Why don't the crane use a thermal relay?
Thermal relay, overcurrent relay two control effect is different, the former for flow after delay, the latter for quick break after flow. In the aspect of security is required with overcurrent relay protection.
Because crane motors are frequently started and reloaded. Fuse or thermal relay all of them are according to the principle of the heating effect of the electric current to control the line contactor action to protect the motor, when the crane application, due to frequent a kick-off meeting for heat accumulation or misoperation. Thermal relay time delay, cause safety accidents, overcurrent relay is fast power-off protection, over-current relay so be used as the short circuit and overload protection. This has become a "hard rule" for cranes.
Security concerns, and now the crane overflows protection has been choreographed to the crane manufacturing standard The heat relay is an accumulation of crane overload to a certain extent that the power will be switched off The overcurrent protection is instantaneous action, and the manual reset switch is added to prevent the inadvertent deployment of a button
