lighting electrickery question

[Deak]

I've fitted a pair of bosch fog lights and I'm puzzled as to why the fuse keeps blowing.

My understanding of electrical stuff is ok but I cant figure out why a 15 amp fuse isn't enough. I've got a live feed direct from the battery through the 15 amp fuse. This goes through a 40amp relay triggered by the low beam circuit via an on/off switch. The feed to the lights is a single line out from the relay then split to the two lights under the headlight pod.

My basic understanding is that the current draw is equal to total watts divided by volts. (2x 55 watt bulbs = 110 watts divided by 12 volts equals 9.16 amps) A 15 amp fuse should easily do it. If I fit a 25 amp fuse it doesn't blow. I know I could run a 25 amp fuse all the time but it still bugs me that I cant work out why this is happening.
All the connections are secure and soldered and nothing earthing out etc. Am I missing something obvious

 

[Deleted account 220427002]

there's always a bigger current draw on start up of things is this when it blows?

 

[Deak]

Nope, they are on usually for about a minute before the fuse blows.

 

[Banjaxed]

Relay?

The maths seems correct - don't know the exact running voltage of your bike, but most 12v systems actually run higher than that (e.g. 13.6v), so a 15amp should be fine (as the amperage will actually be lower). If there are no shorts anywhere, I'd suspect the relay. If it's a spade terminal type, I'd swap in a new one and see how you go. They're cheap enough...

 

[Greg Masters]

Do NOT put a bigger fuse it!!

A 15amp fuse should be fine (and will cope with the surge demand when you turn the lights on).

There is clearly a fault somewhere - maybe a wire chaffed through the outer and perhaps touch a painted surface (with less conductivity than bare metal). Perhaps the relay is full of water and there is a partial internal short.

Get your multimeter out and start checking a few volategs and/or resistances and find the fault. If you resort to putting a bigger fuse in, you might as well take the fuselink out of the circuit altogether!

Greg