What is a critical characteristic of a normally closed contact in a PLC?

A normally closed contact in a Programmable Logic Controller (PLC) is designed to allow current to flow in its default state, meaning when it is not activated or receiving any external force. When an external force is applied—such as pressing a button—it opens the contact, interrupting the flow of current. This behavior makes normally closed contacts particularly useful for safety and control applications where an open circuit is intended to indicate an alarm or a stop condition.

The characteristic of a normally closed contact makes it essential in scenarios where a failure or interruption should be detected. When not activated, it completes the circuit, allowing for normal operations. Thus, the correct answer accurately reflects that a normally closed contact does not permit current to flow when it is activated, serving its purpose as an integral element in controlling machinery or processes within a PLC context.