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Normally Closed Contact (NC)
A Normally Closed (NC) contact is a key part of PLC Ladder Logic programming. It represents a switch or relay contact that is closed (conducting) in its default state. When the associated input condition becomes true (for example, when a button is pressed or a sensor is activated), the contact opens, breaking the circuit and stopping the signal from flowing. NC contacts are often used for emergency stop circuits, guard interlocks, and safety-related inputs where a loss of power or broken wire should turn the system off.
In simple terms, a Normally Closed contact is “on” by default and turns “off” when activated (when the input condition is true). You can use NC contacts to read inputs such as normally closed push buttons or limit switches, which are held closed until an action causes them to open.
Using a NC Contact
Like NO contacts, NC contacts are linked to specific input addresses within the PLC, such as I0.0 for the
first digital input. Each input byte can read eight individual bits (from I0.0 to I0.7), each
representing one input channel. After I0.7, the next byte begins at I1.0 for the next group
of eight inputs.
Behavior Table
The table below shows how a Normally Closed contact behaves:
| Input Condition | Contact State | Output State |
|---|---|---|
| False (Not Activated) | Closed | Signal Flows |
| True (Activated) | Open | No Signal Flows |
Normally Closed contacts are useful in fail-safe designs, because if the input device or wiring fails, the contact will open in the ladder logic — de-energising the output and putting the system into a safe state. This is why NC devices are commonly used for emergency stops and guard circuits.