this results in a high resistance in series with the faults. This not only limits the magnitude of the fault
but also the energy. All this can happen in less than a half cycle.
CLFs are very good at interrupting high currents (e.g., 50,000 A). They historically have had trouble
(General Purpose, and Back-up) with low level fault currents and overloads, where the fuse tube melts
before the element (i.e., these two fuses are not considered to be ‘‘full range,’’ since they do not
necessarily interrupt low currents that melt the element). There are now ‘‘full range’’ CLFs in the market
(see Fig. 23.7).
The three types of CLFs are defined as follows:
.General purpose—A fuse capable of interrupting all currents from the rated maximum inter-
rupting current down to the current that causes melting of the fusible element in 1 h.
.Back-up—A fuse capable of interrupting all currents from the rated maximum interrupting
current down to the rated minimum interrupting current (Fig. 23.8).
.Full range—A fuse capable of interrupting all currents from the rated maximum current down to
any current that melts the element.
23.1.8 Rules for Application of Fuses
- 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- ‘‘Damage’’ curve—75% of minimum melt.
- Two expulsion fuses cannot be coordinated if the available fault current is great enough to
indicate an interruption of less than 0.8 cycles. - ‘‘T’’-SLOW and ‘‘K’’-FAST.
- CLFs can be coordinated in the subcycle region.
- 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.
FIGURE 23.8 Back-up CLF.