j SEMENS DESCRPTON CurrentLimiting Fuses 2.4 and 4.8 Kv for Motor Starting Application The fuse (Fig. ) is a totallyenclosed currentlimiting fuse, designed for motorstarting applications. t is available in basic voltage ranges of 2.4 and 4.8 kv with current interrupting ratings of 8, amperes rms asymmetrical. The fuse consists of silver elements surrounded by inorganic sand, all enclosed in a strong polyester base insulating tube. Circuit connection is made by silverplated copper ferrules on each end of the tube. The silver elements are internally connected to the ferrules. When the fuse is mounted, low resistance is maintained in its supports by the completely silverplated ferrules. A mechanicaltype indicator projects from the lower end of the fuse when the fuse elements melt. The indicator is operated by a mechanism (separate from the fusible elements) that releases the indicator only after the fuse elements have melted. current designations 2R through 36R (Table ) are used to distinguish one size of fuse from another in the same voltage rating. Ampere ratings are not associated with the fuses, since fuse selection is determined by application requirements. These requirements involve both motorstarting current and timecurrent characteristics of the thermal overload relay. FUSE APPLCATON For shortcircuit protection, high voltage motor starters generally employ either fuses or circuit breakers. The standard fuses that are presently available can be classified as current limiting or noncurrent limiting. On short circuit, currentlimiting fuses reduce tne current before the first current peak (Fig. 2) and open the circuit within the first first cycle. The fuse elements melt before the peak value of available shortcircuit current is reached. Noncurrent limiting fuses open the circuit at least one or two cycles after the inception of the short circuit, allowing the peak current to pass. Practically all fuses will melt in considerably less than onehalf cycle on a 6cycle basis when subjected to high values of fault current. Because the arc is the conductor, the current will reach its maximum crest value unless some provision is made to reduce the magnitude of the arc current. When such provision is made, the fuses are classified as current limiting. For the sake of standardization, all fuse interrupting ratings are on the basis of maximum rrns current that will flow in the first cycle after the short circuit occurs (refer to Table ). These are the currents that would flow if the fuse did not limit the current. All types of fuses operate rapidly at, or near, their interrupting ratings. Because of their fast operating time, fuses are generally employed as the last circuit protective device in each voltage level of a primary power system. Typical applications are in motor starters (Figs. 3 and 4). DESCRPTON ceo 683 Page EFFECTVE FEB.. 98 Supersedes issue dated July, 8 Fig. SiemensAllis fuse and fuse clips. Currentlimiting fuses are preferred for motorstarting circuits because t; f their fast operating time and currentlimiting ability. n cases where coordination is not required, noncurrent limiting fuses which have longer time delays may be used. However, in getting this longer time delay, prevention of damage to the circuit when shortcircuit passes is sacrificed. Refer to Fig. 8 for the currentlimiting ability of the fuse. CURRENTf FUSE MELTNG WAVE OF AVALABLE SYMMETRCAL SHORT CRCUT CURRENT Fig. 2 Operating characteristics of currentlimiting fuse. www. ElectricalPartManuals ElectricalPartManuals. com com
Page 2 CURRENTLMTNG FUSES 2.4 and 4.8 Kv for Motor Starting Application EXPLANATON OF TYPE DESGNATONS Current Designation: Example: Fuse Type: r+r+r TYPE FM MOTOR STARTNG FUSES.. MOTOR STARTERS Number of elememts 4R /" Cross sectional area 48FM4R4 / Fuse tube size Nominal voltage kv Size 4 are single barrel fuses. Size 5 are double barrel fuses. Size 6 are triple barrel fuses. TRANSFORMER FUSE Fig. 3 Typical motorstarting fuse application. TO LOAD Current designation Fuse, Motor starting Fig. 4 SpaceMaker motor controller with Size 5 www. ElectricalPartManuals. com com double barrel fuses. }
