fuse time-current curves 1

[fab1961]

Dear all,

I am wondering how can I estimate the change when operating fuses under their nominal load -
all the standard curves show times for cold fuses - I mean for instance for bimetallic protection
devices such thermal overload relays manufacturers show two set of curves, one for cold device
and the other for warmed-up device at normal load - the derating may be quite high, say made
100 the current capacity without tripping you get 25 after 1 hour of current flow at the
nominal value.

Thank you

 

[dpc]

The difference is not as dramatic for fuses, but there is s pre-loading effect. The fuse manufacturer should have data on this. For short circuits, the slightly faster melt time is generally is not a major concern. You can account for it by maintaining some additional time separation between the fuse minimum melt curve and the downstream protective device.