Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 168)

I
1
Fig. 6.1-4
Example of calculation of the effective value for intermittent operation of a motor.
t t t
t I t I t I
I
eff
3 2 1
3
2
3
2
2
2
1
2
1
+ +
⋅ + ⋅ + ⋅
=
t
1
Starting time at starting current I
1
t
2
Service period at operating current I
2
t
3
Interval time at current I
3
=0
t
1
+ t
2
+ t
3
Cycle time
As the operating conditions often deviate from those for determining the conventional thermal
current in the open air I
th
(see above), as a general rule of thumb it is recommended not to
operate devices at over 80 % I
th
. At 80 % current, the current-based heat losses (ohmic losses)
are reduced to around 64 %.
6.1.4.2 Thermal protective devices
In protective devices such as circuit breakers or motor protection relays with narrow adjustment
ranges, the 80 % recommendation can not always be observed as the devices must be set to
the rated current of the load to be protected and often the overlap of the current ranges is
insufficient. As far as possible, a current range should be selected that enables for a setting and
hence operation, in the low to medium range of the scale.
For bimetallic protective devices it should be noted that the heat generated in the bimetal strips
required to provide for the quality of protection is roughly the same for all current ranges of a
frame size. A 1 A bimetal relay at 1 A generates approximately the same heat as a 10 A bimetal
relay of the same size at 10 A.
6.1.4.3 Conductor cross sections
A substantial quantity of heat is removed from the devices via the connected conductors. The
larger the cross section the better is the cooling effect.
During the manufacturer’s temperature-rise tests, attention is paid to compliance with the
temperature-rise limits as of
Tab. 6.1-1, the temperature rise of internal components used within
the devices and their compatibility with the materials used. At increased ambient temperature,
for example when the devices are installed in cases or cabinets, larger cross sections of
connecting conductors are required than those used in the type tests and those corresponding
to the regular installation tables, which are based on an ambient temperature of 30 °C. In
practice, selection of a conductor that is ”one size up” in cross-section is recommended. This
also has the advantage that the heat dissipation in the switching cabinet and the energy
consumption of the installation are reduced because of the lower current density in the conduc-
tor. If necessary, two conductors can be run in parallel.
With bimetal relays and circuit breakers with bimetallic tripping mechanisms, the cross section
of the connected conductor affects the ultimate tripping current. Typically, a larger wire cross
section can, depending on the temperature compensation of the bimetal strips, lead in practice
to an increase of the ultimate tripping current by up to 5 %. From this point of view it is advanta-
geous, rather than choosing the device with the highest current range of a frame size of bimetal
relay or circuit breaker, to choose the next largest frame size.
t
1
t
2
t
3
I
2
I
3
I
t
LVSAM-WP001A-EN-P - April 2009 6-4