6
How does a connection behave
over time?
Cembre's goal in the design and manu-
facture of its connectors is to come as
close as possible to the optimum condi-
tion for continuous, intact conductors.
A correctly executed connection, when
it is made, has characteristics practi-
cally similar to the intact conductor, but
over time it suffers from "ageing", to a
greater or lesser extent, depending on
the types of terminals used and their in-
stallation.
The most obvious effect of this aging
is an increase in electrical resistance,
which corresponds to an increase in
temperature.
This situation does not cause any prob-
lems in a connection up to standard
made to quality terminals that have
been installed correctly.
If, on the other hand, the connection
has been made with poor quality ter-
minals or without respecting their con-
nection capacity, a critical condition is
created that develops over time as it
does not trigger any system protection
intervention (fuses or magnetothermal
switches).
In practice, the connection progressive-
ly heats up until it reaches the critical
temperature of the materials insulat-
ing the cable or terminal .
Connections of different voltages or
currents may come into contact with
each other, giving rise to short circuits
or ignition caused by particles of insu-
lating material at high temperatures in
contact with flammable or combustible
material.
This is what is often diagnosed by fire
fighters as "Fire due to electrical short
circuit".
The method commonly used to evaluate
the behaviour of a connection over time
is to subject it to an ageing test with
thermal cycles.
The diagram and the graph opposite
(Fig. 1 and Fig. 2) refer to a compar-
ative test with thermal cycles between
indirect clamping ZETA terminals and
direct clamping terminals.
In our case, we used PVC insulated con-
ductors of 6 mm² section
, ZETAmini
type Z6-1 indirect clamping terminals
and direct clamping terminals with
brass bushing and steel screw
; the
maximum connection capacity for both
types is 2x6 mm². In the circuit series
a current of 35 A was made to circu-
late, such as to generate
a tempera-
ture of 70 ° C on the reference conduc-
tor
.
For each cycle, the power supply last-
ed 30 minutes, followed by 30 minutes
of forced cooling to room temperature;
the temperatures of the two terminals
and conductor were measured at reg-
ular intervals. Overall, there were 150
cycles.
The graph shows how the direct clamp-
ing terminal
undergoes a significant
temperature increase already from the
first cycles until it reached, after about
100 cycles, the critical temperature of
softening of the conductor's PVC insula-
tion, which is 80°C , and at the end of
the test it had reached about 100°C.