Someone in YouTube asked a question: “Why in a DC circuit, there are two fuses used?”

It’s not terribly traditional to visualize two fuses in an exceedingly DC circuit. 

● The fuse, as we know, it’s AN overcurrent device once there's contact, once the short current is far larger perhaps ten times quite operation current, by this point, we'd like the sturdy breaking capacity， quick acting and current limiting protection, the opposite electrical components cannot reach these three performance at a similar time. Fuse, during this circuit, is that the main protection half, with the superb contact protection performance or unbroken because the contact backup protection device; Fuse is applied to stay the circuit in high responsibleness, longer life, as backup components to shield the most electrical parts(Semiconductors, Diodes, Thyristors, motors etc).

● OK，Let’s see, if it’s in DC circuit, why two fuses are needed? What’s the perform of every fuse?

● The DC circuit scenario is far advanced than AC circuit and it’s with higher demand on each fuses and different components. The fuse must be the DC fuse, with UL or IEC certification.

● Because inductance or capacitance in an exceedingly DC circuit, usually there'll be a line switch machine instant surge current, the surge current is also seven ~ ten times because the traditional operating current, pulse breadth and undulation on line, the surge current within the circuit switch machine can seem when, got to opt for the time delay fuse, looking on the extent of the opposing surge has middle level time delay fuse(M), traditional time delay fuse(T) and longer time delay fuse(TT), generally so as to shield the vital parts, like thyristor, IC, hope there's a fault current fuse quickly open the circuit, then you ought to opt for quick acting fuse(F) or (FF) ultra-fast acting fuse.

● So, if two varieties of fuse ar utilized in DC circuit, the simplest answer ought to be 1PC quick acting fuse(Semiconductor fuse) and 1PC time-delay fuse.

        The semiconductor fuse is employed for overcurrent protection;

        The time delay fuse is employed for overload protection.

Are you continue to confused: one semiconductor fuse ought to be enough, why a time delay fuse is needed?

● Here we'd like to speak concerning the key characteristics of a fuse: Antidelay characteristic

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  It’s: Once Overload current is little, the melting time is long;

       When the overload current is greater and larger, the melting time can a lot of less and fewer. therefore if in an exceedingly sure current scale, once the abnormal current scale back to traditional one, the fuse won't scrap, will still use.

      But, once the present is continuous larger however not reach the quick acting fuse purpose|freezing point|temperature}(Fast acting fuse melting point is 4~7 times of operation current), as an example, 2 times, the parts body will gather the energy, the body is hoter and hoter, this additionally do great harm to components. by this point, a  time- delay fuse can take the responsibility: 

There are several blessings to victimisation these fuses. not like single-element fuses, the Cooper Bussmann dual-element, time-delay fuses may be sized nearer to produce each high performance contact protection and reliable overload protection in circuits subject to temporary overloads and surge currents. For AC motor hundreds, a single-element fuse might have to be sized at three hundredth of AN AC motor current so as to carry the beginning current. However, dual-element, time-delay fuses may be sized a lot of nearer to motor hundreds. for example, it's usually attainable to size Fusetron dual-element fuses, FRS-R and FRN-R and Low-Peak dual-element fuses, LPS-RK_SP and LPN-RK_SP, at a hundred and twenty fifth and one hundred and thirtieth of motor full load current, severally. Generally, the Low-Peak dual-element fuses, LPJ_SP, and CUBEFuse™, TCF, may be sized at one hundred and fiftieth of motor full load amps. This nearer fuse size might give several blessings such as: 

(1) Smaller fuse and block, holder or disconnect amp rating and physical size, 

(2) Lower price because of lower amp rated devices and presumably smaller needed panel house, 

(3) Higher contact protection – less shortcircuit current let-through energy,

(4) Potential reduction within the arc-flash hazard. once the short-circuit current is within the current-limiting vary of a fuse, it's inconceivable for the complete out there short-circuit current to flow through the fuse – it’s a matter of physics. the little restricted parts of the contact component quickly vaporize and also the filler material assists in forcing the present to zero. The fuse is in a position to “limit” the short-circuit current. Overcurrent protection should be reliable and certain. whether or not it's the primary day of the electrical system or thirty, or more, years later, it's vital that overcurrent protecting devices perform underneath overload or contact conditions as supposed. trendy current-limiting fuses operate by terribly easy, reliable principles.

Figure 6. this can be the LPS-RK100SP, a 100A, 600V Low-Peak, Class RK1, dualelement fuse that has wonderful time-delay, wonderful current-limitation and a three hundred,000Ainterrupting rating. inventive liberty is taken parenthetically the interior portion of this fuse. the important fuse features a non-transparent tube and special tiny granular, arcquenching material fully filling the interior house.

Figure 7. actuality dual-element fuse has distinct and separate overload component and contact component.

Figure 9. contact operation: trendy fuses ar designed with minimum metal within the restricted parts that greatly enhance their ability to possess wonderful current-limiting characteristics – minimizing the contact let-through current. A short-circuit current causes the restricted parts of the contact component to vaporize and arcing commences. The arcs burn back the component at the points of the arcing. Longer arcs result, that assist in reducing the present. Also, the special arc conclusion filler material contributes to conclusion the arcing current. trendy fuses have several restricted parts, which ends in several tiny arclets – all operating along to force the present to zero.

Figure 10. Short circuit operation: The special small granular, arc-quenching material plays an important part in the interruption process. The filler assists in quenching the arcs; the filler material absorbs the thermal energy of the arcs, fuses together and creates an insulating barrier. This process helps in forcing the current to zero. Modern current-limiting fuses, under short circuit conditions, can force the current to zero and complete the interruption within a few thousandths of a second.