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Free «Fuses» Essay Sample

Automatic Circuit Recloser

A self controlled device for interrupting light automatically and reclosing the alternating current circuit, with a programmed sequence of closing and reopening is called an Automatic Circuit Recloser. The sequence is often followed by lockout, hold closed or resetting.

Similar to a circuit breaker, light interruption here occurs at a natural current zero but its interruption medium is commonly oil or a vacuum. The insulation medium for this device is commonly SF6, a solid dielectric, air, or oil. The recloser control can always be hydraulic, electronic or electromechanical (when the relay for tripping is electromechanical with an electronic reclosing control). A hydraulic recloser device has springs with a hydraulic system for actuation and timing. The recloser interruption is based on a symmetrically assigned current rating that is steady and does not change in voltage. An exception here lies on some recloser devices with higher interrupting current when operated at a considerably lower voltage than the rating.

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Automatic Circuit Recloser has several distribution applications, it can be found as a feeder interrupter in substation and not as circuit breakers. Following a survey done by IEEE, 51% of stations were found to be using feeder interrupting devices as reclosers (IEEE Working Group on Distribution Protection, 1995). Reclosers are often used in smaller stations while circuit breakers are used more in larger stations. On the main feeder, three-phase reclosers can be used to provide essential protection cover on longer circuits. The three-phase recloser is also essential because it improves reliability. Apart from feeder interrupters and circuit breakers, automatic circuit reclosers can be found as overhead units and padmounted units. Reclosers can also be found as single-phase units that are used on single-phase taps in place of fuses.

Some reclosers have features targeted to distribution target needs because the reclosers are built for distribution circuits. The unit has three phases that operate individually on each face to have a single face fault opening one phase. Sequence coordination is found on some reclosers and is used to enhance coordination between numerous devices.

Expulsion Fuses

Expulsion fuse is a common protective device on distribution circuits because it is a low cost interrupter that is easy to replace especially when in cutout. Interruption here is fast and happens in half a cycle even for larger currents. Expulsion fuse a simple fusible element made up of silver or tin. The fuse often melts under high current. Expulsion fuses are often applied in a fuse cutout in a fuse tube where after the element in the tube melts, an arc will remain. The arc formed contains a considerable energy that causes pressure to pileup. The pressure will force much of the ionized gas out of the bottom of the cutout and thus helps stop the element from reigniting at a zero current. The arc is also cleared at a current zero when dialectic strength of the air is increased through extreme pressures, turbulence and through stretching of the arc in the fuse. An organic fiber liner that melts under the arc’s heat is also present in the fuse tube. The liner emits fresh non ionized gas that helps prevent ignition. Expulsion action always predominates at high current while deionization gases increases the dielectric strength at a lower current. ‘Expulsion’ characteristic of a fuse should be considered when putting a cutout on a structure. Placement of the cutout should be avoided in a place where hot ionized gas blows because the cutout could cause a flashover on other energized equipments or other phases of cutouts. Safety procedures should be implemented and enforced whenever a cutout is switched in. This is because the cutout could be switching into a fault. One should have arc resistant clothing, eye protection clothing and should avoid the bottom of the cutout.

The speed ratio of a fuse indicates how steep a fuse curve is. It is often defined differently depending on the size of the fuse (IEEE Std. C37.40-1993).

Speed ratio for fuse ratings of 100 A and under =     while

Speed ratio for ratings above 100 A =

There are two types of expulsion fuses specified for industries by their standards. ‘K’ link is a relatively fast fuse while ‘T’ is slower. K link has a speed ratio of 10 to 13 and it is the most commonly used fuse for line taps and transformers. T and K fused are standardized such that the fuses are interchangeable among manufacturers for different applications.

Expulsion fuses are published by two time-current curves for the minimum melt curve and the maximum total clear curve. For the minimum, the melt time is at 90% of the average melt time. This accounts for manufacturing tolerance where the total clearance time is the average of average melting time  arcing time + manufacturing tolerances.

Fuse Cutouts

 
 
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In a fuse interrupter, the cutout is an important part because it determines the continuous current capability, the basic lightning impulse insulation level (BIL), the maximum interrupting capability, the load-break capability and the maximum voltage of a fuse. Cutouts are available in a continuous rating of 100, 200, and 300. Most cutouts found are of open variety. The cutouts contain a removable holder put in a cutout with a brushing type support made of porcelain. Apart from open variety, there are also open-link cutouts and enclosed cutouts. Open link contain a fuse link suspension between contacts. It has a much lower interrupting capability of 1.2 kA symmetrical.

Majority of cutouts used on distribution systems do not have load break capability and if the cutout is opened under load, it can draw an arc that will not clear. It is a common practice for maintainers to open cutouts under load and in the event that it draws an arc, the cutout is slammed back in. Cutouts with load break capability are usually able to interrupt 100 to 300 A and usually use an arc chute.

Current-Limiting Fuses (CLF)

This is another interrupter with a unique ability of reducing the magnitude of fault current. CLFs are known for their high fault-clearing ability. Current-Limiting Fuses consist of fusible elements in sand silicon with a symmetrical maximum interrupt rating of 50 kA. This is much higher than expulsion fuses which have typical maximum interrupt ratings of 13kA in cutouts and 3.5 kA in oil. Just like fuse cutouts and expulsion fuses, Current-limiting fuses also contain the arc during operation. But in this case, the arc formation is noiseless with no pressure buildup experience. CLFs are widely used in high fault areas so as to protect equipments. The major reason for its use is safety followed by high fault current in excess of expulsion fuse ratings. To clear high-current faults, current-limiting fuses are normally used. The fuses have much harder time with overloads or low current faults. In low current faults, fusible elements will not melt but would be so hot that it can melt the fuse hardware leading to failure. This gives the reason why most common CLF applications are used as backups in series with expulsion fuse. Expulsion fuse would be used to clear low level faults while CLF will be used to clear high current faults. CLFs have steeper melting and clearing curves than expulsion links. When the fuses are used together, each fuse will have its own 12t rating. There are two stages of operating time in the fuses before they melt. The first stage is known as pre-arcing stage and the other is arcing stage. All the stages combined give the total energy dissipated in the operation to represent a complete clearance of the fault.

During pre-arcing, time is always inversely proportional towards the square of the current. On the other hand, during arcing stage, time is proportional towards the voltage. The 12t of the smaller fuse is normally called minor fuse and that of the larger fuse is major fuse. During performances, to eliminate deteriorations and positive discriminations among fuses, major value should always be higher than minor value.

Coordination approach

The major criterion for making coordination between two fuses is that total clearing time (TCT) for the main fuse should not exceed 75% of the minimum melting time of the backup fuse in the same current (short circuit). This is an important step because it ensures that the main fuse melts and clears before the back-up operates. 75% mark here represents several factors influencing the fuse elements. Such factors include when the system contain downstream fault in one branch making the short circuit current to pass through another fuse that is not supposed to melt. It will affect the backup fuse. The procedure for finding fuses’ coordinated approach (graph method) depends on drawing the time current characteristics (TCC) for two fuses. The drawings here should be made on log-log papers. This method is commonly used to create coordination between fuses instead of tables with data of fuses. Graph method is used to investigate effects of DG on fuses coordination. 

   

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