E-Stops are used to stop machinery or processes quickly.  They usually have one or more  normally closed contacts which are opened when the E-Stop is activated.  If the normally closed contact block falls off the activator, the contacts remain closed, pressing the E-Stop will not stop the equipment.  Of course, a block that is not attached to the activator is not visible from the front of the panel.

The Guardian block from Sprecher & Schuh eliminates this problem.  The contacts in the 2 stage Guardian block are open until the block is attached to the E-Stop activator, at which point they close.  Activating the E-Stop opens the contacts.   If the contact block falls off the activator, the contacts open thus stopping the machinery.

Contact Connectric for more information.