Diapositiva 1 - A. Malignani

Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Istituto Tecnico Industriale A. MALIGNANI - UDINE
Current carrying capacity
of conductors
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
All cables have electrical resistance, so there
must be an energy loss when they carry current
and this loss appears as heat…
…and the temperature of the cable rises.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
The rate of heat loss is a function of the
difference between the conductor and the
surrounding temperatures, so as the conductor
temperature rises, so does the rate at which it
loses heat.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
A cable carrying a steady
current, which produces a fixed
heating effect, will get hotter
until it reaches the balance
temperature where heat input is
equal to heat loss (see figure).
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
The final temperature achieved by the cable will thus depends on
the current carried, how easily heat is dissipated from the cable
and the temperature of the cable surroundings.
P.V.C. is probably the most usual form of insulation, and is very
susceptible to damage by high temperatures. At about 80°C the
material becomes soft, at about 115°C P.V.C. becomes corrosive
to cooper and aluminium.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
It is very important that P.V.C.. insulation should not be
allowed normally to exceed 70°C, so the current ratings
of cables are designed to ensure that this will not
happen; higher temperatures (up to 160°C) are
permitted under very short term fault conditions.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
It should be noticed that the cable voltage drop will be
higher if more current is carried.
Insulation cannot tolerate high temperatures since they
will lead to deterioration and eventually failure.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
There are different Tables and columns for different types
of cables, with differing conditions of installation, degrees
of grouping and so on.
For example, mineral insulation
does not deteriorate, even at very high temperatures.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Methods of cable installation
We have seen in the previous subsection that the rating of a
cable depends on its ability to lose the heat produced in it by
the current it carries.
This ability depends to some extent
on the way the cable is installed.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
For example, a cable clipped to a surface will more
easily be able to dissipate heat than a similar cable
which is installed with others in a conduit.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Determination of the current-carrying
capacity value
According to the type of cable installation, tables T3/1
and T3/2 make it possible to determine the currentcarrying capacities in continuous operation, defined in
the standard as lz, in the copper or aluminium
conductors and cables in free air and in ground
installation in compliance with the IEC 3645-523
Standard.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Determination of the current-carrying capacity value
When there are several circuits close together, or ambient
temperatures other than the standard values (30°C for
installation in free air and 20°C for installation in the
ground), the capacity values shown in table T3/2 must be
multiplied by the correction factors indicated in the tables
of paragraphs 3.2. and 3.3.
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Now I’ll show you a simple example of calculation...
Ing. Stefano Comuzzi I.T.I. Malignani - Udine
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
And now...
Thank you !
Ing. Stefano Comuzzi I.T.I. Malignani - Udine