The fuse and the use of thermal relay in electric circuit

In motor control wiring, the fuse are installed in the main circuit, why add thermal relay? They are all have what effect, can replace each other? In the electric heating and lighting, why only with the fuse without thermal relay installed? A: fuse in main electrical wiring short circuit protection and overload protection seriously, and the thermal relay is mainly used for overload protection. Both can not substitute each other, but can complement each other. If made the motor overload protection fuse, fuse fusing in the boot process, in order to prevent the motor rated current of the fuse melts in general should take the rated current of the motor's 2. 5 ~ 3 times, so that even if the electric machine long time overload 50%, fuse will not fuse, and the motor may burn out due to overload for a long time. So can't over load protection fuse. The heating effect of the current and thermal relay is used to work, because of the influence of the thermal inertia, although when short-circuit current is large, it is impossible to make thermal relay action immediately, thus to extend the time of the influence of fault, will cause harm to power supply system and power equipment. So can't use thermal relay as short circuit protection. For heating and lighting equipment, owing to the nature of the load on the drag load of the motor, generally speaking, they won't load occurred, therefore, is generally not equipped with a thermal relay, and equipped only with fuse, typically a short circuit protection.