It is possible to supply the resistor with a long overload for a certain amount of time in function of the multiplier of the rated power. In the graph, the blue area represents the safe long overload: in this area there is no risk of damage to the component. Outside of the blue area, use of internal fuse or thermal switch (to protect the component against damages) is advised.
The optional “internal fuse” is meant to be fail-safe for the component: in case of opening there is no effect outside the housing. The internal fuse protects the resistor against an overload from 2 to 10 times the rated power. The use of resistor is limited at 250 °C. Once the fuse is open, the resistor cannot be used again.
The optional “thermal switch” prevents the opening of the resistor only when the continuous power is less than 6 times the rated power. The use of the resistor is limited at 160 °C. Once the temperature of the case cools to less than 160°C, the thermal switch contact will close. The thermal switch must be connected to the command circuit in order to exclude the resistor in case of open contact.