Do fuses vary greatly in heat resistance and quality?

[Richard van der Hoff]

On 02/10/17 21:14, Peter Golledge wrote:
> I would check bearings in Fan, likely heat from increased load with
> bearings on way out.
> 
> On Oct 2, 2017 03:53, "Christopher Gharibo" <cgharibo at usa.net> wrote:
> 
>> I have a ‘91 Audi Coupe and one 30 amp fuse to the radiator fan has partly
>> melted but still works. The circuit and the connector to the fuse looks
>> fine. The other fuses looks fine. No burnt spots etc.

Fuses are a crude solution which Edison invented in the 19th century.

It's easy to imagine that a 30A fuse will carry a 29.9A current 
indefinitely, and as soon as the current rises to 30.1A the fuse will 
blow immediately. If you think about how a fuse works, you'll realise 
that's impossible, because a lump of metal thick enough to carry 29.9A 
without melting won't melt immediately unless it's carrying a lot more 
than 30A.

In practice, that means the fuse will happily carry more than 30A for 
some time, and you'll end up with it looking... partly melted. As indeed 
it probably will if it's continuously carrying 25A.

Peter's suggestion to check out the fan bearings sounds like a sensible 
one. Does it spin freely? Does it sound like it's in pain when it's running?

On the other hand, the "jet engine" mode on these fans really does draw 
a large current. It may just be that over 16 years of summers with the 
fan doing its job (are you in a hot climate?) the fuse has gradually 
gone a bit droopy. It might just be worth replacing the fuse with a new one.