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FUSE APPLICATION GUIDE RULES

FUSE APPLICATION GUIDE RULES
Basic points to consider in installing fuses in electrical system network

1)  Cold load pickup -  after 15 minute outage,  200% for.5 seconds
                                                                         140% for 5 seconds
                                       after 4 hrs, all electric  300% for 5 minutes
2)  "Damage" curve - 75% of minimum melt
3)  Two expulsion fuses cannot be coordinated if the available fault current is great enough to indicate an interruption of less than .8 cycles.
4)  "T" - SLOW and "K" - FAST
5)  Current limiting fuses can be coordinated in the sub-cycle region.
6)  Capacitor protection:
  • The fuse should be rated for 165% of the normal capacitor current.  The fuse should also clear within 300 seconds for the minimum short circuit current.
  • If current exceeds the maximum case rupture point, a current limiting fuse must be used.
  • Current limiting fuses should be used if a single parallel group exceeds 300 KVAR.
7)  Transformer
  • Inrush - 12 times for .1 sec.
  • 25 times for  .01 sec.
  • Self protected - primary fuse rating is 10 to 14 times continuous when secondary breaker is used.
  • Self protected - weak link is selected to be about 2 1/2 times the continuous when no secondary breaker is used (which means that minimum melt is in the area of 4 to 6 times rating).
  • Conventional - primary fuse rated 2 to 3 times.
  • General Purpose current limiting - 2 to 3 times continuous.
  • Back-Up current limiting - the expulsion and CLF are usually coordinated such that the minimum melt I squared t of the expulsion fuse is equal to or less than that of the back up CLF.
8)  Conductor burn down - not as great a problem today because loads are higher and hence conductors are larger.
9)  General purpose - one which will successfully clear any current from its rated maximum interrupting current down to the current that will cause melting of the fusible element in one hour.
10)  Back up - one which will successfully clear any current from its rated maximum interrupting down to the rated minimum interrupting current, which may be at the 10 second time period on the minimum melting time-current curve.
11)  CLF - approximately 1/4 cycle operation; can limit energy by as much as 60 to 1.
12)  Weak link - in oil is limited to between 1500 and 3500 amperes.
13)  Weak link - in cutout is limited to 6000 to 15000 asymmetrical.
14)  Lightning minimum fuse (12T-SLOW), (25K-FAST).
15)  Energy stored in inductance = ½  Li^2
16)  The maximum voltage produced by a C.L. fuse typically will not exceed 3.1 times the fuse rated maximum voltage.
17)  The minimum sparkover allowed for a gapped arrester is 1.5 x 1.414 = 2.1 times arrester rating.
18)  General practice is to keep the minimum sparkover of a gapped arrester at about 2.65 x arrester rating.
19)  MOVs do not have a problem with CLF "kick voltages."
 
source: ABB Inc. HARD TO FIND INFORMATION ABOUT DISTRIBUTION SYSTEMS
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