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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.
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.
An NC contact would be shown as normally closed, and an NO contact would appear as a normally open device. All contacts associated with a device will change state when the device is energized. Figure 1 shows a typical relay logic diagram. In this circuit, a STOP/START station is used to control two pilot lights. When the START button is pressed ...
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.
Time-delay relay contacts must be specified not only as either normally open or normally closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts. First, we have the normally open, timed-closed (NOTC) contact.