NO and NC do not define the entire fail-safe strategy

Define the reference condition

Check what the manufacturer means by normal and which process state causes the output to change. The controller must interpret product present and absent correctly. Some configurable devices can invert logic; retain the actual commissioned configuration rather than relying on a generic label.

Consider more than normal operation

Write a state table covering product present, product absent, startup, sensor supply loss and relevant wiring faults. For each state, define the intended PLC indication and process action. Two complementary outputs from one sensor are not automatically two independent protection channels.

A normally closed selection may help a particular circuit detect a loss of signal, but the whole input circuit must support the intended diagnostic behavior.

Test the chain

Verify the indication at the control system and the permitted final action under the plant test procedure. Record delays and latching behavior. Avoid changing output logic without coordinating the PLC program and operator documentation; an apparently harmless swap can reverse an interlock.

What to confirm for your application

• Manufacturer definition of NO and NC
• Actual configured output state
• Power-loss and relevant fault response
• Controller inversion, latching and delays
• Approved test record