Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 44)

class 10 (tripping between 4 and 10 s at 7.2 · I
e
) to trip are considered as heavy-duty starts. In
these cases overload protective relays with slower trip characteristics should be selected. See
also
1.7.1.2.1 .
In addition the load capacity of the switchgear should be checked.
The load capacity of contactors and circuit breakers without thermal release basically depends
on their size (cross-section/mass of the conducting parts). It therefore varies from device to
device. Up to a starting time of around 10 s for direct starting, the load of the devices during
starting needs not to be checked. Furthermore the admissible load capacity can be obtained
from the technical documentation (catalog, RALVET; see example
Fig. 2.3-6 ). Rest times
should be allowed between successive heavy-duty starts that provide sufficient time for the
switchgear (see RALVET) and the motor to cool down before the next loading.
Heavy starting and
regular short-time duty
1
10
100
10 100 1000 10000
Starting current / short-time current [A]
Ma
x
.
s
tar
ti
ng tim
e
/ load pe
ri
od
[s
]
09
/12/1
6
A
23
A
30/3
7
A
43
A
60
/72/8
5
A
95
A
210
A
250
A
300
A
420
A
180
A
140
A
110
A
Contactor size I
e(AC-3)
Fig. 2.3-6
Example of a loading diagram for contactors for heavy-duty starting of squirrel-cage induction motors
2.3.6 Life span
The life span of switchgear basically depends on the size of the load and the number of
switching cycles. Instead of a time span, with electromechanical switchgear reference is usually
made to the number of operations, as the ageing mainly depends on the stress during switching
and less on the on- and off-phases between. The maximum number of operations is usually
determined by the wear of heavily loaded components – in contactors, load switches and circuit
breakers especially of the contact system.
For switchgear, the mechanical and electrical life spans are separately defined. The mechanical
life span states the number of possible operations without electrical loading, while the electrical
life span states the number of operations for a certain size of electrical loading and a certain
utilization category.
In electronic devices, the life span is usually less dependent on the number of operations but
rather on the working temperature. Thus for example electrolytic capacitors (for example used in
power adapters) age more quickly at higher temperatures. This is why it is recommended to
install electronic devices in the cooler parts of switch cabinets.
Ageing is also a problem with fuses, especially in the context of switching of motors. Full-range
fuses (gL, gG) have a soldered joint for tripping in the overcurrent range that may age due for
example to repeated short-term melting. When using them with motors with the latters’ high
starting currents, it should therefore be ensured that the starting current does not raise the
LVSAM-WP001A-EN-P - April 2009 2-16