From users to standards writers, many definitions based on different understandings are given to the reset function. For users, it’s the action of resetting the control system to a run mode, usually after an emergency stop condition. Machine builders and component suppliers often talk about resetting the stop function to allow another mode like run, set-up, etc. Sometimes they talk about the push and twist action of resetting their e-stopping device. Control system suppliers focus on resetting the control system to production mode whether the “reset” of a stop/e-stop was caused by a device or the machine control logic. Finally, standard writers prefer to describe the required steps involved in re-energizing the control system to run production following stopping conditions.
Simply stated, reset in machine safety is the action that consists of testing safety inputs state before allowing the machine to restart. Difference should be made between emergency stop device reset, and the normal restart of the machine, that is a separate and deliberate action from resetting the emergency stop device, thus, it is not about the machine standard functions inputs, but the safety control system inputs (ex emergency stop device, safety switch, etc.).
When the reset button is actuated, the safety control system checks whether all its safe inputs are in 1 state.
- If they are all in 1 state, the safety control system unlocks the safety outputs, the machine can restart.
- If one of the inputs is in 0 state, the safety outputs remain locked and will remain so until all the inputs are first passed to 0 and then all to 1.
Resetting is mandatory. It can be manual or automatic. However, automatic reset involves a voluntary action by an operator to restart the machine.
According to the ISO 13849 : 2015-1 standard, after a stop command has been initiated by a safeguard, the stop condition shall be maintained until safe conditions for restarting exist. The re-establishment of the safety function by resetting of the safeguard cancels the stop command. If indicated by the risk assessment, this cancellation of the stop command shall be confirmed by a manual, separate and deliberate action (manual reset). The manual reset function shall:
- Be provided through a separate and manually operated device within the SRP/CS,
- Only be achieved if all safety functions and safeguards are operative,
- Not initiate motion or a hazardous situation by itself,
- Be by deliberate action,
- Enable the control system for accepting a separate start command,
- Only be accepted by disengaging the actuator from its energized (on) position.
The performance level of safety-related parts providing the manual reset function shall be selected so that the inclusion of the manual reset function does not diminish the safety required of the relevant safety function.
Colors and shape
According to the IEC 60204-1 : 2005, 10.2.1 reset push-buttons shall be BLUE, WHITE, GREY, or BLACK. Where they also act as a STOP/OFF button, the colours WHITE, GREY, or BLACK are preferred with the main preference being for BLACK (according to the NFPA 79-2012, 10.2.2.7. the red color should be used in this case) GREEN shall not be used.
The reset button positioning to have a good visibility, is one of the most important factors to be considered (ex, multiple e-stop and resets, using HMI for reset etc.)
According to ISO 13849 standard, the reset actuator shall be situated outside the danger zone and in a safe position from which there is good visibility for checking that no person is within the danger zone.Where the visibility of the danger zone is not complete, a special reset procedure is required.
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Intervention Prévention concentrates its operations in the field of work safety, offering specialized services following Standards CSA Z462 – Workplace Electrical Safety, CSA Z460 – Control of Hazardous Energy: Lock-out and Other Methods, and CSA Z432 – Safeguarding of Machinery.
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