A guard locking switch ensures that the access door to a hazard zone cannot be opened while the hazard is present, for instance, while a machine is moving. Generally, you must request permission to unlock and open the door. The control system may count down a delay time or wait for a signal from a position sensor or rotary encoder indicating that a hazardous movement has stopped. In most cases, the control system is additionally set up to prevent unlocking of the door even when there is a power failure (guard locking with “power to unlock”).
This brings up a hazard that is sometimes overlooked, because it is secondary: people may be locked in the hazard zone. Of course, you could always climb across the fence. But some fences are 2.5 m high or even higher, making escape quite a stunt. Plus, it is dangerous and forbidden!
Some hazard zones are also additionally roofed or covered by mesh walls meant to catch falling objects. If that is the case, an operator may be trapped inside.
How can you deal with locking-in hazards?
Guard-locking switches have what is called an “emergency release” function, allowing unlocking the switch from outside the hazard zone and thus to open the door. But that requires someone coming to the aid of the locked-in person from the outside. Screaming for help may take too long.
Therefore, many switches can be ordered with an additional “escape release”. This function allows to hit a button or push down a door lever on the inside that will release the door locking. At the same time this will trip the safety-circuit, causing the machine to stop or the hazard to end as quickly as possible.
This is similar to an emergency stop and thus the buttons or handles are colored red in most cases (green in non-interlocked systems – that is the colour for emergency escape and rescue routes). However, the installation of “escape release” is not mandatory.
