● They are not guaranteed operate until twice the rated cur-
rent flowing. This less than the time taken blink eye. The fuse element encased a
glass ceramic tube and secured end-caps which are firmly attached
Screws
securing
fuse element
Fuse
carrier
Fuse
element
FIGURE 12.Overcurrent, short circuit and earth fault protection
241
The British Standard includes ratings 100A and maximum fault
capacities 9kA.1
A semi-enclosed fuse. The fuse element should consist
of single strand plan tinned copper wire having diameter appro-
priate the current rating the fuse.
● The circuit cannot restored quickly since the fuse element requires
screw fixing.
By definition fuse the weakest link the circuit.
● easy identify ‘blown’ fuse. This type fuse was very popular in
domestic installations, but less these days because their disadvantages. Under fault conditions
it will melt when overcurrent flows, protecting the circuit conductors
from damage.
.1.
● The fuse element weakens with age due oxidization, which may
result failure under normal operating conditions.
● There danger from scattering hot metal the fuse carrier is
inserted into the base when the circuit faulty. They are graded according their instantaneous
tripping currents that is, the current which they will trip within
100ms. The fuse element is
connected series with the load and the thickness the element suffi-
cient carry the normal rated circuit current.
Cartridge fuses (BS 1361)
The cartridge fuse breaks faulty circuit the same way semi-
enclosed fuse, but its construction eliminates some the disadvantages
experienced with open-fuse element.
DISADVANTAGES SEMI-ENCLOSED FUSES
● The fuse element may replaced with wire the wrong size either
deliberately accident.
Definition
By definition fuse the weakest link
in the circuit.
● There are mechanical moving parts. When fault occurs over-
current flows and the fuse element becomes hot and melts ‘blows’. Under fault conditions it
will melt when overcurrent flows,
protecting the circuit conductors
from damage.
Semi-enclosed fuses (BS 3036)
The semi-enclosed fuse consists fuse wire, called the fuse element,
secured between two screw terminals fuse carrier.
This type fuse illustrated Fig. 12.
● They have low breaking capacity since, the event severe fault,
the fault current may vaporize the fuse element and continue flow
in the form arc across the fuse terminals.
ADVANTAGE SEMI-ENCLOSED FUSES
● They are very cheap compared with other protective devices both to
install and replace
