6 and fault occurs the appli-
ance, effective discrimination will achieved because the 15A fuse will
operate more quickly than the other protective devices they were all semi-
enclosed types fuses with the characteristics shown Fig. The formula is:
Z R
S Ω
Here the impedance the supply side the earth fault loop. fault current 200A
will cause 15A semi-enclosed fuse operate about 0. circuit designed this way would considered have
effective discrimination.
Earth fault loop impedance ZS
In order that overcurrent protective device can operate successfully,
meeting the required disconnection times, Regulations 411. 12. The value can be
obtained from the area electricity companies, but typical values are 0.4s. If
a circuit arranged shown Fig.
Security supply, and therefore effective discrimination, important
consideration for electrical designer and also requirement the IEE
Regulations. 12.3. Effective discrimination can achieved graded
protection since the speed operation the protective device increases as
the rating decreases.8Ω for
100A
30A
30A
30A
15A
Main
protection
fuse
Distribution
fuseboard
fuse
Local
protection
Faulty
appliance
FIGURE 12.5(b). The value the
earth fault loop impedance may verified means earth fault loop
impedance test described Chapter this book.2.2, that
is, final circuits not exceeding 32A shall have disconnection time not
exceeding 0.3 the IEE Regulations. achieve this, the earth fault loop impedance value mea-
sured ohms must less than those values given Appendix the On
Site Guide and Tables 41.1s, 30A semi-
enclosed fuse about 0.35Ω
for TN-C-S (protective multiple earthing, PME) supplies and 0.0s.6
Effective discrimination achieved graded protection. The
actual value will depend upon many factors: the type supply, the ground
conditions, the distance from the transformer, etc.5(b).2 and 41.4s and 60A semi-enclosed fuse about 5. This can seen Fig.Overcurrent, short circuit and earth fault protection
247
Effective discrimination protective devices
In the event fault occurring electrical installation only the pro-
tective device nearest the fault should operate, leaving other healthy cir-
cuits unaffected. 12.
