Safety locking device

A safety locking device is an interlocking device with guard locking. These are attached to movable guards. With the help of the safety locking device, execution of a hazardous machine function is prevented after the safety gates are opened. Thanks to the safety switch with guard locking, the safety gate is closed and locked for as long as the operator is at risk from the hazardous movement.

The requirements of interlocking devices with and without guard locking are defined in EN ISO 14119. The standard defines principles for the design and selection of interlocking devices. These are divided into four types, based on operating and actuation principles.

• Types 1 and 3 include position switches with uncoded actuators, on which additional protective measures are required for manipulation protection. Type 1 is operated mechanically and type 3 is non-contact. Inductive, for example.
• Types 2 and 4 are interlocking devices with coded actuators. A coded actuator is an actuator which is specially designed to actuate a certain position switch. Type 2 includes mechanical safety locking devices; type 4 includes RFID-based safety locking devices.

ISO 14119 also specifies that, in the case of an interlocking device with guard locking, hazardous machine functions associated with the guard cannot operate until the guard is closed and locked. The guard is only unlocked when the machine is in a safe state or has stopped completely. The safety gate can only be opened when the machine no longer poses any danger. The guard remains closed and locked until the risk due to the hazardous machine functions associated with the guard has ceased.

• Process guarding
  If the time to reach the danger zone is longer than the stopping time, process guarding is sufficient. For pure process protection, guard locking in accordance with the open-circuit current principle is sufficient, for example. Guard locking is achieved via a magnet – the magnet is deactivated again for unlocking.
  
  The operator can unlock the guard at any time. When the unlocking starts, the guard locking device immediately generates a stop command. This is called unconditional unlocking. This is an operating principle offered by the electromagnetic safety locking device PSENslock 2 from Pilz, for example.

• Safe guard locking for personnel protection
  If the access time is shorter than the stopping time, safe guard locking for personnel protection is required. The guard can only be unlocked once the hazardous machine functions have ceased. This is called conditional unlocking.

Two locking principles are used for safe guard locking:

• Closed-circuit current principle
  A spring is used to activate guard locking, while a magnet is used to open the guard locking. The mechanical safety locking device PSENmech from Pilz enables this type of safe guard locking up to PL c, with fault exclusion up to PL d (e.g. when a safety bolt is used).
   
• Bistable principle
  With this design, the position of the guard locking does not change in the event of a power failure. Instead, power must be applied in order to bring the guard locking to another state. As the position of the guard locking does not change when the power is lost, this principle ensures a high level of safety. In the event of a power failure, the last state is maintained.

The dual-channel operation of the guard locking only locks or unlocks when both channels have switched safely. It also detects faults such as short circuit, which cause the OSSD outputs (output signal switching device) to shut down, but prevents the gate being opened unintentionally, even in the event of a fault. At Pilz, this principle is implemented using the safety locking devices PSENmlock and PSENmlock mini.

<span style="font-family:"Arial",sans-serif">Closed-circuit and bistable principles guarantee that guard locking remains closed in the event of a power failure. Guard locking for personnel protection must use one of these two principles.

Safety locking devices are used wherever safeguards need to be secured. These may be accessible safeguards such as larger swing gates and sliding gates, as well as non-accessible safeguards such as covers and flaps. Safeguards are found on almost all plant and machinery, such as packaging machines or machining centres. With accessible safeguards for personnel protection, additional measures are usually necessary to unlock the safety locking device. An escape release, for example, so that people who are accidentally locked in can stop the machine and open the gate immediately.

• An overview of Pilz safety locking devices