Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 111)

4.1.1.3 Complementary protection
The complementary protection effectively provides a third safety net with respect to the protec-
tion against electric shock and offers protection from direct and indirect contact.
Residual current protection equipment with response levels
≤ 30 mA shut down touch currents
before they reach a dangerous level for persons.
Voltage equalizing measures reduce the voltage of accessible parts in the event of a fault.
4.1.2 Protection against overload and excess temperature
Components can be thermally overloaded if they are subjected to operational overcurrents for
extended periods. This can be due to unexpected overloading or because the rated load
capacity of electrical equipment, such as the rating of motors or lines, is too low. Overload
currents do not lead to immediate danger or immediate outage. However they heat the electrical
equipment above the rated temperatures for continuous duty and reduce the life span of its
insulation.
The higher the overload current is, the faster the permissible limit temperature will be reached,
and the shorter is the permissible loading time (
Fig. 4.1-2 a). The limiting loading curve is
obtained by entering the times of permissible loading in a current-time diagram ( Fig. 4.1-2 b).
The task of the overload protective device is to allow operationally occurring overcurrents but to
switch them off timely before the permissible loading time is exceeded.
0
0.5
1
1.5
2
2.5
0 1 2 3 4
t
T
/T
gr
I/I
e
=1.0
1 1.2 1.4 1.6 1.8 2
I /I
e
t
zul
1.6 1.8 2.0
1.4
1.2
1.1
t
1.2
t
1.2
a) Heating at loading with rated b) Limiting loading curve
current (1.0
⋅ I
n
) and at various
overcurrent levels
(1.2
⋅, 1.5 ⋅, 1.7 ⋅, 2 ⋅ I
n
)
T
gr
Rated limit temperature t time
T temperature t
zul
permissible loading time
Fig. 4.1-2
Temperature rise characteristic of a body at various loading levels and limit loading, when its
limit temperature is not exceeded
4.1.2.1 Different loading curves of various kinds of electrical equipment
Various kinds of electrical equipment such as conductors in comparison to motors or electrical
equipment of the same kind with different rated powers have different limit loading curves due to
their differing masses. Thus a conductor with a small cross-section reaches its limit temperature
much quicker than a large motor with compact windings that are imbedded in the iron core.
These differences are also important for the performance at changing load, when heating
phases are followed by cooling phases and the final temperature of one phase represents the
initial temperature of the next phase. Deviations in the replication of the thermal performance of
the protected object by the protective device can thus add up and lead to premature tripping or
failing of the protection (
Fig. 4.1-3 ).
LVSAM-WP001A-EN-P - April 2009 4-3