Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 112)

Fig. 4.1-3
At intermittent operation of self-ventilating motors the simulated temperature rise of a thermal relay lags
behind the actual motor heating as the rate of cooling of a stationary motor slows down.
δ Temperature
1
Heating characteristic of motor and thermal relay
2 Cooling characteristic of the motor
3 Cooling characteristic of the thermal relay
4 Windings temperature in the motor
5 Windings temperature as simulated by the thermal relay
This variety in protective requirements has given rise in practice of technical feasibility and cost
effectiveness to a classification of protection characteristics and protective devices that has
proved itself in a large number of applications and over many decades. This standardization is
laid down in the relevant standards and offers practical and economic solutions without claiming
to be tailor-made for every individual case.
This results in the fact that different protective devices are assigned to different application
areas. Thus fuses are suitable for the thermal (and short-circuit protection) of lines but not for
the thermal protection of motors.
4.1.2.2 Protection in continuous duty and at transient loads
The primary protection requirement with respect to overload/excess temperature is the preven-
tion of long-term overloads and excess temperatures that result in accelerated ageing and in
consequence to premature degradation of the insulation.
Fig. 4.1-4
Reduction of the average life span of motor windings at overtemperatures
Current-measuring protective devices such as thermal (bimetal) or electronic motor protective
devices comply with this requirement by calibration of the tripping current for continuous duty
(ultimate tripping current). The tripping current level can be matched to the object to protect
(motor) by setting on a scale. Correspondingly, for the protection of lines, there are for example
fuses or miniature circuit breakers with defined rated currents that are adapted to the available
conductor cross sections.
LVSAM-WP001A-EN-P - April 2009 4-4