Handbook of Electrical Installation Practice - BeKnowledge

446 Handbook of Electrical Installation Practice
account is taken of the foregoing statement that in the vast majority of applications
the fuse disconnects in times less than 1s. The other important point is that, if protection
is afforded to the protective conductor by a fuse under these conditions, it
is assured on higher fault currents.
Even in motor circuits where the fuse is providing back-up protection and is
therefore usually about twice the rating of the phase conductor, it can be shown that
the use of the adiabatic formula results in smaller sizes of protective conductor than
those specified in Table 54G.
One of the difficulties is applying the adiabatic formula is that it requires a knowledge
of Ze which is the value of earth loop impedance external to the installation.
Where this value is known it is a simple matter to use the adiabatic formula. In those
situations where the precise value of Ze is not known, it can be shown that even
where the value of Ze is as high as 50% of Zs, appreciable economies can be gained
by using the adiabatic formula.
The foregoing confirms that on fuse-protected installations on TN systems, existing
practice usually achieves compliance with the requirements for protection
against electric shock by equipotential bonding and automatic disconnection of
supply. This is applicable for fixed circuits, where the fuse is either giving complete
overcurrent protection to a circuit or back-up protection to motor circuits, and also
to circuits feeding socket oulets, except those socket outlets feeding equipment
outside the equipotential bonded zone.
DOMESTIC FUSE APPLICATIONS
The fuse-links used in domestic installations are those complying with BS 1361 and
BS 1362. Their characteristics are included in BS 7671 together with those of semienclosed
fuses to BS 3036.
Fuse-links to BS 1362 for use in the 13A plug are well established and widely
used both in the UK and overseas, and the type II fuse-links of BS 1361 are the
standard house service cut-out fuse-links used by the Electricity Association (EA).
The BS 1361 fuse-link type I range (5A to 45A), provides a non-interchangeable
range of fuse-links for use in domestic consumer units. They were initially launched
in 1947 but due to lack of availability of replacement fuse-links, and the lack of effort
in educating the general public, who had previously only been used to semi-enclosed
fuses, the system was abused and the anticipated growth of this type of consumer
unit was never achieved. It is interesting to note, however, that the fuse-links to BS
1361 which are used in domestic consumer units, have a breaking capacity of
16.5kA which is the maximum fault level specified for domestic installations in the
UK and discriminates with the 100A house service cut-out fuse.
Furthermore, the BS 1362 fuse-link which is fitted in the BS 1363 plug is apparently
used without any noticeable abuse, because the general public is now well
educated in the use of cartridge fuse-links. The requirements of BS 7671 may well
see an increase in the use of domestic consumer units incorporating cartridge
fuse-links.
With the introduction of BS 7671, which required the determination of the fault
level at the terminals of an installation, the EA declared that the fault level at the