“Locked in a cage together with a lion”

When installing guard fencing, machine manufacturers usually want to prevent people from entering a hazard zone. But when you look at the fence from the inside, it becomes a “cage” holding back a hazard. The cage may become completely closed when a door with guard locking is added and even if the intention is not to be in the area, a person may be locked in the cage together with the hazard. It’s like being “locked in a cage together with a lion”. 

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.  

When should you consider installing “escape release”? 

Automatic warehouses 

The answer is quite clear when it comes to automatic warehouses with rail dependent storage and retrieval equipment (S/R machines).  

The C-type standard for these systems (EN 528) requires that doors in the perimeter guarding (fencing) must open outwards and can always be opened from the inside without a key. From the outside, opening is allowed with a key only (section 4.10.4.2).  Axelent’s X-It lock and the quite new X-It Electric are both available with escape release and are thus generally suitable for such applications. However, an additional proximity switch will be needed to communicate the status of the door to the control system’s safety circuit. 

Manufacturing 

If you run any other sort of manufacturing system behind a fence, you should install guard locking switches with escape release if all or several of the following conditions are true:  

• The area behind the door allows presence of persons. 
• There are hazards in that area. 
• A person inside the cage can accidentally hide from the view of others, so that he/she may be locked in unintentionally (for instance, a person may be out of sight of an operator standing outside the door or at a control panel because part of the machinery or a control cabinet hide him/her). 
• The interlocking and guard locking device on the door does not have a provision that prevents unintentional closing and activation of the guard lock (with current switch designs this will hardly be the case. Most switches allow attaching a padlock to prevent the door from closing. But sadly, few people do so).