OC protection—Fuse
Fuse is a kind of component which is sensitive to current, when the fault current occurs to over the rating current. the fuse will melt to avoid the further damage to the electric equipment. To select an appropriate fuse for a particular application seems very important, we can design a fuse according as below mentioned steps:
Fuse selection-theory:
A. The first consideration, where do you intend to approve your device, For the different design view,there are two kinds of standard system.
1.North America-UL,CSA
2.Europe BS,VDE IEC standards
The fuse characteristic requirements of two of the standards are not com patible,by this case, the two standard compromise as following.
UL standards can lolgo to recognize IEC standards.IEC standards developed UMF standard to meet North America standards.For fuse is a safety protection component,many countries require fuse to be approved by specified safety agencies,other wise it will be for bidden to sell to the relevant market So the agency approvals is one of the quite important factors to design.
B. Construction & Size：
Fuse is a changeable component, it is specified in the relevant standards, the a ppropriate size and construct ion is another
design f actor.
C. Voltage rating (V)：
Normally fuse's power dissipation is quite pity,as well as its voltage drop, but when fault occurs the fuse cutoff, the two sides of fuse will stand the rating voltage, so fuse Voltage rating should higher than supply voltage is another necessary considerati on.
D. Current Rating：
The nominal ampere value of the fuse, it is established by the manufacturer as a value of the current the fuse can carry,based on a controlled set of test conditions.For the different of the two standards (IEC & North America).the rated current (I rated)of an electronic fuse must be equal to or greater than the co ntinuous operating current in the circuit where the fuse is used.
For North Americ a fuse: I rated >= circuit cont inuous operat ing current/0.8 5.
For I EC fuse: I rated >= circuit co ntinuous opera ting current/1 .0.
E. Breaking capacity (interrupting rating) :
The breaking capacity of a fuse is a measure of the maximum fault current which the fuse will safely interrupt without exploding,rupturing,or causing a fire. During a fault or short circuit condition, a fuse may receive an instantaneous overload current many times greater than its rated current. The fuse immediately senses th is in current flow and the element begins to overheat and melt. The fuse must safely interrupt this current flow and subsequently preventany potential arc restrike before a catastrophic falult or short circuit conditions causes physical d amage to the down-stream equipment.The manufacturer will mark Breaking Capacity. User should estim ate the maximum fault current and ensure it alwayss lower than fuse breaking capacity.
F. Ambient temperature ：
For fuse is a kind of temperature sensitive components;the temperature of the air immediately surrounding the fuse,not to be confused with "room temperature". The fuse arnbient temperature is appreclably higher in many casbs.Because it is enclosed (as in a panel mount fuse holder)or mounted near other heat producing components, such as resistors, transformers; etc.if fuse work in different ambient temperature, we should cnsider rerating of temperalure. The l-rate will be adjusled according to tollowing chart(1).
G. Fuse Characteristics：
The characteristic of a fuse design refers to how rapidly the fuse responds to various current overloads. Fuse characteristics
Can be classiffied into four general categories:very fast-acting,fast-acting.Slo-Blo,time delay or surge Tolerant fuses, The distinguishing feature of Slo-Blo fuses is their additional thermal inertia and their ability to tolerate normal initial or start-up
overlo ad pulses.
H. l2t value：
The time -current characteristic curves us ually only provide data down to 10 millisesconds.For blowtime less than 8-10 msec.(which is 1/2 cycle of 50Hz or 60HzAC current),the actual blow time will be significantly affected by the phase angle at which the currentis switched on.l2t is a more meaningful value to represent the blowing characteristic of a fuse at such a short blow time lt is a value proortional to the let-through energy (the integral of square of current against time,with unit A2sec)of a fuse before it blows at high overload currentt with blow tine less rhan8-10ms.
I. Pulse cycle withstand capability：
When the devices turnde on or off,surge current will occur in the circuit and the surge aurrent will do harm to the life of fuse,in order to avoid this damage cauwed by turn on/off,we select the ajppropriate rated current according to the following chart(2).
J. Pulse waveform：
When various switch power supply being invented,the surge form surge current or operating current form will bring out Different energy，the designer should learn waveform of surge current or operating currert in the certain circuit,and then select the proper rating current of fuse for the device chart (3).
