Handbook of Electrical Installation Practice - BeKnowledge

448 Handbook of Electrical Installation Practice
characteristics are somewhat faster at the short time end of the characteristic than
the equivalent rating of fuse-links to BS 88: Part 2. Electricity Authority Standard
EA 12-8 also contains these characteristics as well as information on the coordination
achieved between these fuse-links and the h.v. fuse-links used in the
11kV and 6.6kV ring-main units.
A novel feature is the arrangement of the fuse tags. The fuse-links are accommodated
in fuse-carriers which engage with the fixed contacts of the supply
authority feeder pillar by means of a wedge action.
COMPACT FUSES TO BS 88: PART 6
With the international agreement now reached regarding standardised fuse characteristics,
an innovation which has been introduced into the UK is a range of compact
fuse-links and fuse-holders for applications up to 415V a.c. for use in industrial and
commercial applications.
This range of fuses extends up to ratings of 63A for applications on both 240V
a.c. and 415V a.c. circuits with a breaking capacity up to 80kA. The characteristics
align with the requirements of BS 88: Part 1 and they provide an economically
attractive alternative to fuses to BS 88: Part 2 and to mcbs, for commercial and light
industrial applications.
Figure 16.13 illustrates the dimensional benefits obtained from these compact
designs.
Consideration is being given to the introduction of a larger size of fuse-link and
fuse-holder with a current rating of 125A. This will form part of a revision to BS
88.
FUSE SWITCHGEAR
The combination of a switch and a fuse as a means of providing short-circuit protection,
the switching of loads (both steady and fluctuating) and means of isolation,
dates back as far as the introduction of the fuse itself, but obviously the design of
the modern types of such combinations meets the most onerous service conditions.
The standard covering these products is BS EN 60947–3. It is therefore worth examining
the relevant definitions from these documents.
• Switch (mechanical) A mechanical switching device capable of making, carrying
and breaking currents under normal circuit conditions which may include
specified operating overload conditions and also carrying, for a specified time,
currents under specified abnormal circuit conditions such as those of shortcircuit.
It should be noted that a switch may also be capable of making but not
breaking short-circuit currents.
• Disconnector A mechanical switching device which, in the open position, complies
with the requirements specified for the isolating function. It should be noted
that a disconnector is capable of opening and closing a circuit when either a negligible
current is broken or made, or when no significant change in the voltage