Fusing

[Mosaic]

Hello it seems that in the auto world 7.5A = 9A when it comes to a fuse.

Here's a PDF of an auto fuse series:
**broken link removed**

The graph shows that in excess of 9A is req'd to melt the fast-blo 7.5A fuse.

I was looking for an 8A auto fuse and I couldn't find it....now I realize why they are not common. A 7.5 will work for 8A just fine.

 

[ronsimpson]

Question for you: Do you think a 7.5A fuse is made to blow open at 7.5A or is designed to carry a 7.5A load?
If you think the fuse should carry 7.5A then what margin should the fuse have? To carry 7.5A you probably need a 9A capacity.
Did you see it will blow at 10A in 20 seconds? 20A in 0.3 seconds. 30A in 0.08s.

At 15A its typical time is 0.5S but could be as fast at 0.15S and as slow as 5.S
These are not procession parts!
They change with temperature!
Vibration effects them greatly. What is why car and house fuses are made very differently.

 

[dr pepper]

ATC and ATO blade fuses if I remember right are time delay, so they'll carry the fault current for an amount of time, which will depend on the amount of overload.
As ron says there will be a certain amount of tolerance on them anyway.
If you pull 8 amps then why not use a 10a fuse?, if the application is critical then maybe use a semiconductor fuse in a separate holder.

 

[4pyros]

The europeans used them all the time;

https://www.ipdusa.com/products/5013/105764-8-amp-ceramic-fuse

Look here for more info;

https://en.wikipedia.org/wiki/Fuse_(automotive)#Lucas_type

 

[dr pepper]

I've not seen those for a while, they were in 70's cars.