Fuse location for relayed headlight harness

[Huw Powell]

urq wrote:
> Oftentimes the fuse is located in the individual circuit because the
> operating current flowing in the supply branch is so high that even a short
> circuit in the branch circuit would not be enough to blow the fuse.  In fact
> the larger circuit may be protected by a fusible link.  When I build a relay
> box for headlights I put a fuse of the same rating as the factory put in the
> line to each individual bulb, and then I use an in-line fuse for the feed
> line from the battery.  I've found a blade type in line holder rated at 40A
> which I install near the tap point (usually at the starter/alternator for
> headlight relays) and a small blade fuse panel in the box for each filament.
> You can also incorporate the factory bulb out warning module in the box and
> route the sense wire back in to the relay box.  
> 
> Steve B
> San José, CA (USA)

Agreed, and ideal.  Big fuse at main power tap, smaller fuses in your 
harness for each section.

> -----Original Message-----
> 
> If the purpose of the fuse is to protect the maximum amount of wiring
> harness (and other parts of the car) possible from any short circuits due to
> mechanical wiring faults, it makes sense to locate the fuse as close as
> possible to the power source. If the fuse were right at the designed ground
> point of the circuit it wouldn't blow if a powered wire upstream were to
> contact another ground - the current would go through the short until
> something melted. You want the fuse to blow instead of other wiring melting
> :)
> 
>> I'm getting ready to fab up a relayed headlight harness and now I
>> am wondering where the fuse should really go. I've made them
>> before and always put the fuses between the relay and the power
>> source. Why do we do that rather than put the fuse between the
>> relay and the headlamp? Input appreciated and TIA
> 
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-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/