Fuse on negative vs. positive 12v leads?

[ntsqd]

The whole deal about the actual direction of electron flow means not much in a practical application. The system has an "isolated" side and "common" side. Ignore what the polarity might be labeled as it also further confuses the issue.

It is important that the fuse be as close to the "isolated" side as is reasonably possible. Matt, Jim, Dave, and myself have all said the same thing, just in different words.

In most 12VDC vehicles that means that the fuse/breaker/fusible link wants to be as close as is reasonably possible to the positive battery terminal. Putting another one in the negative wire near the grounding or common connection won't hurt, but it shouldn't be considered to be a replacement for the fuse at the positive terminal.

 

[tuesdayfox]

fuse on negative side is not wise

 

[SoCal Tom]

A short is the most likely cause for fuses blowing. If the vehicle has a negative ground ( ie all metal parts are connected to negative side of battery) then you want the fuse on the other side ( positive).

 

[DaveInDenver]

The fuse in the negative lead (if to the battery neg) is to protect just the negative lead in case the vehicle's negative battery connection gets disconnected. That applies when you intentionally disconnect the negative battery terminal to work on the vehicle's electrics, too!!! Fry that lead and it could fry the vehicle. If the radio's negative lead does not go back to battery negative, the fuse is not required (but still a good idea).

Another scenario is in the event the engine ground strap becomes corroded or disconnected, which could see current through the starter or alternator trying to find a lower impedance path back to the battery and that might be through the radio. The fuse in the negative lead would prevent that. This of course does not replace the positive fuse there to protect the radio harness but to protect from a different type of fault.

 

[Joe917]

A lot of bad advice here.
A dc circuit needs to be fused at the start of the positive run. The fuse only protects the wire after the fuse. If you fuse the ground wire and the positive shorts to ground the fuse will not be part of the circuit and the positive will continue to arc until the wire fails. FIRE.

 

[dwh]

Old dead thread from 2009, revived by a one-liner post.

 

[DaveInDenver]

 

[theksmith]

A lot of bad advice here.
A dc circuit needs to be fused at the start of the positive run. The fuse only protects the wire after the fuse. If you fuse the ground wire and the positive shorts to ground the fuse will not be part of the circuit and the positive will continue to arc until the wire fails. FIRE.

Old dead thread from 2009, revived by a one-liner post.

Joe917 - summed it up well though... both in his comment on the quality of some of the advice in this thread as well as in his response to the original question!

 

[rayra]

If you fuse the ground side, and a short occurs in the device you are trying to protect, the fuse will not open. You will continue feeding electicity to a dead short until something burns up; maybe even the entire truck. The fuse will only open if it is between the source and the load, or the source and the short.

This. 'protect the circuit' is an engineer's / design school answer. The reality is 'to keep your crap from burning up'. So you fuse as close to the power source as practicable. It isn't about 'flow' or 'efficiency' or anything else. It's to de-energize as much of the circuit path as possible.