Normally Closed Contact in Ladder Logic: Definition and Usage

A normally closed contact in ladder logic is a contact that allows current to flow when the controlling input is OFF or false. It behaves like a switch that is closed by default and opens when activated, representing the opposite of a normally open contact.

How It Works

Think of a normally closed contact as a door that is usually shut, letting electricity pass through. When the input controlling this contact turns ON, it opens the door, stopping the flow of electricity. This is the opposite of a normally open contact, which is like a door that is usually open and closes when activated.

In ladder logic, this means the normally closed contact is true (allows current) when the input is false, and false (blocks current) when the input is true. It is used to represent conditions where you want the circuit to be active unless a certain input is triggered.

Example

This example shows a normally closed contact controlling a coil. The coil is energized when the input is OFF, and de-energized when the input is ON.

structured_text

(* Ladder logic example in structured text style for clarity *)
VAR
  Input : BOOL := FALSE; (* Input signal *)
  Coil : BOOL := FALSE;  (* Output coil *)
END_VAR

(* Normally Closed Contact Logic *)
IF NOT Input THEN
  Coil := TRUE;  (* Coil energized when Input is OFF *)
ELSE
  Coil := FALSE; (* Coil de-energized when Input is ON *)
END_IF

Output

When Input = FALSE, Coil = TRUE
When Input = TRUE, Coil = FALSE

When to Use

Normally closed contacts are useful when you want a circuit to be active by default and stop only when a specific condition occurs. For example, safety circuits often use normally closed contacts so that if a sensor or switch fails or opens, the circuit stops immediately.

They are also used in alarm systems, emergency stops, and fail-safe designs where the default state should allow operation unless interrupted.

Key Points

• A normally closed contact is closed (true) when its input is OFF (false).
• It opens (false) when the input is ON (true).
• Used for fail-safe and default-active conditions.
• Opposite behavior of a normally open contact.

Key Takeaways

A normally closed contact allows current flow when its input is OFF.

It opens to stop current when the input turns ON.

Use it for safety and fail-safe circuits that must be active by default.

It is the opposite of a normally open contact in ladder logic.