David Hansen wrote:
On Tue, 14 Sep 2010 16:23:00 +0100 someone who may be Dave Osborne
wrote this:-

The fuse has no idea (and doesn't care) whether the
current flowing is a.c. or d.c. until the point where it ruptures due to
overheating. At the point it ruptures, if the current flowing is A.C.,
then the arc is quenched immediately. If the current flowing is D.C.
then the arcing time will be longer. Therefore, the rated breaking
capacity of a fuse is lower for D.C. than for A.C., but the current/time
characteristic curves are the same.

Within the limits of the discussion I agree, with the minor niggle
that the arc will only be quenched "immediately" on AC if the
current is less than the breaking capacity of the fuse. If it is
more then the fuse cracking, falling to bits or exploding is
possible. The damage if this happens has to be seen to be believed.

But that (the physical destruction of the fuse) demonstrates either a
flawed design or a catastrophic event, does it not?

The prospective short circuit current should in all cases be less than
the breaking capacity of the protective device for all foreseeable,
manageable risks.

For the vast majority of domestic installations, the prospective short
circuit current is an order of magnitude less than the breaking capacity
of the fuse. E.g. The breaking capacity of a BS1361 fuse in a house
service cut-out is rated at 33kA and (I would say) the prospective short
circuit current in the vast majority of dwellings is less than 3.3kA, so
in fact there is a (worst-case) safety factor of 10 on the main supply fuse.


see he http://www.lawsonfuses.eu/lowvoltage.pdf

The difference between AC and DC is dramatic. That reference says

"Breaking Capacity

"The standardized values of Breaking Capacity for fuse-links to
BS88 are 80kA for voltages of 415V a.c., and above, and 40kA for
d.c., applications. The 240V a.c., designs have a breaking capacity
of 16kA minimum."

It's not that dramatic if it merely reduces your worst-case safety
factor from 10 to 5.

Cheers,
DaveyOz