It is essential to know the fuse terminology, before proceeding selecting appropriate fuse for the overcurrent protection of the circuit.
This post will be a reference material during the understanding of fuse operation,analysis and selection by giving us various fuse terminology.
An overcurrent protective device having fusible link that opens the circuit on an overcurrent condition.
[In other words]
A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit.
Current rating of Fuse element:
It is the current which the fuse element can normally carry without overheating or melting. It depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.
It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it. Its value will be more than the current rating of the fuse element.
It is the ratio of minimum fusing current to the current rating of the fuse element.
ie Fusing Factor = [Min. fusing current]/[Current rating of fuse]
Its value is always more than one.
The rms value of the first loop of fault current is known as prospective current.
It is the rms value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance.
It is the maximum value of fault current actually reached before the fuse melts. Its value depends upon
(1) Current rating of fuse
(2) value of prospective current
(3) Asymmetry of short-circuit current
It is the rms value of ac component of maximum perspective current that a fuse can deal with at rated service voltage.
Fast Acting Fuse:
A fuse which opens on overload and short circuits very quickly.
A fuse consisting of a current responsive element inside a fuse tube with terminals on both end of the fuse.
Dual Element Fuse:
It is the fuse with a special design that will utilize two individual elements in series inside the fuse tube. One element, the spring actuated trigger assembly, operates on overloads up to 5-6 times the fuse current rating. The other element, the short-circuit section operates on short circuits up to their interruption rating.
High speed fuses:
These are the fuses with no intentional time delay in the over load range and designed to open as fast as possible in the short circuits. It is mostly used to protect solid state devices.
Time Delay Fuse:
A fuse with a built-in delay that allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits.
The amount of time from the instant the fuse link has melted until the overcurrent is interrupted. ie cleared.
The time required to melt the fuse link during a specified over current.
The total time between the beginning of the overcurrent and the final opening of the circuit at rated voltage by fuse. It is the sum of melting time and arcing time.