Page 966 - Industrial Power Engineering and Applications Handbook

30/916 Industrial Power Engineering and Applications Handbook

m in view of its rigidity and high conductivity. With the
R.m2 easier availability of aluminium and being more viable
economically, aluminium i, now preferred wherever
or - etc. possible. It is employed particularly where the metal has
i2.m to simply carry power such as for the transmission and
The resistivity and conductivity of standard annealed distribution of power at any voltage and as the main
copper and a few recommended aluminium grades being current-carrying conductor in power distribution or control
used widely for electrical applications are given in Table equipment, such as a bus system or a switchgear assembly.
30.1. Their corresponding current-carrying capacities in Similarly, it is also used to feed high currents to an
per cent, with respect to a standard reference (say, 100% induction or a smelting furnace, electroplating plant or a
IACS) are also provided in the table. rectifier plant. For main current-carrying components,
however, as required for switching or interrupting devices
30.1.4 Measuring the conductivity (breakers, switches, fuses, contactors and relays) copper
For this purpose, a simple conductivity meter based on and copper alloys are preferred. The alloys are compact
the principle of eddy current may be used for a direct in size and are a much harder metal, suitable for making
reading of conductivity. The meter operates on the basis and breaking contacts frequently and yet retaining their
of relative variance, in the impedance of the test piece shape and size over long years of operation. Copper is
compared to the reference standard piece of aluminium also used for low ratings, up to 100 A or so, required for
or copper having a conductivity of 100% or 31.9 the internal wiring of power and control circuits in a
m/Rmm2 for aluminium and 58.0 m/Slmm2 for IACS switchgear or a controlgear assembly, where the wires
(International Annealed Copper Standard) in terms of have to bend frequently. Aluminium, being brittle, is
conductivity unit. The test probes, that sense the impedance unsuitable for such applications. The use of copper is
of the test piece, induce an eddy current in the test piece at also recommended for areas that are more humid and
a fixed frequency. The magnitude of this current is directly chemically aggressive which may corrode aluminium
proportional to the conductivity of the metal. This eddy quickly. As aluminium is highly oxidizing and a very
current develops an electromagnetic field around the test susceptible metal to such environments it may loosen at
piece and varies the impedance of the test probe (skin the joints. Typical locations are mines, ships, textile mills
effect). The conductivity is thus determined by measuring and chemical and petrochemical processing units. But
the corresponding change in the impedance of the probe. for such applications also, the latest practice is to instal
Figure 30.2 shows a simple and portable conductivity electrical equipment and switchgears in separate rooms,
meter. away from the affected areas, thus making it possible to
use aluminium. In the following text more emphasis is
laid on the use of aluminium, as it is the preferred current-
30.2 Current-carrying capacity of carrying metal;
copper and aluminium Below we give the recommended sizes and ratings:
conductors
Copper wiredcables refer to Table 13.15 for current
Earlier practice was to use copper in most applications ratings up to 100 A, as recommended for the internal
wiring of a power switchgear or a controlgear assembly.
Copper solid conductors: Tables 30.2 and 30.3 for
general engineering purposes.
Aluminium solid conductors: Tables 30.4, 30.5, 30.7,
30.8 and 30.9 for general engineering purposes.
The following factors must be taken into account while
deciding on the most appropriate and economical sections
of the metal conductors for thc rcquired current rating;
For the same thickness, a smaller cross-section will
have a relatively higher heat-dissipating area compared
to a larger cross-section. The latter therefore will have a
higher deration compared to a smaller cross-section on
account of poorer heat dissipation. This can be illustrated
as follows.
Consider a 25.4 x 6.35 mm conductor with a cross-
sectional area of 25.4 x 6.35 mm2 and a surface area of
2 (25.4 + 6.35) x I (1 being the length, in mm) = 63.51
mm2.
A conductor with twice the width (i.e. 50.8 x 6.35
mm) will have a cross-sectional area of 50.8 x 6.35 mm2,
and a surface area of 2 (50.8 + 6.35) x 1 = 114.31 mm2.
Thus the larger section having twice the cross-sectional
Figure 30.2 Conductivity meter (Courtesy: Technofour) area in the same thickness will have 114.3/(2 x 63.5) or