are designated as branch circuit fuses suitable for protection of distribution
systems, wiring, or equipment.
Because fuses are single-phase interrupters, they provide good protection
for single-phase circuits. However, for multiphase circuits, these single-phase
interrupters can cause problems such as single phasing, backfeeding, and
ferroresonance. Single phasing can be detrimental to motors owing to the
fl ow of negative-sequence currents, which can cause excessive heating of the
motor rotor, causing motor failure or reducing its normal life. The degree of
motor life reduction is a function of motor temperature and elapsed time
between single-phase occurrence and motor de-energization.
The term back feeding is used to describe the condition when fault current
continues to fl ow from the remaining energized phases, most probably at a
reduced value owing to the additional impedance that has been inserted in
the current path. The degree of fault current reduction will determine the
time of response of the fuses in the remaining phases. As fuse interrupting
time increases, the degree of damage also increases. Today’s switchgear
designs employing fuses as overcurrent protective devices use anti-single
phase protection features. The anti-single phase feature in the fused switchgear
open all three phases due to a single fuse blowing, thereby averting the
adverse effects of single phasing discussed earlier.