CURRENTLMTNG FUSES 2.4 and 4.8 Kv for Motor Starting Application Page 3 Currentlimiting fuses have two important advantages over the noncurrent limiting type: () Because of fast operation, they limit the extreme dynamic and electrical stresses associated with the flow of shortcircuit current. (2) They limit the shortcircuit current magnitude to far less than the available shortcircuit current. n selecting fuses, all factors affecting a particular application will be considered. t will be necessary to know the motor fullload current, the motor rotor current and the overload relay characteristics to assure a properly coordinated installation. Other important factors are as follows: Voltage rating Current rating Frequency rating nterruption rating FUSE CHARACTERSTCS Coordination Location Mounting Motor data Most fuses are selfprotecting. This means that they are capable of extinguishing the arc for any value of current within their interrupting capacity rating current limit. Currentlimiting fuses for motorstarting service are de signed to carry low overload currents for long time periods to permit repetitive motor starting or jogging. This type of fuse must be used in conjunction with a control contactor, or similar device, which will interrupt currents that would require over seconds to melt the fuse. f this is not done, the entire fuse unit could become so overheated that it could not interrupt the current when the fuse finally melted. See Table for minimum interrupting ratings of fuses. Figure 5 is an osciuogram showipg the letthrough current, melting and clearing times for a typical interruption. The interrupting ability of FM fuses has been tested in accordance with AlEE and NEMA standards. The tests range from currents near the minimum flowing current to available currents up to 8, rms amperes, all with maximum design voltage. f the fuse had not been in the circuit to interrupt the fault, the current would have reached 8 ka rms(and approximately ka crest value) in the first loop of current. Fuse timecurrent operating characteristics are given in terms of the melting time for a given value of current. Unfortunately, this method has not been universally accepted. Standards do not specify whether they should be plotted on the basis of shorttime, minimum melting, maximum melting, or total clearing time. On the other hand, timecurrent characteristics of the fuses can be derived from given characteristics by adding or subtracting allowances to curves shown in Figs. 6 and. Figure 6 shows total clearing time curves and Fig. shows mimimum melting curves. The currentlimiting characteristics of the fuse is shown in Fig. 8. This curve shows the maximum instantaneous current peaks permitted by each fuse current designation for various available currents. MELTNG TME.3 () NTERRUPTNG TM 4.48 ( www. ElectricalPartManuals. com com Fig. 5let through current, melting and clearing times for typical nterruption.
Page4 ) 6 v looo 6 soo 3 9 6 so 3 : 8. 3 J:.. 9 6 OS o 3 3.tO SO 6 89 o 5 6 " 9 C " Baals for data: CURRENT UMTNG FUSES 2.4 and 4.8 Kv for Motor Starting Application CURRENT N AMPERES.. g g g g g g 88 W Cl a: a: a: a: a: a: N N C") CD ) c c,? ::>.,...,... l l. i CURRENT N AMPERES Fig. 6 Clearing Time.. Tests are made at 55 volts ac at low pf, starting at 25C with no initial load. 2. Curves are plotted to minimum test points. N a: a: CD P"N, l a: CD C") fl. r.. l r N " r... ". www. ElectricalPartManuals ElectricalPartManuals. com com <( L soo OO 3 9 6 5 o 3 9 8 : s m i. = 3 9 8 9 8 6 OS OA ;
CURRENTLMTNG FUSES 2.4 and 4.8 Kv for Motor Starting Application..... CURRENT N AMPERES g g g. 8 g g gg c ). JO OOO 5 6 9 3 4 5 6!9 9( 6 5 JO 2 9 2 9 s ) 2.9..2 9 8 6 5 4 OJ 2.)., 8 9 3 4 5 6 89 Fla. Minimum melting. Baals for data:. Tests are made at 55 volts ac at low pf, starting at 25C with no initial load. 2. Curves are plotted to minimum test points. a: N a: Cf) a: a:a:a: a: a: a: N CD CD "t CD ).... N C"), OwO o o o o 8ooo "l... <...., Q T CURRENT N AMPERES! l. o og g g gggg g ooogo. l l ft, T oo co o o oo oo oooooo.,., e "., o o g D g PageS "" " CD D 9 8 6 5 < 3 2 9 8 6 so < JO 8 :: s i ) = 8 9 8 6 < OJ OJ,. www. ElectricalPartManuals. com com
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.. to 4 to so 4 lo 2 5 3 2.t.....5.4.l.2.....5.4.cu.. 2 2R ll 2 CUrrent n Amperes x T CURRENT CHARACTERSTC CURVES 4R Current Limiting Fuses 24 & 48 Volts (2R24R) Type A2R Current Limiting Fuses 2 Volts (3R8R) T R 9R R! 8R rr. i l T THESE FUSES ARE DESGNED TO NTERRUPT SHORT CRCUT CURRENTS GREATER THAN OR EQUAL TO T(o<A T SHOWN AT THE SECOND MNMUM MELTNG TME PROTECTVE DEVCES N SERES MUST BE COORDN ATED WTH FUSE CHARACTERSTCS TO NTERRUPT LOWER CURRENTS Current n Amperes x SEMENS ENERGY & AUTOMATON, NC. RALEGH, NORTH CAROLNA Current Characteristic CurvesMinimum Melting! 4 to so 4 2 5 J.....5.4.J.2..Oil.oe..5 4 OJ 2 E u U).5 www. ElectricalPartManuals ElectricalPartManuals. com com u E i=
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