Why install a trip and lockout relay? What's the role

Locking refers to "self-locking", which automatically locks the state of the relay after it is energized and keeps the loop connected, even if the control current is turned off at this time, it will not change its state.
That is to say, when the power is turned on and the control current is turned off, it will continue to be in the on state unless the unlock button is pressed. Latching type relay: It can be divided into 1-coil latching type and 2-coil latching type.
(1) 1-coil latching relay: when the coil is energized, the contact will act; when the coil is de-energized, the contact will remain in the previous state. Only when a reverse voltage is applied to the coil, the contact will be reset.