fuses - do they work on negative side?

cumbrian

cumbrian

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Hi,

doing some overdue re-wiring, including adding a multiple glass (fast blow) fuse holder to replace all the individual inline fuses

i've seen a wiring diagram that positions a fuse on the negative side of the device that it's protecting.

this could save a lot of wiring because the multi fuse holder includes a bus bar and could therefore function as a negative bus

but would this protect all of the devices?

thanks

Keith
 
Yes ...... and no. Fuse s protect the wiring more than the device. By the time a fuse goes the electronics is usually in a state that requires a trip to the repairer.

A fuse in the negative line providing power to more than one device will have to be sized large enough for all potential current draws on that bus so if only one device goes south its associated wire could potentially be overloaded.
 
Possible, but not good practise & not something I would contemplate.

The busbar part is for the common connection to the battery feed.

You can buy a separate busbar to common all the negatives, which is a neat way to do it.


What would happen if you had a short on something that is connected to the engine ? say instruments, a fuse in the negative lead would not work, the postive wiring with no fuse would burn out, probably starting a fire in the process.
 
i've seen a wiring diagram that positions a fuse on the negative side of the device that it's protecting.
Click to expand...
Thats fine, no problem ***

but for the reasons given above not in a common negative suppling a whole load of stuff. Similarly not in the positive supplying a whole load of stuff.

As above the fuse should be sized to protect the wiring it supplies. It must be a low enough rating to protect the thinnest wiring connected to it ... or put another way all the wiring it supplies must be heavy enough to safely carry the current that the fuse can supply.


*** but consider the risk of an unfused positive ( unfused because the fuse is in the negative) coming into contact with an unfused negative ( unfused because the fuse is in the positive). I think therefore that the fuses should either all be in the negative or all in the positive. No objection of course to both being fused.
 
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I don't know that much about electrics, but I can follow simple diagrams. The Standard Horizon 180 CP is fused on the negative side.
 
NO NO NO

Don't fuse the negative.

Consider this case, you have fused the negative feed to your VHF radio.

The radio has developed a fault and is drawing too much current, so the fuse blows.

But the radio aerial is still connected, and it happens that the aerial cable passes through a deck mount that happens to be connected to other grounded items.

the radio is therefore still powered via the braid of the aerial cable, so the fuse is ineffective.

Never ever rely on a fuse in the negative to function properly.
 
As ProDave said, a fault condition could rupture the fuse but the equipment may still be connected to battery -ve via ancillaries. It's also worth remembering that the fuse in the +ve line protects the wiring which follows as well..... if the cable gets pinched up against a -ve casing somewhere and goes through the insulation, that fuse will go.
 
Yes, as posted the fuse is there to protect the wire, not the equipment being powered. Electronics often have their own fuses, sometimes in the negative lead and sometimes in the positive. These are generally a lot smaller than the fuse you would use to protect the circuit at the main panel and are there to protect the equipment. The wires to these items should still have fuses at the panel to protect the wire.
 
rhumlady

Fusing the neutral wire in 115 volt ac anything is a bit scary. If the fuse blows 115 volts is still present between the hot and earth wires. If the fuse is in the hot wire the 2 wires remaining intact, neutral and earth will not show any voltage between them.
 
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