or condition. You're concluding the same as I did, which is that an extra fuse the same size isn't protecting against this scenario, making the first fuse smaller is. Fuse always has to be sized smaller than the weakest wire in the circuit.

 

[PaulRainbow]

Lets say the wires to a device are 2.5mm and the positive is fused close to the power source. Somehow, the negative wire comes into contact with a much larger positive wire, the fuse doesn't blow because it's rated too high for the 2.5mm cable. The 2.5mm wire obviously melts.

 

[VicS]

Lets say the wires to a device are 2.5mm and the positive is fused close to the power source. Somehow, the negative wire comes into contact with a much larger positive wire, the fuse doesn't blow because it's rated too high for the 2.5mm cable. The 2.5mm wire obviously melts.

If that can happen it would be a good argument for fusing all the negative wiring in the same way as the positive wiring.
It obviously can happen but far less likely than a positive wire coming in contact with an uninsulated component ( eg engine block) connected to negative
.

 

[B27]

A wire rated for a few amps can carry a lot of amps for long enough to blow quite a big fuse.

If it's likely that a thin black wire will come into contact with unfused or high current fused +ve conductors, there's things wrong which need looking at.

 

[PaulRainbow]

A wire rated for a few amps can carry a lot of amps for long enough to blow quite a big fuse.

For crying out load....... how well do you think a 2.5mm wire will fare if it came into contact with one of the bow thruster cables, or windlass cables, or main starter cables ????

If it's likely that a thin black wire will come into contact with unfused or high current fused +ve conductors, there's things wrong which need looking at.

I didn't say it was likely, i put forward an explanation as to how the event is post #6 could be possible.

 

[MADRIGAL]

It certainly puzzled me. The wires and the fuse were the ones supplied with the Icom VHF set, which I connected directly to the battery. It had worked uneventfully for most of the season. I do wonder about corrosion, as I don't remember seeing evidence that the wires were tinned. I was not aware of a defect such as a break in the insulation when I reinstalled the radio that spring, but of course that cannot be ruled out. Likewise, I was not aware of any contact with another power lead. That too would explain the sudden heating of the negative lead, but I don't have any high-current systems aboard like a bow thruster or a windlass.

 

[MADRIGAL]

Deleted (duplicate)

 

[B27]

Fuses protect the leads as well as the device. I had a fuse on the positive VHF lead when I noticed smoke escaping from the lazarette. A quick check revealed that the unfused negative lead from the battery to the VHF was smoking along its length and the insulation was boiling. Now the negative lead is also fused and an on/off battery switch has been installed.

This sounds pretty weird, because the standby current of a VHF is pretty small.