Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 45)

temperature of this solder joint beyond a certain limit. Fuse manufacturers provide information
on the smallest fuses that can be selected in relation to given motor currents and starting times.
2.3.6.1 Prospective service life
The prospective service life of switchgear is the number of years, months or weeks that it should
complete under the foreseen service conditions in 1-, 2- or 3-shift operation without the re-
placement of
spare parts. It depends on the frequency of operation and the total number of
individual switching operations. For the latter in addition to the mechanical also the electrical life
span of the devices must be selected accordingly (see Section
2.3.6.3 ). The required parame-
ters can be determined by means of the below formulae:
n d h f n
Y Y D S tot
∗ ∗ ∗ =
d h f
n
n
Y D S
tot
Y ∗ ∗
=
d h n
n
f
Y D Y
tot
S ∗ ∗
=
n
tot
Total number of operations (life span)
f
S
Switching operations per hour
h
D
Operating hours per day
d
Y
Operating days per year
n
Y
Number of years (life span)
2.3.6.2 Mechanical life span
The mechanical life span of switchgear is the total number of possible switch operations without
electrical switch loading. It depends on the design, the masses moved, the forces and accelera-
tions occurring. Large load switches and circuit breakers operate with high contact forces and
large masses, and therefore have a comparatively short mechanical life span. On the other
hand, contactors operate with relatively small contact forces and thus achieve longer mechani-
cal life spans.
After the mechanical life span has expired, the devices must be replaced. This life span is only
rarely achieved during the foreseen service life. In a few cases, where the complete mechanical
life span has to be used, it should be ensured that it is not reduced by adverse ambient
conditions, installation position and – in the case contactors – by an excessive control voltage.
2.3.6.3 Electrical life span
The electrical life span for switchgear is the number of possible switching operations under
operational conditions. After this number has been reached, the parts subject to wear must be –
wherever possible – replaced. With small devices, the entire device must be replaced.
Depending on the application, the loading and the resulting erosion of the contacts varies
widely. This is influenced by the following conditions, whereby the first mentioned play the
dominant role:
- Breaking current
- Making current
- Voltage
-
Power factor cos φ with alternating current
- Time constant
τ
with direct current
- Frequency of operation
- Malfunctions in the plant and on other devices (contact chatter)
- Ambient conditions (climate, temperature, vibrations)
Usually the electrical life span determined under test conditions is presented in diagrams as a
function of the rated operational current. These values may generally be used without hesitation
in contactor selection. In practical operation, the loads are usually lower, as the running motor
usually carries a current that is below the rated operational current. In the case of longer inching
LVSAM-WP001A-EN-P - April 2009 2-17