BS EN 1037 advises that locking devices include:
The use of trapped key interlocks are many and varied. The number of keys used will depend on the machine and the number of people you are trying to protect.
If there is a separate isolation procedure (and isolation point) that allows the person accessing the area to lock off the power / energy supply then in these cases one key would suffice, as even if the gate was closed inadvertently the other isolator would ensure safety and the line couldn’t start or equipment move. If there isn’t a secondary point for isolation and if there was a “foreseeable risk” that someone might close a gate with someone inside and out of sight (doing maintenance or cleaning) then the two key system would need to be implemented as a matter of course.
This operates by having an isolation key, (Key 1) located in the operator’s panel. There is also a safety panel, which contains separate keys (Key 2) for each safety gate. The machine is isolated by removing Key 1, which is then inserted into the safety panel to release Key 2. This will allow the safety gate to be opened. Key 2 remains the responsibility of whoever has opened the safety gate and is working within the danger area on the machine. The machine cannot be started until Key 2 has been re-inserted into the safety panel, Key 1 removed and reinserted into the operator’s panel to release the isolation. Note: the system traps each key in the mechanism until the other key has been returned.
If you have a machine where there is more than one person entering the guarded area and they may be working out of site of each other you may require a number of keys. If every person working in the danger area each has their own key it means when the job is completed and the gate is closed you can not restart the machine until everyone who has taken a key has returned it to its respective location.