Manual Reset
The principle that once a safety function has been triggered it can only be cleared by a deliberate action of the operator. In an emergency stop the stop stays active until the latched actuator is pulled or turned; the reset action does not start the motion, it only permits restarting.
The purpose of the principle is for a human being to confirm that the hazard has genuinely been removed. If reset were automatic, the machine would return to the ready state by itself when a fault temporarily disappeared and the person on site would not notice it.
A correctly built reset chain has two steps:
- The latched actuator is reset by hand; this on its own energises no output.
- A separate start command is given; only after this can the contactor coils pull in.
Combining these two steps into a single button is a common design mistake. On a radio-controlled crane what this means in practice is: after the mushroom button on the panel is reset the crane is still in the stopped position, and there is no motion until the operator gives a new command from the handheld unit. Apply the same logic after a loss of supply as well; when power returns the receiver's outputs must stay low and motion must only begin with a new command from the operator.
The context in which this term is used: Wireless Crane Remote Control Selection Guide