RULES FOR APPLICATION OF FUSES BASICS AND TUTORIALS

1. Cold load pickup. After 15-min outage 200% for 0.5 s, 140% for 5 s After 4 h, all electric 300% for 5 min
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 0.8 cycles.
4. ‘‘T’’-SLOWand ‘‘K’’-FAST.
5. CLFs can be coordinated in the subcycle region.
6. Capacitor protection:
. The fuse should be rated for 165% of the normal capacitor current. The fuse should also clear within 300 s for the minimum short-circuit current.
. If current exceeds the maximum case rupture point, a CLF must be used.
. CLFs should be used if a single parallel group exceeds 300 kVAR.
7. Transformer
. Inrush—12 times for 0.1 s.
. 25 times for 0.01 s.
. Self protected—primary fuse rating is 10–14 times continuous when secondary breaker is used.
. Self protected—weak link is selected to be about 2.5 times the continuous when no secondary breaker is used (which means that minimum melt is in the area of 4–6 times rating).
. Conventional—primary fuse rated 2–3 times.
. General-purpose current limiting—2–3 times continuous.
. Back-up current limiting—the expulsion and CLF are usually coordinated such that the minimum melt I2t 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 1 h.
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-s 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 A.
13. Weak link—in cutout is limited to 6,000–15,000 asymmetrical.
14. Lightning minimum fuse (12T-SLOW), (25K-FAST).
15. Energy stored in inductance = ½ Li2.
16. The maximum voltage produced by a CLF typically will not exceed 3.1 times the fuse rated maximum voltage.
17. The minimum sparkover allowed for a gapped arrester is 1.5#1.414¼2.1 times arrester rating.
18. General practice is to keep the minimum sparkover of a gapped arrester at about 2.65x arrester rating.
19. Metal oxide varistors (MOVs) do not have a problem with CLF ‘‘kick voltages’’.

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