When reading through documentation for safety monitoring devices, there are three different types of reset that may come up.
These are automatic reset, manual reset and monitored manual reset.
Automatic reset is straight forward, but what is the difference between manual and monitored manual reset? When should the different reset types be used? The safety update looks at this from the viewpoint of the new AS4024.1-2006 and gives clear answers.
Automatic reset means that when a safety device is re-instated e.g. safety gate closed, and then the safety relay/plc will automatically restart the machine (provided there are no other devices tripped or other start commands required).
A manual reset requires some type of action in order to restart the machine after the safety device has been re-instated, typically human action on N/O pushbutton. This type of reset may automatically reset if the reset pushbutton is constantly held down or shorted out i.e. it can be defeated or fail to danger.
Monitored manual reset:
A monitored manual reset requires a signal to initiate the restart the command, as a manual reset does, but the signal must be seen to change i.e. the actuator returns to its off position – the reset signal is monitored. So the system must see that the resetting device has performed correctly before the safety system will restart.
What the standard says:
AS4024.1-2006 Safety of Machinery, Part 1501: Design of safety related parts of control systems-general principles for design, 6.4 manual reset describes the requirements of a manual reset, should a reset button be deemed necessary for the particular safety function as follows:
After a stop command has been initiated by a protective device, the stop condition shall be maintained until the manual reset device is actuated and safe conditions for restarting exist
The re-establishment of the safety function by resetting the protective device 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 safety-related parts of the control system
- Shall only be achieved if all safety functions and protective devices are operative; if this is not possible the reset shall not be achieved
- Shall not initiate motion or a hazardous situation by itself
- Shall be by deliberate action
- Shall prepare the control system for accepting a separate start command and
- Shall only be accepted by actuation of the actuator from its released (off) position
The category of safety-related parts providing the manual reset shall be selected so that the inclusion of the manual reset does not diminish the safety required of the relevant safety function
The reset actuator shall be situated outside the danger zone and in a safe position from which there is a good visibility for checking that no person is within the danger zone
These translate as follows:
- The safety relay/plc cannot be reset until the protective device e.g. safety gate, light barrier etc. is closed/unobstructed and a reset pushbutton is operated. The reset pushbutton must be a monitored manual reset (can’t be overridden)
- Operating the reset will not restart the machine – a separate start command must be provided e.g. start button, plc etc
- The reset button must not be able to be activated from inside the danger zone
- If the hazardous area has sections that cannot be viewed from outside the area, where an operator/maintenance person may be positioned, other measures and precautions must be taken e.g. sequential, search mode reset
Using automatic or manual reset:
So when should you use Automatic or Manual reset?
AS4024.1-2006 Safety of Machinery, Part 1501: Design of safety related parts of control systems-general principles for design, 6.5 start and restart states that:
A restart shall take place automatically only if a hazardous situation cannot exist
Generally, automatic reset is not allowed if a guard or barrier has full body access, the system should not automatically restart if someone walks through a light barrier into the hazardous area.
If a guard only allows a hand to enter the hazardous area, and the guard cannot be closed while an operator’s hand is in the area, then automatic reset may be used, for example, a lathe with a partial access opening for tool change.
More details are available with Pilz Australia .