Car Battery help needed.....please

No, more like 42A thick per side. (As it uses two circuits.)

IOW, round about the size that might be protected by, ooh let's see, a 100A fuse?

Chris

My understanding of a fuse is that it is rated at 'less' than the current carrying capacity of the circuit it's protecting. Otherwise what's the point? The component or wiring would melt before the fuse blew Mike.

Your understanding is broadly correct, and an arrangement where 2 x 42A rated cables in parallel are protected by a 100A fuse would not meet BS7671 regulations for buildings.

However, it's quite common in car wiring for a fuse to be larger than the capacity of the cable. In this example, if one of the cables became shorted to -ve, the 100A fuse would still blow before the wiring was damaged to a degree that it couldn't continue to be used. The cable carrying capacity is for continuous use; the fault current will be carried for a trivial amount of time.

Chances are, the designers would pick the closest fuse available. There is probably a jump from 60A to 100A in this case, with nothing in between

Chris

Fuses are strange things and depend on many things on how they operate.

Here's a post from another group which will make you ponder about 'open wire' fuses...

*************************************** From: John Rumm Subject: Re: House of Horrors Date: Fri, 01 Jun 2007 22:20 Newsgroups: uk.tech.digital-tv

One of the things that surprises many is just how much current is required to blow a fuse. If you take the example of a rewireable 5A fuse of the type you find in older consumer units (aka fusebox), you can see from:

That it will carry 9A indefinitely, it takes 13A to get it to blow in five seconds, or 45A to get it to go "instantly" (i.e. 0.1 sec or less)

Interesting link. I'm not all that competent when it comes to electrickery as you've probably gathered, but it seems to indicate that a 100A fuse would take considerably more current before it blew. For a heated w/s that suggests the wiring would have to be particularly heavy. AFAIR the heaviest fuse in our 528's is 35A.

BTW Dave, the door mirrors on my 'new' 528, which I picked up yesterday, are heated, self dimming, and fold at the press of a button. Woo! Hoo! :-) Can't really see the point, but there you go. Mike

Going way off topic here, but the current it takes to clear a fault is known as the fusing factor. For a wire fuse, this can be as high as 2, for cartridge fuses it is often around 1.6.

I only have time/current characteristic curves for BS 1361 fuses. (The old-fashioned household rewirables.) The current that will be passed by a

100A one of these under a fault of negligible impedance is 1800A for 0.1s!

Chris

Yes. And in a house this *might* cause problems as the load on any one circuit can vary - which isn't the case in a car, unless things have been modified. So in a car, the fuse really is to protect the wiring in event of a short rather than overload. Some circuits that could be easily overloaded, like a basic electric window where the switch might be held on, can use a circuit breaker.

I find the dimming interior mirror on mine useful - but I can't say I've been dazzled by the door ones. The folding feature is nice, though.

Going way off topic here, but the current it takes to clear a fault is known as the fusing factor. For a wire fuse, this can be as high as 2, for cartridge fuses it is often around 1.6.

I only have time/current characteristic curves for BS 3036 fuses. (The old-fashioned household rewirables.) The current that will be passed by a

60A one of these under a fault of negligible impedance is 1300A for 0.1s!

Chris

THe component normally does melt, they're normally only good for protecting wires and avoiding fires.