Fuses HBC vs LBC

[skcole]

I know what the difference is when it comes to low-power cartridge fuses -

HBC = High Breakdown Current (the highest current the fuse will conduct without shattering, just before it severs. Generally KAmps)
LBC = Low Breakdown Current (the highest current the fuse will conduct without shattering, just before it severs. Generally 10's of Amps)

What I am confused about is why do LBC fuses still exist? Why doesn't everybody just buy HBC fuses and be done with it?

Thanks.

I'm trying to decide whether to spec a HBC fuse on a 19" rack unit, and a LBC fuse on an internal circuit board. As its a question, also, of fuse speeds, and I have heard that generally the speed difference between HBC and LBC is negligible, I don't really understand why LBC fuses still exist?!

As a follow on question, has anyone got any good links to articles on spec'ing fuses so as to prevent a short-circuit tripping a fuse up-stream of the fuse which should blow locally? I get the feeling there should be techniques to checking fusing behaviour, and maybe even being able to choose fuse "families" based where in the chain of circuit-breaker/fuse-sequence the fuse is destined to live.

Thanks for any help on this you can give.

skcole

[wraper]

What I am confused about is why do LBC fuses still exist? Why doesn't everybody just buy HBC fuses and be done with it?

Price, in case of small size fuses, it can also be a size constraint.

the highest current the fuse will conduct without shattering

It's not about shattering but current that fuse is guaranteed to interrupt. Fuse may stay in one piece but be unable to interrupt the current flow.

[Gyro]

What I am confused about is why do LBC fuses still exist? Why doesn't everybody just buy HBC fuses and be done with it?

Price, in case of small size fuses, it can also be a size constraint.

the highest current the fuse will conduct without shattering

It's not about shattering but current that fuse is guaranteed to interrupt. Fuse may stay in one piece but be unable to interrupt the current flow.

There are many places (inside equipment and non mains) where the prospective fault current is low and the interrupt rating of an LBC fuse is perfectly adequate. They also tend to be available in a wider range of (low) current ratings.

EDIT: Fuse manufacturer sites and datasheets are a pretty good source of information on deciding fuse rating and most appropriate delay characteristics.

[ConKbot]

As stated, cost and size.  For example, take a 240v motor for something like a compressor, you feed it with say 30A  rated 50KA interrupting HRC fuses in the disconnect, they are big and chunky. But the contactor inside needs a fuse for the coil and control circuit. Could you put a 1A rated HRC in there? Sure, but the fuse in the control panel only needs to handle a few hundred A before the upstream HRC is well on the way to blowing quickly. Perfectly safe, despite being significantly cheaper.