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For fail-safe operation, the emergency stop button is a normally closed switch, which ensures that a broken wire will not prevent it from being activated, but may accidentally activate the emergency stop. In the European Union, most types of machinery are required to be equipped with an emergency stop according to the Directive 2006/42/EC ...
This circuit shows a push button that closes a door and an obstruction detector that senses if something is in the way of the closing door. When the normally open push button contact closes and the normally closed obstruction detector is closed (no obstruction detected), electricity is able to flow to the motor which closes the door.
To make a PLC fail-safe the system does not require energization to stop the drives associated. For example, usually, an emergency stop is a normally closed contact. In the event of a power failure this would remove the power directly from the coil and also the PLC input. Hence, a fail-safe system.
1 - Master Element; 2 - Time-delay Starting or Closing Relay; 3 - Checking or Interlocking Relay, complete Sequence; 4 - Master Protective; 5 - Stopping Device, Emergency Stop Switch
A normal relay uses a wire coil and the mechanical movement of the metal contacts to switch the load on and off. The metal contacts may weld shut after repeated operation cycles. If this happens, the machine would continue running if the operator pressed the emergency stop pushbutton. This would be hazardous for the operator.
The crew attempted to turn the plane around and make an emergency landing at the airport, but crashed in a nearby rice patty and skidded for a quarter of a mile. The plane broke up into four parts ...
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