Safety fuses: general information
Thermal action of current is widely used for protection of electrotechnical devices and lines of transfers against razrushitelyyony action of short circuits and long overloads.
Are sewn up with the simplest device of such thermal the safety fuse is. In it serves mute the basic element of protection the fusible insert — the replaced part of the predokhrayonitel melting when current intensity in the protected chain prevyyosit the defined value. In essence, it is the short site of the protected electric circuit, rather easily razrushayeyomy thermal action of current. To receive such ponizhenyony thermal stability, the fusible insert is made of material with rather high specific soprotivleniyoy (for example, alloy of tin and lead) or of well provodyayoshchy metal (for example, silver, copper), but with rather mayoly section.
At burn-out of the safety lock in it is mute not there has to be the arch, therefore, the fusible insert has to have length, soyootvetstvuyushchy to the switched-off tension; for this reason on safety locks, except rated current, it is specified as well tension.
In the constructive relation safety fuses are carried out in the most different way.
Approximately up to 60 A and nizyoky tension use plug safety locks (fig. 1) at which the fusible part is absolutely closed to currents. In the basis of 1 safety lock ввертывается the insert 2 replaced at burn-out, the so-called stopper; current passes through the screw thread of the stopper, the fusible wire 3 in it and the contact screw 4 in day of foundation.
In houses safety locks focus on gruppoyovy guards. The last serve as the place of the branching of wiring: from them wires disperse in separate constructions, parts of the building or room of the apartment. Each line is protected here by otdelyyony pair of safety locks (fig. 2). Such device facilitates supervision of safety locks and change of traffic jams at their burn-out.
Sometimes instead of the fused stopper insert several copper veins which number in the bolyyoshinstvo of cases undertakes accidentally into the basis of the predokhraniyotel.
In such conditions at korotyoky short circuit in the apartment the fire or from pereyogrety current of the wiring cord which is not protected by the safety lock, or from spraying of metal around the safety lock can break out.
Lamellar safety locks (fig. 3) are widely used to plants of low tension. In them the fusible part, the insert 2, consists of several pro-reducing dies (or plates) supplied with contact nakonechniyoka 3 which serve for fixing of the insert by means of screws 4 on the isolating fire-resistant basis 1. For protection against splashes of the melted metal (at the insert peregorayoniya) the insert is closed by the casing 5.
In installations of high tension trubchayoty safety locks in which the melting wire is placed in the porcelain tube are used and has znachitelyyony length. The arch arising when melting the wire in the tube quickly is broken off because of draft of air in the tube, at the same time the tube does not give the chance to be sprayed to the melted metal.
Characteristic size for the fusible predokhraniteyolya is its rated current intensity, i.e. that preyodelny current intensity which the safety lock has to vyderzhiyovat indefinitely long time, without collapsing. This value of current intensity is specified on the safety lock insert.
What kayosatsya current intensities of melting of the safety lock, that its value zayovisit from the variety of reasons and first of all from nayogruzka duration current and safety lock refrigerating conditions.
On the dependence of time of burn-out of the fusible insert on so-called frequency rate of current, i.e. on the relation of the actual current intensity to its rated force is provided.
The safety fuse has to protect reliably installation wires from the dangerous overload, however it has to maintain short-term pushes of current at launches of engines, etc. Therefore at the choice of the safety lock it is necessary to know not только the rated current intensity of loading of the line, but also its character. If the line feeds engines, then it is necessary to take mean value of the starting current which approximately in 5 — 7 times more rated value of current of the engine as the basis for the choice of the predokhrayonitel.
Duration of starting process usually is about 5 — 10 sec.; during this time, according to fig. 23, the predoyokhranitel has to maintain the current intensity exceeding it rated approximately by 2,5 times. Therefore, the nomiyonalny current intensity of the safety fuse has to be equal (or more) 40% of the average starting current intensity of the engine, i.e.
In=1/2.5 x Ipusk =0.4 Ipusk.
However, when the rated znayocheniye of current intensity of fusible inserts chosen thus significantly exceed dopuyostimy long loadings of the protected conductors, then safety locks protect wires from short circuits, but not from long overloads.
If the line bears the quiet not oscillating load, without starting pushes, the rated current intensity of the safety lock of the dolzhyon to be equal to labor of current of the line. Though the fusible predokhrayonitel maintains also the heavy load, but is long nagruyozhenny over rated current intensity it strongly heats up and therefore it is unreliable in work.Top